PROPOSED FCAS CALCULATION CHANGES IN TASMANIA

Transcription

1 PROPOSED FCAS CALCULATION CHANGES IN TASMANIA FOR THE NATIONAL ELECTRICITY MARKET PUBLISHED JUNE 2014 IMPORTANT NOTICE

2 IMPORTANT NOTICE Purpose AEMO has prepared this document to provide information to how AEMO proposes to change the calculation of the contingency frequency control ancillary service (FCAS) requirements in Tasmania (for non Basslink trip contingency events), as at the date of publication. Disclaimer This document or the information in it may be subsequently updated or amended. This document does not constitute legal or business advice, and should not be relied on as a substitute for obtaining detailed advice about the National Electricity Law (NEL), the National Electricity Rules (NER), or any other applicable laws, procedures or policies. AEMO has made every effort to ensure the quality of the information in this document but cannot guarantee its accuracy or completeness. Accordingly, to the maximum extent permitted by law, AEMO and its officers, employees and consultants involved in the preparation of this document: make no representation or warranty, express or implied, as to the currency, accuracy, reliability or completeness of the information in this document; and are not liable (whether by reason of negligence or otherwise) for any statements or representations in this document, or any omissions from it, or for any use or reliance on the information in it. Copyright Australian Energy Market Operator Limited. The material in this publication may be used in accordance with the copyright permissions on AEMO s website Page 2 of 16 AEMO June 2014

3 VERSION RELEASE HISTORY Version number Release date Author Comments June 2014 Ben Blake Minor editing changes. Updated Figure 7.1 and 7.2. Added section on Tamar Valley CCGT. Added appendices for coefficients and plain English example. Updated to new AEMO template December 2013 Andrew Groom Updated Figure December 2013 Ben Blake Initial version AEMO June 2014 Page 3 of 16

4 CONTENTS IMPORTANT NOTICE 2 VERSION RELEASE HISTORY 3 1. INTRODUCTION Issues with the existing FCAS calculations 5 2. MODELLING BASSLINK S CONTRIBUTION TO TASMANIAN FCAS Raise Contingency FCAS Lower Contingency FCAS Tamar Valley CCGT outage 9 3. IMPACT OF THE CHANGE 9 4. SLOW FCAS (60 SECOND) FREQUENCY LEVEL IMPLEMENTATION 12 APPENDIX 1 FCAS COEFFICIENTS 12 APPENDIX 2 PLAIN ENGLISH EXAMPLE OF R6 CONSTRAINT EQUATION 13 TABLES Table 4-1 Frequency operating standards in Tasmania 11 Table 4-2 Frequency levels used by the constraint equations 11 Table 6-1 Coefficients for Raise FCAS constraint equations 12 Table 6-2 Coefficients for Lower FCAS constraint equations 12 FIGURES Figure 2-1 Tasmanian raise 6 second requirement comparison - for loss of a Tasmanian generator 7 Figure 2-2 Tasmanian lower 6 second requirement comparison for loss of a Tasmanian load 9 Figure 3-1 Duration curve for current and proposed raise calculation 10 Figure 3-2 Duration curve for current and proposed lower calculation 10 Page 4 of 16 AEMO June 2014

5 1. INTRODUCTION A recommendation from AEMO s Wind Integration studies 1 is to model the effect of the Basslink frequency controller in the calculation of Tasmanian contingency frequency control ancillary service (FCAS) requirements. Under most operating conditions this change will reduce the requirements for Contingency FCAS services in Tasmania and therefore reduce the costs of FCAS to Tasmanian consumers. Tasmania currently has the highest cost for FCAS in the NEM and if the proposed changes had been implemented in the first half of % of the cases requiring raise 6 second (R6) for loss of the largest Tasmanian generator would have had a 0 MW requirement. It is proposed to implement this change in mid-2014 after consulting with Transend, Hydro Tasmania and the NEM Wholesale Consultative Forum. This change will mean the FCAS calculations more closely model the observed behaviour of the Tasmanian frequency following either a load or generator contingency event in Tasmania Issues with the existing FCAS calculations AEMO currently calculates contingency FCAS requirements in the Tasmanian region taking into account post contingent Tasmanian inertia, contingency size and Tasmanian demand. Analysis by AEMO as a part of the wind integration studies indicates that the calculation should be updated to consider the behaviour of the Basslink frequency controller and the performance of Tasmanian wind farms following AC faults. Under conditions of low power system inertia in Tasmania the required quantity of R6 Contingency FCAS in Tasmania can increase rapidly, increasing by more than 2 MW for a 1 MW increase in contingency size. To ensure the accuracy of the calculation used to determine Contingency FCAS requirements in Tasmania, AEMO engaged a consultant to independently evaluate the calculations for a range of power system conditions, with particular emphasis on low inertia conditions. The consultant provided the following observation: The way Basslink is accounted for has a large impact on the accuracy of the FCAS Calculator, particularly for large generator and load contingencies. Relative to the response time over a 6 second period of a generator governor, the Basslink response tends to be quasi-instantaneous. The response of the Basslink frequency controller to a change in Tasmanian frequency is significantly faster than the response of the governors of hydro generation, and the Basslink frequency controller will continue to rapidly respond to changes in Tasmanian frequency until either a Basslink limit is reached, or until the frequency deviation is corrected (within the controller s dead band of ±0.01 Hz). This behaviour means that it is very conservative to assume that Basslink will not respond to a generation contingency ahead of hydro governors when determining the requirement for Contingency FCAS in Tasmania. It also means that under most operating conditions AEMO is procuring more R6 FCAS from generators in Tasmania than is actually required for control of power system frequency. However, for a small number of operating conditions characterised by very low inertia, high Basslink import into Tasmania and high wind farm generation the current contingency FCAS calculations yield results which are too low. Following an AC fault in Tasmania the power electronic devices on Basslink and the 3 existing wind farms in Tasmania 2 will dramatically reduce their power output to nearly zero as a part of their fault ride through (FRT) sequence. This has the effect of transiently increasing the contingency size by the summation the pre-contingency outputs of Basslink and the three Tasmanian wind farms, until such time these devices 1 AEMO, Wind Integration Studies Report. Available at: Energy. 2 Bluff Point, Studland Bay and Musselroe AEMO June 2014 Page 5 of 16

