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Solar integration in the UK and India: technical barriers and future directions
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Batzelis, Efstratios, Zakir Hussain Rather, John Barton, Bonu Ramesh Naidu, Billy Wu, Firdous Ul Nazir, ONYEMA SUNDAY NDUKA, et al.. 2021. “Solar Integration in the UK and India: Technical Barriers and Future Directions”. Loughborough University. https://doi.org/10.17028/rd.lboro.14453133.v1. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS Report of the UK-India Joint Virtual Clean Energy Centre (JVCEC)
A collaboration of the Joint UK-India Clean Energy Centre (JUICE), the India-UK Centre for Education and Research in Clean Energy (IUCERCE) and the UK India Clean Energy Research Institute (UKICERI) AUTHORS GLOSSARY
Authors: ADN Active Distribution Network Efstratios Batzelis, Zakir Hussain Rather, John Barton, Bonu Ramesh Naidu, Billy Wu, Firdous Ul Nazir, AID Anti-islanding detection Onyema Sunday Nduka, Wei He, Jerome Nsengiyaremye, AS Ancillary Services Bandopant Pawar, Diane Palmer CERC Central Electricity Regulatory Commission, Contributors: India Bikash Pal, Murray Thomson, Chandan Chakraborty, Prabodh Bajpai, Saikat Chakrabarti, Mark Dooner, DER Distributed Energy Resource Marcus King, Jihong Wang DG Distributed Generation
Any inquiries should be directed to the first author: DNO Distribution Network Operator Dr Efstratios Batzelis ([email protected]) DR Demand Response
EVs Electric Vehicles
FRT Fault Ride Through ACKNOWLEDGEMENTS IBR Inverter Based Resources LCOE Levelized Cost Of Electricity
This report builds on research carried out within the LoM Loss of mains UK-India Joint Virtual Clean Energy Centre (JVCEC), within which, the UK partners have received funding LVRT Low Voltage Ride Through from the Engineering & Physical Sciences Research NWP Numerical Weather Prediction Council (EPSRC) for a project entitled “Joint UK-India (for solar radiation forecasting) Clean Energy Centre (JUICE)” with grant reference EP/ P003605/1, and the India partners have received funding PLL Phase Locked Loop from the Government of India’s, Department of Science and Technology (DST) for projects entitled “India-UK PV Photovoltaic Centre for Education and Research in Clean Energy Renewable Energy Sources (IUCERCE)” and “UK India Clean Energy Research Institute RES (UKICERI)”. Two authors also received grant funding from RoCoF Rate of Change of Frequency the Royal Academy of Engineering under the Engineering for Development Research Fellowship schemes SCL/SCR Short Circuit Level/Ratio (RF\201819\18\86, RF\201819\18\89). The views expressed in this report are those of the authors, and do not SOC State of Charge (for batteries) necessarily reflect the views of all JVCEC collaborators or TSO Transmission System Operator of the funding bodies. V2G Vehicle to Grid To follow the consortium’s work and publications, please visit http://www.juice-centre.org.uk/, http://www.ukiceri.com/, http://www.iucerce.iitb.ac.in/.
Published: April 2021 PARTNER LOGOS
SPONSOR LOGOS 02 | THE SOLAR PICTURE TODAY
EXECUTIVE SUMMARY
In the past few years, the world However, global experience shows and Research in Clean Energy has been mobilised in the fight that solar installations start to (IUCERCE) and the UK India Clean against climate change, with level off in countries with certain Energy Research Institute (UKICERI) more and more countries making integration levels. The UK saw rapid – a total of ten UK universities and ambitious commitments for a net- solar uptake during the first half thirteen Indian institutes. Active since zero emission energy sector. Even of the previous decade which has 2016, the consortium aims to tackle during the COVID-19 era, major now stalled. India has experienced key challenges in the development economies envision a recovery that lately a rapid growth in solar and deployment of clean energy has climate actions and energy photovoltaic deployment with 37.5 technologies in India and the UK, transformation at its core. In this GW installed capacity by the end with a focus on integrating solar, context, the solar photovoltaic of 2020, but it has a long way to energy storage and network technology has experienced go till the 100 GW solar target of technologies. The JVCEC consortium remarkable boom in the past 2022. The rate of solar deployments is jointly funded by UK Newton decade, in part due to falling solar is linked to a variety of reasons Fund (via EPSRC) and Department photovoltaic (PV) technology such as financing, regulations and of Science and Technology (DST), prices, being today among the market mechanisms, but also a set Government of India to a total of cheapest generation technologies of technical limitations arising from around £10 M over five years. worldwide. the nature of solar generation. The aim of this report is to take a deep The consortium has promoted look on the technical challenges collaborative research and that act as barriers to further solar supported researcher visits from integration and develop a roadmap India to the UK and vice versa. with technology innovations and These visits have enabled exchange transformations that will allow of research ideas and initiated a high-solar future. The UK and research investigations that have India are the focal points of this led to many co-authored journal investigation, examining closely how papers and specialist conference the challenges and future directions presentations. The aim of the relate or differ between the two report in-hand is to build on that countries. complementary expertise of the consortium to present the topic in This report arises out of the UK- a more wholistic manner, bringing India Joint Virtual Clean Energy together established knowledge Centre (JVCEC) which is a research with more novel discussion of issues consortium comprising the Joint UK- arising in the progress of both India India Clean Energy Centre (JUICE), and the UK towards a solar-powered the India-UK Centre for Education low-carbon future. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 03
CONTENTS
Executive Summary 2 • A.8.2. Technical barriers to integration of batteries 35 A.9. Other barriers to PV technology 36 Contents 3 • A.9.1. High Temperatures 36 The Solar Picture Today 4 • A.9.2. Dust and soiling 36 Solar status in the UK 5 • A.9.3. Opportunity costs of land use 36 Solar status in India 5 Part B: Roadmap to High Solar Integration 38 International experience and lessons learnt 6 B.1. Rethinking the power system operation 39 • Solar PV overgeneration in California 6 • B.1.1. More flexibility and improved generation • Loss of 1200 MW of solar in Southern California 6 dispatch 39 • Experience from roof-top solar in India 7 • B.1.2. Better solar forecasting 40 Part A: Technical Barriers to Solar Integration 8 • B.1.3. Transmission/Distribution systems A.1. Balancing & flexibility challenges 9 coordination 41 • A.1.1. Variability of solar generation 9 • B.1.4. Improved voltage control in the distribution • A.1.2. Uncertainty of solar generation 10 network 42 • A.1.3. Solar resource data 12 • B.1.5. Rethink the protection of distribution A.2. Power system stability challenges 13 networks 43 • A.2.1. Common instabilities caused by solar 13 • B.1.6. Improve the power quality in distribution • A.2.2. Falling system inertia 14 networks 44 • A.2.3. Diminishing short circuit level 16 • B.1.7. Continuous evolution of grid codes and • A.2.4. The power cut of August 2019 in the UK 17 ancillary service products 44 A.3. Voltage challenges 18 B.2. Demand response 45 • A.3.1. Voltage Rise 18 • B.2.1. Examples of flexible electrical loads 46 • A.3.2. Reverse Power Flows 19 • B.2.2. Incentives and market mechanisms 48 • A.3.3. Voltage Flicker 20 • B.2.3. Future Challenges and Strategy 49 • A.3.4. Distribution and Transmission System B.3. Energy storage 50 Interaction 20 • B.3.1. Future prospects 50 A.4. Protection challenges 20 • B.3.2. Location of energy storage 51 • A.4.1. Protection in the transmission system 20 • B.3.3. Electric vehicles as storage devices 54 • A.4.2. Protection in the distribution system 21 B.4. More grid-friendly solar PV systems 55 • A.4.3. Fault Ride Through 22 • B.4.1. Ancillary services by solar 55 A.5. Anti-islanding protection 24 • B.4.2. Fault-ride-through and Anti-islanding A.6. Power quality issues 25 by solar 56 • A.6.1. Harmonics 26 • B.4.3. Grid-forming control in PV systems 58 • A.6.2. Voltage unbalance 26 • B.4.4. Curtailments and power reserves in A.7. Grid codes and ancillary services 28 PV systems 60 • A.7.1. The evolution of grid codes 28 Part C: Conclusions 64 • A.7.2. The ancillary services in India 30 • A.7.3. Limited services potential by PV systems 30 References 68 A.8. Energy storage as enabler for high solar levels 31 Appendix A: Fault Ride Through (FRT) Operation 78 • A.8.1. Electrochemical energy storage technologies 32 Appendix B: Islanding 81 04 | THE SOLAR PICTURE TODAY
THE SOLAR PICTURE TODAY
Renewable energy is being rapidly (LCOE) lower than the cheapest to 2019), which includes around integrated to power systems across fossil fuel-powered generation 37.5 GW in India (Dec 2020) and the world, with solar photovoltaics (IRENA, 2020b). In 2021, IRENA 13.5 GW in the UK (Jan 2021), the (PV) and wind power being the forecasts that utility solar will be world is expected to exceed 1.2 front runners. Solar PV in particular the cheapest source of electricity TW solar capacity by 2022 (Solar has experienced remarkable among all renewable energy Power Europe, 2020). The rapidly deployment worldwide in the sources (RES) and will have a LCOE reducing PV technology prices last decade, adding in 2019 more less than the marginal costs of even and the commitments towards capacity than all fossil fuel and existing fossil fuel plants, i.e. less energy decarbonisation in the nuclear plant additions combined than just the fuel and service costs global scene have led to ambitious and more capacity than all other ignoring the initial capital costs. In commitments for very high levels renewable technologies combined addition, in contrast to other RES, the of solar integration into the power (Solar Power Europe, 2020). This solar PV system is probably the most systems of many countries. noteworthy increase in solar modular grid-connected resource, Furthermore, massive investment deployment is in part due to the ranging from small few-kW roof- in solar and other RES is seen today falling price of PV technology that top installations to large multi-MW as a promising way for sustainable recently reached the landmark of open-field farms, connected to both development in the post-COVID19 1 US cents per kWh in some parts the transmission and distribution era, for reasons such as cheaper of the world (Solar Power Europe, network. electricity, lucrative investment 2020). According to the International and easy financing, as well as Renewable Energy Agency (IRENA), With more than 630 GW of global creating many job opportunities 40% of utility solar installed in 2019 cumulative solar PV installed to mitigate the jobs lost due to the had a levelized cost of electricity capacity (counting from 2000 COVID-19 pandemic. Notable such examples are EU’s ‘Next Generation EU’ programme (EU Commission, 2020), as well as Malaysia’s and Switzerland’s economy stimulus packages (Solar Power Europe, 700 2020).
600 However, the global solar growth 500 is now guided mainly by Asiatic countries as shown in Figure 1, while
GW GW 400 the European installations that used to lead have now slowed down. 300 Besides all the benefits that solar
200 PV brings, massive solar integration introduces several challenges in 100 planning and operation of the power system, which may work as 0 barriers to further solar deployment. 2011 2017 2012 2013 2001 2010 2015 2014 2019 2018 2016 2007 2002 2003 2000 2005 2004 2009 2008 2006 The solar status in the UK and India is briefly described below, followed by lessons learnt and international Europe Amer APAC China MEA experience on solar integration.
Figure 1. Global total solar PV installed capacity (Solar Power Europe, 2020). SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 05
SOLAR STATUS IN THE UK 30 The UK now has a solar capacity of more than 13.5 GW (Jan 2021) spread 25 across over 1 million installations (BEIS, 2021). That is a significant 20 part of the 78 GW total generation capacity (17% solar integration) Output GW 15 (BEIS, 2020). It is worth noting that 10 about half of this capacity comes from large PV plants (>5 MW), 5 even though 93% of the number of installations are of small-scale 0 (<4 kW) (BEIS, 2021). The share of 00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00 solar energy has been steadily increasing, recently achieving a Coal Gas Nuclear Offshore wind Onshore wind Hydro Biomass Solar Other record of 30% of the total electricity produced instantly during the prolonged coal-free periods in Daily profile of the electricity generation mix in Great Britain on 6 May Britain (Apr 2020) (Grundy, 2020). Figure 2. 2020 (http://electricityinfo.org/fuel-mix-archive/#data) The electricity mix of 6 May 2020 in Britain is depicted in Figure 2, which is indicative of the sunny spring periods: solar undertakes almost 1/3 of the demand at midday, along Presently (Dec 2020) India has with nuclear, gas and biomass for SOLAR STATUS IN INDIA installed about 37.5 GW of solar (10% the rest; interestingly there is no coal India is a growing economy with integration), which includes around online at all. a total generation capacity of 375 6 GW of rooftop installations (Bridge GW (Dec 2020) (Central Electricity to India, 2020). The generated However, solar deployment has Authority, 2020). Among the first electricity from solar was about 13.5 stagnated lately, mainly due to states in India to set solar targets TWh in 2016-17, 25.8 TWh in 2017-18, closure of the Feed-in-Tariff scheme was Gujarat with a rooftop solar 39.2 TWh in 2018-19 and 50.1 TWh in in March 2019, and concerns are programme back in 2009 and 2019-20 (Central Electricity Authority, raised regarding the actions needed Gandhinagar aiming to be a solar 2020), indicating a steady growth in to allow for further solar integration city. However, the big step towards solar PV integration in the country. in view of the recent commitments solar integration in India was made Although these numbers come for net zero UK carbon emissions by by the government in 2010 with the from large PV plants, rooftop PV 2050. Notably, National Grid UK– the Jawaharlal Nehru National Solar systems are also expected to grow UK transmission system operator Mission programme (IEA, 2020). The substantially in the coming years (TSO) – estimates that solar has initial target was 20 GW of solar due to improved financing and the potential to be one of the most installations by 2022, which was falling PV technology prices. significant generation technologies then revised in 2015 after the COP21 It is worth mentioning that India is in the UK in the future, reaching up to Paris agreement to a much more home to two of the largest solar 66 GW by 2050, but only if coupled ambitious target of 100 GW solar PV plants in the world, Bhadla solar with storage and other changes power by 2022, including 60 GW of power park of 2245 MW capacity in the power system for increased utility scale plants and 40 GW of in Rajasthan, and Pavagada flexibility (National Grid UK, 2018a). rooftop solar systems (NITI, 2015). Solar Park of 2050 MW capacity in Karnataka state. Bhadla solar park 06 | THE SOLAR PICTURE TODAY
located in Bhadla village, Jodhpur INTERNATIONAL curtailments and dictating more district, Rajashtan is spread across power system flexibility to cope with 2 EXPERIENCE AND more than 56.6 km of land, and LESSONS LEARNT the high upward ramps towards the the park has been developed by sunset. CAISO reported curtailment multiple entities (Sanjay, 2019). Solar PV overgeneration in of 187,000 MWh of solar and wind The Pavagada Solar Park located California generation in 2015, which rose to in Karnataka’s Tumakuru district California Independent System over 308,000 MWh in 2016 (CAISO, has been jointly developed by the Operator (CAISO) has been 2017). Several mitigation measures Solar Energy Corporation of India experiencing high PV penetration in have been explored, such as (SECI) and Karnataka Renewable their system for about a decade. The effective participation of energy Energy (KREDL); recently completed well-known ‘duck chart’ published by storage, enhancing demand- in January 2020, it spreads across CAISO in 2013 (Denholm P. et al., 2015) response, time-of-use rates, flexible a staggering 53 km2 of drought-hit showed how overgeneration from electric vehicles (EVs) charging land (Ranjan, 2020). Karnataka state PV systems can lead to curtailments and exploring policies to reduce is the solar leader in India, having due to technical constraints that minimum operating levels for integrated 7.34 GW of solar and significantly reduce the benefits existing generators. generating more than 60% of its from high solar deployment. Figure electricity from renewable energy 3 illustrates the net load (actual LOSS OF 1200 MW OF sources in August 2020 (SRLDC, load minus solar power generation) SOLAR IN SOUTHERN 2020). which features a “belly” during the CALIFORNIA midday when the solar PV systems operate at full capacity. This curve On 16 August 2016, the blue cut fire becomes steeper over the years in Interstate 15, California resulted with increasing PV penetration in thirteen 500 kV line faults and levels, posing a risk for power two 287 kV line faults in a single day leading to outage of generation, including tripping of a prominent 1200 MW of solar at 11:45 AM Pacific time (NERC, 2017). The investigation Net load – March 31 performed by the North American 28,000 Electric Reliability Corporation 26,000 (NERC) revealed that some PV
24,000 inverters tripped immediately after reading erroneous grid frequency 22,000 measurements that were caused by 20,000 2012 (actual) transients following the faults rather
18,000 2013 (actual) than actual frequency fluctuation. ramp need Megawatts 2014 Additionally, the majority of the 16,000 ~13,000 MW 2015 in three hours inverters installed at that time 2016 14,000 2018 2017 were configured to immediately 2019 cease operation when the terminal 12,000 2020 overgeneration risk voltage was out of limits (0.9-1.1 per 10,000 unit), which led to massive tripping 0 during the fault-induced voltage dip 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00 with delayed recovery afterwards. This instantaneous tripping of inverters has been identified as Figure 3. The CAISO duck chart (Denholm P. et al., 2015) an undesirable function and the recent guidelines advise either SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 07
against it if technically possible, or require momentary cessation – immediately restoring output after the disturbance.
EXPERIENCE FROM ROOF- TOP SOLAR IN INDIA
India has set a target of 40 GW of rooftop solar installations by 2022 (NITI, 2015). A study on large-scale rooftop PV integration at the Indian distribution system concluded that the network at the rural areas with weak distribution feeders is likely to experience overvoltages at solar penetration levels of more than 40% (Gaebler, 2017). On the other hand, the investigation on urban distribution systems, considering feeders in the cities of Delhi and Bhopal, indicated higher solar hosting capacity due to feeders with shorter length. These issues can be mitigated by reactive power absorption by PV inverters, but not in highly loaded feeders that do not have the available margin for additional reactive power flow. Other mitigation measures include active power management strategies, such as limiting the PV generation to 70-75% of the inverter capacity or peak shaving using batteries, which result in improved voltage control and avoid feeder overloading. 08 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
The increasing amount of Apart from the relevant balancing Finally, embedding solar generation renewables, such as solar and limitation, this also contributes to locally into the distribution network wind, in the power system has the falling system inertia which is an and closer to the load provides recently started to cause concerns emerging source of stability issues in many benefits energy-wise, but it about the reliable operation of the the power system. also challenges the way voltage electricity network. Many countries control and protection schemes are in Europe and North America that Grid codes and regulations are designed and operate, requiring pioneered in RES integration in the often slow in embracing new great efforts to accommodate more past few decades, were also the concepts and ideas, and that may localized solar. In addition, small first to see signs of the limitations impede the transition of the power roof-top solar systems are often and challenges brought by these system. As solar PV systems have connected to a single-phase, which unconventional power sources. limited inherent ability to provide is a source of voltage and current The aim of this chapter is to take a ancillary services, the grid codes unbalance in the network. deep and holistic look on the major need to strike a delicate balance technical barriers faced when between grid support requirements It is worth noting that many of deploying high levels of solar PV and investment viability and these challenges can be alleviated into the grid, drawn from the UK- attractiveness. or completely addressed when India expertise of the consortium solar PV systems are physically and other international experience. Interfacing the grid via power or virtually coupled with energy The primary objective is to identify electronic converters (inverters) storage, especially the balancing, the roots of these challenges and allows for much faster response stability and voltage barriers. For this main differences between the UK compared to electrical machines, reason, this document also includes and India, to provide complete but this comes with very limited a section dedicated to the key understanding of the subject matter overcurrent capacity. This means technology of energy storage and and draw a roadmap to high solar than an inverter-based resource its own challenges, as an enabler to integration. (IBR) can respond very quickly to high solar integration levels. a power imbalance in the network Figure 4 provides a graphical and safeguard the frequency from These and many more challenges illustration of the most important fluctuating too much, but only within are discussed in the following technical barriers and how they the limited capability of the inverter. pages, presenting wherever possible relate to technology limitations and Not being able to inject the much- examples and evidence from case- power system challenges. Probably needed high currents during faults studies in the UK and India. one of the most restrictive and well- contributes to a gradual lowering known limitation of solar technology of the grid strength (or short circuit is the variable and uncertain nature level), which is considered a big of solar generation, which makes challenge for the stability and balancing supply and demand protection of the network. Synthetic in the system more challenging, inertia emulation by PV grid- but also poses risks for instability following inverters remains inferior and voltage out of bounds in the to a rotating machine’s mechanical network. But this is only a small part inertia, as it relies on actual inertia of the story. from other sources to function and is limited by the inverter’s capacity. Another major limitation of the solar Power electronic converters also technology is not having inherent inject harmonic distortion into the energy storage capacity to allow grid that may cause power quality adjustment and time-shifting of issues at high penetration. the plant’s output when needed. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 09
this mismatch has to be met by
Limitations Challenges Barriers other sources, rendering the tasks Solar Balancing of scheduling and dispatch more & flexibility generation challenging. In fact, National Grid variability & challenges uncertainty UK estimates that balancing wind Stability from Scotland, solar from England No energy issues storage in and EVs in London will become quite PV systems Falling inertia complex and challenging in the future (National Grid UK, 2018a). Grid Voltage Limited ancillary regulations issues services by solar Although there is no solar output
Power for several hours a day, there is electronics Diminishing Protection temporal proximity between the interface challenges short circuit level solar peak around noon and the Localised peak demand usually found in the solar Power generation quality afternoon towards evening hours. issues To investigate this correlation, National Grid UK has forecasted the summer net power demand seen at Figure 4. Technical barriers to solar integration, illustrating how technology the transmission level, affected by limitations relate to different challenges and barriers. the embedded solar generation at the distribution network, for various solar integration scenarios in the shadow), which in turn requires near future (Figure 5). For more A.1. BALANCING & information on the four scenarios, FLEXIBILITY CHALLENGES higher flexibility from the power system. These are among the main please see (National Grid UK, 2016). One of the most restrictive barriers reasons why system operators Evidently, the net minimum demand towards high solar integration is maintain a limited portion of is shifted from the early morning the variable and uncertain nature solar in the generation mix and hours in 2016 to the midday in 2025 of solar generation that poses a occasionally curtail solar generation due to the solar contribution; the series of balancing and flexibility at high penetration levels. The morning pick-up has also vanished challenges to the power system. higher the solar integration, the in most scenarios in 2025 for the Balancing is matching supply and more the curtailments, which raises same reasons, but there is now demand in the grid in a scheduled the levelized cost of electricity much higher load ramping in the manner, and flexibility is the ability and essentially puts a limit on afternoon to evening hours due to of the power system to resolve the amount of solar that can be retraction of solar (National Grid UK, any mismatches that happen in accommodated into the power 2016). These observations are quite real-time. Since solar generation is system. similar to the duck chart and the variable and non-dispatchable, it solar overgeneration phenomenon varies during the day and is often A.1.1. Variability of solar generation in California discussed in the unavailable when needed (e.g. no The power output of solar PV previous section. power at night), which makes it an systems follows the solar radiation ineffective balancing source. At that greatly varies during the day, This variability renders displacement the same time, it is also uncertain with no generation at night, small of conventional dispatchable and stochastic, meaning that the values at morning and evening generation by solar more actual solar generation delivered hours, and peak output at midday. challenging. The power system may substantially differ from the Since the electricity demand does has to keep online very flexible planned value (e.g. clouds casting not quite follow the same trend, units, along with solar, with low 10 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
technical minima and high ramping capabilities as discussed below, 2016/2017 2020/2021 2025/2026 but also perform very accurate 60 60 60 forecasting of solar generation 50 50 50 towards credible scheduling. Although solar forecasting has 40 40 40 seen lately much improvement 30 30 30 through technologies like artificial 20 20 20 intelligence and machine learning among others (Su, Batzelis and 10 10 10 Transmission demand (GW) Pal, 2019), there is still room for 0 0 0 improvement depending on the 00:00 12:00 00:00 00:00 12:00 00:00 00:00 12:00 00:00 forecast horizon and time resolution; forecast errors entail balancing Gone green Slow progression No progression Consumer power mechanisms and more flexibility online to resolve any supply- demand deviations. Figure 5. Electricity demand at the transmission level in Britain on a summer day with low demand for four scenarios of solar integration (National Grid UK, 2016). Another critical aspect when accommodating variable power sources in the system is scheduling. Historically, most countries used to A.1.2. Uncertainty of solar solar is generally lower than onshore schedule at 1-hour intervals in the generation wind, but it still necessitates more past, but now there is a need for Solar generation is also uncertain, flexibility in the system and sets a more frequent dispatch to account meaning that the actual solar limit to the permissible amount of for more up-to-date system output may differ a lot from the solar online, which in turn entails information and for more accurate forecasted value, and stochastic, solar curtailments (Denholm, Clark forecasts. In Britain the market meaning that it varies from minute and O’Connell, 2016). operates at 30-min time slots to minute in a non-deterministic (National Grid UK, 2016), whereas in manner. The primary reason More flexibility in the system the US many regions have moved for the solar stochasticity is the means, among other things, that to 5-min scheduling intervals volatile weather, for which the there are other sources online to (Denholm, Clark and O’Connell, UK is renowned, with variations accommodate any solar mismatch; 2016). To accommodate higher in incident solar irradiance due thermal and hydro plants have levels of solar and other variable to moving clouds and in the PV been traditionally used as sources generation, minute-level scheduling modules temperature because of flexibility in the past. A crucial is highly recommended. of changes in the weather. This limitation, however, is the technical uncertainty not only may cause a minimum level of such sources (i.e. mismatch between expected and the lowest permissible output while delivered generation that has to be online), which can be as high as met by other sources, but it is also 55% for many conventional plants in often coupled with intermittency India and the UK. For example, if the (i.e. fast power fluctuation that can operational range of such a source sometimes exceed 20% per second is 55%-100% and the scheduled level (Batzelis et al., 2019)) that requires is 70% to have both upregulation very high ramping capabilities from (headroom) and downregulation the balancing sources. The level of (footroom) capability, there is stochasticity and intermittency in limited flexibility of -15%/+30% SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 11
to support a supply/demand mismatch in the system (National Grid UK, 2016). This limitation comes into the picture mainly at the low-demand/high-solar hours at midday of the summer weekend when the net load to be undertaken by conventional plants is low (see 2025 plot in Figure 5), keeping online only a few of such units and thus having reduced flexibility.
