The value of reducing minimum stable generation for integrating wind energy

1* 2 3 4 M. L. Kubik , P. J. Coker , Mark Miller and J.F.Barlow

1,4 Technologies for Sustainable Built Environments, University of Reading, United Kingdom 2 School of Construction Management and Engineering, University of Reading, United Kingdom 3 AES Kilroot, Carrickfergus, County Antrim, United Kingdom

* Corresponding author: [email protected]

ABSTRACT

The integration of wind energy is a major driver toward grid decarbonisation in a number of electricity systems. However, no large-scale electricity grid is able to operate without some minimum level of conventional generation, which is required for both system security and to maintain power quality. This minimum stable generation level caps the amount of wind energy that can be used to satisfy system demand, and any excess must be curtailed if it cannot be stored. The curtailment of wind generation is undesirable for wind developers as it reduces their economic viability and increases costs for the system. It is also undesirable for the goal of reducing the carbon intensity of the grid as zero-carbon generation is held back in order for fossil fuel based conventional generation to run. With increasing wind capacity this problem becomes more severe. Certain modifications can be made to conventional generation to reduce their minimum stable generation levels, with differing cost implications. This paper examines the system benefits of reducing the minimum stable generation level of Kilroot power station for the Northern Ireland electricity system under the 40% wind penetration level planned for 2020. Keywords: Variability, intermittency, conventional generation, minimum stable generation

1. INTRODUCTION

With a growing proportion of wind generation capacity in many world electricity markets, periods where wind generation exceeds system demand will become increasingly frequent. Without methods of demand matching, storing or exporting this energy, wind generation has to be curtailed in order to balance supply and demand and maintain an uninterrupted supply of power. Modern large scale electricity systems require a minimum level of conventional generation that is kept online at all times to provide system security, balancing, and power quality services (such as inertia against unexpected trips, automatic voltage regulation and frequency response). A large-scale electricity system could not fully satisfy demand requirements using wind energy alone, as wind generators cannot normally provide the full range of services required. Curtailing wind generation is undesirable from a system operator perspective. Wind is a zero cost fuel; when it is curtailed, not only is the cheap electricity it generates not utilised, the system operator also has to pay conventional generation to remain on and reimburse the wind generators for their curtailment. Both of these aspects increase the wholesale price of electricity. It is also counterproductive to meeting carbon emissions targets, as a zero-carbon generation technology is held back to allow carbon intensive fossil fuel generation to run. The minimum amount of generation that a conventional power plant unit can safely operate at is known as its Minimum Stable Generation (MSG) value. By reducing this value, more of system demand is free to be met by wind generation and reduces the need for curtailment. This paper examines Northern Ireland, a small system with a high future wind penetration forecast for 2020, where 40% of electricity is expected to be met from renewable resources. The pattern of curtailment in 2020 is simulated under different MSG scenarios, drawing upon experience of power plant operators and projections of demand, capacity and network developments by 2020 (EirGrid & SONI 2010). Further work that will advance the understanding of curtailment levels is also identified.

2. BACKGROUND

2.1. Northern Ireland’s renewable targets

The island of Ireland is currently heavily dependent on conventional fossil fuels (Howley et al. 2009), but is amongst the most gifted in Europe in terms of renewable wind, wave and tidal resources (Rourke et al. 2009). The UK government has set an ambitious target for Northern Ireland to meet 40% of its annual electricity consumption using renewable sources by 2020. The transmission capacity forecast by the System Operator for Northern Ireland (SONI) anticipates that by 2020 the renewable resource will be largely composed of wind, with onshore and offshore making up 74% of the installed 2020 renewables capacity (EirGrid & SONI 2010). Wind is an inherently variable renewable resource and this presents challenges for system balancing (Laughton 2007; Milborrow 2009; Gross et al. 2006). A number of existing and future technological developments, such as energy storage and demand side management, have the potential to address the impacts of a more variable generation pattern. However, flexible operation of conventional generation plant is expected to play a significant role in maintaining security of supply, especially in the short to medium term (Kubik et al. 2012). Many of the existing thermal generators in Northern Ireland will still be operational in 2020, and no new plants are forecast to be constructed in the interim. It is therefore particularly important to understand how their operation can affect the integration and curtailment of wind. 2.2. Northern Ireland’s MSG capabilities

Power plants have an MSG level below which they cannot maintain stable combustion conditions. This is limited by the rate that fuel can be combusted without leading to instability issues. Different types of generator have different MSGs, as shown in Table 1.

