Putting a Spin on the Role of Inertia
Traditional electric power grids use the energy stored by inertia in spinning generators for grid reliability—renewables like wind and solar bring no inertia so new ideas are needed.
Because the U.S. electric power grid, based upon spinning alternating current (AC) generators powered largely by steam turbines, has been around for more than 100 years, certain features of traditional electric power generation are baked into the system. They have become useful in designing and maintaining safe and reliable grid operation. One such is the energy stored as inertia in large rotating generators.
Imaged used courtesy of Pixabay
A traditional power grid comprises AC generators powered by coal or natural gas combustion, nuclear, or hydroelectric power generation. The AC generators are synchronized to run at the same speed, producing AC electricity at a frequency (in the U.S.) of 60 cycles per second (hertz). If one generator goes down, the others on the grid will increase their output slightly to maintain the proper grid frequency.
But here is the thing. Because of the inertial energy held within the spinning generator, there is some significant time (typically a few seconds) between the failure of the generator system and when it stops producing electric power—time that can be spent using the other AC generators on the grid to detect the problem, responding to the failure, making up the difference, and maintaining the proper grid frequency.
Inverters Have No Inertia
The adoption of renewable energy sources presents new challenges. Photovoltaic (PV) solar and wind turbines generate direct current (DC) energy and require an inverter to convert DC to the AC used by the grid. Inverters do not provide the inertia that a spinning generator does—if it fails it does so nearly instantaneously. So, a question of how to maintain inertia on the grid—or even if it is necessary—has emerged.
"We find that replacing conventional generators with inverter-based resources, including wind, solar PV, and certain types of energy storage, has two counterbalancing effects," said Paul Denholm, National Renewable Energy Laboratory (NREL) principal energy analyst in a news release. "First, it's true that these resources decrease the amount of inertia available on the system. But second, these resources can reduce the amount of inertia actually needed—and thus address the first effect. In combination, this represents a real paradigm shift in how we think about providing the grid services that maintain system reliability."
Work at the Pacific Northwest National Laboratory (PNNL) has shown that it is possible to operate a microgrid using inverters that have no reliance on synchronized power generators. Scaling this up to a national power grid is the next step. In addition, commercial battery-based energy storage systems (ESS) installed in places like Australia have proven to provide nearly instantaneous energy to help stabilize grid frequency should an inverter fail.
"Ultimately, although growth in inverter-based resources will reduce the amount of inertia on the grid, there are multiple existing or possible solutions for maintaining or improving system reliability," Denholm said. "So, declines in inertia do not pose significant technical or economic barriers to significant growth in wind, solar, and storage to well beyond today's levels for most of the United States."
NREL has produced a video that explains inertia and how it changes when renewable energy comes into play.