Wind turbine with advanced controls used in grid-forming test by NREL and GE

General Electric (GE) and the National Renewable Energy Laboratory (NREL) demonstrated grid-forming technologies in the Type-3 wind turbine, a step in long-term grid modernization efforts as the rapid deployment of renewable technologies transforms the electric power system.

Grid-forming technologies provide functions that are traditionally provided by synchronous machinery. They allow solar and other inverter-based energy sources to restart the electric grid independently.

In grid-forming mode, a generator can set grid voltage, frequency and, if necessary, operate without power from the grid. This includes the ability to restart power following an outage, restabilize after a transient electrical event and to generally form the grid as baseline power resources.

Instead of large, traditional spinning generators, inverter-based resources like wind, solar, and batteries are being primed for this role in multiple U.S. Department of Energy (DOE) projects. As renewables make up a larger share of the power supply, they will also need to take on more responsibility as stewards of grid stability and reliability.

In this particular demonstration, GE and NREL teams deployed controls for the 2.5 MW Type-3 wind turbine to provide primary frequency and voltage support, restabilizing the surrounding grid by adjusting its power in response to momentary electrical variances.

Researchers said Type-3 turbines are especially complex for developing grid-forming controls. These turbines use a generator that is directly connected to the grid, with the turbines’ electricity output controlled by power electronics components. Grid-forming controls could allow the turbine to make up for fewer conventional sources of stability on the grid, such as natural gas-fired generators.

A 5 MW research dynamometer served as prime mover in the mock power system, allowing the researchers to emulate different grid dynamics and observe the turbine’s performance. The team found that with GE’s grid-forming controls, the turbine can contribute inertial and phase jump power in similar ways as a synchronous machine, which is a key feature to adding stability to the grid.

NREL used Advanced Research on Integrated Energy Systems (AIRES), an energy systems integration platform which allows at-scale experimentation in a replica grid environment. AIRES is a step-up from previous platforms, allowing for research at the 20 MW level. Helping to power and scale up the technology is NREL’s 8-petaflop supercomputer.

“In this work we have found that the grid-forming turbine serves underlying stability in cases where it’s needed: in systems with many inverter-based resources and few conventional forms of stability,” said NREL Chief Engineer Vahan Gevorgian.

Such capabilities are generally not available with grid-following controls in today’s inverter-based resources, which rely on externally generated voltages by synchronous machines to operate. Grid-following technologies, which exist in most solar plants and battery storage systems, typically produce power that closely follows the grid frequency and voltage of the larger electric system. With grid-following, inverters will shut off power when there is a large disturbance or outage on the grid and wait for a signal that the disturbance has settled and it is safe to restart.

The GE-NREL effort is the first of several federally funded wind technology demonstrations as part of the Energy Technologies Office project, “Wind as a Virtual Synchronous Generator,” which aims to research wind and storage inverter controls that electronically imitate conventional generators. Research teams will continue to study how the grid-forming turbine interacts with other devices on the power system and whether the grid-forming mode results in greater mechanical stress on the turbine.

In the 2020 Research Roadmap on Grid-Forming Inverters, researchers from NREL, universities, and the DOE’s Solar Energy Technologies Office outlined a plan to use renewable energy to jump-start the grid by taking advantage of inverters, which provide the interface between the grid and energy sources like solar panels, wind turbines, and energy storage.

The present power system (a) has historically been dominated by synchronous generators having large rotational inertia. Future systems (b) will have a significant fraction of inverter-based generation resources. This implies a need for next-generation grid-forming controllers that ensure grid stability at any level of penetration with inverter-based resources. (Source: Research Roadmap on Grid-Forming Inverters, NREL)

Researchers noted in the report that inverter controls now are 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 increases, the researchers said.

They recommended the “review of regulatory and technical standards and the development of advanced modeling techniques” for grid-forming inverters.