The contribution of renewable energy resources and energy storage systems to the North American electric grid is increasing with power system connections through power electronic inverters rather than spinning electromechanical machines. The challenges posed by such growth in inverter-based resources are addressed in a technology roadmap developed by researchers from U.S. National Renewable Energy Laboratory, U.S. Lawrence Berkeley National Laboratory, University of Washington, U.S. Sandia National Laboratories, University of Wisconsin and U.S. Department of Energy Solar Energy Technologies Office.
When there is a large disturbance or outage on the grid, conventional inverters will shut off power to these energy sources and wait for a signal from the rest of the grid that the disturbance has settled and it is safe to restart — known as “grid-following.” As wind and solar account for increasing shares of the overall electricity supply, it is becoming impractical to depend on the rest of the grid to manage disturbances. Grid-forming inverters are an emerging technology that allows solar and other inverter-based energy sources to restart the grid independently.
The research roadmap focuses on grid-forming inverter controls, their impact on grid stability, and evaluating crucial system interactions for both electromechanical and inverter-based resources. Transitioning to a grid with more inverter-based resources presents major challenges as the operation of future power systems must be based on a combination of the physical properties and control responses of traditional, large synchronous generators and those of numerous and diverse inverter-based resources.
The roadmap recognizes that inverter controls are now predominantly grid-following and that future power systems will involve a mix of inverter-based resources with both grid-following and grid-forming control capabilities. Growth over time will depend on how well grid-forming inverters perform and what advantages they bring as penetration levels of inverter-based resources increase. The roles and requirements of grid-forming inverter-based resources, including solar photovoltaics, wind generators and energy storage, are considered. A review of current research is accompanied by an outline of research needs related to frequency control, voltage control, system protection, fault ride-through and voltage recovery, and modeling and simulation.
The present power system (left) has been dominated by generators with large rotational inertia. Future power systems (right) might have a significant fraction of inverter-based resources, which implies a need for next-generation controls to ensure stability. Source: U.S. National Renewable Energy Laboratory