In addition, keeping some power -1,000 -800 -600 -400 -200 0 200 400 in reserve is not sufficient if that Mean ramp rate (MW/minute) power cannot be deployed quickly enough to compensate for the rate August 2020 January 2021 of change of supply and demand. The speed by which a power plant can up- or down- regulate its Figure 6. Distribution of forecasted residual ramp rate capability in 2020-2021 in output is known as ramp rate or Britain for the “Gone Green” scenario (National Grid UK, 2016). simply ramping. Figure 6 illustrates a histogram of the residual ramp rate capability in Britain (i.e. the maximum up- or down- ramping This is among the reasons for the the power system can support gradual phasing out of coal in the as a whole) for a scenario with UK electricity mix and the recent more solar than today. Apparently, record of several-week coal-free both downward and upward durations. Nuclear plants are also ramping capability is much lower considered as inflexible since their in the summer than in the winter, output must change very slowly because of the lower demand at for safety reasons. On the other the transmission level that entails hand, gas-fired units are typically fewer dispatchable generation much more flexible with short units online. This happens frequently start-up times and high ramping overnight and during the sunny rates, and are expected to act as summer days when the high replacement for coal generation embedded solar generation and provide some baseload power effectively suppresses the local by 2050 in the UK (National Grid UK, load, resulting in low demand in the 2018a). Other sources of flexibility transmission level (National Grid UK, that have yet to be fully explored 2016). are demand response and energy storage. Increasing the flexibility Coal plants have been generally in the system, leveraging new designed to act as baseload units technologies and extending the and not to deliver flexibility, which requirement to small generators, is why they feature relatively high will be crucial to allow for more technical minimum levels and low intermittent generation like solar into ramping capabilities. These plants the grid. are seen as very expensive to retrofit and convert to more flexible sources. 12 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
A.1.3. Solar resource data of output predictions. Higher missing values and require regular Despite its famous rain, the UK is frequency data (i.e. hourly rather cleaning. eighth in the world in PV deployment than daily or monthly) gives a better after China, Japan, the US, Germany, indication of weather variations Most satellite-based solar radiation India, Italy and Australia (IRENA, and improves accuracy of energy models were developed using 2020a). India has great solar modelling. data relating to Europe and North potential, rendering solar energy America (Purohit and Purohit, an excellent solution to the nation’s There are certain similarities 2015). Data based on long-term energy needs. After the revision of between the UK and India on the Typical Meteorological Year (TMY) its solar targets in 2015, India has availability of the solar resource. smooths out anomalies but hides quickly increased its ranking being Solar radiation is intermittent in the trends in short-term and long-term now the fifth largest PV deployer in two countries, while both experience variation. These TMY datasets were the world (IRENA, 2020a). peaking of solar generation at not developed for photovoltaic midday when most people are applications, thus functioning poorly A critical component to further solar working away from home and in this context and are only used in deployment is accurate knowledge the demand is low. The UK has a pre-feasibility studies. TMY data is of the available solar resource, temperate maritime climate; it is available for both countries from which is essential during both the an island, situated between the Meteonorm, PVGIS, NASA’s Surface development and operation phases continent of Europe and the Atlantic Meteorology and Solar Energy data of PV projects. It is necessary for Ocean, which leads to one of the set, and RETScreen. preliminary studies, site location, most changeable and volatile design, feasibility analysis, weathers in the world. On the other In the UK, ground station bankability, and for performance hand, the Indian subcontinent has measurements of hourly global evaluation and monitoring of the a predominantly tropical climate horizontal solar irradiation are established system. Financing with both dry and humid areas; the the responsibility of the UK of solar projects, in particular, weather in India is strongly linked Meteorological Office. Between 80 can be promoted by producing to a jet stream (south-westerly and 100 weather stations have been reliable estimates of profits on summer monsoons) which follows operating in recent years, with the investment based on accurate the latitude, like in the UK. However, first record in 1947. These sensors solar irradiation data. The Ministry of the weather patterns only slightly are spread unevenly throughout the New and Renewable Energy of the change with the latitude (Müller country, with one station per 3,000 Government of India have described et al., 2017), as opposed to the UK, km2 on average; only two stations reliable solar radiation data as where they strongly do so (Palmer report at one-minute intervals. In vital for the success of solar energy et al., 2018). India, ground-based measurements installations across the country. are taken each minute by the Indian In both the UK and India, solar Meteorological Department and It is also essential to monitor and resource data is available from National Institute of Wind Energy at forecast the solar generation in either ground-based sources or 123 irregularly distributed sites (one real-time for scheduling, control, satellite derived values, usually in pyranometric station per 30,000 safety and efficiency purposes in the form of long-term time series km2). Measurements date since 2011. the electricity network. Uncertainty data of monthly averages of 10- or Interestingly, interpolation of hourly in estimating solar radiation is often 20-year periods. Arguably, ground- global horizontal irradiation values regarded as the largest contributor based measurements are the most gives a similar accuracy (nRMSE to the uncertainty of the system accurate, because unlike satellite 38%) (Palmer et al., 2018) for both output for any solar PV plant. values, they are measured directly countries, despite the much higher Accurate ground measurements in and do not depend on cloud or density of weather stations in the UK. the form of long-term time series aerosol data. On the other hand, The UK’s weather is changeable, and are required to lower the uncertainty ground sensors may be subject to India’s is relatively more stable, but SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 13
its measurement stations are further irradiation databases was noted is crucial to permit universal apart. Therefore, both have similar for 23 representative locations replacement of the conventional errors for interpolated data. (Purohit and Purohit, 2015). To this power plants by solar systems and day, accuracy and availability of achieve high solar integration levels. Commercial satellite databases solar resource data at various time also are available for both countries frames remains an open issue. A.2.1. Common instabilities from Solargis, 3Tier and Solcast. caused by solar These are of an accuracy that A.2. POWER SYSTEM High penetration of solar energy matches ground data but at STABILITY CHALLENGES has the potential to substantially 15-minute intervals only. Publicly affect the dynamic behaviour of available, lower accuracy satellite Replacing synchronous generation the power system due to a variety values for 15- to 30-minute intervals (e.g. thermal plants) with of reasons related to the highly are available from CAMS (Wald, intermittent solar generation different electrical and dynamic 2016) in the UK and from SARAH interfacing the grid via power characteristics of solar compared (Pfeifroth et al., 2017) in both India electronic converters gives rise to a to synchronous generation. Some and the UK. These are produced series of operational challenges in of the most common stability from Meteosat instruments. India the power system. A main challenge issues are (i) transient stability, (ii) also has data from NREL (NREL, 2020) is the uncertain nature of solar small signal stability, (iii) frequency and from the Indian Space Research power – not guaranteed – especially stability and (iv) voltage stability. Organisation (ISRO), based on the in cloudy days, which generally While transient stability (large Kaplana-1 satellite. This has yet to be dictates more operating reserves disturbances) is mainly affected extensively validated. When sub- often provided by conventional by the dynamic response and hourly data is required to account sources (e.g. thermal and hydro). protection settings of PV inverters, for the irradiance variability, it is Other increasingly prevalent small-signal stability (small normally synthetically generated challenges come from the solar disturbances) is primarily influenced from hourly values using Markov- plants not having inertia – absence by the PV inverter controller Chains, e.g. (Richardson, Thomson of rotating masses – and the power parameters. Frequency stability, on and Infield, 2008). converters’ low overcurrent capacity the other hand, is dictated by the – their ability to withstand currents total system inertia and the ability The aforementioned datasets only slightly over the rating. These of PV systems to withstand and are of varying quality, spatial and limitations, apparent to some extent support frequency disturbances. temporal resolution and standard to other renewables as well (e.g. Finally, dynamic voltage stability of validation. Obtaining a proper wind), contribute to the reduction of is negatively affected by the database for a particular solar the power system inertia and short insufficient reactive power injection project is a challenging task. Cole circuit level (SCL) and pose risks and current contribution during et al. found that a 2% variation of instability in the power system. faults due to the PV inverters in net annual system output/net Furthermore, solar plants cannot overcurrent limitations. The risk of annual irradiation was possible provide the same ancillary services each instability heavily depends on across 10 sites in the UK, depending traditionally offered by conventional the structure of the power system, on which of five irradiation sources power plants, such as frequency i.e. its size, the network strength, and was used to model the output operating reserves to contain the generation mix. As expected, (Cole et al., 2018). Aggregation power imbalances in the network these parameters vary during the of hourly irradiation values to (e.g. loss of generation). New day, which renders time as another yearly sum results in substantial ancillary services will be needed to critical risk-related factor; for error smoothing. Similarly, in handle high PV penetration, such example, the risk is higher at high- India, significant variation in as fast upward and downward PV/low-demand periods, such as at PV yield estimation calculated power ramping, grid flexibility, fast midday on a sunny day, or during using different global horizontal frequency support, etc. Tackling, or evening periods when the solar at least mitigating, these challenges 14 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
generation declines steeply and the Voltage instability comes primarily high, there is a lot of stored energy demand increases rapidly. from reactive power imbalance. to feed a sudden generation loss Although the synchronous machines or load tripping, thus limiting how The negative impact of high solar can support the voltage during fast the grid frequency changes; penetration on the frequency faults by injecting additional current under low inertia conditions, a small stability of the power system has up to 6-8 times the nominal, the power imbalance can lead to high arisen lately as a topic of concern PV inverters are limited to an frequency excursion with cascaded for the system operators. Although overcurrent capacity of 1.2-1.6 implications (e.g. generators it is a challenge of the future times the nominal value. Therefore, tripping, load shedding or even rather than of today, replacing replacing synchronous generation system collapse). more and more conventional with inverter-based solar results power plants with solar PV systems in lower short circuit strength and Figure 7 shows how the frequency will lead to lower system inertia dynamic reactive power resources typically changes when a and reduced capacity to absorb in the network. A more thorough generator is lost: the initial period large power imbalances in the analysis in this topic follows. Other of decline is the most critical, which system. This aspect is discussed reasons for solar-induced voltage determines the rate of change in more detail in the following instability are undesirable tripping of frequency (ROCOF) and the section. In addition, solar plants of PV systems at voltage dips and minimum frequency value (nadir), are less reliable sources under controller interactions among both directly related to the total severe disturbances compared neighbouring generators. inertia of the power system. If the to conventional generation, which frequency falls below a certain level, entails higher level of power A.2.2. Falling system inertia some load is automatically cut off reserves to compensate for sudden The system inertia is the total energy (under-frequency load shedding loss of solar. For example, Southern stored in the rotating masses of – UFLS) to protect the system from California witnessed an aggregated machines in the power system collapse; also, when the frequency loss of 900 MW of solar power on 9 (generators and motors) and is the changes too fast (high ROCOF) October 2017 due to a voltage dip first line of defence during power some generation may trip due to triggered by two transmission line imbalance and fluctuation of the protection settings, which may faults caused by the Canyon 2 Fire. grid frequency. When inertia is trigger further frequency drop and eventually load shedding.
Another illustration that shows how different levels of inertia affect the frequency is given in
Inertia Primary Secondary Tertiary Figure 8 (simulations of the IEEE response control control control 9-bus testbench in DIgSILENT PowerFactory). The same disturbance results in widely different ROCOF and frequency nadir ROCOF values: the more inertia, the better. Frequency (Hz) This is why replacing synchronous Nadir generation by solar risks frequency UFLS instability if not accompanied by proper countermeasures. Time (sec)
In the UK, the major inertial
Figure 7. Typical system frequency response at loss of generation. contribution comes from conventional power plants and SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 15
to a much lesser extent from 50 smaller synchronous generators, synchronous demand and induction 49.9 motors (National Grid UK, 2016).
Technologies that do not have 49.8 rotating mass, such as solar PV plants, do not inherently contribute 49.7 to system inertia. National Grid UK has been recently paying significant 49.6 attention to the falling system inertia Frequency (HZ) 49.5 of the Great Britain power system due to the uptake of renewables, 49.4 including solar, which is estimated to decrease inertia from 220 GVAs to 49.3 about 100 GVAs on average in a few 0 1 2 3 4 5 years (Figure 9). Notably, the cost Time(s) for managing the maximum rate of change of frequency (ROCOF) System inertia ≈ 3GWs System inertia ≈ 1.5GWs System inertia ≈ 0.75GWs to prevent undesirable tripping of loads and generators in the system has more than doubled lately, Figure 8. IEEE 9 bus system frequency under a disturbance for different system from £60 million in 2017/18 to £150 inertia values. million in 2018/19 (National Grid UK, 2019b). For this reason, National Grid UK is currently exploring advanced methods to monitor the system inertia in real-time via various projects, which was simply estimated in the past (National Grid The most common UK, 2019b). In order to achieve high value is 220 GVA.s solar integration levels, new sources of inertia should be explored, e.g. from demand response or synthetic The minimum and The area under maximum values inertia provision by solar plants. the curve shows are 100 GVA.s the distribution and 350 GVA.s Presently there is no similar of system inertia concern in India, where the power in 2016/2017 system has sufficient synchronous generation. However, by achieving the envisioned RES targets it is 0 50 100 150 200 250 300 350 400 not unlikely to start seeing such System inertia (GVA.s) problems in the Indian network as well in the future. 2016/2017 2020/2021 2025/2026
Figure 9. Distribution curves of system inertia in the UK (National Grid UK, 2016) 16 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
A.2.3. Diminishing short circuit of solar plants (and other inverter- anticipates phase-locked-loop (PLL) level based resources – IBRs) have very instabilities in parks connected to Short circuit level (SCL) or ratio limited short term overcurrent North Scotland by 2030 (National (SCR) or simply grid strength, at a capacity (typically up to 120-160% of Grid UK, 2018b). Other problems particular node of the network is the nominal current), which leads to arising from a low SCL and weak the ratio of short circuit current over reduced contribution to a fault and grid are control interactions among the rated current of a generator to the SCL of the neighbouring network. neighbouring systems (e.g. fighting be connected. SCL is a measure for voltage control on the same of the grid strength at a node of Along with the inertia, SCL is nodes, reactive power oscillations interest: high SCL means “strong” considered a critical stability between a solar park and HVDC grid that can withstand line faults indicator, expected to continue or STACOM) and difficulties in with limited oscillations in the falling rapidly in the UK in the energizing large transformers by voltage; low SCL implies a “weak” coming years (Figure 11). Low SCL IBRs (IEEE/NERC, 2018). For these grid that will experience significant implies risk of control instability, reasons, India introduced in 2019 a voltage oscillations during faults stringent fault-ride-through (i.e. requirement for at least SCR = 5 at and poses a risk for voltage staying connected to the grid the point of interconnection of new instability (see illustration in Figure during faults) and more challenging DGs as a precautionary measure 10). SCL is primarily influenced by anti-islanding (i.e. tripping when the (Central Electricity Authority, 2019). synchronous generation, due to the network is islanded) in IBRs (IEEE/ Since SCL is a “regional/local” metric, inherent ability of such machines to NERC, 2018). In fact, SCL is usually in contrast to the “global” inertia, it withstand as high as 6-8 times the lower in remote regions where solar is imperative to explore how solar nominal currents for short durations. and wind parks are often installed, plants can more actively contribute In contrast, the power converters which is why National Grid UK to the strength of the local grid.
SCL Effect on the system
High SCL In a high SCL system during a fault voltage will fall, but it will quickly recover once the fault is fixed. The voltage will
settle down to normal condition quickly Voltage with only small oscillations.
Time
Low SCL In a low SCL system the voltage will still fall during a fault, however it will experience oscillations as it starts to recover. The voltage will settle down to normal conditions but this will happen slowly and voltage will Voltage fluctuate during this period. Time
Very low SCL In a very low SCL system, the voltage behaves in a similar manner to the low SCL system, however instead of recovering to normal conditions the
oscillation will continue or increase. Voltage
Time
Figure 10. Impact of SCL on voltage stability during faults (National Grid UK, 2018b) SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 17
A.2.4. The power cut of August 2019 5 in the UK On the 9th of August 2019, a series
0 of events resulted in a large power cut in the UK that caused major disruption to about 1.1 million -5 customers: a rare event that has not happened in over a decade -10 (National Grid UK, 2019c). A lighting strike on a transmission circuit and a technical fault in a steam turbine -15 led to more than 1.1 GW generation Percentage change in short circuit (%) loss from the Hornsea offshore -20 wind farm and the Little Barford gas
2019 2021 2023 2025 2027 2029 power station; this consequently led to almost 500 MW embedded Year generation tripping and eventually significant load shedding upon subsequent faults in the Little Figure 11. Estimated change in SCL in the UK (National Grid UK, 2018b) Barford plant (Figure 12).
