With increasing usage of renewable energy, or inverter-based resources (IBRs) such as wind and solar, inverters with grid-forming capabilities (GFM) are needed, according to engineers with the North American Electric Reliability Corp. (NERC).

Inverters convert energy generated by renewables into alternating current that flows through the transmission system. “The power system’s heartbeat is a 60-Hertz, or cycles per second, current to which all generation on the grid must synchronize,” said Mark Lauby, senior vice president and chief engineer at NERC. “As long as variable renewables’ inverters synchronize their generation with that 60-Hertz heartbeat, they can deliver power into the system without concern about system faults.”

Without unified standards for inverters, fluctuations in the system’s “heartbeat” threaten reliability. Traditional power plants feature synchronous machines to stabilize fluctuations, but they’re retiring. Thus, deployment of GFM is needed with new IBRs to correct faults that cause fluctuation. But first, pilot tests are needed to track GFM performance. Based on those results, NERC and system operators should set standards defining GFM requirements to assure reliability.

In its November 2020 GFM Roadmap, the National Renewable Energy Laboratory explained that coal, natural gas, nuclear and hydropower plants are designed to drive spinning turbines, which generate electricity with a 60-Hz heartbeat rate. The spinning mass of traditional synchronous machines has a momentum, or inertia, that corrects frequency fluctuations. But as these machines are replaced with IBRs, system inertia is reduced. This increases the risk of frequency swings. While IBRs using grid-following inverters don’t address those changes, GFM detects them and adjusts power flow to limit them.

Because GFM is “human-programmed through power electronics, highly controllable and can provide very fast responses” to frequency fluctuations, said Mahesh Morjaria, executive vice president of plant operational technology for utility-scale solar project developer Terabase Energy, Berkeley, Calif., it can replace synchronous machines.

GFM provides reliability through frequency control to manage fluctuations and voltage control to maintain system stability by increasing or lowering output. It also provides other protections, such as stability services that detect abnormal grid operating conditions, which it can mitigate by disconnecting failed parts.

GFM can also perform a system “black start” using IBR-supplied energy to restart generation and build normal operations back up piece by piece. But that adds operational challenges, requires energy to drive the restart and is more expensive.

There may be other costs involved for integrating GFM into IBRs; however, possible market opportunities could offset those costs. With the possibility of market incentives, IBRs with GFM “could provide cost reliability services that create additional revenues because they use zero-cost fuel,” Morjaria said.

There’s also a cost of inaction. Fortunately, the Department of Energy is investing tens of millions of dollars and making GFM a “huge priority,” according to Becca Jones-Albertus, director of the DOE’s solar energy technologies office. Deploying utility-scale batteries takes advantage of the policy-driven clean energy build-out and enables GFM at scale, but more research and pilots are needed.