Grid-forming-inverter-backed big batteries can raise renewables hosting capacity

US-based professional body the Energy Systems Integration Group (ESIG) has published a study about the benefits of grid-forming (GFM) BESS. The report is based on research conducted by ESIG’s GFM BESS project team, which evaluated the benefits of a GFM BESS installed on an electricity transmission network.

ESIG reports GFM controls can provide grid-stabilizing characteristics that support the reliable operation of a grid under increasing levels of inverter-based resources (IBR) such as solar and wind power. While “GFM controls can be implemented on new solar … and wind plants, with some limitations,” ESIG said, battery energy storage is “particularly low-hanging fruit” for the implementation of GFM controls.

Solar, wind, and energy storage sites without GFM controls use grid-following (GFL) inverters.

The project team found using GFM instead of GFL BESS in an electricity transmission system improved grid hosting capacity for solar and wind power generation.

Real grid conditions

The team reached that conclusion by studying part of an actual transmission network – a weak area with limited transmission that was “saturated” with 780 MW of solar generation capacity featuring GFL inverters and paired with 225 MW of GFL BESS. The weak patch of grid studied is actually connected to a strong surrounding transmission network but the researchers modeled it in isolation to replicate “the weakest grid conditions possible.”

While the use of GFL inverters had the local grid “marginally stable,” swapping in GFM systems for the 225 MW BESS enabled the accommodation of an additional 125 MW of GFL-inverter-based solar capacity. That demonstrated “increased IBR hosting capacity was obtained before hitting a stability limit,” the researchers said.

The team reported GFM BESS had a “small incremental cost” over GFL BESS “when designed-in up front” but had a “dramatically lower cost than other solutions,” such as synchronous condensers – direct-current-activated free-spinning motors which exist solely to adjust grid conditions.

The ESIG study recommended TSOs consider adopting GFM BESS “system-wide to improve stability and to maximize IBR [instant backup recovery] hosting capacity.” Another recommendation was to develop GFM BESS technical requirements for integration into grid interconnection requirements.

“Don’t wait,” the study said. “Take advantage of BESS interconnections today” because “retrofits are expensive.”

The study dubbed GFM BESS a “do no harm” solution, in part because it provides stability benefits in weaker grids and operates stably and reliably in strong networks.

The researchers found GFM BESS are interoperable across equipment supplied by, unbranded, original equipment manufacturers and with GFL inverters; they can help defer more costly alternatives; and can serve as a bridge measure for long-lead-time solutions.

The project team plans to publish a brief on the benefits of GFM BESS for policymakers, regulators, and other decision makers, and intends to conduct outreach to key stakeholders.

The project team’s core modeling and study efforts were conducted by Elevate Energy Consulting, Electranix Corporation, and ESIG, in partnership with the American Transmission Company utility.

From pv magazine USA.