Circuit fault Eaton Fault cleared Hornsea loss Little Barford ST Increase in transformer Socon-Wymondley [16:52:33.564] of 737MW trip 244MW loadings (Loss of mains) [16:52:33.490] [16:52:33.835] [16:52:34] ~500MW [16:52:34] 50.4 Frequency is restored to 50Hz [16:57:15] 50.2
50
49.8 Frequency Little Barford GTla response recovers trip 210MW 49.6 frequency to 49.2Hz [16:53:31] [16:53:18] 49.4
Frequency (Hz) Little Barford GTlb 49.2 trip 187MW [16:53:58] 49 ESO National Control instruct 1,240 MW of actions to restore 48.8 frequency to operational limits and restore frequency response and reserve services. 48.6 -50 0 50 100 150 200 250 300
Time(s) Circuit closed Frequency Embedded Frequency on DAR fall arrested gen. loss 200 breaches 48.8Hz [16:52:53] at 49.1Hz MW @49Hz triggering LFDD [16:52:58] [16:53:49.398]
Figure 12. Frequency variation during the power cut of 9th August 2019 in the UK (National Grid UK, 2019c). 18 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
The Hornsea wind farm deloading by and frequency distortions. There is a – keeping the voltage within limits at 737 MW was caused by oscillations need to fully explore the decoupled all nodes – more challenging. New in the grid voltage that resulted potential of solar and other IBRs to loading patterns, the variable nature from the fault in the transmission come up with better interconnection of solar power and interaction circuit; critical factors were the standards. of PV inverters with conventional controller’s settings and the weak voltage regulating devices (e.g. grid that emerged by clearing the This incident highlights the new type on-load tap changers (OLTCs), fault (National Grid UK, 2019c). This of challenges the UK power system step voltage regulators, switched highlights the control instability and faces with the massive uptake of shunt capacitors) have rendered low-SCL risks that come with IBRs, distributed generation. Another today’s voltage control a concern such as wind or solar farms, and two frequency dips below 49.6Hz to distribution system operators raises the need to consider more were apparent in May and July 2019 (Borghetti, 2013; Jabr, 2019). seriously these new IBR-related (without any load shedding), which challenges. have sparked a debate on the A.3.1. Voltage Rise reliability of the power system in the One major impact of distributed Furthermore, the 500 MW embedded UK while heading towards the net- solar generation on the network generation that tripped due to zero carbon future of 2050. is voltage rise – having voltage vector shift or high ROCOF was exceeding the limit at certain nodes. unnecessary for some sources, A.3. VOLTAGE This undesirable phenomenon especially the power-electronics- CHALLENGES is more severe at higher solar based IBRs like solar: the IBRs do not penetration levels and during the low have synchronised rotating masses High PV penetration in the demand times (Masters, 2002). It is a and technically are capable of distribution network renders well-known challenge to distribution withstanding severe grid voltage controlling the voltage of the network network operators (DNOs); the UK Power Networks reported a voltage rise of more than 2% in some 255 feeders in 2015 due to residential PV installations (UKPN, 2015b), while 250 similar voltage rise issues were found in the Peterborough Central 245 grid in the UK (UKPN, 2015a). An example of voltage rise in the low- 240 voltage network in the UK is shown in the daily profile of Figure 13: there 235 are several times at midday when the PV generation peaks and the Voltage (V) 230 customer demand is low, resulting in voltage higher than the upper limit 225 of 253 V (230 V +10%). Voltage rise is made worse by the voltage set point 220 of distribution transformers, that is often set close to the upper limit. As 215 0 5 10 15 20 25 described in the next section, this is Time (Hrs) done on the assumption that power only flows from the higher voltage Measured voltage Lower limit Upper limit levels to the lower voltage levels, and from the transmission grid to the Figure 13. Voltage rise in the low voltage network in the UK. distribution grid. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 19
These issues are more prominent in the distribution network, whose lines Pl+jQl Il are typically more resistive than Distribution Substation R+jX inductive (high R/X ratio) in contrast PPV+jQPV VPV to the transmission network. This V0 entails that the active power flows PV in the distribution network affect the voltage to a great extent, unlike in the transmission system. Figure 14 Figure 14. Thevenin equivalent looking from the PV node. shows the operating principle via a simple Thevenin equivalent circuit; if the substation voltage V0 is close to nominal (1∠0o per unit), the voltage the distribution lines where the However, embedded solar magnitude at the PV node would be inductive part is usually lower (or generation into the distribution (Strbac et al., 2002): much lower) than the resistive part. system does not only result in These challenges are expected to reverse power flows, but also
|VPV |≅V0+(PPV – Pl )R + (±QPV – Ql)X be more severe in rural grids with multidirectional variable flows: weaker and lengthier feeders, such power may flow in opposite When the voltage approaches the as in rural India. To this day, optimal directions in different subsections limit, the standard approach is to coordination of all these actions of the feeder, a condition that is not use voltage regulating devices, such towards efficient and voltage- static but also varies from minute to as on-load tap changers and shunt secure network operation remains minute. This complicated situation capacitors, to reduce the voltage an open challenge for DNOs and the interferes with how tap changers back to an acceptable range. An scientific community. function, resulting at times in additional tool towards the same worsening of the voltage rise, rather goal is to leverage the reactive A.3.2. Reverse Power Flows than alleviating it, during reverse power capability of the PV inverters Conventionally, the power has power flows. In addition, some tap (inductive mode) which can also been flowing in one direction from changers and voltage regulators reduce the voltage at the PV nodes the main grid towards the load may have a lower than the nominal depending on the inductive value of connected across the distribution power rating at reverse power flow, the line. feeder. This is why the distribution which complicates their operation system has been traditionally even further. However, these options have treated as a passive load and the important limitations too. The voltage control has been designed In the UK, we have started seeing voltage regulating devices are considering unidirectional power these issues in the network. A meant to operate occasionally flows only (Sun et al., 2019). For technical report on the changing only a few times a day, which does example, the voltage control of nature of the transmission and not fit well with the intermittent distribution systems has been distribution boundaries carried out solar generation, especially during historically carried out based on the by the Scottish Power Network (SP partially cloudy days, that may concept of line drop compensation, Energy Networks, 2017) describes the require minute-level voltage which assumes that voltage case of the Dunbar grid supply point regulation. More frequent than decreases with distance as we that exhibits high levels of reverse normal switching of these devices move away from the substation power flow. The case study of 2015 results in additional stress and along the feeder. Based on showed peak demand of only 30 deterioration of their life expectancy. this principle, the tap changers MVA while having installed about Additionally, the reactive power traditionally adjust their settings 110 MW of distributed generation, consumption by the PV inverter using local current measurements resulting in exporting power to the may not be that effective in to estimate the voltage drop across transmission grid for 63% of the the feeder. 20 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
time. Similar concerns arise for A.4. PROTECTION A.4.1. Protection in the transmission India, where the 40 GW rooftop CHALLENGES system PV installations target by 2022 is The protection scheme of the UK expected to lead to similar issues for With the increase of solar PV transmission system incorporates several distribution feeders in solar- integration in the UK power network overcurrent, distance and rich states. at different voltage levels, it has differential protection mechanisms become more challenging for the (National Grid UK, 2016). The grading A.3.3. Voltage Flicker traditional protection schemes and setting of these protection Another voltage regulation to function properly and protect settings is based on an assumed challenge for the DNOs related to the network from faults (National range of short circuit level (SCL). high solar penetration is power- Grid UK, 2016). These newly High levels of solar in the network will line flicker. Flicker is the condition brought challenges are mainly negatively impact the SCL, which in when the voltage varies periodically, related to how solar PV systems turn may impair the effectiveness resulting in visually noticeable respond during faults. In contrast of these protection mechanisms, as changes in brightness of light and to synchronous generators, solar briefly explained in Table 1. power quality deterioration that plants have very limited overcurrent interferes with the electronic devices capacity (like other IBRs) and their Furthermore, with low SCL in the on the network. It may be caused by current contribution during faults system, it may be difficult to fast variation of the PV generation is little and slow in response (IEEE, discriminate between actual faults due to movement of clouds (Barker 2019). These may have a negative and normal events of high inrush and De Mello, 2000). impact on the post-fault grid current caused for example when voltage recovery, as well as on the starting-up motors (without soft A.3.4. Distribution and current measurement of protective starter) that may require up to 6-10 Transmission System Interaction relays and their effectiveness in times their rated current for a short Another emerging issue at high solar detecting and isolating faults time (National Grid UK, 2016). To integration levels is the interaction (ENTSO-E, 2016; H. Urdal et al., 2016; address these challenges, some between the transmission and Li et al., 2016). In addition, in order technical reports (H. Urdal et al., distribution network when controlling to support a fault, the PV system 2016; Tuladhar and Banerjee, 2019) the voltage and exchanging has to have fault ride through (FRT) recommend to give more weight reactive power. Autonomous capability – to stay connected to to differential protections over the voltage control by PV inverters may the grid despite the disturbance – alternative schemes. This approach, compete with other regulation which is not a trivial task for an IBR. however, may malfunction as well equipment and it may also affect Furthermore, embedded solar plants at low SCL when the fault currents the dynamic interactions of reactive still face challenges in the anti- are lower (National Grid UK, 2016). power between the two layers of islanding protection, occasionally It remains a challenging task to the power system. A more holistic tripping when they should not or keep the protection scheme of the voltage control framework will be failing to trip when they should transmission system up to date with necessary to coordinate all devices during major disturbances and the increasing IBRs, implying that in the transmission and distribution islanding events in the network. fundamentally different approaches network simultaneously, while taking With the anticipated high solar and a complete restructuring may into account the different ways in integration levels in the UK and India, be required for the inverter-intensive which the reactive power and active operators are increasingly expected power system of the future. power affect the voltage in the two to face a series of challenges in layers. the protection of transmission and distribution systems. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 21
PROTECTION SCHEME OPERATING PRINCIPLE IMPACT OF LOW SCL
Differential Compares the current infeed and If the difference between the currents is very Protection output from the equipment; if the small, it may not be detected by the relay. The difference between the two is bias may need to be set comparatively high at greater than the bias current, the times of low SCL to avoid maloperation. relay is set to trip.
Distance Protection Calculates the impedance at the Not affected if the ratio of voltage to current relay point and compares it with the decreases following the short circuit. This ratio reach impedance; if the measured however will be affected by the significantly impedance is lower than the reach different volumes of synchronous generation impedance, the relay is set to trip. at peak and minimum demand and may drive additional settings.
Overcurrent Protection The operating time of the relay This type of protection is the most likely to be is inversely proportional to the affected by low SCL. However, these schemes magnitude of the short circuit are mainly used for back-up protection and current. therefore the consequences may not be severe, provided that main protection schemes are not compromised.
Table 1. Operating principle and impact of low SCL on UK transmission protection schemes (National Grid UK, 2016)
A.4.2. Protection in the distribution system Installing solar generation at the end (a) nodes of the distribution system provides transmission-like features Relay NB 5 MW PV to the otherwise radial unidirectional Es EB Zs Zp Ep network, with bidirectional power flows that are usually found in ZB 13.2kV IS Ip meshed grids. Along with the low Source Fault fault current of PV systems, these pose a real risk to the traditional protection of distribution networks (b) 7,800 Amps (overcurrent relays and fuses), Fault close to breaker whose operation heavily depends
5 MW PV on the magnitude of the fault 240 Amps Close to breaker current. An example is shown in Figure 15: the current flowing through a relay or breaker during a fault may 13.2kV be either lower or higher due to a Source nearby solar plant, potentially leading to relay desensitization or equipment damage (Bryan Figure 15. Examples of (a) lower and (b) higher fault current flows due to embedded Palmintier et al., 2016). Of course solar in the distribution network (Bryan Palmintier et al., 2016) these challenges refer mainly to three-phase solar plants connected 22 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
at the medium voltage level, since the protection schemes, as faults into the future (Roscoe et al., 2017; the impact of single-phase low- are much harder to detect in EURAMET, 2018). voltage PV systems is quite limited, islanded operation (National even negligible for faults not close Grid UK, 2019a). For microgrids in A.4.3. Fault Ride Through to the solar plant (Bhagavathy et al., particular, the current practice is The duration of faults is usually 2019). to design protection schemes on very short (in the range of tens to a per case basis, entirely tailored hundreds of milliseconds), either Unfortunately, these challenges are to that particular network (Shiles et because they are of temporary not simply overcome by applying al., 2018), which indicates that there nature or because the network the transmission protection may be no place for “generic” and isolates them through the protection schemes to the distribution network “universal” protection schemes in equipment discussed previously. (Hooshyar and Iravani, 2017). The the distribution network of the future. The risk for solar plants and other limited fault current contribution DGs is to trip at such short-term by solar is comparable to a heavy Another source of concerns relates disturbances, either intentionally load condition, thus rendering the to unnecessary tripping of solar or unintentionally, triggering overcurrent protection scheme generation during disturbances, cascaded implications that extend ineffective. This fault current is also which has been seen to take place to a much larger time scale. The configured to be symmetrical even in quite a few power cuts in the UK desirable capability of a power at unbalanced faults, which may and elsewhere (Roscoe et al., 2017; plant to withstand these faults and seriously challenge directional National Grid UK, 2019c; E3C, 2020). remain connected to the network relays that are based on negative/ For example, the 2016 fire incident during faults is denoted as fault zero voltage sequence or on voltage in California led to disconnection ride through (FRT). Voltage FRT polarising (IEEE, 2019). The latter may of 400 MW of solar due to ROCOF and frequency FRT are nowadays also affect the distance relays that and vector shift protections even common requirements for large are based on voltage and current though there was no islanding DGs by most standards and grid measurements which depend (Roscoe et al., 2017). Since then, codes. Nevertheless, the track on the fault type and impedance the protection settings have been record of fault incidents that led (Hooshyar and Iravani, 2017). revised several times, now having to disconnection of significant banned the vector shift protection distributed generation in the past It is worth noting that massive and relaxed the ROCOF cut-off limits few years in India, the UK and the deployment of solar in the from 0.125 Hz/s to 1 Hz/s with a delay US (Table 2) indicates that there is distribution network not only affects of 0.5 s for all DGs in the recent more to be done. the distribution system protections, Engineering Recommendation G99. but may also compromise Similarly, the recent power cut of protection mechanisms in the August 2019 in the UK had about transmission level and the operation 500 MW of embedded generation of other network equipment such trip, either due to high ROCOF or as circuit breakers, lines, and vector shift, which was to some transformers in similar ways (H. extent unnecessary (National Grid Urdal et al., 2016). UK, 2019c). Erroneous high ROCOF readings may also be recorded due Furthermore, it is likely that many to noise, off-nominal frequencies distribution feeders will evolve and interharmonics (EURAMET, into microgrids in the future, 2018). All these recent examples supporting both grid-connected clearly show that misconception and islanded mode of operation, of islanding and unintentional towards increased reliability and generation tripping is expected to resilience. This will further challenge remain a challenge for DGs well (Figure 16(b)). that lastedforalmost10minutes frequency overshootinthesystem in India,whichturnledtoa the aftermathof2016incident load loss.Suchalosswas 2019 30 secondspost-fault( voltage recovery’,maylastupto referred toas‘fault-induceddelayed after thefault.Thisphenomenon, voltage recoveryupto5seconds fault, butitalsoexhibitsdelayed until theprotectionscleared features adipforabout1.88seconds 2014 2016 isshowninFigure16( the NorthernpartofIndiainJune consecutive transmissionfaultsat Another relevantincidentwithtwo Table 2. SOLAR INTEGRATIONINTHEUKANDINDIA: August 2019 October 2017 August 2016 July 2014 DATE ). ThevoltageinFigure16(a) ) andcanleadtofault-induced
Trip eventsofwindandSolarPVgeneratingunitsduetonetwork faults. G. Lammert, power plantfaults,GreatBritain,UK Lightning strikeontransmissionlines(400kV)and faults (220kVand500kV),LosAngeles,US Canyon 2fireincidentcausedtwotransmission on 500kVline,SouthernCalifornia,US Blue cutfireincidentledtoanormallyclearedfault Tuticorin TPSline,Tamilnadu,India Delayed clearanceoffaultin230kVKayathar- DETAILS OFINCIDENT CERC,
TECHNICAL BARRIERSANDFUTUREDIRECTIONS (b) (a) (a) VoltageatDadristation,(b)System frequency Figure 16.
Frequency (Hz) Voltage (kV) 49.9 50.4 50.3 50.2 240 200 230 50.1 220 160 180 190 150 210 170 50
Fault responseintheIndianelectricity gridduringtheeventofJune 2016.
14:25:30.440 14:27:15.000 Fault Initially Y Phase Three PhaseFaultstartedatt=0sec 14:27:15.960 14:26:18.440 14:27:16.920
14:27:06.440 R Phase 14:27:17.880
14:27:18.840 14:27:54.440 14:27:19.800 cleared aftert=1.88sec Three phasefault 14:28:42.440 14:27:20.760
14:27:21.720
14:29:30.440 voltage afterthefault Slow recoveryofthe Y Phase
14:27:22.680 1691 –variousDG 900 -solar 1200 -solar 920 -wind LOSS OFGENERATION(MW) 14:30:18.440 14:27:23.640 Voltage exceededitsinitialvaluet=5sec 14:31:06.440 14:27:24.600 (
Shukla 14:27:25.560 14:31:54.440 14:27:26.520 B Phase 14:32:42.440 et al 14:27:27.480
14:27:28.440 ., 2017 14:33:30.440 14:27:29.400
) 14:34:18.440 14:27:30.360
14:35:06.440 14:27:31.320 | 23 24 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
The Blue cut fire in California in 2016 60.00 is indicative of how simultaneous A A tripping of many solar plants can 59.95 lead to other types of frequency B B distortions. The faults at the 500 59.90 kV transmission lines caused Frequency (HZ) C C disconnection of about 1,200 MW 0 100 200 300 60 80 100 120 140 160 of solar, which was reflected in a frequency dip of 130 mHz shown Time(s) Time(s) in Figure 17(a). Figure 17(b) depicts (a) Value A: 59.979 Value B: 59.92 Point C: 59.8669 the post-fault solar power output: about 450 MW of solar recovered in a ramp-like manner within two 110 Zone I Zone II minutes (Zone-I); another 350 MW 100 that tripped due to PLL inaccuracy 11:45:15 am 11:52:16 am 2,885.05 MW 2,483.20 MW reconnected after five minutes 90 (Zone-II); the remaining 400 MW 11:47:15 am ~2,200.05 MW deliberately remained offline for 80 safety reasons and resumed the 70 11:52:15 am next day. 2,191.64 MW % of solar generation 60 11:45:16 am This experience clearly shows that 1,705.70 MW reliable FRT function in PV systems 50 is critical to allow large-scale solar 11:44:00 11:49:00 11:54:00 deployment. There is a need to (b) Time develop new means and methods so that solar plants and other DGs Figure 17. Blue cut fire incident in California 2016 leading to disconnection of 1,200 MW of solar generation. (a) Frequency dip (b) solar generation profile. can withstand a wider range of voltage and frequency disturbances.
A.5. ANTI-ISLANDING conflicting challenges (Weitzl and phase reclosing of embedded DGs, PROTECTION Varjasi, 2010; Dietmannsberger and resulting in excessive currents and Schulz, 2016; Ropp et al., 2016; Li and torque loads (Mahat, Chen and Bak- When a part of the network that Reinmuller, 2018). Jensen, 2008). The set of conditions does not have conventional under which AID fails to detect LoM generation is cut off from the main Failure in detecting LoM is referred to as the non-detection grid due to a fault (islanding), then During a LoM event, the anti- zone. the islanded network experiences islanding protection of a DG may a loss of mains (LoM). During LoM fail to act and cease operation, events, it is essential that any especially when the embedded embedded DG ceases operation generation and load in the islanded for the safety of equipment and network happen to balance out. people in the distribution network. This poses a series of hazards: Anti-islanding detection (AID) is electricity distribution workers the software and hardware in a safety is at risk; power quality may DG dedicated to detecting such be compromised in the islanded islanding events. However, this is not part of the system; instantaneous a trivial task and faces two main reclosing can result in out-of- SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 25
Because of the conservative of temporary disconnection of DGs Historically, such unbalance was approach of most operators, there are described in (Li and Reinmuller, caused by uneven loading among have been recorded only a few 2018). Another event of false tripping the three phases when connecting incidents of accidental islanding. has occurred in the UK due to vector single-phase consumers. However, An incident of undesirable reclosure shift LoM protection on 22nd May a similar situation arises in the of a wind farm in Canada resulted 2016 (National Grid, 2017). These case of connecting single-phase in large inrush currents (Li and instances show that anti-islanding PV systems to the three-phase Reinmuller, 2018). There have also protection remains to this day an network (e.g. in kW-scale rooftop been failures of anti-islanding open issue. solar systems), which may lead to systems in solar photovoltaic uneven current flows and voltage plants in an Iberdrola grid (Pazos A.6. POWER QUALITY in the three phases. Voltage or et al., 2013) and a risk assessment ISSUES current unbalance in the network of potential islanding of solar PV in is detrimental to utility assets, with India (Joshi and Pindoriya, 2013). We have power quality issues when more significant impact in three- the waveforms of the current and phase systems with grounded False tripping voltage in the network deviate from neutral (Karthikeyan et al., 2009; In order to avoid any undetected what they should be; common Smith et al., 2017). islanding, the anti-islanding such problems are harmonics, schemes are occasionally found to voltage unbalance, flicker and The intensity of these issues erroneously trip when they should voltage rise. These issues are more depends on a range of factors, such not. With reduced inertia and grid prominent in the low-voltage level as the network conditions prior to strength in the power system, large of the distribution network, where PV connection, the technology of changes in grid voltage, phase PV systems are more common as the PV inverters and the standards angle or ROCOF may give the compared to other RES (Nduka, they comply with, and of course impression of a false LoM resulting Kunjumuhammed, et al., 2020). the amount and location of solar in DG disconnection. This in turn generation. The type of network, may cause cascaded implications, Any distortion in the current and i.e. urban, semi-urban or rural, disturbing further the frequency voltage waveforms (due to the is of particular importance due and voltage in the network, leading presence of other frequency to widely different electrical and to further DG disconnections in a contents) that differentiate them physical characteristics (Smith et domino-like manner and possibly to from the ideal sinusoidal signals is al., 2017; Nduka, Kunjumuhammed, power cuts. referred to as harmonics or inter-/ et al., 2020). When comparing India sub-/supra-harmonics depending to the UK, these challenges are The Blue cut fire in California in 2016 on the frequency components of the greater in the former case that and the power cut in the UK on 9th distortion (Smith et al., 2017). These has been already experiencing August 2019 are such examples harmonics may come from PV high levels of voltage/current of false tripping. In both cases, inverters or other power electronic distortion and voltage unbalance embedded generation including devices, whose produced voltage (Karthikeyan et al., 2009; Rajesh et solar disconnected due to high and current usually contain other al., 2016); integrating lots of solar ROCOF and vector shift protections frequency components apart from into networks that already exhibit triggered by faults, even though the fundamental grid frequency (50 such issues may be tricky and there was no LoM. Afterwards, the Hz in UK and India). risks further worsening of these operators relaxed the protection conditions. On the other hand, the settings to avoid such phenomena The voltage unbalance is another UK generally uses underground in the future, but this is not really the problematic condition, under cabling within its cities in contrast solution: by relaxing the settings too which the voltage magnitude to India’s overhead lines. The much, an actual islanding event in the three phases is not equal, capacitance of the underground may go unnoticed. Other examples resulting in asymmetrical voltage cables can theoretically interact and current flows in the network. to thereducedfundamentalcurrent. harmonic currentsarecomparable solar poweroutputislowandthe times, evenreaching45%,whenthe as aratiotakesveryhighvaluesat seven days.Notunsurprisingly,THDi frequency (e.g.50Hz)-THDi)for the currentatfundamental of harmoniccomponentsover distortion inthecurrent(ratio plot depictsthetotalharmonic the UKareshowninFigure18.This from householdmeasurementsin emissions ofPVinvertersobtained Field dataontheharmonic A.6.1. Harmonics the UK. comply withregulatorystandardsin on PVsystemsthatwerecertifiedto on fieldmeasurementsconducted The findingspresentedarebased with theIndianelectricitynetworks. owned PVunitsanddrawparallels distribution systemswithcustomer- have beenobservedintypicalUK power qualitychallengesthat In thefollowing,wediscuss plasma TVs,laserdevicesetc. of customers’sensitiveloads,e.g. assets andsometimesmaloperation accelerated ageingoftheutility insulation andburnoutoffuses,thus result indeteriorationofthecables’ ( voltage orcurrentexcursions phenomena andundesirable PV inverter)andleadtoresonance (e.g. transformerorgrid-sidefilterof with impedancesinthenetwork 26 Nduka andPal,2017 |
PART A:TECHNICALBARRIERSTOSOLARINTEGRATION ). Thismay different inverters Figure 19. at sevendays. Figure 18. h3 current (A) 0.6 0.8 0.9 0.4 0.5 0.3 0.2 0.7 Current THD (%) 0.1
0 40 50 30 20 1 10 0 9
Magnitude ofthethirdharmonic current (absolutevalue)forfive Total harmonicdistortion(percentage)inthecurrentforaUKhousehold 0 July 29 July 24 HS23 10 10 July 25 July 30 HS07 11 11 Time (hrs) Time (hrs) July 26 12 12 HS16 13 July 27 13 HS19 14 14 July 28 HS35 15 15 SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 27
1.2 For more detail, Figure 19 and Figure
1 20 show the third and fifth harmonic components in absolute values
0.8 (Amperes) of five PV inverters for one day (July the 30th), which are 0.6 fully acceptable. Interestingly, there is widely different emissions level h5 current (A) 0.4 among the five certified inverters, some units injecting more than 0.2 double the harmonic current than others. This demonstrates how 0 9 10 11 12 13 14 15 the inverter and the connection point affect the harmonic Time (hrs) emission. Although the absolute current values are low for each HS23 HS07 HS16 HS19 HS35 individual household, interaction of neighbouring units with the same Figure 20. Magnitude of the fifth harmonic current (absolute value) for five different harmonic behaviour may lead inverters. to aggregation of the harmonic currents and undesirable effects to the utility assets, especially at low- voltage transformers and cables.
1 While the harmonic emission by PV systems at household level is sufficiently low in the UK, it is
0.8 difficult to draw parallels with the Indian networks due to the cheaper power electronic converters used in
0.6 developing nations including India. Moreover, given the existing power quality issues in Indian distribution
0.4 networks – particularly phase
Voltage unbalance (%) voltage unbalance and existing harmonic levels, it is expected to
0.2 face more such challenges with the rapid uptake of rooftop solar in the Indian low-voltage networks.