Table 1 - Summary of MSG sent out capabilities of conventional generation units in NI

Location Plant typei Units Capacityiii/unit MSG/unit

Ballylumford B Steam plant 3 170 MW 54 MW (Owner: AES) (Natural Gas)

Ballylumford C Single shaft 1 101 MW 63 MW (Owner: AES) CCGT (Natural Gas)

CCGT/OCGTii 2+1 2x160 MW + 180 68/113 MW (Natural Gas) MW

Coolkeeragh Single shaft 1 404 MW 260 MW (Owner: ESB) CCGT (Natural Gas)

Kilroot Steam plant 2 220/260 MW 93/45 MW (Owner: AES) Coal/HFO i. CCGT = combined cycle gas turbine, OCGT = open cycle gas turbine. HFO = heavy fuel oil. ii. Ballylumford is able to operate in open cycle and closed cycle modes. The lower MSG is for open cycle operation. iii. Capacities given are maximums (actual ratings may vary, as GT capacities are weather dependant).

Single shaft CCGTs such as those at Coolkeeragh and Ballylumford C are designed for efficiency and baseload output, and struggle to reach low MSG values because they are unable to uncouple their gas turbine generation from the steam turbines. Although Ballylumford C’s single shaft unit has a small absolute value of MSG, it is also Northern Ireland’s smallest non-peaking unit. It is therefore not commonly used for system security, as it is not able to provide enough power to cover the loss of another large unit tripping.

The units at Ballylumford B offer one of the lowest MSG levels per unit and one of the lowest turndowns (MSG as a percentage of its capacity), making them valuable for system security. However, these units are the oldest in Northern Ireland and are anticipated to be decommissioned in 2016 due to opting out of the Large Combustion Plant Directive (European Parliament 2001).

Ballylumford C’s 2+1 CCGT is able to operate in both open cycle and closed cycle modes. However, the lower value, achieved through open cycle operation is less efficient and more expensive.

The MSG that can be achieved by Kilroot power station, which normally runs on coal, depends on the quality of the fuel blend. However, Kilroot’s units are also able to run on heavy fuel oil rather than coal, which is a more expensive, but a more homogenous fuel and able to burn at much lower mass flow rates without loss of stability. This paper focuses on quantifying the benefits of reducing Kilroot’s MSG by switching to oil during times when minimum stable generation levels force the curtailment of wind.

2.3. System security constraints

Northern Ireland is a system with a small number of units and is therefore sensitive to trips or rapid changes in generation. SONI has a “3 generator unit” rule for system security purposes, of which one should be from Kilroot power station (EirGrid & SONI 2012). At any one time, three units must be kept running so that if one were to suffer an unplanned outage the other two could ramp up their production to meet the system demand. An increased level of wind generation presents a challenge in keeping generators online to meet the 3-unit requirement without curtailment. Although this requirement might be revised by 2020, given the planned construction of a new tie-line to the Republic of Ireland relieving constraints (EirGrid 2011), there is no clear indication whether this will occur. The current market framework for bidding generation into the Single Electricity Market1 (SEM) stipulates that the price of generation of electricity increments have to be monotonically increasing. This means that a bid for more generation from the same unit has to be for a larger amount of money than for less generation. The framework therefore restricts Kilroot from starting up on oil (a more expensive fuel) and moving to (cheaper) coal when a stable generation level is reached (Figure 1), as it is not accepted by the scheduling software.

