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 return the system frequency to normal. 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 isolate 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 maintenance. Wind and solar generation respond best to grid frequency increases, which require a drop in power generation. 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 or by capturing more energy from the wind. Solar arrays, which lack a large rotating mass, would need auxiliary storage to provide inertial response.
Because most wind and solar plants are not equipped with frequency response capability, NREL is researching the effects of displacing conventional generation with significant quantities of wind and solar generation.
NREL is addressing frequency response issues on the transmission system through its work on active power controls.