0 0 5 10 15 20 25
Time (Hours)
Figure 21. Assessing voltage unbalance due to PV uptake 28 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
A.6.2. Voltage unbalance Grid code The statutory voltage unbalance regulation limit stipulated by International Electrotechnical Commission (IEC) is 2% (Karthikeyan et al., 2009). planning Operation Market code code code Figure 21 depicts simulation result of the voltage unbalance at the point of connection of a single- phase PV system integrated into Steady state Dynamic operation operation the UK distribution network. The computational tool developed by the authors and published with the Active Reactive Frequency Voltage Black Forecast & power/ power/ Power ride ride start/ IEEE (Nduka, Yu, et al., 2020) has been dispatch frequency voltage quality through through restoration used for the simulation. As expected, control control the level of unbalance varies over the day, peaking at midday when Low High Low voltage High voltage frequency frequency the solar generation is maximum, ride through ride through ride through ride through but not exceeding the 2% limit. On the contrary, measurements conducted in India indicate that Figure 22. Broad classification of grid code regulations voltage unbalance is already an issue in the regional networks (Karthikeyan et al., 2009; Rajesh et al., 2016), which highlights the need the operator commands, response greatly varies among countries for careful integration of single- to grid disturbances and power as shown in Figure 23, with some phase rooftop PV systems that could quality standards. This code can countries demanding even zero- further worsen this condition. be further split into the steady state voltage ride through. The shape of and dynamic operation codes, the these curves depends on a series A.7. GRID CODES AND former focusing on the power plants of factors, such as the grid strength ANCILLARY SERVICES behaviour during steady state (SCL) and power system size, the operation and the latter covering generation mix and RES penetration The main purpose of grid code the response of the power system level, the dynamics of the system regulations is to discipline the grid, components to grid dynamics. load, the reactive power capacity of and thus ensure secure and stable Market regulation guides the market the system and any HVDC links. operation of the power system for trading electricity between under the increasing levels of RES generators and consumers. A.7.1. The evolution of grid codes integration. Grid code regulations The need for comprehensive grid – or simply grid codes – can One of the most critical codes emerged with the massive be broadly classified as shown requirements when it comes to uptake of renewables in the past few in Figure 22. The planning code solar PV systems and other DGs is decades, as they introduced a series primarily covers planning aspects, the fault ride through (FRT) function, of technical challenges to the power such as demand forecasting, and especially the Low Voltage Ride system due to their intermittent and system reserves, transmission line Through (LVRT) requirement. The non-synchronous nature. At first, planning, generation planning, grid LVRT function not only requires that solar and other renewables were connection requirements, related the power plant stays connected treated on a “must-take” basis and licensing etc. On the other hand, the to the grid during a dip in the grid were permitted to operate in an operation code refers to the actual voltage, but it also supports the grid “ON/OFF” fashion, i.e. to disconnect operation of the power system, at that time by supplying reactive freely during disturbances at the such as the generators’ response to and active power. This requirement network. Under low renewable the world with renewables has its the worldwithrenewableshas its Since then,almosteverycountry in Denmark andGermanyinthe 1990s. grid coderegulationsforRES were these challengesandintroduce Historically, thefirstcountries tonote even resultinginablackout. implications inthepowersystem which inturncanhavecascaded under severegriddisturbances, led inthepasttomassivetripping “ON/OFF” operationforREShas up andrunning.Permittingasimple burden ofkeepingthepowersystem structured mannerandsharethe these sourcestobehaveinamore mix increases,thereisaneedfor such powerplantsinthegeneration plants. However,astheproportionof the powersystemandRES is effectiveandbeneficialforboth integration levels,thissimpleregime SOLAR INTEGRATIONINTHEUKANDINDIA: Figure 23.
Retained voltage (pu) 0.6 0.8 0.9 0.4 0.5 0.3 0.2 0.7 0.1 ENTSO-E LowerBand UK (HVDCConnection)[at3min,V=0.9pu] Germany UpperLine Ireland UK (Onshore&Offshore-HVACConnection)[at3min,V=0.9pu] 0 1
LVRT curvesindifferentcountries 0.5 India (at10sec,V=0.9pu) 1 1.5 Germany LowerLine ENTSO-E UpperBand
TECHNICAL BARRIERSANDFUTUREDIRECTIONS Time(s) 2 countries, it has been gradually countries, ithasbeengradually on relevantregulationsfromother while atfirstitwaslargelybased Commission (CERC)backin2010; the CentralElectricityRegulatory grid code(IEGC)wasfirstissuedby more stringent.TheIndianelectricity updated andbecomemore own gridcodes,whichareregularly were recently published by CERC were recentlypublishedbyCERC May 2019,whoserecommendations an expertcommitteeformed in This reviewisbeingundertaken by RES targetsinthecomingyears. implementation oftheambitious particular focusonthesmooth another roundofreviewwitha 2019). Currently,itisundergoing six times(2012;2014;2015;2016;2017; the Indiancodehasbeenamended the Indianpowersystem.Since2010, characteristics andchallengesof adapted andalignedtothespecific 2.5 Hydro Quebec(at30sec,V=0.9pu) Spain (at15sec,V=0.95pu) 3 Denmark 3.5 4 Alberta
50.05 Hz. Other modifications include 50.05 Hz.Othermodificationsinclude reduced from49.9-50.05Hzto49.95- allowable frequencyrangewas was reformedsignificantly,andthe Additionally, theplanningcode and ComplianceOversightcode. Cyber Security,andiii)Monitoring and CommissioningCode,ii) new codes:i)Protection,Testing report aretheintroductionofthree changes inthedraftIEGC2020 in JulyandAugust2019.Major consultations withstakeholders in January2020afteraseriesof operation, as well as new ancillary operation, aswellnewancillary the electricitymarketinto grid are theeffectiveintegration of Valuable toolstowardsthisgoal defeating theirverypurpose. less attractiveinvestments,thus of renewablesbymakingthem disincentivise furtherdeployment system, atthesametimethey reliable operationforthepower regulations aimatsecureand stakeholders. Whilemorestringent balance amongtheinterestsofall successful gridcodestrikesa It isworthhighlightingthatthe p.u. forupto140msduration. online forvoltagegreaterthan0.15 in theUKarerequiredtoremain control area.Forexample,RESplants the relativelysmallersynchronous cross borderinterconnectionand mainly duetoitsnonsynchronous and advancedgridcoderegulations, The UKhasrelativelymorestringent and solar. of must-runpowerplantslikewind leading tospillage)inthecategory plants (incaseofexcesswater date, andclassificationofhydro used bythesystemoperatorupto every fiveyearstokeepthemodels mandatory fieldtestingofmachines | 29 30 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
service products such as fast PRIMARY SECONDARY TERTIARY TERTIARY frequency response, ramping SLOW FAST products, dynamic reactive power support and other services. Reserves 4000 MW 3623 MW 5218 MW Pilot level amount Furthermore, the high level of Response Few sec to 30 sec to 15 min to 5 min to differentiation of the grid codes time 5 min 15 min 60 min 30 min among countries requires costly redesign and customization of Manual / Automatic Automatic Manual Manual products in the solar power industry. Automatic Harmonizing these regulations is Paid / Mandated Paid Paid Paid expected to strengthen the solar Mandated PV/Wind power industry and foster further deployment of renewables. Table 3. Earmarked power reserves by CERC for frequency response services (CERC, 2018)
A.7.2. The ancillary services in India India has recently experienced are regional entities and their tariff electricity markets, this being a a rapid increase in integration for full capacity is determined by barrier towards fair competition and of renewables, mainly solar PV, CERC; they get compensated by the unlocking their true potential. CERC which led to an update of the system operator. All the participating has recognised these limitations ancillary services regulations in generators must share information and is currently working to this end. 2015 by CERC. At the moment, the on their technical minimum and frequency support services in India ramp rates (up and down) along A.7.3. Limited services potential by are classified into (i) primary, (ii) with unit-wise installed capacity, PV systems secondary, (iii) fast tertiary, and (iv) start-up time (hot and cold), and It is now generally accepted that slow tertiary services. The required eventual module constraints. Finally, high levels of renewables into power reserves for each service are tertiary fast services have been the power system implies more given in Table 3 (CERC, 2018). tested only at a pilot level. advanced ancillary services. However, the solar PV system Primary frequency response is While the existing ancillary services as a generator has very limited provided by all central power framework has been beneficial capacity for the provision of such plants, called Inter-State Generating to the Indian system, it also faces services and has to overcome Stations (ISGS) in India. The certain design and operational lots of technical hurdles to impact secondary frequency control – or challenges. One such issue is the the grid to a similar extent as the Automatic Generation Control availability of the tertiary reserves synchronous generators. Some of (AGC) – is applied independently to (RRAS) for deployment during hours the most prominent such limitations the five regional grids of India and of high demand. While all eligible are briefly outlined below, using the it has not been implemented to the generators’ unscheduled capacity European ENTSO-E 2013 (ENTSO-E, entire Indian system as a whole yet. is considered available, only the 2013) classification of ancillary The 3623 MW secondary reserves running machines can really deploy services into (a) voltage regulation, include 1000 MW in the Southern them in short notice in contrast (b) frequency regulation, and (c) region, 800 MW in the Western to machines that are offline. The robustness/restoration services: region, 800 MW in the Northern non-dispatched margin can be region, 660 MW in Eastern region, also very small at high demand and and 363 MW in North-Eastern region. unnecessarily large at low demand Tertiary slow reserves – or Reserves hours. Furthermore, the ancillary Regulation Ancillary Services (RRAS) services procurement mechanism is – are required by all generators that not yet technology-neutral in India, as opposed to most developed SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 31
(A) VOLTAGE REGULATION SERVICES A.8. ENERGY STORAGE AS • Provision of reactive power based on commands issued by the ENABLER FOR HIGH SOLAR grid operator may be very slow due to inadequate communication LEVELS infrastructure (IEEE/NERC, 2018). Many of the technical challenges • The dynamic voltage support during faults is restricted by the inverters’ brought by solar can be mitigated low overcurrent capacity, which may lead to low SCL and weak grid, as or completely addressed by well as relays failing in detecting and isolating the fault (IEEE/NERC, 2018). leveraging the transformative technology of energy storage. (B) FREQUENCY REGULATION SERVICES When coupled with a PV system either physically or virtually, energy • Frequency response by non-standard sources, including solar, was storage can effectively convert the found to suffer from high latency in communication in the UK (National variable solar energy source to a Grid UK’s EFCC project) (National Grid UK, 2019d). fully dispatchable power station • Provision of under-frequency and inertial response implies up- (Nikolaidis et al., 2016). Based on the regulation of the output power, which is not possible in solar systems if form of energy stored in the system, they already harness the maximum available power; the solar plant has energy storage can be categorised to be either coupled with energy storage (e.g. batteries) or curtail some into (Luo et al., 2015): mechanical power to keep reserves (power headroom). This service is still optional in (pumped hydroelectric storage the latest standard IEEE 1547-2018 (IEEE, 2018). (PHS), compressed air energy storage (CAES), and flywheels), (C) ROBUSTNESS/RESTORATION SERVICES electrochemical (conventional rechargeable batteries and flow • Although the fault-ride-through (FRT) requirements are regularly revised, batteries, hydrogen storage about 500 MW of embedded generation, including solar, tripped in the with fuel cells), electrical recent power cut of August 2019 in the UK due to vector shift or high (capacitors, supercapacitors, and RoCoF (National Grid UK, 2019c). This was unnecessary to some extent, superconducting magnetic energy especially for IBRs like solar, which is why National Grid UK is currently storage), thermochemical (solar changing the protection settings of installed DERs to avoid unnecessary fuels), and thermal energy storage. tripping in the future. Figure 24 provides a graphical • Solar PV parks conventionally cannot establish voltage and frequency illustration of each technology in the system (grid-following sources), thus not being able to support capabilities in terms of power rating, islanded networks and must trip. Islanded operation capability (grid- energy capacity and discharge rate. forming) would increase the resilience of the system.
• Black start function (i.e. the capacity to restore the network in the event of a blackout) is not required by solar PV systems and other DERs, thus having to maintain lots of conventional generation in case of a blackout (IEEE/NERC, 2018). A National Grid UK’s project concluded that black start from wind and solar is technically possible, but it will be weather- dependent and should come from an aggregation of various sources (National Grid UK, 2019a). technology has alsoshowngreat DOE, 2020 Energy StorageDatabase( 2020 accordingtoUSDOEGlobal of theglobalenergystorage inFeb constituting about180GWor 95% capacity amongalltechnologies, longest historyandlargestenergy constraints. Pumpedhydrohasthe with aseriesofgeographical scale applicationsandcome are mostlyapplicabletolarge- cost andpercycle,butthey options intermsofbothcapital air technologiesarequitelucrative pumped hydroandcompressed power arefullydecoupled).The is onlyindicative,asenergyand capital costoffuelcellstechnology commercialized technologies(the typical valuesforsomeofthemost efficiency, withTable4providing cost, lifetimeandroundtrip considered include:thecapital with solar,figuresofmeritoften When couplingthesetechnologies 32 | energy capacityanddischargetimeatnominalpower( Figure 24.
Power rating PART A:TECHNICALBARRIERSTOSOLARINTEGRATION 100MW 100kW 10MW 10kW 1MW 1GW 1kW Supcapacitor 0.1kWh ). Thecompressedair A listofenergystoragetechnologiescomparedintermspower rating, SMES 1kWh SMES SMES Li-ion Small CAES Small CAES FES Small CAES FES 10kWh US SMES VRB 100kWh Rated energycapacity Li-ion Flywheel Lead-acid Lead-acid 1 sec ZnBr 1MWh VRB technologies havebeenalmost electrochemical energystorage it comestosolarPVsystems, and Wang,2018 reliability forseveraldecades( 91.2–99.5% startingandrunning consistent goodperformancewith (110MW, builtin1991),delivering in 1978)andtheMcIntoshplant the Huntorfplant(290MW,build potential, withtwolargeplants, mature ofthese technologies electrolyser/fuel cells.Themost batteries, redoxflowbatteries and lead acidbatteries,lithium-ion to stationarygridapplications: technologies havebeenapplied energy storagemethods,fourkey Within thefamilyofelectrochemical storage technologies A.8.1. Electrochemicalenergy applications. appropriateness forsmall-scale to theirscalability,efficiency,and exclusively usedinstead,mainlydue ZnBr ZnBr VRB VRB FES VRB NiCd Li-ion Lead-acid Li-ion 10MWh VRB Li-ion NaS
VRB Lead-acid Luo 1 min NaS Lead-acid* NaS Lead-acid 100MWh et al AA-CAES* TES ). However,when ., 2015 PHS PHS NaS * Underconstruction Large CAES Large CAES 1GWh ) 1 hr PHS 10GWh 1 month He 1 day lifetime, respectively( due tohighcostsandlimited attention ingrid-scalestorage not receivedmuchcommercial manganese oxide(LMO),have cobalt oxide(LCO)andlithium electric vehicles,suchasthelithium for consumerelectronicsandearly chemistries whichhavebeenused alternative solutions. been motivatingtheindustrytofind efficiency andpoorlifetimehave system islow,thelowround-trip whilst thecapitalcostof in stationaryapplications.However, rendered thistechnologyaleader of scalesandcostreductionthat up systemsdrovetheeconomies This earlyadoptionforenginestart- services inautomotiveapplications. is commonlyusedforancillary is theleadacidbattery,which oxide (NCA)( and lithiumnickelaluminiumcobalt manganese cobaltoxide(NMC) iron phosphate(LFP),lithiumnickel considered typesarethelithium scale storage,themostcommonly terms ofcomposition.Forgrid- the cathodecandiffergreatlyin anode ismadeofgraphite,whereas In thevastmajorityofcases anode andcathodematerials. components ofabatteryarethe construction. Themostcritical by thematerialsusedintheir of differenttypescharacterised ion batteries,thereisalsoavariety Within thesub-categoryoflithium- electronics andelectricvehicles. their widespreaduseinconsumer cost andimprovinglifetimedueto driven bytheirrapidlydecreasing ion batteries.Thishasmostlybeen integration projectsutilisinglithium- increasing numbersofsolar More recently,therehavebeen Hesse et al Schledde ., 2017 ). Other et SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 33
TECHNOLOGY PUMPED COMPRESSED LEAD ACID LITHIUM-ION ELECTROLYSER/ REDOX FLOW HYDRO AIR BATTERY BATTERY FUEL CELL BATTERY
Capital cost 10-100s 10-100s 10-100s 100s 1000s 100s ($/kWh)
Cost (c/kWh/ <1-10 <1-10 10s-100s 10s-100s 100s-1000s 10s cycle)
Round trip 70-85 50-75 65-85 80-90 <40 65-85 efficiency (%)
Table 4. Comparison of different energy storage technologies for the grid. Adapted from Few et al. (Few et al., 2016)
al., 2018). LFP in particular is seen are potentially more favourable for days when the battery is fully as an attractive option for grid longer duration storage. discharged to prevent formation applications due to the low cost of zinc dendrites and failure of the of the materials, long lifetime due Redox flow batteries are seen as battery (Weber et al., 2011; Wang to the very limited volume change a potential solution for storage et al., 2013). This reconditioning during operation, and improved systems requiring about 4-10 hours’ cycle is thus a real challenge for safety of the chemistry. However, LFP worth of storage due to their ability applications in solar PV systems, exhibits a strong voltage hysteresis to decouple power and energy. In a which is why the all-vanadium during cycling which makes state- lithium-ion battery, the power and alternative has received more of-charge (SOC) estimation quite energy are often coupled, which focus with several systems already challenging for control applications implies overrating power in long- deployed at the scales of multi-kWh (Xing et al., 2014; Crawford et al., duration applications. In contrast, and multi-MWh (Alotto, Guarnieri 2018). Furthermore, the anode-side a redox flow battery commonly and Moro, 2014; Cunha et al., 2015). graphite is often found to be the stores energy through changing the In theory, these flow battery systems electrode with the most severe oxidation state of liquid electrolyte can have a lower cost and longer degradation, which has motivated stored in external tanks. These lifetime than lithium-ion batteries, the use of alternative anode electrolytes are then pumped however they have not received as materials such as lithium titanate through a flow battery stack where much research and development (LTO) that is much more stable albeit they can be charged or discharged. as the latter. Furthermore, these at additional cost (Zhao et al., 2015; Therefore, if more energy is required, benefits are only achieved once Reniers, Mulder and Howey, 2019). the tanks can be made larger, a certain system size is achieved, whereas if more power is required which means that system cost Despite the prevalence of lithium- then a larger battery stack can remains a major challenge. ion battery technology, various be used. There are a number of Despite the advantages of the lead studies have shown that as the different flow battery chemistries acid, lithium-ion and redox flow scale and storage capacity of the emerging, however the two batteries, these technologies are not storage system increases, the cost technologies that have attracted appropriate for inter-day storage of a lithium-ion battery-based commercial uptake are the all and beyond. For longer storage system becomes less attractive vanadium flow and zinc-bromine capacity, an electrolyser/fuel cell due to intrinsic material costs. flow batteries (Skyllas-Kazacos system is a better alternative. An Thus, efforts are being made to et al., 2011). Several kWh-scale electrolyser can convert excess develop alternative technologies zinc-bromine systems have been electricity into chemical fuels, such as redox flow batteries and demonstrated, but a reconditioning such as hydrogen, which are then electrolyser/fuel cell systems, which cycle is often required every few converted back to electricity at a 34 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
later time by passing through a While the cost of these technologies the characteristics of the required fuel cell; however, the conversion do vary significantly, Schmidt et al. energy storage system. efficiency of these systems results used experience curves to show in a roundtrip efficiency of less than that regardless of technology, costs 40%. Alternatively, the hydrogen are on a trajectory towards 340 $/ can be used in a power-to-gas kWh for an installed system and 175 system, where the chemical fuel is $/kWh at the (battery) pack level, injected into a natural gas system once 1 TWh of installed capacity for combustion. For electrolysis, is reached (Schmidt, Hawkes, et three main technologies are used: al., 2017). However, the capital cost the alkaline, proton exchange should be considered along with the membrane and solid oxide device lifetime, with Schmidt et al. electrolysis cell. Here, the main later evaluating the levelized cost of barrier is the capital cost which is electricity (LCOE) for various storage forecasted to vary within 700-1400 technologies (Schmidt et al., 2019). €/kW, 800-2200 €/kW and 2500- They show therein that lithium-ion 8000 €/kW respectively for the three batteries are most likely to be the types (Schmidt, Gambhir, et al., 2017). most cost-effective solution for These numbers are predicted to fall nearly all stationary applications in 2030 to 700-1000 €/kW, 700-1980 by 2030. However, beyond that €/kW and 750-6800 €/kW, although time frame and as technology with economies of scale they can innovations carry on, this may well be reduced even further. Fuel cells change, and different storage have similar cost challenges to technologies may find niches within electrolysers, which combined with various application areas. Table the hydrogen storage costs currently 5 shows six different commonly limit wider industrial uptake of this considered grid applications and energy storage technology.
FREQUENCY REGULATION DEMAND CHARGES DEMAND RESPONSE
Power/energy ratio 4:1 to 2:1 1:1 to 1:3 1:2 to 1:4
System size 100 kW to 40 MW 20 kW to 1 MW 100 kW to 10 MW
Response time <20 ms 1 to 5s <10 mins
Grid domain* T, D, BtM C&I, BtM C&I, BtM
DOMESTIC SOLAR INTEGRATION TIME-OF-USE SHIFTING ENERGY ARBITRAGE
Power/energy ratio 1:1 to 1:3 1:2 to 1:4 1:3 to 1:10
System size 5 kWh to 30 kWh 5 kWh to 1 MWh 1 MW to 100 MW
Response time <1 min <5 mins <5 mins
Grid domain* BtM C&I D, T
*T: Transmission, D: Distribution, BtM: Behind the meter, C&I: Commercial and industrial
Table 5. Comparison for figures of merit for different stationary energy storage applications. Adapted from (Lux research Frankel, Laslau and Torbey, 2016) SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 35
A.8.2. Technical barriers to Degradation Degradation integration of batteries Cause Effect mechanism mode With lithium-ion batteries forecasted to be the most likely technology Time SEI growth to enable solar integration, there SEI decomposition are still a number of issues to be High temperature resolved, of which lifetime is one of Electrolyte Loss of lithium decomposition inventory the largest. This is mostly due to the High Vcell/SOCcell Binder highly usage-dependent and non- decomposition Capacity fade Graphite exfoliation linear degradation of lithium-ion Current load batteries, as illustrated in Figure 25. Structural Loss of active disordering material
Low temperature Lithium plating/ Operating regimes that accelerate dendrite formation Power fade Loss of electric degradation include high Stoichiometry contact temperatures; low temperatures Electrode particle Loss of active cracking cathode material with high charging currents; and Mechanical stress Transition metal high SOC, which implies that the dissolution Corrosion of Low Vcell/SOCcell charge/discharge control of the current collectors battery system is a critical factor for the longevity of the battery. Model- based control is seen as a promising Figure 25. Cause and effect of degradation mechanisms and associate degradation way forward. For instance, Patsios modes in lithium-ion batteries (Birkl et al., 2017) et al. developed an integrated framework for the analysis and control of grid-connected energy storage systems, whereby a physics- need to consider the battery control decay relationship, which makes based model of a lithium-ion strategy alongside the objectives of temperature control even more battery was combined with models the PV system (e.g. extraction of the critical in hot environments. Typical of the power electronics and a grid maximum power) (Reniers, Mulder thermal management systems (Patsios et al., 2016). At high SOCs, and Howey, 2019). can range from active air cooling the available energy is maximised to liquid cooling, but in both cases, and the resistance of the battery Another challenge in integrating these incur additional cost and is minimised, which increases the batteries to solar systems is the parasitic load that reduces the round-trip efficiency. However, alignment of component lifetimes. round-trip efficiency. prolonged operation at high SOCs The widely adopted polycrystalline leads to accelerated degradation and monocrystalline PV panels In addition, from the power through the formation of a solid- exhibit lifetimes in excess of electronics perspective, the voltage electrolyte interphase layer on the decades, whereas lithium-ion of a lithium-ion battery varies as a anode that leads to power and batteries struggle to achieve function of its SOC. The magnitude capacity fade, an effect intensified such durations in a cost-effective of this variation depends on the at higher temperatures. Conversely, manner. Given the importance specific chemistry but is commonly if the batteries are charged at low of levelized cost of electricity, between 4.2-2.7 V for an NMC/ temperatures, this can lead to maximising the lifetime of these graphite cell. When translated into formation of lithium dendrites which MWh-scale batteries often a large-scale system, assuming can also accelerate degradation requires thermal management. for example 200 cells in series, this or cause catastrophic failure. The The correlation between lifetime results in an 840-540 V fluctuation, complexity of these constraints on and operating temperature which poses an input voltage battery operation thus highlights the typically follows an exponential challenge for the power electronics. 36 | PART A: TECHNICAL BARRIERS TO SOLAR INTEGRATION
Relevant to the previous challenge decreases at high temperatures. A parts of India during the summer. is the cell balancing in a large typical temperature coefficient of Most PV cleaning schemes use battery stack. A lithium-ion battery power output is 0.41% drop for each water to wash modules, and system of kWh and MWh scale °C, such that the real PV module while the relevant energy cost consists of several thousands efficiency can drop from 15% at is not significant compared to of cells connected in series and 25°C to 12.85% at 60°C for the same the energy output of the PV, the in parallel; slight differences irradiance value (Skoplaki and logistical difficulty of obtaining among the individual cells during Palyvos, 2009). water and the labour costs are manufacturing may result in important barriers to cleaning uneven charging/discharging In practice, high levels of irradiance PV (H. Qasem, 2013). Automatic and deterioration of the cells, usually accompany high ambient PV cleaning systems are being which directly impacts the entire temperatures. In addition, the developed and commercialised battery pack that is as strong as its solar irradiance causes surface to make cleaning easier (Mondal weakest cell. To address this, battery heating in the solar panels such and Bansal, 2015; Jaradat et al., packs often include a battery that module temperatures can be 2016), while hydrophobic coatings management system that monitors as much as 25 °C higher than the in the PV modules may be a good the voltage of the individual cells ambient temperature (Faiman, option in the future to reduce dust and seeks to balance them out. Two 2008; Muzathik, 2014). They can accumulation (Quan and Zhang, main cell balancing regimes exist: easily reach and exceed 60 °C on 2017; Womack et al., 2019). the passive and active method; the hottest days in India. Another passive balancing involves resistors temperature-driven effect of more A.9.3. Opportunity costs of land use that slowly discharge overcharged permanent nature is degradation A solar installation occupies space, cells in a passive manner, which is a due to high temperatures and and in so doing, it precludes other simple and the most widely adopted humidity. Module degradation land uses to a great extent; if solar is method to this day; active balancing rates can be widely variable, the only use of an area of land, then transfers charge from one cell to with observed rates from 0.22% the value of energy must be higher another in a selective and optimal to almost 3% per year (Ndiaye et than the value of alternative uses for fashion through power electronics, al., 2014). Therefore, for India, it is that land. Loss of agricultural land resulting in higher roundtrip important to use PV modules with is sometimes cited as a reason for efficiency albeit at additional cost good resistance to tropical climate opposition to solar PV (Stronberg, and complexity. and to monitor their performance 2019). Only low-value land, such as over time with regular replacement poor grazing land, brown-field sites A.9. OTHER BARRIERS TO of deteriorated and damaged and desert land, is unquestionably PV TECHNOLOGY modules. suitable for such exclusive use by PV farms. When such sites A.9.1. High Temperatures A.9.2. Dust and soiling are separated from centres of The performance of solar PV Dust accumulation heavily depends population by long distances, they panels is certified under standard on rainfall and atmospheric incur high network expansion and test conditions (STC), but those levels of dust. In the UK context, electricity transmission costs. It is conditions are never encountered it is rare for PV power output to worth noting, however, that there in the real world. Standard test be significantly reduced by dust, are great examples of low-value conditions include a temperature but in hot dry climates, power land close to centres of population, of 25 °C, an irradiance of 1000 W/ output can be reduced by as such as brown-field land in cities, 2 m and a light spectrum equivalent much at 34% (H. Qasem, 2013). In poor moorland close to Sheffield to 1.5 air mass (Muñoz-García et Kuwait, the optimum PV cleaning and Manchester in the UK (Natural al., 2012; Ndiaye et al., 2014). While frequency was calculated as England, 2010) and a non-rice the power output of the PV panel about once every 8 days. Similar growing area close to Jaipur in India increases with high irradiance, it weather conditions occur in some (Xiao et al., 2006). While installing SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 37
rooftop solar in towns and cities is now well established (Wiginton, Nguyen and Pearce, 2010), it can only provide a small portion of the local energy use on its own, especially in densely populated cities (Peng and Lu, 2013).
