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AN INTRODUCTION TO DIRECT CURRENT DISTRIBUTION GRIDS

Initiation of: DC – Road to its full potential JB Woudstra1 , P. van Willigenburg1 , BBJ Groenewald2 , H. Stokman3, S. De Jonge4,5, S. Willems4,5

1The Hague University of Applied Sciences (THU), Delft, Netherlands

2Cape Peninsula University of (CPUT), Cape Town, South Africa

3DC Current b.v., Aalsmeer, The Netherlands

4GROUP T – University College Leuven, Leuven, Belgium

5CORE cvba-so, Leuven, Belgium

ABSTRACT

Electricity consumption worldwide is and regional operating Small and Medium continuously on the rise due to the load Entrepreneurs (SME’s) to improve their requirements for the of innovative capacity and maintain their transportation, houses, offices, factories and competitive advantage in the Netherlands and many other facilities. However, the use of worldwide. sustainable in urban surroundings is on the rise: for example, solar panels on roofs of The paper will conclude by arguing for the buildings. creation of low DC distribution grids to avoid DC-to-AC and AC-to-DC conversions, These loads and sustainable electricity sources stressing the perceived advantages of low voltage have one thing in common - Direct Current. Solid grids in terms of sustainability. State appliances define the new electric world. The introduction of these DC loads and Keywords: knowledge transfer; industrial sustainable DC electricity sources to the current partners; efficiency; material saving; lifespan grid could contribute to changes in the behavior 1. INTRODUCTION of the current electricity grid – although not all these effects are beneficial to the grid. The The continual growing demand for electricity current grid provides a very stable and very necessitates the consideration of natural resources to reliable supply for all its users. This may not be a generate electricity. However, it is necessary not certainty in the near future, especially in densely only to generate electricity but also to consider the populated areas, such as The Netherlands and losses and cost involved with the different conversions between AC and DC power sources and Belgium, where AC transportation and loads.. The Netherlands has seen growing demand distribution grids are facing more and more for electricity due to, for example, the increase in: stability and reliability issues. Their common is the re-discovery of Direct Current and • Electrical transport: trains, cars, motor its applications. bikes and scooters • Cloud storage and data centers This paper aims primarily to illustrate the need • Use of Air-conditioning to consider Direct Current Grids in the In the future, a trend towards full electric is Netherlands. Additionally, it describes the expected. But also an increase in energy sources ensuing research and educational programs providing DC power is expected. being developed around Direct Current Grids to support this need. Finally, it promotes the creation of strong networks to transfer knowledge from knowledge institutes to local The growth of plants in urban and frequency and connection to the grid is only possible industrial areas creates new possibilities and new via a power converter (AC-DC-AC). If almost all problems (challenges). Among the possibilities is loads need DC as a supply voltage, why do we use the alleviation of electricity supply from current (AC) for the transportation and generators. One of the challenges is the introduction of DC into the current AC grids. This may cause an distribution of ? oversupply of DC leading to possible harmonic distortions and instability in the AC grid. Where It therefore appears to be logical to reconsider the does one start and what does one do? The common possibility of creating a low voltage DC distribution factor is Direct Current at the origin and at the load. grid to avoid conversions between AC/DC. The advantages of low voltage DC grids are higher Low voltage Direct Current grids are not something efficiency, material saving and longer lifespan of new. Thomas Alva Edison patented in 1880 the first apparatus. incandescent light bulb and initiated the construction of the first DC- in Pearl Street, New Due to this renewed interest in DC the authors and York, including a network that supplied 110 V DC fifteen members of various industries in the to 59 clients. In that time it was not possible to Netherlands met to decide on a strategy to revive DC transform DC to a high voltage level to make it grids. To do this, the consensus was to introduce a possible to transport DC-energy over a long new research and educational program to consider distance. the advantages and disadvantages of scaled DC grids. This new program aims to generate Nikola Tesla conceived the concept of alternating knowledge and strong networks to transfer current in 1886, together with the concept of using knowledge from knowledge institutes to local and to step up the voltage. This causes a regional operating companies (SME’s), improving proportional reduction in current, therefore inducing their innovative capacity and maintaining their voltage drops and losses. This was the beginning of competitive advantage. the development of AC-grids.

