ERCOT said in a market notice that it has identified an error in how Mitigated Offer Caps (MOCs) were calculated for certain Operating Days (ODs) over five months in 2017 and 2018.

ERCOT said that, during months in which ERCOT made mid-month updates to Verifiable Costs, ERCOT’s Settlements system incorrectly doubled or tripled the value of Operations and Maintenance (O&M) costs above the Low Sustained Limit (LSL) for certain Resources with Verifiable Costs. As a result, these Resources were under-mitigated in affected Security-Constrained Economic Dispatch (SCED) intervals. MOC components other than O&M were not impacted. O&M costs used for startup and Minimum-Energy Offer Caps, and for Reliability Unit Commitment and Day-Ahead Make-Whole Payments, were likewise not impacted.

ERCOT has determined that the error impacted the following Operating Days:

• August 2, 2017 – August 31, 2017;

• October 4, 2017 – October 31, 2017;

• February 14, 2018 – February 28, 2018;

• May 2, 2018 – May 31, 2018; and

• December 5, 2018 – December 31, 2018.

ERCOT said in a market notice that, "Although this issue may have affected prices, and dispatch to some extent, ERCOT is unable to perform price corrections because it cannot determine accurate prices, as any effort to recalculate new prices would require ERCOT to make significant assumptions concerning the operating behavior of individual Resources."

Pending implementation of updates to impacted production systems, ERCOT has instituted manual processes to correct the error going forward. Production system fixes are expected to be implemented during the release scheduled for April 2 - 4, 2019, ERCOT said in a market notice