It is generally preferred to combine an open-field solar installation with other land uses. Agriculture is a good such option, also known as agrivoltaics. Grazing under PV arrays is very common, but systems of arable cropping under PV arrays have also been established (Dupraz et al., 2011; Dinesh and Pearce, 2016; Lane et al., 2017; Santra et al., 2017; Aroca-Delgado et al., 2018), mostly in experimental settings and greenhouses. 38 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
PART B: ROADMAP TO HIGH SOLAR INTEGRATION
Higher solar integration into the grid overcoming the main barriers of There are also two key technologies entails mitigating the challenges solar integration, organized into four in this regard that are worth it brings in terms of balancing, directions as shown in Figure 26. mentioning separately: demand stability, voltage and protection The major direction in this roadmap response and energy storage. in the power system. There is no is the ongoing transformation and Demand response has great golden rule on how much solar can rethinking of the power system potential in tackling flexibility and be accommodated to a network towards higher flexibility and more balancing challenges by better without interventions, and there RES-receptiveness. These actions aligning supply and demand over are examples in several countries involve: more flexible generation the day, and also in mitigating that have experienced very high with higher ramping rates; more stability issues that may arise with solar and other RES penetration frequent security-constrained generation intermittency. Similarly, for short times. However, a future economic dispatch (e.g. as often as energy storage may substantially power system with solar generation every 5 minutes); redesigning the assist in balancing the system levels that exceed other sources electricity market and updating grid and in voltage regulation of the on a regular basis has to seriously codes to account for new and more network; if collocated with the consider the challenges of the relevant ancillary service products; solar plant, these benefits are previous section or risk the reliability more credible forecasting of solar maximized by converting the plant and security of the grid. generation; cooperation between to a dispatchable power station with the TSO and DSOs in managing additional ancillary services that This chapter discusses a wide the embedded solar; improved makes a tangible contribution to the range of actions and interventions voltage control and redesigning power system stability. in the power system that assist in the protection schemes in the distribution network. Furthermore, apart from what the power system can do to allow more solar, there is also what solar can do for itself by becoming more grid-friendly. This entails a wider range of services to the grid,
Directions Barriers especially fault ride through (FRT), Rethinking Balancing and operating in grid-forming mode the power & flexibility to assist in challenges related to system challenges operation stability, voltage and protection. To unlock this full potential, the Demand response Stability future solar plants should be able issues to maintain some power in reserve, Energy by collocating energy storage or storage Voltage curtailing power or both. issues More grid-friendly solar PV Protection systems challenges
Figure 26. Roadmap to high solar integration, showing how each direction addresses individual barriers SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 39
B.1. RETHINKING the reactive and active power will probably incur prohibitive costs, THE POWER SYSTEM capabilities of solar and other although there a few pilot projects OPERATION DERs. The protection regime of ongoing that explore faster ramp the distribution network may rates and lower technical minimums A limited amount of solar can be also require a restructuring, from coal plants (GTG-RISE, 2020). readily integrated into the power deploying new types of relays Going for gas-fired plants instead system with minimal interventions. and telecom infrastructure, much seems a better alternative for With increasing levels of solar like in microgrids. Furthermore, to thermal power stations, as they penetration, however, come higher avoid deterioration of the power enjoy very low technical limits, short needs for flexibility and more quality in low and medium voltage start-up and shutdown times, and advanced operation regimes. networks, harmonic emissions of high ramping rates; this is one of the Most importantly, there is a need PV inverters should be regulated reasons that the UK’s strategic plan for more flexible generation with even at low generation. Finally, involves mainly gas and no coal at lower technical minimum limits and the grid codes and regulations all as thermal energy sources into higher ramping rates to support need to be constantly updated to the electricity mix from 2025 and the variable and intermittent account for changes in the system onwards (National Grid UK, 2018a). solar generation. Promising and technology advances, always options include gas-fired plants maintaining a balance between Hydro power plants are also a in place of coal, more hydro and grid security and incentives for solar very good source of flexibility and interconnections where possible, deployment. already play a dominant role in the and fully deploying the new Indian energy mix, although the technologies of demand response B.1.1. More flexibility and improved relevant geographical constraints and energy storage. This more generation dispatch do not render hydro a universal versatile electricity mix should Increasing the flexibility of the solution for every power system. be coupled with more frequent power system is one of the major In the UK, interconnections with generation dispatch and more requirements to allow for more neighbouring countries will continue reliable solar forecasting to minimise variable and intermittent generation to be a very important source of the flexibility requirements. Relevant like solar into the grid. An essential flexibility, supporting ramp rates to solar forecasting, although step towards this goal is tapping of more than 50 MW/s and being further improvement in accuracy is the flexibility potential of existing able to alleviate many ROCOF and expected with recent advances in energy sources by lowering their downward regulation constraints machine learning, attention should technical limits and boosting their (National Grid UK, 2016). It is also be paid also to the balancing ramping rates. This is a challenge expected to increase the available mechanism for the deviations for most thermal stations that have flexibility by extending these services between forecasts and actual been traditionally designed as less- to smaller generators and DERs generation. flexible base-load units, although via aggregators. Furthermore, there are relevant successful apart from the aforementioned In addition, it has become examples in the global scene, such conventional options, the new important that the TSO and DSOs as technical minimum of 10% in technologies of demand response start to effectively coordinate Kauai, Hawaii. In India, the technical and energy storage are very their actions to exploit ancillary minimum requirement for central promising sources of flexibility, services by embedded solar and power plants is currently 55%, but it discussed in more detail in the support upward power flows to the is expected to be reduced in view following sections. transmission network. More effort of the ambitious RES targets by should be put into the voltage 2022; the net load ramp-up rate is control of distribution networks, also forecasted to reach 32 GW/ leveraging new optimization hour. Converting coal-based units techniques and fully exploring to meet these new flexibility needs 40 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
Another tool towards dispatch is accurate short-term 2016). An accurate and reliable solar decarbonization of the energy forecasting of the solar generation, forecasting tool essentially removes sector that has attracted a lot of as discussed in a separate section the uncertainty and stochasticity interest lately is sector coupling below. Such enhanced power features from the solar generation, across several energy vectors, scheduling should be followed by converting it into a semi- such as electricity, heating, a flexibility market framework that deterministic, albeit variable, energy cooling, transportation and gas. will incentivise sources of flexibility source with higher capacity credit. Historically, there has only been to support the solar generation at limited coupling of electricity and times of high penetration. Two major families of solar heat in combined heat and power forecasting methods exist today: the (CHP) plants in some countries, Last but not least, to actively physical methods and the statistical but a universal cross-sector multi- include the solar PV systems in methods (Sobri, Koohi-Kamali energy coordination is yet to be the power dispatch, the visibility and Rahim, 2018). The physical implemented on a national basis. and telecommunication access methods employ meteorological This approach usually involves to these systems have to be measurements and models to infer substantial electrification of the upgraded. An improved monitoring the expected operating conditions energy sector and allows for self- and communication framework of the PV system. They are further reliance on a local level, forming would be beneficial to allow the divided into Numerical Weather energy islands. For example, massive operator to have minute-resolution Prediction (NWP), Sky Imagery and electrification of the transportation visibility of the power output and to Satellite-Imaging models. NWP would bring an additional source of communicate power commands usually adopts meteorological flexibility to the system, in terms of to both large and small PV plants models and involves radiative balancing, reserves and frequency in real-time (e.g. reactive power transfer equations of the solar flux services, as well as reducing commands for voltage regulation). and atmospheric dynamics; it is the carbon emissions of the Regardless of this control regime a well-established approach that transportation sector. Multi-energy being centralised or decentralised, produces its best results for periods sector coupling is seen today as one having some kind of bidirectional from 6 hours to two weeks and is of the most promising ways forward communication between the PV almost exclusively used by the US to increase flexibility and renewables plants and the operator is seen and European weather services. penetration in the power system. as a credible way to facilitate The Sky Imagery method requires a coordination of the various ground-based digital camera that In terms of system balancing, there components within the network. captures a hemispheric image of is also a need for more frequent and the sky to extract cloud presence, effective generation scheduling. B.1.2. Better solar forecasting speed and direction by generating Unit commitment and economic The impact of the solar generation cloud motion vectors from dispatch that used to be performed uncertainty on the power system consecutive images; this is quite with an hourly or half-hourly time can be mitigated to some extent useful for sub-hourly predictions. step in most countries, should be by a good forecast; knowing with Satellite Imagery also captures carried out more often, every few confidence what the solar profile cloud patterns, but from above, minutes, to account for changes will be in the next few minutes to employing cloud motion vectors in both supply and demand. For hours permits effective generation as well. This method is very good example, ERCOT moved from a scheduling and balancing in the for forecasting horizons between 15 15-min zonal market to 5-min system, successful voltage control in minutes and 6 hours. nodal market in 2010, which led to the distribution network, and keeping higher integration of renewables in the right amount of reserves and the ERCOT system (Adib, Zarnikau flexibility online towards secure but and Baldick, 2013). An essential also economical system operation component of more effective (Brancucci Martinez-Anido et al., SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 41
The statistical or data-driven regions caused by local weather The traditional distribution networks methods, on the other hand, depend phenomena are somewhat filtered have evolved into Active Distribution on past data of meteorological out in the transmission level, having Networks (ADNs) that may utilise parameters and solar generation little effect on tasks like balancing. the embedded generation for more and aim to extract the relation On the other hand, voltage economic, reliable and secure between the two. Regression and regulation is a local optimization operation of the entire power system autoregressive and moving average in the distribution network and the beyond the distribution level. Locally functions (ARMA or ARIMA) models regional solar intermittency needs installed DERs have the potential are such examples. Spatio-temporal to be accounted for. Therefore, for a wide range of system-wide kriging allows predictions to be the various control operations in ancillary services (AS) that extend made at one or more locations the power system have different to the transmission layer, such as between and beyond observation solar forecasting needs in terms frequency and voltage support, times. Another group of time- of accuracy, forecasting horizon, congestion management and series functions are the Artificial temporal and spatial resolution. balancing, flexibility and black-start Intelligence (AI) methods which Deviations between forecasted and services. are learning approaches. Popular delivered solar generation lead to representatives are: (i) k Nearest a balancing gap that is currently To fully unlock this potential, Neighbour (kNN), a classification addressed in different ways however, an appropriate layer of and voting technique; (ii) Support worldwide. Some countries penalise real-time communication and Vector Machine (SVM), another the variable generator for deviations coordination between the TSO and classification method; (iii) Artificial above a certain limit, which implies the DSOs has to be in place, in order Neural Network (ANN), a computer an additional financial burden for to align requirements and resolve network which mimics human the investors. In other countries, potential conflicts. Harmonizing the learning; (iv) Genetic Algorithms the generator operator is required way these operators work is not a (GA), which carry out selection to fill the gap from the balancing trivial task. A recently completed based on fitness, i.e. forecasting market, which again introduces European project has concluded in accuracy; and (v) hybrids of the important financial constraints and five different potential cooperation above methods. These statistical risks at times of high irradiance schemes given in Table 6, exploring methods need several months of uncertainty. Co-locating solar and various market mechanisms data collection prior to deployment. energy storage (e.g. batteries) is and alternatives for the level of They deliver accurate results when seen as a solution that resolves responsibility among the TSO and used for sub-hourly forecasting but short-term forecasting errors, DSOs (SmartNet project, 2019). Given also for time horizons of a few hours albeit with additional installation the current regulations in the UK or more. Each method has its strong and service costs. The take-home and India, the centralized AS market points and weaknesses, so the latest message here is that accurate solar seems to fit better to these needs, research combines techniques from forecasting has become vital for having the TSO undertaking and different families to achieve robust many stakeholders in the power leading the role in this coordination. estimation at different horizons system, from the solar plant owner (Diagne et al., 2013; Su, Batzelis and to the DNO and TSO. Pal, 2019). B.1.3. Transmission/Distribution An important aspect in solar systems coordination forecasting is the spatial diversity The constantly increasing of the solar resource and the penetration of DERs in the inherent smoothing out of the local distribution level, including solar, intermittency at the transmission has rendered the cooperation system layer. Forecast errors between the two layers of the from individual plants or small power system an emerging need. 42 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
TSO/DSO COORDINATION MODEL MECHANISM Centralized ancillary services The TSO manages the AS market for both distribution and transmission sources with (AS) market minimal DSO involvement Local AS market The DSO first addresses the local constraints and then offers the remaining services to the TSO Shared balancing responsibility The TSO and DSO agree on a common schedule Common TSO-DSO AS market Both operators aim at reducing AS costs either via a common market operated jointly or by effective integration of TSO and DSO local markets
Integrated flexibility market A common market for both regulated and deregulated players run by an independent market operator
Table 6. TSO/DSO coordination models proposed in the SmartNet project (SmartNet project, 2019).
In the UK ‘Power Potential project’, diversity of the resource that function can mitigate to some National Grid UK (TSO) and UKPN provides an inherent smoothing extent the voltage fluctuations (DSO) explored the potential for mechanism to an otherwise induced by the PV system itself, but reactive power/voltage support intermittent generation. From the it is also an excellent local source and active power support from the TSO’s perspective, treating all solar of reactive power for loads in the DSO through a model similar to PV systems in the network as an vicinity, which reduces the reactive the local AS market, aiming mainly aggregated less-intermittent power power flows from the substation at resolving voltage and thermal plant will lower the balancing and and the relevant power losses. It is constraints at the transmission flexibility requirements. Although it is indicative that the IEEE 1547 standard level (National Grid UK, 2020b). The commonly accepted that the TSO/ initially expected a unity power distribution system is considered DSO cooperation is a key technology factor from PV plants (IEEE, 2003), but by the TSO as a virtual power to high RES integration, there are it now requires participation of the plant with the capacity to provide still many operational and market solar systems to voltage regulation dynamic reactive and active power challenges to be overcome to fully by reactive or active power injection support to the transmission level. deploy this concept. (IEEE, 2018). The ‘Power Potential For this purpose, a digital platform project’ in the UK also explored this facilitates the power exchange B.1.4. Improved voltage control in concept in the south-east part of between the two layers of the the distribution network England (National Grid UK, 2020b). power system, exploring three Maintaining the voltage within There are many ways that this scenarios of support services: (a) limits has become increasingly voltage support is implemented reactive power consumption by challenging in the distribution in a PV system, most of them the distribution system during network due to the integration of employing droop characteristics overvoltage at the transmission solar and other variable generation. of voltage-reactive power, active network (e.g. during light loading of The intermittent and fast-changing power-reactive power, voltage- the transmission lines), (b) reactive generation makes it difficult for active power, or more simply power injection during voltage dips conventional regulation devices, constant power factor or constant at the transmission level (e.g. due such as on-load tap changers reactive power value (IEEE, 2018). to a fault), and (c) active power (OLTCs), step voltage regulators and The challenge that arises is how support for managing congestion at switched shunt capacitors, to control to coordinate all the regulation the transmission system. the voltage throughout the network. equipment of the network (OLTCs etc.) along with the PV systems in an Another reason for TSO/DSO A new tool explored lately to this efficient and optimal manner. There coordination when focusing end is the PV inverters to provide are basically three big families of on solar generation and other voltage support services by injecting improved voltage control which are variable renewables, is the spatial or consuming reactive power. This currently under investigation and development, detailed in Table 7. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 43
METHOD FUNDAMENTALS
Centralised These methods rely on extensive communication and employ a single central controller that coordinates Methods all devices in the network. They employ mathematical models of the network and real-time measurements to form an optimization problem that is solved centrally (Ul Nazir, Pal and Jabr, 2019). Advanced versions of these techniques consider the dispatch of PV inverters along with conventional devices (Dall’Anese, Dhople and Giannakis, 2014) and the uncertainty of solar generation (Kekatos et al., 2015).
Distributed These methods distribute the optimization effort over various controllers, each controller being Methods responsible for a separate zone of the network (Dall’Anese et al., 2014). Allowing proper coordination among these controllers facilitates the optimality of the final solution. In addition, these distributed solvers also resolve communication bottlenecks, permitting parallelization of the solver and limited information exchange, and increase the resiliency against cybersecurity threats.
Decentralized Under these methods, each device regulates the voltage autonomously following its own control Methods rule. This rule may differ among the devices and is either fixed (hard-wired in the local controller) or transmitted and updated regularly by a central controller (Jabr, 2018).
