Ancillary Service Methodology - CIMView

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Ancillary Service Methodology

The Gist

Every year ERCOT looks at historical data to determine how much Ancillary Services (reserves, or ‘AS’)  it needs to procure over the coming year.  Currently it does this for four reserves; Regulation Up (REGUP) , Regulation Down (REGDN) , Non-Spinning Reserves (NSRS, or NonSpin), and Responsive Reserves (RRS).  The current (as of 1/1/2022)  methods for determining these reserves are as follows:


For each month take the larger of A) the 95th percentile of REGUP deployments for the same month of the previous two years and B) the 95th percentile of the positive net load changes for the same month of the previous two years.


For each month take the larger of A) the 95th percentile of REGDN deployments for the same month of the previous two years and B) the 95th percentile of the negative net load changes for the same month of the previous two years.

For both REGUP and REGDN ERCOT will adjust amounts throughout the year based on how much wind and solar gets added.  The adjustment tables which detail MWs of REGUP and REGDN change per 1000MW of added Wind, and same -different table- for Solar, are included in the methodology document .

Also, if the CPS1 score (a metric for determining how well frequency is in control) drops below certain thresholds ERCOT will increase the amount of REG it purchases.


NonSpin is based on analysis of four hour blocks of the same month in the previous three years.  The main component is based on “Net Load Uncertainty”.  For each 5 minute interval in an hour the net load is determined by subtracting the uncurtailed total wind and solar output from the load.  The highest of those 12 net loads for an hour is compared to the 6 hour forecasted value of the net load and the difference is the Net Load Uncertainty.  The main component of NonSpin is either the 85th percentile (for low risk intervals) or the 95th percentile (for high risk intervals) of the net load uncertainty.

A second component of the NSRS is calculated as the 75th percentile of the historical intra-day forced outages of thermal resources (since midnight) for six hour blocks of each month in the last three years.

A third component is a variable (up to 1000MW) quantity of NonSpin that ERCOT may buy if it feels an upcoming day is high risk.

Finally, Like REG, NonSpin amounts will be adjusted throughout the year based on the amount of additional installed wind and solar capacity.  Tables detailing these adjustments (per 1000MWs of additional installation are in the Methodology document.

New for 2022: 

  1. Basing Net Load Uncertainty on a 6 hour forecast (previously 4 hour forecast)
  2. The forced outage compnent
  3. The variable additional 1000MWs (introduced mid year 2021)


RRS is based on the amount that ERCOT studies are needed to maintain frequency during sudden shocks to the  system based on expected load, solar and wind patterns.  Over the peak hour blocks at least 2800MW of RRS will be purchased.  At all time at least 40% or 1,420MW of RRS, whichever is greater, must be provided by resources using Primary Frequency Response (PFR, usually conventional generation with governors).  The most RRS that can be provided by Fast Frequency Response resources (usually batteries) at any given time is 450MW. 

New for 2022: The 2800MW floor over peak.

Important Actions

  • 1/1/22 Implemented
  • 2/24/22 ERCOT clarifies that the additional 1000MW of NonSpin is not just for high variability days but also for high risk days (e.g. high winter load)


2022 AS Methodology (ERCOT)

Meeting Notes

  1. 2/24/22 WMWG

    • Executive Summary

      Today’s Wholesale Market Working Group meeting was short and efficient.  The group quickly discussed the new Reliability Unit Commitment report, NPRR1100 (Tesla Microgridding), NPRR1108(ERCOT to approve or deny all resource outages), NPRR1100(Black Start Reform), and that Nitika Mago is now the contact person for discussions about battery testing for NonSpin.

    • Agenda

      ERCOT Meeting Page

    • Notes

        • Reliability Unit Commitments- ERCOT (Zhengguo Chu) gave a presentation on RUCs for January and the first half of February (this will be a regular monthly report going forward).  
        • February 1-15 had more RUC hours than January 1-31 (due to cold weather event).
        • ERCOT will present RUC process to WMWG in the future.
        • Extra NonSpin Procurement procedure- ERCOT (Nitika Mago) talked about how ERCOT determines if it needs extra NonSpin.
        • If ERCOT determines that there is a lot of variability in the net load forecast it might get up to 1000 MWs of extra NonSpin OR
        • If ERCOT sees tight conditions due to cold weather, it also might get up to 1000 MWs of extra NonSpin (this is what happened in the earlier February event).
        • The only place you can see the extra NonSpin procurement is the DAM Ancillary Service Plan.
        • NPRR1100(Tesla microgridding) – Tesla will meet soon with ERCOT to iron out details and then publish new comments; hopefully they will report back with an agreed upon document by the next WMWG.
        • NPRR1108(ERCOT to approve or deny all outages)- ERCOT (Fred Huang) talked through the relatively minor changes in ERCOT’s latest set of comments.
        • Report on all available outage MWs (60 month look ahead) twice daily.
        • Single phase automatic implementation.
        • The most recent numbers for max MWs out for next 60 months to be posted by ERCOT soon to the January WMWG meeting page
        • NPRR1110(Black Start Reform) – ERCOT (Freddy Garcia) talked through ERCOT’s latest thoughts on this NPRR.
        • The Black Start Agreement is now ERCOT Critical Electrical Infrastructure Information.
        • ERCOT is still undecided on whether the procurement period should be two, three, or four years.
        • NonSpin Testing for Batteries- Nitika Mago is the ERCOT point person for any ideas people have on how to better handle batteries providing NonSpin.


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