Figure 1 - Schematic showing bid profile for Kilroot on oil and coal 2.4. Curtailment

The oldest 80MW of installed wind capacity in Northern Ireland did not have control requirements when they were constructed. However, SONI has the ability to curtail the generation of all other existing wind farms as they are added to the system. When wind generation levels are too high to operate the system securely, the SONI control centre at Castlereagh House has the ability to remotely curtail or shut down a wind farm completely. However, this has financial implications both for the system as a whole and for the individual wind farm. 3. METHOD

3.1. The model

A process flow diagram of the model used to determine curtailment is shown in Figure 2. The model uses five input parameters: an annual half-hourly time series for onshore and offshore wind, demand, Moyle Interconnector flow to Great Britain, and interconnector flow between the north and south. Together these form an input scenario against which various MSG conditions could be investigated.

1 Northern Ireland and the Republic of Ireland have a single common market for electricity, but are operated by two separate system operators; SONI and EirGrid. Onshore wind Offshore wind Demand series Interconnector Interconnector Won Woff D Ins Im

Input scenario (annual time series)

Apply MSG condition Gmsg

For each time step, balance generation: Gb = D – Won –Woff –Ins –Ins -Gmsg

For each time step, determine curtailment: If (Gb < 0) Gc = Gb Else Gc = 0

Outputs

Curtailment 2D/3D Load duration curve visualisation (demand, demand net array plots wind)

Figure 2 - Process flow of the simulation approach used to calculate curtailment Four MSG input conditions were considered, as indicated in Table 2. MSG-H1 and MSG-H2 represent the two most common (high) configurations of minimum stable generation under the current transmission constraint requirements in Northern Ireland. The two (low) conditions, MSG-L1 and MSG-L2, represent the reduced MSG level that may be achieved by running Kilroot’s units on heavy fuel oil instead of coal. Combinations including Ballylumford’s B station were not included as it is expected to shut down in 2016 due to European emissions restrictions (EirGrid & SONI 2010). Table 2 – Summary of input scenarios considered in the curtailment model

Name Scenario MSG‐H1 Coolkeeragh + Kilroot 1 (coal) + Kilroot 2 (coal) MSG‐H2 Coolkeeragh + Kilroot 1 (coal) + Ballylumford C (2+1) MSG‐L1 Coolkeeragh + Kilroot 1 (oil) + Kilroot 2 (oil) MSG‐L2 Coolkeeragh + Kilroot 1 (oil) + Ballylumford C (2+1) ON1030 Onshore wind, 30% capacity factor, 1030MW OFF600 Offsore wind, 40% capacity factor, 600MW D‐M Demand (medium projection) M‐Z Moyle interconnector (zero transfer) M‐MI Moyle interconnector (max inflow) M‐MO Moyle interconnector (max outflow) NS‐Z NS interconnector (zero transfer) NS‐FMO NS interconnector (future max outflow) The onshore and offshore wind scenarios, ON1030 and OFF600, were developed from EirGrid and SONI’s (2010) projections of installed capacity of wind. Wind generation simulated from meteorological records, as applied by Pöyry (2009), Sinden (2007) and Sinclair Knight Merz (2008), were used to simulate the time series. However, rather than surface station records of wind speed used in these approaches, MERRA reanalysis (a 4- dimensional data assimilation approach for creating a time series of atmospheric properties) was used to generate a proxy time series for wind generation. Medium case year-by-year growth projections from EirGrid and SONI’s generation capacity statement were assumed in order to arrive at a projected demand level, D-M, for 2020. The Moyle interconnector to Great Britain’s electricity network is commercially operated and the system operator has little control over the size or direction of the flows it produces. Three cases were therefore considered: maximum export capacity M-MO (-295 MW), maximum import capacity M-MI (450 MW) and zero capacity M-Z (0 MW). The interconnection between the north and south of Ireland is influenced by the generation dispatched by SONI and EirGrid. By 2020 this interconnection is expected to be strengthened by the construction of a new 400kV tie-line between Tyrone and Cavan (EirGrid & SONI 2010). Further interconnection between the Republic of Ireland and Great Britain is expected to allow this generation to be exported into the rest of the European electricity market. As dispatch of generation is coordinated between the north and south, it is assumed that flows into Northern Ireland will not be imposed during times when wind generation is high. Hence the upper and lower bounds to consider are future maximum export, NI-FMO (-950 MW) and zero transfer, NI-Z (0 MW).

