Site loading image

Regulatory Update

NERC to Modify Standard and Develop Compliance Guidance to Accommodate Inverter-Based Generation Technologies

Dylan Achey | February 20, 2019

Renewable energy systems have dramatically changed the power generation resource mix. These new generation technologies no longer involve directly coupled rotating generators which were once standard in the industry. Now, inverters that change Direct Current (DC) electricity to the Alternating Current (AC) electricity suitable for delivery via AC transmission systems are becoming more prevalent, raising reliability concerns which NERC is addressing through modifications to its mandatory standards and compliance guidance.

NERC Standard PRC-024: Generator Frequency and Voltage Protective Relay Settings

On February 20, 2019 NERC approved a Standard Authorization Request (SAR) to initiate revisions to PRC-024 that address gaps related to inverter-based resources.

Based on recent lessons learned from disturbance analyses and the PRC-024-2 Gaps Whitepaper, the NERC Inverter–Based Performance Task Force (IRPTF) identified potential modifications to PRC-024-2 to help inverter-based generator owners, operators, developers and equipment manufacturers understand how to respond to electric grid disturbances and continue to support the reliable operation of the Bulk Power System. The SAR proposes to revise PRC-024-2 as follows:

  • Modify the region outside the “No Trip” zone of the ride through curves so that registered entities do not interpret this area as a must trip zone.
  • Clarify the “Off Nominal Frequency Capability Curve” and the “Curve Data Point” tables on pages 8 and 9 of PRC-024-2 to reconcile the 0.1 sec time delay in the frequency capability curve with the curve data point table that allows instantaneous operation.
  • Clarify the language in point 5 of the Curve Details found in the Voltage Ride-Through Curve Clarifications to eliminate confusion about whether the curves pertain to RMS (Root Mean Square) or crest values.
  • Remove inconsistency regarding per unit voltage and nominal operating voltage by correcting point 1 of the Curve Details found in the Voltage Ride-Through Curve Clarifications.
  • Clarify the implied functionality of cumulative time by explicitly specifying the conditions for when cumulative values for low and high voltage curves start, stop, and reset.
  • Clarify whether the voltage and frequency protection functions that can trip the inverter are subject to the standard’s requirements.
  • Clarify the definition and whether the use of momentary cessation for inverter-based resources within the “No Trip” zone of PRC-024-2 is acceptable.

Depending on how long standard development takes, it is possible for the modified standard to become effective sometime in 2020, subject to FERC approval.

NERC Standard PRC-019: Coordination of Generating Unit or Plant Capabilities, Voltage Regulating Controls and Protection

Industry guidance is under development within NERC to address the application of PRC-019 considering inverter-based resources operating modes. The standard verifies coordination of generating unit facility voltage regulating controls, limit functions, equipment capabilities and protection system settings. The standard was developed based on traditional rotating generation technology and calls for a five-year cyclic review of voltage regulating system controls (including in-service limiters and protection functions) with the applicable equipment capabilities and the settings of the applicable protection system devices and functions.

When released, the PRC-019-2 Compliance Implementation Guidance will help develop a common understanding and identify methods for the coordination of voltage regulator controls, limit functions, and protective relay settings with respect to the standard. It will also establish reasonable assumptions for the calculations to meet the intent of the standard.

Next Steps

Utilities should stay informed regarding these two NERC initiatives to better understand and prepare for future compliance expectations.

Because your company may have limited technical resources in certain areas of planning and design, TRC can supplement your existing staff and programs, helping you develop internal initiatives to proactively respond to the changing demands of NERC compliance.  Our experts provide assessment, corrective action and program implementation solutions for commissioning procedures, protection system design coordination with generating assets, including new renewable generating technology.


This regulatory update is a service to TRC’s utility clients, helping keep you informed of issues that impact your company’s electric system reliability and resilience risks along with related topics regarding future regulatory developments so you can achieve your business goals. 

Dylan Achey

Dylan Achey is TRC’s Manager of Generation Engineering Services. He has been leading the effort with TRC generation clients on evaluating and providing updates/information so that clients can meet applicable NERC standards. His highly technical staff perform NERC compliance standard evaluations as well as studies for both generation and transmission clients that need assistance on technical issues concerning NERC compliance. Contact Dylan at

Tim Farrar, PE

Tim Farrar is a licensed professional engineer and works as the Protection & Controls Chief Engineer in TRC’s Augusta, Maine office. He is also a Certified Control System Technician (CCST) and Licensed Electrician with an Associate Degree from Eastern Maine Technical in Electrical Power Technology. Tim has 28 years of experience in protection and controls systems engineering for electric utilities and power generation industries including 10 years at Central Maine Power Company and 18 Years in the consulting engineering business. He has held several positions at TRC as an Engineer, Supervisor of Automation and Controls and Electrical Engineering Manager prior to his current position as Chief Engineer.

Contact Tim at

By clicking "Accept", you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Read our Privacy Policy.