6 recover to their pre-fault active power output. This behaviour can be modelled using existing (but currently unused) functionality in the FCAS calculator for modelling network control scheme actions. 2. MODELLING BASSLINK S CONTRIBUTION TO TASMANIAN FCAS AEMO s proposed change is to discount the original contingency size, based on the available headroom on Basslink available for use by the Basslink Frequency Controller. The contingency size which remains after discounting is the amount that needs to be managed via the dispatch of Contingency FCAS. An additional change will be made to model the energy loss due to the fault ride through behaviour of Basslink and the relevant wind farms, though as mentioned in the previous section, it is expected that under most operating conditions the effect of this particular change is expected to be small. Safety factors and margins will be applied to the available Basslink headroom, to ensure the available Basslink headroom is considered in a secure manner. These margins are required to allow for changes in available Basslink headroom during a dispatch interval due to Basslink moving to new dispatch target, and movement in the Basslink operating point due to frequency regulation. To model the Basslink frequency controller action AEMO is proposing to discount the Tasmanian contingency size by 80% of the available headroom on Basslink, with this capped at a maximum of 100. These settings are believed to be conservative but sufficiently large enough to approximately model Basslink s actual contribution to credible contingency events occurring in the Tasmanian region (see Figure 3-1 and Figure 3-2 for a comparison of 100 and 200 MW caps). This setting could be adjusted further based on observed Basslink operation and the assessed performance of the region in comparison with the Tasmanian Frequency Operating Standards Raise Contingency FCAS The data used in the analysis below was retrieved from January to September 2013 as well as half hour periods where Basslink was importing into Tasmania during December 2011 to February 2012 and November 2012 to December Using this data set the current Raise 6 second FCAS calculation for the loss of the largest generator and largest inertia was compared to the proposed calculation. This was done using 80% the available Basslink headroom, subsequently capped at both 100 and 200 MW. The analysis (see Figure 2-1) indicated a large difference between the FCAS requirements calculated using the existing methodology (Blue) and proposed new headroom-based calculations (Green and Red respectively). For most cases there was little difference between the Basslink headroom cap of 100 MW (Green) and 200 MW (Red), indicating that the lower, more conservative 100 MW cap on the available Basslink headroom will provide most of the potential benefit. Overall, the Raise 6 second FCAS requirement is greatly reduced compared to the existing method. This can be seen in the scatter plot below as most of the 100 MW cases (green) are covered by the 200 MW cases (red). Page 6 of 16 AEMO June 2014

7 Figure 2-1 Tasmanian raise 6 second requirement comparison - for loss of a Tasmanian generator This change means that up to 100 MW of the Tasmanian contingency event is transferred to the mainland, where it will be covered by mainland load relief. In reality, more of the contingency event may be transferred out of Tasmania to the mainland, depending on the available headroom on Basslink and the frequency change that results in Tasmania. A 100 MW change on Basslink would be covered by the normal load relief on the Mainland, as a mainland demand of only 6,600 MW would be sufficient to provide this. However, to ensure this behaviour is reflected in the FCAS design, new constraint equations will be constructed to model the load relief for the Mainland regions, as well as several islanding scenarios for Victoria. The constraint equation for Mainland Raise 6 second would be of the form: Where: Mainland (Raise 6 sec) >= Min (Basslink Headroom Calculation, 100) Mainland Load Relief Basslink Headroom Calculation = 80% x (If Basslink current flow > 50 MW Basslink Bid availability Basslink current flow Abs (Basslink current flow) 50 Else if Basslink current flow < -50 MW Else 0) Mainland load relief = x Mainland Demand The constraint equations for Tasmanian Raise 6 second will be similar to the current Basslink trip constraint equations (such as F_T+NIL_BL_R6_1) and most likely will be of the form: AEMO June 2014 Page 7 of 16