With the invention of high voltage power electronic 2. DC: BACK TO THE FUTURE devices, direct current has made a comeback. Long high voltage direct current (HVDC) lines and cables The electricity timeline is shown in figure 1. Before are on the rise. Direct current is also present in 1880 electricity was purely experimental. With the distribution grids from 230 V to 50 kV. Photovoltaic invention of the light bulb in 1879 it became more (PV) panels produce a DC output voltage and when relevant; everyone wanted to use electrical lighting. connected to current grids the

Figure 1 Electricity time line

DC output must be converted to AC. Wind turbines Batteries were no longer enough to satisfy the produce an AC output voltage with variable energy demands, raising the need for DC-generators. 2.1 + Sustainable sources and users are working mostly with DC Thomas Alva Edison (1847 – 1931) initiated the + Lower losses with transport on high powers construction of the first DC-power station in Pearl (HVDC cables) Street, New York, including a network that supplied + Fewer conversions, on higher frequencies 110 V DC to 59 clients. In many other places DC + Possible to transport more power over a cable or networks were used as well. Soon after the line with the same cross section + No reactive power realization struck: DC power lines are limited in + No problems their length due to voltage drop constraints. + Reliable power line + Longer lifespan loads and components Nikola Tesla (1856 – 1943) conceived the concept + of alternating current in 1886, together with the No power concept of using transformers to step up the voltage. + Reduced use of raw materials This causes a proportional reduction in current, − Safety (still) not standardized therefore inducing voltage drops and losses. An − Mechanical switching is difficult impolite campaign raged in the late 1880’s between − Standardization only for high voltage DC and Tesla and Edison, the so-called “War of the not for low voltage DC currents.” Tesla emerged as winner, marking the − Not commonly known beginning of the AC-grid.

2.2 DC Comeback

After the invention of the transistor in 1947 by John Bardeen, the electronic revolution started. The invention of high voltage power electronic devices in particular has made a comeback for direct current possible. Long high voltage direct current (HVDC) lines and cables are now used more often. In addition to HVDC, direct current is present in residential and urban grids from 230 V to 50 kV. PV-panels produce a DC-voltage. Wind turbines produce an AC output voltage with variable frequency; connection to the grid is only possible via a power converter (AC-DC- AC). Almost all loads need DC as supply voltage. In figure 2 some DC sources and users are shown.

2.3 Advantages and disadvantages

Below are some advantages and disadvantages of AC and DC.

2.3.1 Alternating current Figure 2 Some DC-Sources and Users

2.4 Saving, efficiency and lifespan

+ Efficient transformers This research program aims to search for savings of + Switching without sparks material and energy. Furthermore it is important to + Securing can be selective discover the lifespan of the DC tools. The lifespan + Widespread use of AC equipment is better defined.. Is it necessary − Reactive power that the lifespan of DC equipment is as long as some − Power factor AC-equipment (40 years)? Perhaps it is cheaper to − Losses with high power transport renew DC installations every 10 years. In this case, it is important that the source materials are easily − Short circuit power needed (certainly with obtainable and recyclable. decentralized energy production) − Difficult to connect different grids with each Figure 3 shows a normal three phase, 3 kVA, 50 HZ other AC 400/20V 150A with a weight of40 kg and a DC transformer 350/16V 200A 3200W. 2.3.2 Direct Current The transformer used in the DC/DC converter weights 0,4 kg, the frequency is 30 kHz. The electronics and housing weights 2,6 kg, for a total of Table 2 DC world 3kg. PV ±25 DC 350 DC

panels – transport – load

350 V 20 V

DC DC

DC transport is more efficient (±2,5%) than AC transport due to the higher voltage used: 350 V instead of 230 V. The overall efficiency is ±92%, resulting in an energy savings of around 10%. This figure gives the energy savings possible for a household, and considers only internal energy distribution. Additional savings are expected from Figure 3 AC and DC transformer of 3 kVA the used materials. Because DC systems require less current than AC systems, less copper, is needed.

