In general, the U.S. bulk power system operates on an alternating current. For reliability and interoperability, that current must maintain its frequency within predetermined boundaries above and below 60 Hertz. An interconnected grid’s ability to arrest and stabilize frequency deviations within those boundaries after a sudden loss of generation or load is called "frequency response." A grid's frequency response characteristics are affected by factors including inertial response (as spinning generators speed up or slow down when load changes), primary frequency response, and secondary frequency response. Historically, most primary frequency response has been provided by baseload synchronous generators as an ancillary service.
But the U.S. electric grid's energy mix is changing. In a Notice of Inquiry released on February 18, 2016, the Federal Energy Regulatory Commission notes that changes to the U.S. electric supply portfolio likely mean that fewer resources are now primary frequency response. In particular, the U.S. has seen broad retirement of coal-fired baseload synchronous generators, some of which provide primary frequency response, while some have been replaced with variable energy resources such as wind and solar which do not typically have primary frequency response capabilities.
In response, FERC solicited public input on whether and what action is needed, including whether to:
- Amend the pro forma Large Generator and Small Generator interconnection agreements to require that all new generation resources have frequency response capabilities as a precondition of interconnection;
- Implement primary frequency response requirements for existing generation resources; and
- Establish procurement and compensation mechanisms for primary frequency response.
FERC has docketed the matter as RM16-6-000, Essential Reliability Services and the Evolving Bulk-Power System — Primary Frequency Response. Comments on the Notice of Inquiry are due 60 days after publication in the Federal Register.