8 Tasmania (Raise 6 sec) + Basslink MW >= Constant + A x Tasmanian Inertia + B x Tasmanian Demand + C x Woolnorth + D x Musselroe + E x (MW of largest Tasmanian generator - Max (Basslink Headroom Calculation, 100)) Basslink s initial response to the contingency occurs in the millisecond timeframe. Following this response Basslink s frequency controller can continue to provide a conduit for Mainland FCAS services in the 6/60 second and 5 minute timeframes. Therefore Basslink will appear on the left hand side (LHS) of the constraint equations in the same manner as the existing constraint equations to model this behaviour Lower Contingency FCAS The same methodology as outline above will also be applied to the Tasmanian contingency lower calculations. A similar comparison was performed on the existing Lower 6 second FCAS calculation for loss of the largest load against the proposed calculation, using 80% of the available Basslink headroom. Again, a cap of both 100 and 200 MW was examined. The analysis (see Figure 2-2) indicated a notable difference in the FCAS requirements between the current (Blue) and proposed new headroom-based calculations (Green and Red). In the contingency Lower case the difference was not as large as for the Raise calculations and there was a wider scatter in the data. For the 100 MW headroom cap scenario (Green) the vast majority of cases were for a Lower 6 second requirement above 50 MW. In line with the raise methodology a headroom cap of 100 MW will be applied initially for lower calculations. This will be reviewed once the constraint equations have been running for one to two months and some operational experience is available to draw upon. Page 8 of 16 AEMO June 2014

9 Figure 2-2 Tasmanian lower 6 second requirement comparison for loss of a Tasmanian load 2.3. Tamar Valley CCGT outage The impact of outages of the Tamar Valley CCGT (the largest MW and inertia in Tasmania) on the proposed contingency FCAS calculations was also examined as to whether a separate equation or terms were required. While outages of the Tamar Valley CCGT reduced the total inertia in Tasmania these only had a minor impact on the calculation of contingency FCAS. No extra terms or equations will be constructed for outages of the Tamar Valley CCGT. 3. IMPACT OF THE CHANGE The effect of this change would be to make around 70% of historical cases examined have a zero Raise 6 second FCAS requirement (for loss of the largest Tasmanian generator) in Tasmania and in over 99% of cases a reduced requirement from the current calculation, when Basslink is in service (see Figure 3-1 below). The L6 contingency FCAS requirements (for loss of the largest Tasmanian load) would also be reduced from the current methodology but not as much as the raise requirements. Only 40% of cases were zero (for the 200 MW headroom) and for the 100 MW headroom case the requirements would be much lower than the current calculation (see Figure 3-2 below). For the past 3 years, the 6 second raise and lower services in Tasmania have incurred the greatest costs of any NEM region, and Tasmanian FCAS prices are consistently the highest of all NEM regions. AEMO June 2014 Page 9 of 16

10 Figure 3-1 Duration curve for current and proposed raise calculation Figure 3-2 Duration curve for current and proposed lower calculation Page 10 of 16 AEMO June 2014

11 AEMO expects that this change will reduce the calculated FCAS requirements in Tasmania for both raise and lower services when Basslink is in service. This change reflects the actual behaviour of Basslink in transferring some or all of Tasmanian contingency events into the mainland, where they are absorbed by mainland load relief. The contingency FCAS requirements will be the same as the current calculation when Tasmania has low inertia and Basslink is out of service or Basslink has no available headroom. Under conditions of high Basslink import into Tasmania, high wind farm output and low available headroom on Basslink, the contingency FCAS requirements will be the same or higher than the current calculations, though to date this specific operating condition has not occurred. 4. SLOW FCAS (60 SECOND) FREQUENCY LEVEL The Tasmanian Frequency Operating Standard for loss of the largest load or generator does not specify a frequency stabilisation level for the 60 second FCAS. In these cases AEMO sets the 60 second service to the same frequency level as for the 6 second services. The way the existing FCAS calculator determines the 60 second requirements is to multiply the 6 second service by an empirical factor of determined when the inertial FCAS calculations were originally introduced. Table 4-1 Frequency operating standards in Tasmania Condition Containment Stabilisation Recovery Generation event, load event or network event 48 to 52 Hz to Hz within 10 minutes For large contingencies, the calculated values of R60 have been shown to be very conservative. Simulations show that after the frequency reaches its apex and begins to recover, generator outputs tend to return back toward their initial output (since many governor controllers are approximates to proportional control). However the FCAS calculator assumes a linearly increasing governor response and produces a governor output at the 6th second which is larger than what is really required. This number when multiplied by 125 % results in very large values for R60. AEMO proposes to change this methodology and use a frequency level for the 60 second service that is 0.01 Hz different to the 6 second service. This effectively means that empirical factors will not be used to calculate R60. Initial analysis indicates this will provide a reasonable reduction in the 60 second service requirements whilst maintaining the frequency standards. Table 4-2 Frequency levels used by the constraint equations Condition Containment Stabilisation Recovery Generation event, load event or network event 48 to 52 Hz to Hz to Hz within 10 minutes 3 This is labelled in the Tasmanian FCAS calculation spreadsheet as lambda AEMO June 2014 Page 11 of 16