4. RESULTS AND DISCUSSION 4000 3500 (GWh) 3000 (1) NS‐FMO, M‐MO 2500 (2) NS‐FMO, M‐Z 2000 curtailment (3) NS‐FMO, M‐MI 1500

wind (4) NS‐Z, M‐MO

1000 (5) NS‐Z, M‐Z 500 (6) NS‐Z, M‐MI Required 0 MSG‐H1 MSG‐H2 MSG‐L1 MSG‐L2 Minimum Stable Generation Scenario

Figure 3 - Graph showing required wind curtailment for each interconnector scenario combination under each MSG condition With three input conditions for the North-South interconnector and two for the Moyle interconnector, six interconnector combinations were possible (labelled 1 to 6 in Figure 3). The graph shows the required levels of wind curtailment for each of these combinations under the four MSG conditions. The severity of curtailment is strongly correlated to the constraint scenario, with very little curtailment required when both interconnectors able to export at maximum capacity (scenario 1), and heavy curtailment required if this tie line is out of commission and the Moyle interconnector is importing electricity (scenario 6). Table 3 - Table summarising the maximum reduction of curtailment under each interconnector scenario

Scenario GWh wind saved Reduction (1) NS‐FMO, M‐MO 0.6 100.0% (2) NS‐FMO, M‐Z 23.0 67.4% (3) NS‐FMO, M‐MI 108.2 35.6% (4) NS‐Z, M‐MO 185.5 30.9% (5) NS‐Z, M‐Z 332.3 25.1% (6) NS‐Z, M‐MI 536.4 19.5% In all cases, the lower MSG conditions reduce curtailment requirements, although the reduction of curtailment requirements diminishes as the scenarios worsen. This is emphasised in Table 3, which shows the absolute amount of wind energy saved from curtailment by moving from the highest minimum stable generation (MSG-H2) to the lowest (MSG-L1). In the most favourable conditions, where the north-south and Moyle interconnectors are both able to export wind at their rated capacity (scenario 1), a reduction in MSG would negate the need for wind curtailment completely, whereas in the worst case (scenario 6) it reduces curtailment requirements by 19.5%.

Figure 4 – Array plots showing when curtailment is required for the different interconnector scenarios (1-6) under MSG-H1 A series of 2D array plots is shown in Figure 4, illustrating the progression of curtailment requirements through the six interconnector combinations. Time of year is given on the vertical axis and hour of the day along the horizontal. The colour bar indicates the strength of curtailment required. It is clear that as the interconnector combination becomes more constraining, the frequency and strength of curtailment both grow. Curtailment is first required for night periods when system demand is low (10 p.m. – 6 a.m.), but curtailment during the day also becomes necessary when constraints are more severe.

5. CONCLUSIONS

This paper has shown that level of wind curtailment in Northern Ireland under the three unit rule is strongly influenced by the assumptions surrounding the interconnector. Curtailment was found to be initially required during night periods of low system demand, but may be required any time of day if interconnector flows constrain the demand profile further. Reducing the MSG condition has shown a substantial absolute reduction of wind curtailment, from 0.6 GWh saved from curtailment in scenario 1 (with the most favourable interconnector flows), up to 536.4 GWh in scenario 6 (with the least favourable interconnector flows). However, the relative reduction in curtailment diminishes from 100% (i.e. no curtailment needed) to 20% in these same scenarios. Although a reduction to wind curtailment could be made by switching Kilroot’s units to run on oil instead of coal, this is not currently allowed under the existing market framework. Further work is required to quantify the value of such a reduction in MSG, in terms of net carbon reduction and distributed system costs. A future scenario worth exploring further is the saving and benefit of reducing Northern Ireland’s MSG requirement to two units. ACKNOWLEDGEMENTS

The authors would like to extend their thanks to SONI and NIREG for their time in answering questions related to this research. They also wish to thank members of Kilroot and Ballylumford power stations for their invaluable insight.

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