Electric power system operators must consider the frequency response of renewable energy generation. Wind and solar generation use significantly different technologies from conventional power plants; therefore, their electrical characteristics and performance are different.
When a large generating plant shuts down, the frequency of the electric power system drops because of the imbalance between generation and load. The frequency decline is checked in the first few seconds by conventional synchronous machines, which contribute stored inertial energy to the system. Over the next few tens of seconds, synchronous machines equipped with governors increase their power output in an effort to stop system frequency decline and stabilize it at a certain level. Synchronous machines can also respond to frequency increases caused by large losses of load. This frequency response—both inertial and governor—could change with significant levels of variable generation.
Most modern wind turbines and solar arrays connect to the grid via power electronics-based converters. These converters decouple the wind and solar generation from the grid and its frequency excursions. When equipped with governor-like controls, the converters can also allow the renewable generation to contribute to grid frequency stabilization. They can provide governor response to frequency drops only when they are operating in a curtailed condition.
In addition, wind turbines can provide an inertia-like response by contributing power to the grid from their own stored kinetic energy. Solar arrays, which lack a large rotating mass, would need auxiliary storage to provide inertial response.
Because wind and solar generation primary and inertial responses differ from those of conventional generators, and this is not entirely understood, NREL is researching the effects of displacing conventional generation with significant quantities of wind and solar generation, including how this will affect the system frequency response at different penetration levels.
NREL is addressing frequency response issues on the transmission system through its work on active power controls. For more information, see Role of Wind Power in Primary Frequency Response of an Interconnection.