12 5. IMPLEMENTATION AEMO performed the regression analysis and constraint equation construction in December 2013 and January The constraint equations were implemented in AEMO s Pre-Production systems in late January 2014 and their performance monitored for several months. Further analysis has been conducted to determine the performance of the regression equations versus the FCAS calculator as well as determining the impact of outages of the Tamar Valley CCGT. The revised constraint equations are proposed to be implemented in Production on 1 July APPENDIX 1 FCAS COEFFICIENTS Table 5-1 Coefficients for Raise FCAS constraint equations Term R6 Factors R60 Factors R5 Factors Intercept Basslink headroom Musselroe MW Bluff Point MW Studland Bay MW Gen MW at risk Tas Demand Tas Inertia Table 5-2 Coefficients for Lower FCAS constraint equations Term L6 Factors L60 Factors L5 Factors Intercept Basslink headroom Load at risk Tas Demand Tas Inertia Page 12 of 16 AEMO June 2014

13 APPENDIX 2 PLAIN ENGLISH EXAMPLE OF R6 CONSTRAINT EQUATION Below is the plain English version of the proposed constraint equation (dispatch only) for raise 6 second for loss of the largest Tasmanian generator or largest Tasmanian inertia. Constraint: F_T++NIL_MG_R6 LHS= MW flow north on the Basslink DC Interconnector + RAISE6SEC Tasmania region RHS Default RHS value= 50 Dispatch RHS= if Generic Equation: X_FCTRL_RESPONSE_R <= 0 Max ( Tas FCAS calc for Max Inertia generator for Raise 6 sec, Tas FCAS calc for MG for Raise 6 sec ) {Intercept} x [Musselroe wind farm] x [Bluff Point Wind Farm SCADA MW - Stage 1 & 2 of Woolnorth wind farm] x [Studland Bay Wind Farm SCADA MW - Stage 3 of Woolnorth wind farm] x [Tasmania largest generator] AEMO June 2014 Page 13 of 16

14 x [Tas Scheduled Gen + Non Scheduled Gen - Basslink Export from TAS. Equal to TAS Native Load] x ( Tasmanian Inertia - Tasmania largest contingent generator inertia) x [Generic Equation: X_FCTRL_RESPONSE_R] + Generic Equation: X_BL_TAS_MINAVL_ON Equation: X_BL_TAS_MINAVL_ON if if if MW flow north on the Basslink DC Interconnector <= 0-1 x [Maximum bid availability from VIC1 to TAS1 Basslink DC Interconnector (MNSP)] 50 {NoGoUpper} - MW flow north on the Basslink DC Interconnector <= if {PrevDI_Threshold} - Absolute( Basslink previous dispatch interval target (MW)) <= 0 0 Page 14 of 16 AEMO June 2014

15 1 + if {CurrentFlow_Threshold} - Absolute( MW flow north on the Basslink DC Interconnector) <= if 1 - BassLink NEMDE Switch - taken from same SPD_ID for SPD_Type 'S' <= <= 0 if MW flow north on the Basslink DC Interconnector <= 0-1 x [Maximum bid availability from VIC1 to TAS1 Basslink DC Interconnector (MNSP)] 50 {NoGoUpper} {Swamp} AEMO June 2014 Page 15 of 16

16 Equation: X_FCTRL_RESPONSE_R Min ( 100 {Response_Cap}, 0.8 x ( if MW flow north on the Basslink DC Interconnector <= 0 MW flow north on the Basslink DC Interconnector + Maximum bid availability from VIC1 to TAS1 Basslink DC Interconnector (MNSP) MW flow north on the Basslink DC Interconnector - 50 {No_Go_Zone}) ) Page 16 of 16 AEMO June 2014