Incident Response – Inverter Grid Response

Subject: ERCOT May 9th Event and Grid Stability Preparedness

On May 9th, 2021; a 345kV lightning arrestor faulted at a substation in Odessa, TX. The fault lasted less than three seconds but still caused a cascade of events resulting in nearly 1,299 MW of lost PV generation across three sites in an instant.

Soon afterwards, ERCOT and NERC began an investigation into the response to the lightning arrestor fault at these PV facilities to understand site specific causes and prevent a similar drop in generation from occurring in the future and creating a grid stability problem.

Site Responses

At one site, inverters detected an undervoltage condition and rode through as expected and responded by injecting reactive current. As the grid voltage recovered, the system overshot and pushed the internal site voltage higher than intended, and some of the units saw AC voltages over the 1.25 pu threshold. These inverters tripped on instantaneous overvoltage.

At a second site, inverters detected the underfrequency condition resulting in a “loss of synchronization” trip. This inverter protective function was intended to identify faults internal to the inverter but resulted in an unintentional trip after a modest excursion in grid frequency.

On the last site, inverters detected the change in frequency and responded with momentary cessation and recovered within four cycles. These inverters had the ideal response to the grid event, however the power plant controller (PPC) overshot the frequency response after initial recovery resulting in several swings in generation.


The following recommendations for Generator Owners / Operators may help to mitigate site specific risk and prevent lost generation under the same conditions:

  • Ensure that inverter software configuration is up to date by working with inverter OEMs to implement yearly checks as a part of a preventative maintenance plan.
  • As new control setpoint guidance is released from inverter OEMs, review PPC settings for potential improvements.
  • Work with inverter OEMs to understand boundary conditions for momentary cessation. If equipment limitations permit, use momentary cessation instead of faulting for faster recovery.
  • Use manufacturer-provided inverter dynamic models to run transient stability analysis to determine site-specific inverter dynamic voltage control settings
  • Determine whether overvoltage settings at the inverters can be set at value higher than the ride through limit at the POI to prevent inverter overvoltage trips

If you have questions or concerns about how to prepare and respond to a potential threat, please don’t hesitate to reach out to our team – we would love to help support your efforts. – Radian Generation Risk Management Team