The electronics needed for the conversion steps in In conclusion, the DC transformer is much smaller the DC-world are also much simpler than in the AC- and is using much less material than the AC world. It is probably possible to skip the electrolytes transformer. As there are many transformers in the with their relatively short life span. This cause the distribution grid, the impact of a transition to a DC life span of loads to become much longer. grid would be significant. It is clear that a DC-world could save large amounts of copper and iron. When 2.5 Trends in DC grids one also considers that fewer conversions are needed in a DC world, the impact becomes even Considering the grid as a whole, one can see that DC more clear When locally produced sustainable power is already present as HVDC in the energy is used, a significant number of conversions transmission grid and that in the distribution grid are necessary in the AC grid system (see Table 1). LVDC is coming in the future. Figure 4 shows the electricity grid now and in the future. Table 1 AC world In the present , DC power is used PV ±25 D AC A ±30 DC mostly to transport energy over long distances and C C 0 high . Take, for example, the HVDC 500 panel - transpor loa km cable connecting the Netherlands and Norway. s - t - - d 320 In the future we expect that in the transmission grid V A D 20 V HVDC usage will rise. The energy from offshore C C parks must be transported over long DC DC distances. A prime example is Germany; the offshore wind turbine parks are north of Germany while most (industrial) loads originate in the South of Germany. Lately, DC power is also coming to The first conversion is a DC–DC step where the play an important role in the distribution grid.. In the relatively low voltage of the pv-panels is current (centralized) grid there are few sources in the transformed to a higher value. The efficiency of this distribution grid, but in the distribution grid of the conversion is ±96%. The next step is a DC–AC future (decentralized) a conversion with an efficiency of ±93%. Transportation of the AC energy is done with efficiency of ±95%. The next conversion is from AC–DC at an efficiency of ±95%. A final conversion transforms the high voltage (300V) to the needed low DC voltage with±97% efficiency. The overall efficiency of all those conversion is ±80%.

In comparison, the number of conversions required for a solely DC system is small (see Table 2).

Figure 2 Electricity grid: Now and in the Future

Figure 4 Electricity grid: Now and in the Future more solid state components. These DC loads and large number of mostly sustainable sources (wind sources change the behavior of our electricity grid, and sun) will be connected in the urban areas. and not all effects are beneficial. Grid operators are Knowing that almost all loads are working with DC- urged or sometimes forced to invest in the grid, to voltage and that all sustainable sources are provide access to those prosumers, and to maintain producing DC-energy - directly or indirectly, - it is safety, reliability and usability. Direct Current and its logical to also distribute the energy via a DC- applications can be seen as instigators. A need to Distribution Grid to avoid AC/DC conversions. study the effects of more DC components or even a full DC grid to prevent disruptions from occurring has 2.6 Present-Day Research been identified.

Present-day research is focused on HVDC In the DC-world it is not the voltage level that is applications (>400 kV) and on the very low voltage dangerous, but the amount of current. A high DC (12/24 V) applications: boats, campers and caravans. current is difficult to off mechanically. Bad Almost no research has been performed in the connections can also become very dangerous, as voltage range of 100–1500V. Looking at the new sparks can cause fires. Working with 24V in domestic developments of decentralized sustainable electricity installations is not recommended because of the high production one can see that an increasing number of current. The transportation losses are relatively high consumers have become “prosumers”. They produce and one needs thick copper wires. The installation as their own energy, sustainably sourced, to fulfill their a whole will become very expensive and own needs. Usually that means selling and trading a unsustainable. Even to supply low currents it is surplus of energy as well. Our energy system is necessary to work with higher voltages if large evolving into a multi-directional distributional amounts of power are required.. Looking around in system, for which it was not designed. the DC-world reveals that DC voltage levels are unstandardized, as the following voltages are These small and often sustainable power plants (CHP, reported: 12, 24, 48, 56, 60, 110, 220, 350, 380, 400, Wind, Solar) have a few things in common. Their 500, 600, 700, 900, 1000, 1100, 1200 and 1400 V. sustainable nature is one. Their unpredictability Research has to be performed to discover what causes disturbances in the grid. The other ‘end’ of the voltage levels are preferable. It is important to stay grid, that of the consumers, faces similar issues. below 1500V as that marks the border between low Electric loads include increasingly voltages and high voltages in Europe. Above 1500V mark different regulations are in use.