Table 7. The different philosophies of voltage control in the distribution network
B.1.5. Rethink the protection of One of the popular methods is to harmonics and transients. There is distribution networks use directional overcurrent relays also another alternative based on The protection challenges and adapt their settings according voltage measurements and Park experienced recently in the to the microgrid state or topology transformation (Loix, Wijnhoven distribution networks were found first (Mahat et al., 2011; Laaksonen, and Deconinck, 2009), but this in isolated microgrids. Bidirectional Ishchenko and Oudalov, 2014; Coffele, again struggles at short lines, power flows, meshed grid and Booth and Dyśko, 2015; Tummasit, has a strong dependence on the limited overcurrent capacity by Premrudeepreechacharn and network topology and has high the inverters are no strangers to a Tantichayakorn, 2016). This continuous computational complexity. microgrid, which is why the relevant adaptation is implemented through research and experience is a source local measurements or a central One of the most promising of inspiration in restructuring the unit. The main challenge in these directions is the differential protection of distribution networks. protection schemes is a need for protection-based techniques A summary of major directions in prior knowledge of all possible (Sortomme, Venkata and Mitra, 2010; microgrid protection is given in network configurations and heavy Dewadasa, Ghosh and Ledwich, 2011; Table 8, all mandating additional computational execution time Sortomme, Ren and Venkata, 2013; sensors or telecom infrastructure. whenever the network changes. That Xu et al., 2016; Kar, Samantaray and might be an issue when extending Zadeh, 2017). These methods are It is worth noting that most of these this approach to the distribution very accurate in detecting any type schemes are tailored and derived network. of faults, as they are not affected for a particular microgrid, i.e. certain by bidirectional flows, current topology, generation mix, location, Distance protection schemes levels, the number of DERs, weak size and operating conditions inspired by the transmission infeed or the microgrid operating (Hooshyar and Iravani, 2017; IEEE, system employ admittance or mode. They also remain effective 2019), and do not necessarily remain impedance relays (Dewadasa in meshed microgrids and fully appropriate and effective if the et al., 2009). They can be applied support the LVRT function of solar or system changes substantially. This is to both medium and low voltage other DGs. The main weak point is a major limitation when translating levels but are not very effective the communication infrastructure these approaches to the distribution in detecting phase to phase and required, which entails significant network, which is much more volatile earth faults or high-impedance additional costs especially when and evolving. faults, and they are susceptible to appropriate redundancy is admittance measurement errors, considered for safety reasons. 44 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
PROTECTION SCHEME ADVANTAGES DISADVANTAGES Adaptive Protection Dynamic change features in settings Expensive due to communication infrastructure between relays • Communication Complex, involves heavy computational simulations and improves the speed of operation • Can can compromise real-time operation • Communication be effective in grid-connected and dependant which can impact reliability islanded mode of operation
Directional Effective in bidirectional current flows Expensive, as it requires voltage and current Overcurrent Good selectivity • Can allow local or measurement devices • Prior knowledge of all possible Protection central implementation system configurations Distance Protection Can provide fast operation • Can be Expensive, as it requires voltage and current measurement suitable for both grid-connected and devices • They may not be effective for short line and islanded mode of operation DG-intensive systems due to high sensitivity to DG fault contribution • Inability to detect high impedance faults • Susceptible to harmonics and transients
Voltage Effective fault detection Coordination issues for short lines • Requirement of measurements voltage measurement devices • Network structure dependence • May not detect high impedance faults
Differential High fault detection accuracy • Expensive communication system • Communication Protection-based Immune to bidirectional power flow, dependent • Requirements for synchronised techniques changing current levels, number of DGs, measurements for long lines • Traditional differential system topology and operation mode protection may fail in islanded mode of operation
Table 8. Summary of protection schemes proposed for microgrids
B.1.6. Improve the power quality in much higher as a percentage, should be a plan from the network distribution networks which poses a risk for high distortion operator and a coordination with the The power quality issues brought by levels in the network at times. In PV installers to evenly distribute these type-tested (certified) embedded addition, these requirements are systems across the three phases in solar PV systems to the distribution more relaxed for small kW-scale PV the same area, aiming at balanced networks are not significant at the systems and are not quite followed power flows throughout the year. moment, but there are concerns for in some developing countries. Going for three-phase connections the near future especially related There is a need for universally in smaller PV systems is also an to the 40 GW rooftop solar target in more stringent requirements on effective measure to this goal. India. These issues are mainly solar- the harmonic emissions from PV induced harmonic distortion and inverters across the entire power B.1.7. Continuous evolution of voltage unbalance, but also voltage range. There is today mature grid codes and ancillary service rise/swells and flicker. technology on multi-level converters products and advanced filter design that The grid code is a key factor for the Although there are regulations can meet these targets and thus integration level of renewables into on the total harmonic distortion these should be encouraged in the the power system. The successful of PV inverters, these usually solar system design and installation grid code takes the concerns of all refer to power levels close to the industry. stakeholders into consideration, nominal power rating, i.e. when while maintaining grid security and solar generation is high, and the Voltage unbalance is already an stability as the primary objective. proportion of harmonic distortion issue in rural India and there are While lenient regulations may is relatively low. However, at lower concerns that it may be further potentially compromise grid security generation conditions, usually in amplified with the rapid uptake of and place extraordinary burden the morning or on a cloudy day, small-scale rooftop solar connected on the system operator, stringent the solar-induced harmonics are in a single-phase fashion. There codes may disincentivise further SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 45
solar deployment thus defeating renewables. It is indicative that power system needs, is not entirely their very purpose. It is worth noting EirGrid in Ireland and National Grid new, but it has been applied so far that for many renewables the cost UK in the UK have already introduced only in selected industries and it has of technology is closely linked to the fast frequency services in their yet to be fully deployed to system- grid codes compliance, this being respective control areas. In EirGrid wide commercial and household a critical factor for the pace and in particular, seven new system loads. extent of a particular renewable services were implemented in 2018, penetration in a market. The grid with the market value around € 235 The DR technology has great codes should follow a technology- million per year, as opposed to € 50 potential in mitigating the balancing neutral and realistic approach million the year before. and flexibility challenges brought that strikes a balance among the by solar. Time-shifting part of the diverging interests of all involved It is important to also monitor demand to coincide with solar stakeholders. technology developments in generation (e.g. at midday) makes the field and new challenges scheduling easier and more Furthermore, these regulations brought. For example, the smart economic, while cutting off some should be regularly revised and grid technology has created load in the same way that solar updated to account for changes cybersecurity risks, while the generation changes (e.g. solar in key factors in the network, such grid-forming technology has the retraction towards evening) reduces as the grid strength (SCL), the potential for increased grid support the ramping requirements by other generation mix and RES penetration by solar and other IBRs; these sources and the flexibility needs. DR levels. For example, a large share advances should be taken into can be also found useful in power of old-type wind generators with account in the new grid codes to system stability issues brought by induction machines indicates a guide easing or tightening parts solar and other intermittent sources. weak grid and slow post-fault of the regulations accordingly. Committing part of the demand voltage recovery, thus necessitating Moreover, the ancillary services to be adjustable and interruptible stringent LVRT requirements to market and the grid codes need may effectively absorb unforeseen ensure grid stability after a severe to be reviewed simultaneously fluctuations of solar generation (e.g. fault. However, these old and and in a coordinated fashion to due to clouds), thus limiting their directly coupled wind turbines will avoid inconsistencies or conflicts impact to the transient stability and be replaced with newer variable- between the incentives for voluntary frequency stability of the power speed ones in the future, resulting involvement driven by the former system. This technology along in stronger grid that permits relative and the mandated requirements set with energy storage has a great easing of the LVRT requirements. out by the latter. potential in tackling some of the most fundamental barriers to solar In addition, new ancillary service B.2. DEMAND RESPONSE integration. products will be needed to account for the new generation mix with In the traditional electricity system, Electricity is not an isolated form of lower synchronous generation, the energy demand has been energy but must be seen as part of reduced system inertia and grid treated as inelastic except for an integrated low-carbon energy strength. For example, fast frequency stability reasons, thus putting all system, as depicted in Figure 27. response over the first few seconds the burden to match supply and DR can be given by a multitude of of a disturbance in the power demand on the generation side. This electrical loads that provide some balance will be key in a low-inertia may not be an option anymore with service or commodity that can power system. Introducing new the massive uptake of renewables be either interrupted with minimal ramping products of various rates like solar, which are to a great extent impact on users or stored for later by both conventional and new variable and intermittent power use. A discussion on such loads generation is also expected to assist sources. The concept of demand follows. Other terms associated with with the variability of solar and other response (DR), i.e. modifying the DR and used interchangeably or with load consumption according to the overlap in meaning include: 46 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
• Demand-Side Management (DSM) Primary Energy Transformations Final Energy Uses or Demand Management (DM)
which can also mean demand Solar Energy Heat Homes reduction and reduction in peak storage for pumps Coolth electricity demand • Demand-Side Response (DSR) Commercial Heat and other • Demand-Side Participation (DSP) Wind buildings Electricity • Flexible Demand (FD) Combined heat and Electric Electricity power vehicles B.2.1. Examples of flexible electrical Hydro loads Electrolosis hydrogen Industry The following are a few examples Hydrogen and Wave / Tidal production of common flexible loads from agriculture a very wide range of energy use Synthetic Long Biomass Synthetic fuel distance sectors that can be used for DR (Gils, and waste fuels production transport 2014). This list is indicative and not exhaustive by any means.
Water pumping. Water pumping Figure 27. Energy flows in an integrated low-carbon energy system. can be done at any time of the day because water can be stored in water tanks. Stand- alone solar PV-powered water demand for a few minutes, perhaps PV power at high power levels when pumping is becoming increasingly for over 1 hour (Short, Infield and surplus electricity is available. Heat economically viable for irrigation Freris, 2007; Lakshmanan et al., 2016), pumps are more expensive and and drinking water in rural locations without noticeable impact in their require a low-grade heat source, (Odeh, Yohanis and Norton, 2006; performance. Freezers can provide and although more efficient than Chandel, Nagaraju Naik and DR for several hours, even running resistive heating (COP>1), their Chandel, 2015; Engelkemier et al., on solar PV power alone (González higher cost and lower heat delivery 2019), but grid-connected water et al., 2018). Air conditioning of temperature make them less pumps can also provide DR services indoor spaces can provide DR with popular for water heating. (Ma et al., 2013; Menke et al., 2016). a duration of about an hour (Zhang et al., 2013), but disruption of any Space heating. Heat pumps are Refrigeration and air conditioning. longer duration affects the thermal the leading technology to replace Cooling delivers lower temperatures comfort of building occupants, fossil fuelled space heating and can to a space and materials within so it is appropriate for short- be readily converted to a flexible that space. There is usually a range term interruptions. A promising load. However, like air conditioning, of acceptable temperature and enhancement is air conditioning the opportunity for modulating a substantial thermal mass of with ice storage, with already a few the heat input only lasts for up to material that allows flexibility of products available (Kosi et al., 2016), about 1 hour. Although buildings the timing of the cooling process but the uptake has been slow. have significant thermal mass, (Wai et al., 2015). Refrigeration the occupants’ tolerance on air and air conditioning are large Water heating. Water has a very temperature for thermal comfort is and growing sectors of electricity high specific heat capacity, is easily quite narrow, so space heating has demand and can provide significant stored, and hot water is widely used, great potential for minutes-duration amounts of DR. Domestic and both domestically and industrially. interruptions (like air conditioning) small commercial fridges can Resistive water heating is very cheap that can be used to absorb solar easily provide a 25% reduction in to install and can make use of solar intermittency due to passing clouds. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 47
Electric vehicle charging. The potential for time-shifting DR and the plant capacity should increase transport sector must also acting as flexible generators in the to afford less-than-maximum decarbonise, and the leading network. production at times. Assuming a technology to replace fossil fuels required average industrial output in surface transport over short Energy intensive industries. Some of 100 units per hour in this example, distances is electric vehicles industrial output can be ramped up three different approaches are (EVs). This is expected to create or down, interrupted or shifted to shown: a very large increase in demand times of the day when solar power • ‘Flat Output’ does not provide any for electricity generated from is more abundant. Energy intensive DR and requires a production low-carbon sources, but also an industries that could make large capacity of 100 units per hour opportunity for flexible electricity contribution to demand flexibility • ‘Peak Lopped’ ramps up and down use. The batteries onboard EVs are include aluminium, chlorine, paper production according to solar a form of energy storage that allow production and recycling, steel and generation and entails a 33% separation of EV charging from EV cement (Gils, 2014). capacity increase to 133 units use. The flow of energy is mostly per hour from the electric grid to the vehicle, Most of these industries currently • ‘Solar Powered’ strictly follows but new EVs have also the capacity operate 24 hours a day and any the solar generation profile and to provide ancillary services and variation in output has a knock-on necessitates an increase over inject some power back to the grid effect on capital cost. For a given 300% in capacity to 412 units through the so-called vehicle-to- total output per day, the plant per hour grid (V2G) mode. In this regard, EVs capacity is inversely proportional can be treated as mobile energy to the full-power hours of operation Therefore, there is a trade-off storage units that have great per day as shown in Figure 28, i.e. between electricity cost and capital cost of the plant: whether to invest in manufacturing capacity, invest in onsite electricity storage, or to buy in somebody else’s demand response. 450
400 Water desalination. Water scarcity 350 is forcing many countries to use desalination to provide enough 300 drinking water for their population. 250 The leading technology for new 200 desalination plant is reverse osmosis
150 (RO), which is electrically driven. The RO desalination installation is Output rate, units per hour 100 expensive, but the electrical energy 50 used to run the plant is also quite 3 0 large at typically 5 kWh/m of 0 3 6 9 12 15 18 21 24 water. RO desalination is therefore Hour of day a particularly important example of an energy-intensive industry, which has potential for time-shifting and Flat output Peak lopped Solar powered DR with proper oversizing and water storage facilities. Figure 28. Manufacturing output rate for three flexibility scenarios. Peak capacity increases as operating hours reduce. Solar irradiance time series generated by the CREST demand model (Barton et al., 2017; Barton, 2018). 48 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
Hydrogen and synthetic fuel production. In order to decarbonise all sectors of the economy, low carbon alternatives must be found for all uses of energy outside of the electricity sector, including, for example long-distance transport Power fuels, fertiliser production and iron smelting. These sectors cannot be directly electrified and must use a fuel, which may be synthesised. Synthetic fuels also provide a means of long-term or inter- Time seasonal energy storage. Examples of synthetic fuel include hydrogen, RE Supply Electrical demand ammonia, methane (synthetic Electrolysis (grid balancing and hydrogen fuel production) natural gas), methanol and synthetic long-chain hydrocarbons to substitute for petrol, aviation fuel and diesel. Figure 29. Hydrogen fuel production becomes a load management technique for a zero-carbon electricity grid (Gammon, 2007). The industrial route to all the above synthetic fuels is via hydrogen. While low-carbon hydrogen can is that hydrogen production can services. National Grid UK buy the be produced from fossil fuels be directed to absorb the solar following services (National Grid UK, with carbon capture and storage variability as shown in Figure 29, thus 2020a) specifically related to DR of (CCS), such hydrogen will never serving as an excellent source of active power (so excluding reactive be completely carbon neutral due flexibility and balancing to the power power for voltage support and black to fugitive emissions of methane system. start etc.): and incomplete CO separation 2 • Demand side response (DSR) and storage. In a truly sustainable B.2.2. Incentives and market • Demand turn up (DTU) energy system, hydrogen is made mechanisms • Enhanced frequency response from sustainable feedstocks and (EFR) energy sources. Unless there are United Kingdom • Fast reserve big technological breakthroughs in In the UK, the existing market • Firm frequency response (FFR) areas such as direct photochemical mechanisms for demand response • Mandatory response services water splitting, electrolysis will be consist of reduced unit prices for • Short term operating reserve (STOR) the principle source of hydrogen interruptible electricity supply, high and an excellent flexible load. In a charges for consumption at times National Grid UK currently zero-carbon economy, given the of peak demand (triad charges), contracts with just 19 companies large amounts of fuel required charges based on peak monthly as aggregators of balancing by industry and long-distance demand, and half-hourly metering. services, and some of these transport, the renewable energy The details of these schemes are aggregators have contracts with generation will be much larger than specific to the contracts between smaller providers of DR. The market the purely electrical demand, so consumers and their electricity is changing, and new trading as to support hydrogen production supply companies but they are platforms are being created for intended for other energy uses. A bundled together and sold to DNOs and aggregators to buy and great opportunity in this approach National Grid UK (TSO) as ancillary sell flexibility services, for example SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 49
‘TraDER’ and ‘Piclo Flex’ (Schittekatte automatically. Tertiary response with turn-up and turn-down and Meeus, 2020). and generation scheduling are signals despatched manually on longer • Aggregation into dispatch units Until recently, such market timescales. that are large enough to sell mechanisms have been available services into the ancillary services only to large commercial and India’s states effectively buy and sell market (Lipari et al., 2018) industrial consumers. Domestic electricity from each other at a price • The acceptability of more volatile customers could have simple based linearly on system frequency electricity prices, including much Time-Of-Use tariffs (TOU) tariffs. (CERC, 2017b), but this mechanism higher prices at times of peak However, DR pricing is now available does not extend to suppliers or demand and low supply (Octopus to all smaller consumers through a customers. India’s electricity system Energy, 2020) Dynamic Time-Of-Use (dTOU) tariff regulator, the Central Electricity • Implementation of local and offered by one electricity supplier Regulatory Commission (CERC) variable pricing mechanisms in the UK (Octopus Energy, 2020), recognises that the market for (Albadi and El-Saadany, 2008) and to some EV owners through ancillary services needs to expand • Issues of trust and buy-in (Albadi a specific supplier (Honda, 2020). and develop and therefore published and El-Saadany, 2008) Various dynamic electricity pricing a discussion paper in 2018 (CERC, • Market stability and price-finding tariffs have been created in other 2018). The provision of ancillary (Nguyen, Negnevitsky and De countries and research trials carried services depends on dispatched Groot, 2012) out (Schofield, 2015; IRENA, 2019; generators operating at less than • Investment in smart appliances Hussain and Torres, 2020). rated power and therefore able • Weather dependence and to quickly ramp-up power output. unpredictability affecting business India The transmission system operator, planning India has an ambitious target of known as Power System Operation installing a total renewable energy Corporation Limited (POSOCO), Schemes for implementing DR fall generating capacity of 175GW by recognises that this surplus into two types: direct control and 2022 (CERC, 2018; Tongia, Harish capacity is in short supply at times real-time pricing. Direct control and Walawalkar, 2018), but India’s of high demand. CERC propose methods (Nguyen, Negnevitsky and ancillary services market is more a transparent market in ancillary De Groot, 2012; Lipari et al., 2018) limited and traditional than in the UK, services in which ‘all technologies use a system of offers and bids with almost all services provided by and services should be able to by market participants in bilateral gas-fired, coal-fired and large hydro compete for Ancillary Services on a contracts, often to control individual power plants (Priolkar, 2015; Kumar ‘level playing field’ and ‘regardless of flexible loads, with immediately et al., 2018; Singhvi, 2019). Ancillary size or type’ (CERC, 2018). verified change in demand. In services are used for both grid contrast, real-time pricing strategies balancing and overcoming network B.2.3. Future Challenges and (Lu and Nguyen, 2005; Dutta and constraints. Balancing services, Strategy Mitra, 2017) employ a broadcast called ‘Reserves Regulation Ancillary For DR to achieve its full potential, price signal, or use properties of Services’ (RRAS) are despatched by the markets for ancillary services the network itself (voltage and the Regional Load Despatch Centres in both India and the UK need to frequency) to this end. Real-time (RLDC) for each of the five grid continue to reform to enable all pricing appears to be simpler and regions. RRAS are provided by just 67 available technologies to participate easier to implement and requires large power plants spread across at all physical scales. There are much less communication between India (CERC, 2018). Ancillary services many challenges in the contractual parties. Real-time pricing relies on are provided at sizes of 1000MW complexity, communication and an aggregated average response or more. Inertia, primary response aggregation of DR providers at all but is more appropriate to smaller and secondary response (response scales (Rajabi et al., 2017), namely: consumers and smaller flexible times up to 15 minutes) are provided • The need to prove compliance loads. 50 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
B.3. ENERGY STORAGE Many technologies exist for large- into the grid (e.g. summer days) scale energy storage, but smaller to be deployed at other times Energy storage is undoubtedly a kW-scale systems employ mainly of the year (Hunt, de Freitas and key technology for the integration of batteries. The battery technologies Junior, 2017). Typical round-trip solar and other variable renewables, are more appropriate for collocation efficiencies are 70-85% for pumped exhibiting lately great potential within the solar plant and have also hydro (Ibrahim, Ilinca and Perron, with the advances in battery recently enabled a mobile version 2008) and 45-70% for compressed technologies. Coupling solar with of energy storage in the form of EVs. air (Budt et al., 2016), (Zavattoni et energy storage essentially converts The following sections discuss the al., 2017). In the UK, pumped hydro the variable solar source to a fully future and placement of energy currently dominates the energy dispatchable power station that storage in regard to solar, as well storage sector with a total capacity has grid-support capacity similar as the role of EVs as mobile energy of about 26.7 GWh and provision to conventional plants. This pairing storage devices. of about 3 TWh in 2018 (Table 9). can alleviate most of the balancing There is also some potential in and flexibility challenges brought B.3.1. Future prospects converting dam hydro into pumped by the variable solar radiation, while National Grid UK considers energy hydro plans (Scottish Renewables, it paves the way for a full range of storage as a key factor for all 2016). However, this technology ancillary services and contributes to future scenarios of a high-RES UK has important geographical, size system inertia, thus tackling several power system, identifying mainly and scale constraints, and cannot stability issues in the power system. four technologies: pumped hydro, be collocated with PV systems, The dispatchable operation is also compressed/liquid air, batteries and which puts a limit on how useful it useful in the voltage regulation of hydrogen (National Grid UK, 2018a). can be in supporting further solar the distribution network, allowing Pumped hydro and compressed deployment into the grid. to adjust the power flows in the air are large-scale technologies network to address voltage rise with great peak-shaving and Although the UK does not have phenomena caused by intermittent time-shifting capabilities that operational compressed air plants solar generation. complement nicely the daily at the moment, there are good variation of solar generation. These geological resources underground This coupling can be made either systems have also storage potential for air storage. Using saline aquifers, physically with collocation in the for more prolonged periods of this technology could be used as same site, or virtually by pairing the many days or even months, storing inter-seasonal storage (e.g. 77-96 solar generation with energy storage solar power when it is abundant TWh, about 160% of the national installed elsewhere in the network. and cannot be accommodated electricity consumption for January
NAME STORAGE CAPACITY DISCHARGING POWER YEAR OF COMMISSION [GWH] CAPACITY [MW]
Ffestiniog 1.3 360 1963
Cruachan 10.0 440 1965
Foyers 6.3 300 1974
Dinorwig 9.1 1728 1984
Table 9. Pumped hydro energy storage plants in the UK (Scottish Renewables, 2016; BEIS, 2020) SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 51
and February) with a relatively low When small-scale distributed are the most commonly applied, cost ($0.42-4.71/kWh) (Mouli-Castillo energy storage is considered, benefitting from their uptake in et al., 2019). In addition to saline batteries dominate the field. There is automotive systems. LFP/graphite aquifers, salt-mine deposits can be a perfect match between batteries has the potential for lower cost and also used to store compressed air. and PV systems for collocation higher safety over NMC/graphite Currently several gas facilities (more in the same site, a coupling that systems, but challenges related to than 66 million cubic meters) have reduces installation costs, treats strong voltage hysteresis renders been built and used for natural gas solar intermittency at its very source, SOC estimation a more laborious storage, which could be potentially and supports local energy systems. task (Xing et al., 2014; Crawford et al., used for compressed air storage as Batteries in a solar plant can assist 2018). well (Parkes et al., 2018). Such a 268 in voltage regulation of the network MW plant was scheduled to be built and increase the local consumption Degradation of batteries strongly in Larne, Northern Ireland to use the of solar energy (Von Appen et al., depends on how they are operated underground caverns created within 2014); can smooth out fluctuations (SOC, depth of discharge, currents geological salt deposits on the in solar generation (Li, Hui and Lai, magnitude) and on the cell Islandmagee peninsula, but recently 2013); can defer upgrades in the temperature. These points should be the planning application has been distribution network to host more seriously considered when designing unexpectedly withdrawn despite solar capacity (Tant et al., 2012); and the control and operational funding having been secured by a can provide a wide range of other framework of the batteries’ energy £77 million EU grant. technical and financial benefits storage system. Beyond the cells (Ekren and Ekren, 2010; Sahraei et themselves, the thermal and In India, integration of energy al., 2018; He et al., 2020). Currently, battery management systems storage into the grid is low at the the UK has about 700 MW of energy also add significant costs, but with moment, but it is expected to storage apart from pumped hydro, intelligent control and engineering increase rapidly with the massive most of which being lithium-ion optimisation there is a potential for electrification of isolated rural batteries installed in the last few substantial cost reduction. Battery communities and the ambitious years. packs currently cost about 340 $/ targets for 30% EVs by 2030 (World kWh with the ancillary components Energy Council, 2019). At present, Overall, lithium-ion batteries are making up half of the system costs. Lithium-ion batteries are the most expected to be the most likely commercially viable technology technology to facilitate solar B.3.2. Location of energy storage in India due to their competitive integration in the coming years, The size and location of an energy price and improving performance due to the versatile and effective storage system is an important parameters (India Smart Grid Forum, nature of the battery storage factor to the associated costs and 2019). Pumped hydro compares system and the falling prices. Lead value to the network. Although in quite favourably as well, with about acid batteries, whilst cheap, suffer technologies like pumped hydro and 5,757 MW capacity (operational from poor lifetime and roundtrip compressed air these parameters and under construction) (Buckley, efficiencies. All-vanadium redox are pretty much determined by the 2019; IEEFA Australasia, 2019). flow batteries have the potential for geographical constraints and there The compressed air technology a lower capital cost and LCOE at is little room for manoeuvre, battery has more limited potential scale, but they have yet to achieve energy storage can be placed though, due to low availability of economies of scale. Electrolyser/fuel almost anywhere in the network and suitable geological formations cell systems have potential for intra- at any size required. The following for underground storage caverns day and longer timescales of energy dialogue aims to inform the optimal (Aghahosseini and Breyer, 2018). storage, albeit at a prohibiting cost location for energy storage. at the moment. Among the various lithium-ion battery chemistries, NMC/graphite and LFP/graphite cells 52 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
Should storage be placed close to solar generation or close to centres of demand? 50MW solar farm Town, average The cost of grid reinforcement demand 10MW dictates that energy storage is more valuable when placed close to the greatest source of variability. Solar PV generation is more variable than demand during the day, and so energy storage should be placed closer to the PV plant rather than the centre of demand. This is illustrated with an example solar farm and a town in Figure 30, the town having the same average electricity Figure 30. Illustrative solar farm and town connected by a transmission line. demand as the solar farm’s output.
Plotting the output of the solar farm over a 24-hour period (Barton et al., 2017; Barton, 2018), and a typical aggregated demand profile on the Indian electricity system (Gaur 45 et al., 2017), Figure 31 shows that 40 the solar farm generation is much more variable than the electricity 35 demand. Therefore, if the energy 30 storage is placed adjacent to the 25 town, the required power rating of 20 the transmission line should be 41
Power flows, MW 15 MW. However, if the energy storage is placed next to the solar farm, 10 the required power rating of the 5 transmission line will be only 11 MW, 0 i.e. a 73% saving. 0 3 6 9 12 15 18 21 24 -5 Hour of day Ground-mounted solar farms have been built in the UK with energy storage since 2014 to enhance the Demand Solar PV value of electricity supplied and to overcome network capacity constraints without paying for grid Figure 31. Solar PV generation and power demand for the example of Figure 30. reinforcement (Anesco Limited, 2014). They are now being built without subsidy (Stoker, 2019). SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 53
Are there economies of scale in a These long time-constant batteries Can energy storage provide larger energy storage system? do exhibit economies of scale. In ancillary services to the electricity For most conventional battery the example of Figure 31, energy network? chemistries, e.g. lead acid or lithium storage capable of supplying all Energy storage provides flexibility ion, the time constant (energy/ the demand using PV alone would to active power flows and can power ratio) is only about 1 hour, require a charging power capacity thereby participate in the provision and energy is stored in the plates of 31 MW, a discharging power of of ancillary services, such as fast of the battery at room temperature. 11 MW and an energy capacity of frequency response and short- Theoretically there are no physical 152 MWh. The hot batteries have term operating reserve (Renewable constraints governing the size of the larger volume/area ratios at larger Energy Association, 2015; National battery, although practically there scale and hence slower heat loss Grid UK, 2020a). Again, by placing are economies of scale relating rates. The flow batteries have the energy storage upstream of the solar to manufacturing processes and same complexity of pumps, valves inverters, the inverters can provide industry learning rates. and control systems regardless these ancillary services, increasing of storage capacity, and this fixed the value for the solar farm operator. When considering diurnal energy overhead cost is relatively smaller storage, however, a longer energy/ at larger scale. For these reasons, Is the local electricity network power ratio is required, typically diurnal energy storage is better reliable? 6 hours. Other battery types have placed in a larger industrial setting In many countries like India, rural lower cost per kWh and are therefore than in a domestic setting. Again, distribution networks suffer more more suitable for these longer-term this suggests that solar farms are lost customer minutes per year storage applications. No clear winner good locations for energy storage. than urban networks (Chowdhury has yet emerged, but the candidate and Koval, 2000; Yu, Jamasb and battery types fall into two broad Can the cost of inverters be Pollitt, 2009; Harish, Morgan and categories: minimised by prudent location of Subrahmanian, 2014). For this reason, 1. Hot batteries such as sodium- energy storage? local energy storage is more useful nickel-chloride (Benato et al., 2015) Solar PV electricity is first generated to rural consumers than urban ones. or sodium-sulphur (Sudworth, as direct current (DC) and it is then If the consumer is also a generator 1984; Kamaludeen et al., 1991). The converted to alternating current of electricity (a prosumer), then high temperatures enable a very (AC) to be exported to the grid via the ownership of energy storage reactive metal (sodium) to be the inverters. By placing energy can enhance the value of that stored in liquid form. storage within the solar farm and generation, maximising the value of 2. Flow batteries (Weber et al., 2011; upstream of the inverter, the inverter exports and minimising the cost of Shibata et al., 2013; Wang et al., capacity can be minimized due to imports. 2013; Alotto, Guarnieri and Moro, the smoother exporting profile (lower 2014; Cunha et al., 2015) such as peak) similarly to the transmission Is there a large, local, intermittent vanadium redox flow battery, line capacity reduction. electricity demand for which vanadium bromide, zinc bromide, energy storage might reduce the iron chloride, zinc-cerium, Maximum Power-Point Tracking cost of grid reinforcement? lead–acid (methanesulfonate), (MPPT) is still required on the output Rapid EV charging points (40kW manganese-hydrogen and of PV arrays, but a simpler and or more) will be required in very others. Energy is stored in cheaper DC-DC converter can be large numbers to enable EVs to be separate tanks that enable the placed between each PV string and adopted as the principle technology energy capacity to be arbitrarily the energy storage. The location of for road transport. Their installation large and independent of the storage upstream of the inverter also is sometimes limited by electricity power rating. achieves some of the advantages network constraints. For those of Generation Integrated Energy locations, energy storage would Storage (Garvey et al., 2015). enable new charging points without 54 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
grid reinforcement. Considering B.3.3. Electric vehicles as storage The unique feature of EV battery the times and locations of rapid devices storage as opposed to other EV charging, it is most required Electric vehicles (EVs) are one of the batteries, is its mobility: vehicles during long, inter-city journeys. This key transformative technologies in charge in one location and transport suggests that there may be another greening the transportation sector. the stored energy to another. For synergy with large solar PV farms in The number of electric vehicles example, 86% of EV owners in the UK the countryside and thus another on both British and India roads is currently charge at home overnight. income stream for solar PV farm expected to increase rapidly in the When stationary and partly or fully operators. coming years. In the UK, the target charged, the vehicle’s battery may is set to 50%-70% of new car sales also provide supply at times of Conclusion being EVs by 2030, with sales of pure peak demand, e.g. when the owner In an electricity system dominated petrol and diesel cars being banned has returned home in the evening by solar PV generation, the from 2030 (HM Government, 2020). from work. Nissan and Renault are best location of most bulk grid- National Grid UK estimate the EV currently undertaking European connected energy storage is numbers between 2.7-10.5 million by trials to enable EV owners to obtain upstream of inverters, inside ground- 2030 and up to 36 million by 2040 payment for sharing energy from mounted solar PV farms where it (Hirst, 2020). India aims at a 30% their vehicles when they are not in provides the following benefits and EVs by 2030 as well (World Energy use, without compromising mobility services: Council, 2019). needs or causing deterioration of • Defers some grid reinforcement vehicle components. • Provides ancillary services to the To accelerate this uptake and grid become the norm in place of However, despite the potential • Enhances the rural network cheaper fossil-fuelled vehicles, the opportunities of vehicle-to-grid reliability EVs should deliver an additional (V2G) applications, there are • Minimises the cost of diurnal advantage. Their use as storage concerns around the potential energy storage units for clean energy during periods reduction in lifetime of the • Minimises the capacity and cost of of peak demand could be this automotive battery if used in this inverters advantage, having the potential to way. In general, increasing energy • Enables highway EV rapid charging reduce the Total Cost of Ownership throughput in a battery can result in of EVs via utility incentives. accelerated degradation due to the Some residential and other small There is a range of services that volume expansion/contraction in the consumers in rural locations may EVs can deliver to the network battery active material. However, if also benefit from battery-backup operators in terms of balancing, controlled in the right way V2G has energy storage. This conclusion frequency, voltage and congestion the potential to extend the lifetime of is confirmed by National Grid UK management much like other types an automotive battery as shown by which forecasts that embedded of energy storage. Exploring such Uddin et al. (Uddin et al., 2017). This generation, including solar, will financial incentives is particularly is due to other degradation modes, exceed Britain’s demand quite often important for India, as batteries such as operation at high SOCs, in the future and that collocation of generally degrade faster and exhibit which, if avoided through intelligent storage with PV plants will play an poor charging performance in control during the idle phase of important role in time-shifting the hot climates. In that case, it may vehicle use, can maximise the excess generation (National Grid UK, be more beneficial to use more battery lifetime whilst providing grid 2018a). expensive and temperature-tolerant services at the same time. batteries (e.g. LFP over NMC) for longer life, while offsetting the cost overhead with such grid-support services. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 55
B.4. MORE GRID-FRIENDLY energy storage or operating below important that the future PV systems SOLAR PV SYSTEMS capacity and curtailing power. The provide the widest possible range details of this roadmap to unlock the of ancillary services and that this The previous sections of this report full grid-friendly potential of solar flexibility is extended also to plants explore how the power system has to plants follow. of smaller size beyond the MW-scale change as a whole to accommodate solar parks. more solar generation; the remaining B.4.1. Ancillary services by solar part of this roadmap focuses on The Indian target for 175 GW of Types of services what solar generation can do for renewables by 2022 is expected to The grid codes and standards itself towards the same goal. To result in very high instantaneous worldwide require now a wide facilitate solar integration, a PV penetration to the electricity mix, range of ancillary or grid-support system should evolve to be a more momentarily reaching 88% in the services by DERs that somewhat friendly and compatible power southern and 75% in the western mimic the way that synchronous station to the electrical network, i.e. region (USAID and Government of generators support the power become more ‘grid-friendly’. This India, 2017). At such high integration system during disturbances. These entails relieving some of the burden levels, issues in stability, voltage standards greatly differ from country solar places upon the power system control and protection of the power to country and among the various in terms of balancing, stability, system brought by solar power are types of DERs and their power rating. voltage control and protection, as likely to pose risks for the security of Some of the most common and well as providing additional flexibility the electrical network, which may fundamental services required are and services to the network during work as a potential barrier to further shown in Table 10, classified into three disturbances. High levels of solar solar deployment. main categories according to the integration most certainly require co- European code ENTSO-E (ENTSO-E, evolution of the power system and A very promising way to mitigate 2013): PV systems together. these challenges is to equip the solar PV power stations with ancillary At the moment, only a minimal A grid-friendly solar plant should services support capability for secure number of these services are have the capacity to provide a wide and stable grid operation. These mandatory for solar PV plants range of ancillary services to tackle services may include frequency and mainly due to the variability of the stability and voltage issues in the voltage support functions, as well as solar resource (non-dispatchable), network. A particularly critical service robustness and restoration services and when they do apply, they is fault ride through (FRT) that avoids much like the synchronous machines usually refer to large plants of MW unintended disconnection of solar of conventional plants. Some of these scale. Among the three categories and relevant stability implications, services aim to counterbalance an in Table 10, the voltage stability while it also contributes to the grid inherent limitation of the PV system, services are generally the easier to strength (SCL). In addition, a new such as synthetic inertial response facilitate in a PV system by injecting/ transformative technology in IBRs to make up for the missing physical consuming reactive power, which is is ‘grid-forming control’, which inertia, or fault ride through (FRT) a straightforward task for the power paves the way for 100% IBR network, to curb severe disturbances and converter of an IBR. This is why most encourages local energy islands, maintain the grid strength (SCL) standards require these functions and allows for services otherwise not within limits. Other services, however, for a wide range of PV installations, possible, such as black-start and extend to the network beyond the quite often alongside appropriate islanded operation. To gain these solar plant and may provide an inverter oversizing of the order of 5% additional functions, a solar plant additive value to the system, such to minimise bottlenecks between has to keep some power in reserve to as injecting reactive power to assist active and reactive power. deploy when providing these services in local voltage regulation or fast and at grid-forming operation, a frequency response to support feature gained by either installing the network at generation loss. It is 56 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
VOLTAGE STABILITY SERVICES FREQUENCY STABILITY SERVICES ROBUSTNESS AND RESTORATION SERVICES Reactive power injection/ Dispatchable operation Fault-Ride-Through consumption Primary frequency response (voltage and frequency) Dynamic reactive current injection (Synthetic) inertial response Black start capability Oscillations damping Islanded operation
Table 10. Common ancillary services requirements by DERs (ENTSO-E, 2013)
On the other hand, the frequency connection) implies that the PV protection, i.e. to detect an islanding stability services require both up- system works in grid-forming mode and LoM event and cease operation. and down- regulation of the active (i.e. acts as a voltage source and Staying connected to the grid during power, which entails a need for a can establish voltage and frequency a fault entails that the PV system power headroom to allow increase in the network), which again is not supports the network with much of the output in the former case (e.g. the case for today’s PV systems. Most needed fault current that limits the to support under-frequency events). solar systems run at grid-following voltage and frequency distortion, as This is not straightforward in solar PV (or feeding) mode, which aligns well as it sustains the grid strength systems that have been traditionally better with the uncertainty of solar (SCL) and helps the protection operating at their maximum power irradiance but cannot establish and scheme of the network to function point and generally do not have the support the grid in the same way. properly. provision of dispatchable primary resource or energy storage. The only To allow for a wide range of ancillary Grid regulations on FRT way a solar PV plant could provide services by solar power plants, The FRT service includes the voltage a full range of frequency services is there are two key technologies to FRT function, i.e. withstanding sags to maintain some power in reserve, be explored that will unlock the or swells in the voltage magnitude, either by means of energy storage full grid-friendly potential of PV and frequency FRT that refers to or by operating in a deloaded systems: keeping power reserves, fluctuations in the grid frequency. manner to establish a power either through energy storage or In the former case the PV plant can headroom (National Grid UK, 2019d). power curtailment, and operating further support the grid voltage by With the falling of system inertia due in grid-forming mode. The specifics injecting additional reactive current to the increasing number of IBRs in and potential of these emerging (dynamic reactive current injection the network, this kind of service is technologies are discussed later in service), while in the latter it may expected to be key in the future of this section. be possible to adjust its active solar integration. power output to support the system B.4.2. Fault-ride-through and Anti- frequency (frequency response Among the robustness and islanding by solar service). More technical details on restoration services, FRT is commonly Fault Ride Through (FRT) capability these mechanisms may be found in required by most grid codes albeit of a PV system, i.e. to remain Appendix A. with significant differences on the connected to the grid during tolerance limits and time duration. major disturbances and faults, is The FRT service was the first That may be the most critical service usually a mandatory grid code requirement for DGs introduced in stability-wise and is discussed in requirement; it guarantees there will the grid codes. It is indicative that more detail in the following section. be no domino-like disconnections the Indian grid code introduced The capability for black start (i.e. to due to faults and it is an essential a voltage FRT requirement for the restore the power system in case requirement to allow a massive solar first time in 2014, referring to the 66 of blackout) and islanded off-grid fleet into the network. This service is kV level to account for high wind operation (i.e. isolated with no grid strongly related to the anti-islanding integration in the southern states SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 57
of Tamil Nadu and Andhra Pradesh. It was then extended to 33 KV 1.3 recently in 2019 to include utility- May disconnect scale solar PV stations. The voltage 1.2 FRT characteristic of the Indian grid 1.1 code is depicted in Figure 32: as long as the terminal voltage of the 1.0 PV system lies within the two lines it 0.9 has to remain connected, otherwise 0.85 it is permitted (but not obliged) to (pu)
pcc Stay connected
disconnect. These clearing times V and limits vary with the voltage level of the network. May disconnect During that time, the DG should supply reactive power as a high priority, meaning that it may curtail 0.15 some active power to make room for the reactive power, but in that 0 0.2 0.3 2 3 10 case the output has to be restored to 90% of the pre-fault level within Time(s) 1 second of the voltage restoration (Central Electricity Authority, 2019). This dynamic reactive power service Figure 32. Voltage FRT characteristics adopted by the Indian grid code is crucial in sustaining the voltage (Central Electricity Authority, 2019) until the fault is cleared and supports the SCL of the local grid.
As for the frequency FRT function, the Indian code requires the voltage FRT (Steck, 2017; Pukhraj (Distribution Code Review Panel, generator to stay connected within Singh, 2020). Such initiatives are 2020) applies to all users of the a frequency range of 47.5 Hz to 52 needed for the solar PV plants as electricity distribution systems: G98 Hz and feed in the nominal active well. for generators up to 3.68 kW at 230 power within 49.5 Hz and 50.5 Hz. V (ENA, 2019), superseding G83; and There are also requirements for Status of anti-islanding protection G99 for generators over 3.68 kW dispatchable operation, droop As the protection settings (ENA, 2020b), superseding G59. Under frequency control and ramping limits and tolerances of DGs on grid these rules, smaller generators, up to for generators of 10 MW capacity or disturbance are relaxed to improve 3.68 kW are required to temporarily more connected at 33 kV and above FRT, and as more IBR are deployed, disconnect from the grid in response (Central Electricity Authority, 2019). failing to detect correctly islanding to a disturbance and then reconnect events is increasingly becoming when the grid is restored. Larger It is important that these functions an issue (Noor, Arumugam and generators are required to have are tested and certified for all Mohammad Vaziri, 2005; Mahat, specified levels of FRT but still relevant DGs to make sure that Chen and Bak-Jensen, 2008; have to detect LoM. Following a they comply with the regulations. In Dietmannsberger and Schulz, 2016; consultation in August 2017, the India, there are some industries that Dyśko, Tzelepis and Booth, 2016; electricity industry recognised the provide testing facilities to validate Tzelepis, Dyśko and Booth, 2016). danger of false tripping by LoM the compliance of wind plants to In Great Britain, the distribution code detection equipment (National 58 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
Grid, 2017) and implemented new Eastern region (ER), Southern region being deployed in the GB network rules retrospectively on medium- (SR), and North-eastern region to enable improved real-time sized and larger generators. Vector (NER) (CERC, 2017a). These settings monitoring and early fault detection shift protection is now prohibited were last raised in 2013 prior to the (Ashton et al., 2012). PMUs or because it is more likely to cause synchronization of the Southern ‘synchrophasors’ allow for improved false tripping and less sensitive region with rest of the grid. In LoM detection by measuring the to genuine islanding than RoCoF addition to UFLS relays, df/dt (RoCoF) phase angle difference between protection (Dyśko, Tzelepis and relays are also installed in NR, WR, an islanded network and the main Booth, 2017). The maximum tolerated and SR networks. In NR and WR, df/dt grid (Laverty, Best and Morrow, RoCoF rate is increased from 0.125 relays are set to get armed at 49.9 2015). The benefit is this case is that Hz/s to 1.0 Hz/s with a time delay of Hz to shed load automatically if the anti-islanding protection becomes 0.5 s. The increase in the allowable rate of fall of frequency is faster than a responsibility of the network, with frequency slew rate is due to the 0.1, 0.2, or 0.3 Hz/s (i.e. three stages). no dedicated hardware or software reduced system inertia because In SR, however, the frequency at in the PV system, which may bring of more IBRs in the electricity mix. which UFLS is armed and the rate down the implementation time and It was estimated that the cost of thresholds are 49.5 Hz & 0.2 Hz/s, 49.3 cost (Dutta et al., 2018). Advanced keeping the maximum RoCoF below Hz & 0.2 Hz/s, and 49.3 Hz & 0.3 Hz/s digital signal processing techniques 0.125 Hz/s exceeded £100 million/ for the three stages, respectively like Discrete Fractional Fourier year (National Grid, 2017). Therefore, (CERC, 2017a). Transform (DFrFT) and machine it was cheaper to change the learning methods may prove to be a settings on LoM protection, including In Ireland, large generators are potential tool for islanding detection an Accelerated LoM change required to remain connected to (Dutta et al., 2018), especially Programme with subsidies paid to the system with RoCoF rates of up when combined with impedance DG operators to make the necessary to 0.5 Hz/s in the Republic of Ireland measurements to harmonics of the changes (ENA, 2020a). Methods and up to 1.5 Hz/s in Northern Ireland grid frequency (Malakondaiah et al., other than RoCoF and vector shift (Eirgrid and Soni, 2012). The tolerance 2019). More technical details on anti- detection are not prohibited in to greater RoCoF in Northern islanding methods are given in the the distribution networks of Great Ireland is so far considered not to appendix. Britain, but they are not specifically be in significant danger of causing mentioned in the grid code. accidental islanding (Dyśko, Tzelepis B.4.3. Grid-forming control in PV and Booth, 2016; Dyśko et al., 2018). systems In the Indian network, the generating EirGrid, the TSO of the all-Ireland The dynamic response of an units are required to abstain from electricity grid, has set a limit of 75% inverter-based resource like solar to back-energization of the network of generation from wind power and a disturbance in the grid is governed in the event of LoM. At the moment, other IBRs or induction generator mainly by the inverter controller there are no specific requirements sources at any time to maintain rather than physical parameters, on Anti-islanding protection in the sufficient inertia for stability, referred as it does not have any rotating Indian grid code. In the event of to as the System Non-Synchronous mass like synchronous generators. large-scale loss of generation or Penetration limit. The likely instability The control scheme of the inverter insufficient generating capacity, that would occur if the limit is plays a crucial role on how the system frequency control is exceeded remains a barrier to the plant perceives the disturbance performed by load shedding based Irish grid achieving 100% zero-carbon and supports the grid during that on low or falling frequency. Presently, generation. time. There are mainly two different there are four stages of Under- philosophies for this control scheme: Frequency Load-Shedding (UFLS) Future trends in anti-islanding grid-following and grid-forming. A relays which are set at 49.2 Hz, 49.0 involve smart meters and phasor schematic diagram illustrating the Hz, 48.8 Hz, and 48.6 Hz in Northern monitoring units (PMUs). An fundamental concepts of these region (NR), Western region (WR), increasing number of PMUs are schemes is shown in Figure 33. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 59
At grid-following mode, the inverter (a) (b) is locked to the grid frequency ifm by means of a synchronizing unit ifl L f (phase-locked loop – PLL), and it Grid Grid + vfl vfm - delivers active and reactive power Cf L f Cf according to input setpoints. The inverter behaves essentially as a vfm controllable current source that vfl Synchronising Closed loop ifm Closed loop vfl 0 Unit (PLL) Controller f Controller requires an already established 0 ifl v voltage in the grid to function, ref ref ref ref formed by other sources such as P Q P Q synchronous generators. This may be a barrier to a future high-RES power system with lots of IBRs, such Figure 33. Schematic diagram of (a) grid-following and (b) grid-forming inverter as solar, wind and batteries. (Milano et al., 2018)
Most of today’s grid-connected inverters are operating in grid- following mode under the 33). Therefore, it can form the grid emulates the dynamics of the assumption that the grid is stiff (high without the need from other sources, synchronous machine; ‘matching SCL) and the PLL can accurately which is why this approach has been control’ that produces the frequency track the voltage frequency and used in battery-inverters of stand- signal looking at both the ac and dc angle, due to a sufficient number alone microgrids for years. With the side of the inverter; ‘virtual oscillator’ of synchronous machines in increasing RES integration, it may that treats the plant like a non- the generation mix. However, be necessary to bring this control linear oscillator. There may be small IBRs displacing synchronous philosophy to the power system differences in the transient response generation reduces the grid as well. In addition, the dynamic of these techniques, but all schemes strength, which alongside delays response of a grid-forming inverter manage quite well to support the and higher impedance lines in the to a disturbance is instantaneous grid during disturbances. regional network, may lead to PLL and guaranteed, not involving any malfunction at severe disturbances sensing or synchronization delays, With the increasing renewables that risk disconnection from the which makes it a credible source in penetration, there will be a need for grid. Furthermore, whilst staying weak grids (low SCL). This is another robust synchronization, dynamic connected, a grid-following reason to consider grid-forming voltage and frequency support source can support the grid only control in solar PV plants as the IBRs and even black start capability in a limited fashion, suffering from integration increases. by inverter-based renewables, synchronization and control delays services that can be best, or only, that do not allow it to deploy its full The simplest grid-forming strategy provided by grid-forming inverters potential. is ‘droop control’, which adjusts the (Milano et al., 2018). A major inverter active and reactive power as a limitation that remains though is In the grid-forming mode, the linear function of the grid frequency the limited overcurrent capacity, inverter generates the voltage and voltage, much like the droop i.e. overcurrents up to 120%-160% magnitude and frequency on its speed control of governors in the nominal value, due to the own, without the use of a dedicated synchronous generators. Other more inherent limitations of the inverter synchronization unit; it acts as sophisticated grid-forming control switches (e.g. IGBTs). Furthermore, a controllable voltage source schemes in the literature include the dynamic interactions of the connected to the grid through (Tayyebi Ali et al., 2018): ‘virtual grid-forming sources with the rest a low series impedance (Figure synchronous generator’ that closely of the system are yet to be fully 60 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
understood, which means that projects to see how grid-forming B.4.4. Curtailments and power there should be caution in wide can convert variable inverter-based reserves in PV systems adoption of this technology until a generation to be more grid-friendly Curtailing (rejecting) energy from proper coordination framework is (National Grid UK, 2019b). solar plants or other renewables developed. This should be performed is a common practice followed by alongside new pricing and market It is worth noting that a fundamental the network operators for security mechanisms to reward and requirement for a grid-forming purposes. The solar-related incentivise grid-forming sources. source is up- or down- regulation challenges in balancing, stability, of its active power automatically voltage and protection of the Potential of grid-forming PV in response to a disturbance. This power system may put a limit on systems implies ability to increase the output the maximum solar penetration The ancillary services a grid-forming of the input primary source (e.g. PV at any time. Although it is easy to source can provide to the grid are arrays, batteries etc.) beyond the maintain the curtailments to low summarized in Table 11 (CIGRE/ pre-disturbance operating level, levels when RES integration is limited CIRED, 2018), presented here as a i.e. to keep some power in reserve. with simple interventions in the comparison to the grid-following. Although this is straightforward at power system, at higher integration Services that refer to normal and dispatchable IBRs, like battery energy levels this is not the case as seen in steady-state operation, such as storage systems, solar PV systems many countries such as Ireland and voltage control or harmonics have been operating traditionally at Spain. Recommended measures compensation, are equally a maximum power tracking mode include improved forecasting and performed in the two modes. (MPPT), which means that normally scheduling, enhancing transmission However, services to support the they cannot further upregulate capacity, increasing the system grid during disturbances in a rapid their output. For the solar plants to flexibility, installing energy storage and dynamic manner, such as gain power reserves and operate in and other directions discussed in this frequency response at generation grid-forming mode, there are mainly roadmap; however, it is unlikely that loss or dynamic reactive current two options: either installing and solar curtailment will ever be exactly injection at faults, are provided collocating energy storage in the zero. more effectively and robustly with plant, or operating below capacity at grid-forming control. Finally, services all times (e.g. 90% of the maximum Keeping power reserves in a PV required from the plant to establish power – curtailing energy) to have system voltage in the network and form a a headroom for upregulation. These A PV system with power reserves is grid, like operation in islanded mode methods are discussed in more much more grid-friendly, being able or system restoration after blackout detail in the following. to provide a wide range of ancillary (black-start), are only possible with and balancing services to the a grid-forming source. To explore network and operate in grid-forming this potential, National Grid UK is mode. To have this option, either currently carrying out a number of energy storage has to be installed
AVAILABLE IN BOTH SCHEMES BETTER IN GRID-FORMING ONLY POSSIBLE IN GRID-FORMING
Steady-state voltage control Inertial response Islanded operation Power oscillation damping Primary frequency response Act as reference for grid-following Harmonic voltage compensation Dynamic reactive current injection sources Black-start capability
Table 11. Ancillary services provided by grid-forming inverters as compared to grid-following. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 61
(a) Power reserves by curtailing power Operating below capacity (or
MPP power To the grid curtailing power or deloaded operation) results in lower energy yield but allows for immediate Surplus upregulation of the power output when needed to provide services – + (e.g. dispatchable operation, under- Grid support frequency primary response, inertial response, grid-forming control etc.). (b) With the drastic fall of PV module prices and the very low LCOE of solar To the grid in many countries nowadays (Solar Power Europe, 2020), keeping power Curtailed reserves through curtailment may
Power be a viable solution in the near future reserves for a quick conversion of existing PV Grid support systems to more grid-friendly power Lost stations.