3. RESEARCH AND EDUCATIONAL

PROGRAMME

Direct Current (as a theme) has presented itself as an educational and research opportunity. In one of our

earlier research and entrepreneurial programs, KITE120, the THU has contributed to the development of DC-driven assimilation lights in the Horticultural Greenhouse Sector. Results were very leaders have been identified and have been invited promising: economical benefits are present for to participate. Involved project partners include ten involved SME’s and potential buyers, and ecological SME’s, three non SME’s and three other benefits have also occurred due to the reduction in educational and research partners. energy usage. Partnering SME’s have very different backgrounds. The THU has recognized this potential and has This project brings together three electrical initiated a new two year research programme. This firms, two design studios, two private research and educational programme focuses on educational suppliers, one supplier in greenhouse SME’s and their chances at developing state-of-the- lighting, one Dutch supplier of windmills in Kenya art products and services, based on new knowledge and one consultancy firm specializing in concerning Direct Current. Ten SME’s, two industrial installations for the build environment. parties (with more than 250 employees) and three other Universities are participating in the The list of non-SME-partners contains two very programme. In this new programme, we will be large companies involved in the Dutch electricity modeling, simulating and partly realizing three sector and one development agent in greenhouse different levels of electricity grids, our so-called DC- horticulture areas. The research and educational distribution-grids. partners are the Delft University of Technology, GROUP T (part of KU Leuven) and CPUT. Our programme is written together with trade and industry partners. The reduction of energy usage, a longer lifespan and material savings are playing a key role in their motivation to participate. 3.2 Micro–DC Grid

The initial questions from the industry are: The definition of a micro–DC grid is a grid with a power consumption of less than 5.000 kWh a year. • Can a DC distribution grid contribute to a This value represents an average consumption of a substantial reduction in use of energy and household in the Netherlands over the course of a in materials without dramatic change in year. functionality and costs? • What is the lifespan of a product when Research questions are: supplied with DC? • What is the optimal size for a sustainable • What are the used DC voltage levels? household power plant? Financial indicators, safety and standardization are • How can one optimally make use of the also issues to look into. sustainable power sources, design energy management systems and design energy The research will focus on three different storage? distribution grids (Micro–DC Grid, Mini–DC Grid, • Is it easy to convert AC installations to DC and Midi–DC Grid), creating new systems and installations? renovating existing installations Both short-term • What is the efficiency of the whole and long-term considerations are taken into installation and individual users? account. • What are the economical and ecological consequences? Research methods: • Are DC-grids safe to use in households?

• Exploring technical possibilities (proof of To demonstrate the results, a demo laboratory will concepts, proto types, beta-releases) by be built at the university. Solar panels on the roof of modeling, simulation, and laboratory tests. the building will be used as energy source. A student • Comparing AC and DC lineups by looking apartment will also be used as a demo. at energy consumption and the use of materials. • Testing safety, usability, and reliability, standards, 3.3 Mini–DC Grid

3.1 Industrial, Research and Educational For a mini–DC grid we will study a small residential Partners area of roughly 10 households, an office building or super market with a power consumption of less than One of the first steps in the programme initiation was 50.000 kWh a year. Almost the same questions as to build a consortium of industrial, research and for the micro-grid can be asked. It is interesting to educational partners. Key players and opinion- compare a centralized sustainable power plant with decentralized power generation at each location. • contribution to the standardization of voltage Should one use a central storage unit or does every levels household require one? • smart plugs • wall socket 3.4 Midi–DC Grid • fuses • A midi–DC grid is a residential area of about 100 • houses, a small industrial area or a greenhouse new household products complex with an energy use of more than 500.000 • du/dt detection kWh a year. Currently, several industrial partners • di/dt detection have initiated a project to develop a DC-grid, to • leakage current detection connect two to three greenhouses and several power • DC grid design and more products sources. • communication protocols • educational tools During this research and educational programme, • demo labs modeling and simulating the above case will be done • demonstrator for in house DC-grid simultaneously in order to compare results and to • models and simulation results of distribution prepare the educational system for DC-engineering. grids.