Figure 34. Keeping power reserves by (a) collocating energy storage and (b) Large MW-scale plants comprising operating below capacity (curtailing some power). a multitude of identical PV clusters (PV arrays and individual inverters) may easily realise this curtailment function by instructing some of these and collocated in the plant, or it must The alternative is to operate below clusters to reduce their power whilst operate below capacity and curtail capacity, i.e. deliberately track a keeping the remaining modules some energy on a constant basis. suboptimal power level (e.g. 90% of at MPPT mode (Loutan et al., 2017), the MPP), so that there is a power as shown in Figure 35a. This way, The benefits of energy storage, margin to deploy should the need the MPPT-clusters monitor the especially when installed upstream arise (Figure 34b). In this case, varying maximum power available of the inverter in a solar park, have there is no additional equipment or and extrapolate it to the entire already been discussed. Batteries hardware costs, but some power plant. This approach is simple but are the most appropriate technology is curtailed and the solar capacity requires identical components for this purpose, connected to the is not fully utilised. In that sense, and conditions, which is not always PV generator usually in a parallel the solar plant is treated as a guaranteed in MW plants (e.g. during fashion: the PV arrays continue dispatchable power station with cloud coverage (Gevorgian, 2019)) to produce the maximum power a zero-cost fuel (solar radiation). and is uncommon in smaller-size available (MPPT), the excess part The barriers in this approach is plants of the kW scale (e.g. small being diverted to the batteries only that a market mechanism has to number of PV clusters or even a to be fed back into the grid later be developed to compensate for single one). when needed (Figure 34a). This the unutilized capacity, and that option paves the way for a truly grid- the maintained reserves are not friendly energy source much like a perfectly guaranteed in volatile conventional power station, but it weather. comes at the cost of the additional investment and maintenance overheads of the energy storage system. 62 | PART B: ROADMAP TO HIGH SOLAR INTEGRATION
A more universal method that applies to any kind of PV system is 100% 60% 100% 60% 100% 60% the entire plant to operate at an off- MPPT mode, i.e. to shift the operating 60% 100% 60% 100% 60% 100% point of all PV clusters away from the
MPP to a reduce power level (Figure 100% 60% 100% 60% 100% 60% 35b). An example is illustrated in Figure 36: conventionally the PV 60% 100% 60% 100% 60% 100% system would aim to operate at the MPP to extract the maximum (a) available power (MPPT mode), but now the target is a reduced 80% 80% 80% 80% 80% 80% power level that creates a power margin (reserves, headroom) that 80% 80% 80% 80% 80% 80% can be deployed later if needed.
This technique is currently under 80% 80% 80% 80% 80% 80% development by the scientific community, with open questions on 80% 80% 80% 80% 80% 80% monitoring the varying maximum power while away from the MPP, (b) on the control scheme stability and operation at partial shading Figure 35. Keeping power reserves by operating below capacity: (a) some PV clusters conditions (Batzelis, Kampitsis and in MPPT and some in off-MPPT, (b) the entire plant in off-MPPT mode. Papathanassiou, 2017; Batzelis, Papathanassiou and Pal, 2018).
Although keeping reserves by curtailing power is a simple method to provide additional grid-friendly potential to existing plants without additional investment, there are important barriers to commercial Maximum power application. An appropriate market Reserves MPP mechanism should be developed (headroom) to compensate for the unused PV Curtailed power level capacity and unextracted solar Operating point energy, possibly through purchase Power of ancillary and balancing services. With such a market framework and the falling prices of the PV technology, it may be viable to oversize a PV plant by 5% or 10% and commit that capacity portion only Voltage for reserves and services.
Figure 36. Power-Voltage curve of a PV system operating at off-MPPT mode and keeping power reserves. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 63
Another major challenge in this approach is that the maintained reserves are weather-dependent and are not entirely guaranteed (Gevorgian, 2019). Coupling this method with accurate short-term solar forecasting is crucial to establish credibility and usefulness of the service, although it is highly unlikely to ever reach the reliability levels of alternatives like energy storage. It is possible that the solar PV systems of the future would need to implement a hybrid approach, installing some battery storage but also curtailing some power occasionally, to strike a balance between the costs and benefits of keeping power reserves and being more grid-friendly. 64 | PART C: CONCLUSIONS
PART C: CONCLUSIONS
This report explores the current and combined their complementary The main findings of this report are future technical barriers against high expertise to draw a roadmap summarized in Table 12 and Table 13 solar integration in the UK and India. with promising directions towards with a special focus on the synergies A team of researchers within the UK- achieving high levels of solar and differences between the two India consortium for clean energy generation into the power system. countries.
BARRIERS SEVERITY COMMON CHALLENGES IN THE UK COMMENTS FOR THE UK COMMENTS AND INDIA FOR INDIA
Major • Overgeneration of solar at midday • Increasing electricity • Developing grid • High ramping needs for solar mix of variable solar, code and ancillary retraction at evenings and wind and EVs services market Balancing intermittency on cloudy days • Highly volatile • Need for robust & flexibility • Need for high flexibility from other weather leads to solar ancillary service challenges sources intermittency and system • Conventional generation has high forecast errors • Lots of inflexible coal technical minimum limits and low units ramping rates
Medium • Lowering of short circuit level • Expected low SCL in • Relatively low • Frequency and voltage stability remote regions like regional grid issues North Scotland strength Stability • Limited services by solar and poor • Falling of power • Largely weak grid issues communication infrastructure system inertia infrastructure in • Tripping of embedded solar at • Massive tripping of rural areas disturbances embedded generation • Stability issues in recently in August 2019 RES-rich states
Medium • Voltage rise phenomena • High levels of reverse • More severe voltage • Reverse power flows power flows issues in regional • Deterioration of regulation devices networks Voltage (OLTCs etc.) due to frequent issues operation
Medium • Difficulties in detecting faults • Unintentional DG • FRT performance • Low fault currents by solar and disconnection due to certification is not other IBRs high ROCOF or vector ironclad Protection • Need to redesign protection of the shift challenges distribution networks • Unintentional tripping of solar at faults
Minor • Harmonic emission by solar • Potential resonance • Already voltage • Risk to worsen voltage unbalance due to capacitive distortion and with single-phase solar systems underground cables unbalance in low- Power quality voltage networks issues
Table 12. Technical barriers to high solar integration in the UK and India SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 65
CHALLENGES COMMON DIRECTIONS FOR THE COMMENTS FOR THE UK COMMENTS FOR INDIA UK AND INDIA
• More flexible generation like • Phasing out coal • Mainly hydro for flexibility hydro and gas over coal and increasing • Electricity market should New ancillary services and interconnections evolve faster Balancing & flexibility markets • Extending demand • Demand response only challenges • Generation scheduling more response to more partially explored often, e.g. every 5 minutes residential and • High potential for pumped • More accurate solar commercial customers hydro energy storage forecasting and access to • Energy storage also by • Batteries storage also very solar resource data compressed air competitive • Strengthening the role of • High potential for EVs as demand response mobile energy storage • More energy storage by pumped hydro, batteries and EVs • Multi-energy sector coupling
• Continuous update and • Services by batteries and • Strengthening the regional harmonizing of grid codes EVs energy storage grid • More ancillary services and • Going forward with TSO/ • Faster evolution of the Stability issues grid-forming operation by DSO coordination grid codes and electricity solar • Black start by solar market • Collocate solar and batteries, to assist in case of a • Islanded and grid-forming or oversize solar for power blackout solar operation for reserves via curtailment resilience to power cuts in • Turn solar curtailment into an rural areas opportunity • Improved telecom and visibility to solar • TSO/DSO coordination
• Improved voltage control • Co-optimization of • Strengthening grid • Leveraging the reactive power regulation devices and infrastructure, especially in of solar and other DERs DERs at a TSO/DSO level rural areas Voltage issues
• Redesign the protection of • Revisiting the • Extending FRT requirement distribution networks disconnection settings of to low voltage networks • Robust FRT by solar existing solar and lower power levels Protection challenges
• More stringent harmonic • Limit solar harmonics • Ensuring harmonic emission requirements at both high and low requirements are met in generation kW-scale PV systems Power quality issues • Improving voltage balance between phases in distribution networks
Table 13. Roadmap for high solar integration in the UK and India 66 | PART C: CONCLUSIONS
BALANCING & STABILITY ISSUES FLEXIBILITY CHALLENGES
The primary barrier for a solar- The stability concerns related to There is a need for the PV systems to intensive electricity mix is the solar are linked to the diminishing evolve into more grid-friendly power variability and intermittency of solar grid strength in both countries, stations, providing a wider range of generation that dictates higher especially in remote areas, and ancillary services and operating in flexibility in the power system not lowering of the power system inertia grid-forming mode. The black start readily available from conventional mainly in the UK, which make the function is seen as highly beneficial power plants. That is expected to be power system more susceptible to to the UK power system to minimize a concern in the near future in the UK disturbances. In India in particular, restoration costs after a blackout, with lots of wind in the north, lots of the regional grid in rural areas is while islanded operation will be solar in the south-east and a highly already weak with regular power desirable to rural areas in India changeable and unpredictable cuts and low SCL that may cause suffering from regular power cuts. weather. India on the other hand has trouble in accommodating the solar These services are available only still a more traditional generation deployment foreseen. To this day, in grid-forming mode. This grid- mix with lots of inflexible coal plants the risk for massive unintentional friendly potential can be unlocked and developing grid codes and disconnection of solar and other by placing battery storage within market. embedded generation remains, as the solar plant or/and by oversizing recently seen in the power cut of the PV system and operating below Recommendations shown in Table August 2019 in the UK. Furthermore, capacity to keep power reserves. 13 include more flexible generation, the limited services potential by solar Especially for the solar curtailments, more energy storage and demand plants and poor communication/ it is time we treated them as response, more frequent generation visibility render solar generation capacity under-utilization, rather scheduling and new ancillary more of a burden than an asset to than just energy loss, and explore the service products to draw from the the grid operator. opportunities given the appropriate available flexibility in the network. market framework. Furthermore, the The UK has rightly set out complete UK should go forward with the TSO/ phase out of coal and increase of DSO coordination currently in pilot interconnections to neighbouring phases, while India should work countries, but these should be towards strengthening the regional coupled also with residential-level grid infrastructure and update the demand management and effective grid codes, balancing grid security integration of the large anticipated and incentives for solar deployment. EVs fleet. India should leverage the high hydro potential for flexibility and energy storage, but also make efforts to modernise and adapt faster the electricity market and grid regulations to the new reality. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 67
VOLTAGE PROTECTION POWER QUALITY ISSUES CHALLENGES ISSUES
Connecting solar generation to The protection schemes in The power quality is one of the nodes that used to have only load distribution networks are expected minor issues related to high solar in the distribution network leads to to face serious challenges in generation at the moment, but it has transmission-like power flows and detecting and isolating faults, the potential to cause deterioration gives rise to voltage rise and reverse because of the irregular fault current in utility assets if it worsens in the power flow phenomena. The UK flows in the network and low fault future. PV inverters inject harmonic already experiences a considerable current contribution by solar and distortion into the network, which amount of reverse power flows other IBRs. There may be a need to although kept low at nominal upstream through some substations, redesign the protection regime of generation levels, it may dominate while India sees significant voltage the distribution network, possibly by when the solar output is reduced. rise in some areas. There is a need mimicking mechanisms of microgrid There is nowadays the technology for improved voltage control in the protections. In addition, unintentional to allow for more stringent harmonic distribution network to optimize the tripping of embedded generation regulation at the kW level. Another operation of conventional regulation at faults remains an issue in the UK, power quality issue is the voltage devices (e.g. OLTCs, voltage with ongoing efforts to revisit the phase unbalance, more commonly regulators, switched capacitors) with disconnection settings. In India, the in India, which may be adversely the reactive power capacity of solar FRT regulations have become more affected by the connection of single- and other embedded generation. stringent recently, but they should phase rooftop solar if not done This is important for efficiency be extended to lower voltage levels properly. and security reasons, as well as to and make sure that the existing solar avoid excess wear and tear of the plants comply fully. regulation devices due to continuous operation. This optimization should be carried out simultaneously for the transmission and distribution system, especially in the UK where the TSO/ DSO coordination concept is already being explored. In India, these actions should be accompanied also by upgrades in the network infrastructure especially in remote rural regions. 68 | REFERENCES
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APPENDIX A: FAULT RIDE THROUGH (FRT) OPERATION
A generator that has fault ride Whilst staying connected to the FREQUENCY FRT IN SOLAR through capability can withstand grid, the solar system can further PV SYSTEMS distortions in voltage and frequency support the network by injecting and remain connected to the grid, reactive current to sustain the A frequency distortion is the result while it may be possible to further voltage levels. In order to achieve of a power imbalance in the support the network with active and a quick restoration of the system system followed usually by loss reactive power adjustment. voltage, the network operator needs of generation or load. It may also to inject significant amounts of result from undesired massive VOLTAGE FRT IN SOLAR reactive power during and after tripping of embedded generation PV SYSTEMS the fault; leveraging the reactive like solar in response to a fault in power capability of the solar the network that should have been The voltage FRT function plants is a highly effective and ridden through, such as in the dictates that the generator stays economic solution, over alternatives power cut of August 2019 in the UK synchronized to the grid when like sending massive amounts of or the Blue cut fire in California in the terminal voltage gets out of reactive power from the substation 2016. It is imperative therefore for limits, either above or below. The or installing additional reactive all DERs to withstand frequency fundamentals of this mechanism power compensation devices within distortions so as to avoid domino- are shown in Figure 37 for a two- the network. like disconnections and wider stage PV system (dc-dc converter implications in the system. and inverter). Following a variation Injecting reactive current by in the terminal (PCC) voltage, the the PV system is nowadays a An IBR like solar does not have any predominantly effected parameters straightforward task implemented inherent limitation on the frequency are the DC bus voltage and the by the inverter control. A main operating range, as machine-based inverter current, thereby affecting limitation though is that the total generators do, so theoretically can the power injected to the grid. At current of the inverter (active function in frequencies much higher high voltage, the output current will and reactive) should not exceed or lower than 50 Hz. However, the be reduced for the same power the nominal rating by more than main challenge for solar PV systems intake with no further implications. usually 20% to 60% even for short operating in grid-following mode During a mild low-voltage fault, times or the power switches will be is the sensing mechanism that we will have a proportional current irreversibly damaged. That may extracts the grid frequency (PLL); the increase within the inverter’s current result in a bottleneck between the PLL is found to be prone to distorted rating alongside a short-term DC solar generation (active power) voltage waveforms during faults and bus voltage overshoot. At a severe and grid support (reactive power); may mistakenly detect frequency low-voltage fault, however, the common solutions are oversizing change when there is not. While inverter cannot handle the over- of the inverter and/or active power the PLL is still being improved and the-limit current, which will lead curtailment during the fault. It is developed to become invulnerable to uncontrollable DC bus voltage worth noting that active power at these distortions, there is also the excursion and tripping of the system curtailment is not a panacea alternative of grid-forming control if no additional measures are taken. though, since in resistive networks that does not require a PLL during Common such measures to avoid lowering the active power too much normal operation and features an tripping include: off-MPPT mode in favour of the reactive power may inherent immunity to frequency and solar curtailment, a DC crowbar lead to the opposite effect in the disturbances. circuit to direct the excess energy, or voltage. energy storage to absorb and store the power surplus (Shukla et al., 2017). SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 79
While withstanding a frequency Solar PV DC-DC Inverter Transformer Utility Grid fluctuation, the PV system may also System Converter assist the network by adjusting its Filter DC Bus PCC active power towards balancing the power gap in the system, i.e. to provide primary frequency response. At over-frequency events the plant can easily reduce its output, but to upregulate its output at under- Vh Ph frequencies there is a need for Vnom power reserves. These aspects are Pnom discussed in detail in a previous PCC Voltage Vl Injected power Pl section. t Time Time
I h CASE STUDY AT IIT Vdc,h KHARAGPUR CAMPUS NETWORK IN INDIA I nom Vdc,nom DC bus voltage To highlight the importance of FRT, Inverter current I l Vdc,l t a case study on the distribution 0 t1 t2 0 t1 t2 network of Indian Institute of Time Time Technology, Kharagpur (IIT KGP) campus is shown here. The Response to low voltage at PCC Response to high voltage at PCC distribution network operates at 11kV, 50Hz with solar generation and load conditions as depicted in Figure Figure 37. Voltage FRT in a PV system. (a) Power circuit of a grid-connected PV 38, while the upstream network system. (b) PV system’s response to terminal voltage fluctuations. beyond the PCC is represented with a Thevenin equivalent circuit of an X/R ratio equal to 5. The response of the test system is simulated to a capacitor bank of the grid-side filter symmetrical three-phase voltage remains connected and provides sag at the PCC for three different some much-needed reactive power operating modes of the solar plant: to the network. Much improved no voltage FRT function, voltage voltage profile is apparent when the FRT without reactive power support, PV plant has FRT, with expectedly and voltage FRT with reactive power better performance when the PV support. system injects additional reactive power as well. These results indicate Clearly in the no-FRT case the PV how important staying connected system trips, resulting in further to the grid during disturbances is for lowering of the terminal voltage solar plants, and how critical the FRT during the fault and a predominant function is to allow higher levels of delayed voltage recovery after solar integration. the fault. Such disconnection may lead to cascaded implications in the power system and pose stability risks. Here it is assumed that although the PV system trips, the 80 | REFERENCES
6.75MVA 11/0.22 kV PCC Yg-D1
0.4746+j2.373Ω
11 kV 6.75MW 8.25MVA, 11 kV, 0.96pf
1 1.01 0.9 1.005 0.8 1 0.7 0.995 0.6 0.5 0.99 Voltage (pu)
0.4 Frequency (pu) 0.985 0.3 0.98 1 1.5 2 2.5 3 3.5 4 4.5 5 1 1.5 2 2.5 3 3.5 4 4.5 5 Time (s) Time (s)
6000 2500 2000 4000 1500 1000 2000 500 0 0 -500 Real power (kW) -2000 -1000
Reactive power (kVAR) -1500 1 1.5 2 2.5 3 3.5 4 4.5 5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time (s) Time (s)
no voltage FRT voltage FRT without reactive power voltage FRT with reactive power
Figure 38. Simulated response of the IIT KGP campus network to low voltage on the upstream network. (a) Thevenin equivalent circuit of the IIT KGP campus distribution network with solar penetration. (b) PCC voltage. (c) System frequency. (d) Real power output from the PV system. (e) Reactive power output from the PV system. SOLAR INTEGRATION IN THE UK AND INDIA: TECHNICAL BARRIERS AND FUTURE DIRECTIONS | 81
APPENDIX B: ISLANDING
Islanding in the network can be The most common local and passive INTENTIONAL ISLANDING unintentional or intentional. Un- method of AID is using the rate-of- intentional islanding is the scenario change of frequency (RoCoF) (Jia et Intentional islanding generally in which islanding occurs without the al., 2014). However, RoCoF relays can is not an issue and embedded prior knowledge of the utility supply lead to block tripping. An inter-lock DGs can theoretically continue or the power producer. Unintentional method (Jia et al., 2014) can improve to be online as long as everyone islanding events mostly occur due the RoCoF performance by applying working on the system is aware and to network and switching actions of a system of impedance estimation, the power quality is maintained loads and generators. Intentional demonstrated though only on during the islanding and after the islanding is primarily implemented rotating generators rather than IBRs. resynchronization. Microgrids can for safety reasons and system Other local passive AID methods be designed to transition smoothly maintenance purposes. include detection of voltage change, between grid-connected mode high/low frequency, high/low voltage and stand-alone mode (Achlerkar UNINTENTIONAL (Hobbs, 2009), voltage vector shift et al., 2018; Qaiser Naqvi, Kumar and ISLANDING (Noor, Arumugam and Mohammad Singh, 2019) with now commercially Vaziri, 2005) and more sophisticated available equipment. However, a There are many local and remote methods including detection small microgrid is not expected schemes to detect islanding (Noor, algorithms (Dietmannsberger and to have FRT capability in its grid Arumugam and Mohammad Schulz, 2016). One drawback of local connection. Vaziri, 2005; Balaguer et al., 2008; passive methods is that the NDZ Mahat, Chen and Bak-Jensen, 2008; cannot be entirely eliminated. Dutta et al., 2018). Local detection methods may be active, passive or Local active AID methods involve hybrid. Remote anti-islanding (AID) injection of some disturbance, schemes function by detecting for example a small-amplitude changes or differences across a second harmonic current signal network, for example: the opening (Malakondaiah et al., 2019) or an of breakers; or the grid phase angle added inductance (Murugesan difference between the DG and and Murali, 2020) and measuring other locations on the grid using its effect on voltage in order to synchrophasors (Laverty, Best and measure system impedance via the Morrow, 2015). Remote methods measured voltage signal. Both active require communications between and passive impedance-based the utility and generators but can local islanding detection methods virtually eliminate the Non-Detection exist in the literature and are proven Zone (NDZ), the conditions under to be effective at detecting the which local generation and demand occurrence of islanding (Liu et al., are so closely balanced prior to the 2015; Mohamad and Mohamed, LoM that an islanding event is not 2018). detected.