3.5 Educational and Research ambitions

Our ambition is fourfold: 5. REFERENCES

1) as an educational and research institute: to gather [1] Prof. Lou van der Sluis: Future Generation, and capture new knowledge and to initiate more research in The Netherlands. ISBN programmes on direct current. 978-9461860088 2) for our students: to provide excellent, relevant and up-to-date projects to participate in and to [2]] Arjan van Voorden: Power Balancing in help provide them with job opportunities fitting Autonomous Systems. their new skills and competencies. ISBN 978-90-71287-23-7 3) for our partners: to aid them in overcoming obstacles, to develop new products and services; [3] Sue Roaf: Ecohouse 2. ISBN 0-7506-5734-0 becoming more successful entrepreneurs and innovators. [4] J.B. Woudstra, B.B.J. Groenewald: Future 4) to contribute to the cause of sustainable energy IPP’S can benefit from the Dutch system of net sources, by using the electric energy that will be metering for its national electricity supply grid. generated in the future more efficiently. Proceeding ICUE 2012, Cape Town.

In conclusion, we expect to stimulate innovative [5] J.B. Woudstra, H. Stokman: Direct Current is behavior and hope to improve performance of our the future. Proceeding DUE 2012, Cape Town SME’s, contributing to regional development and the innovative capacity of the Netherlands. [6] Papaefthymiou, G.: “Integration of Stochastic Generation in Power Systems” Future 4. CONCLUSIONS Generation - Smartgrid research in the Netherlands, TU Delft Library, October 2011, In this paper an introduction of a research and education programme for the development of direct pp. 10-11, ISBN: 978-94-6186-008-8. current distribution grids is given. Sustainable DC sources are increasingly common in urban areas. [7] M. Reza.: “Stability Analysis of transmission Consumers are becoming “prosumers”. systems with high penetration of ” Future Generation – Smartgrid A consortium of industrial, research and educational research in the Netherlands, TU Delft Library, partners is built up. In the consortium are electrical engineering firms, design studios, private October 2011, pp. 16-17, ISBN: 978-94-6186- educational suppliers, a supplier in greenhouse 008-8. lighting, a wind turbine supplier for Kenya, a consultancy firm and several universities. A research programme is written and the ambitions of 6. AUTHORS this programme are: Principal Author: • a good motivation for the used voltage levels Johan Woudstra holds a Master degree in Electrical from the Technical University of Delft, combines his master studies with a postgraduate in The Netherlands. He is presently Entrepreneurial Engineering Experience. In this a senior lecturer specializing in program he participates in the CORE cvba-so where Power Engineering at The Hague he is involved in the development of a DC grid for University of Applied Sciences. student residences.

C0-author & Liaison South Africa: Co-author: Ben Groenewald holds a MSc in Electrical Albertand van Albert van Oudheusden. Liaison person for projects Electronic Engineering from the handled between the Netherlands University of Cape Town (UCT). and South Africa. He is currently the Head of the Albert holds an Electrical and Electrical Engineering Mechanical Bachelor degree and Department at the Cape Peninsula a M.Com Degree from UNISA. University of Technology His back ground comes from his (CPUT). He is currently studying work in the Electrical and towards his PhD in the Commercial field in South Optimization of wind and solar Africa. energy resources used in micro grids.

Presenter: Co-author: Pepijn van Willigenburg holds a Bachelor Degree in This paper is presented by Johan Woudstra. Business Engineering from The Hague University of Applied Sciences. He is presently a researcher specialized in innovation and entrepreneurship. Furthermore, he is Greenhouse Horticultural specialist within THU.

Co-author: Harry Stokman is general manager of Direct Current. Harry has 25 years’ experience in engineering DC applications. Furthermore, he is chairman of the Direct Current Foundation in the Netherlands.

Co-author: Stijn De Jonge holds a Masters degree in Chemical Engineering from GROUP T - University College Leuven. Since 2006 he holds the position of Assistant Professor at GROUP T - Chemical Engineering department where he is lecturing courses related to (nano)-chemistry and advanced materials. Besides he is also the general manager of CORE cvba-so, a student cooperative working on sustainable energy.

Co-author: Simon Willems is a master student electromechanical engineering at GROUP T University College Leuven in Belgium. He