On May 9, 2021 at 11:21 central time near Odessa in West Texas, a combined-cycle gas-fired generating plant tripped off line, due to what a reliability expert described as “a small, ordinary line-to-ground fault.”
The result was entirely unexpected: solar photovoltaic generation over a 200-mile section of the
Electric Reliability Council of Texas grid also went offline. As Troy Dahlgren, a reliability specialist at the North American Electric Reliability Corporation, wrote, “At the time of the disturbance, solar resources accounted for nearly ten percent of the online generation supplying the net demand of 47,434 MW in ERCOT. Almost 25 percent (1,112 MW) of that solar generation tripped offline in response to the disturbance and took about eight minutes to return to predisturbance operation.”
A similar but smaller event in ERCOT occurred on June 26, according to NERC. Other anomalous solar grid disconnections have also occurred in California.
The event, now known as the “Odessa Disturbance,” may have implications for reliability far beyond ERCOT as solar and wind make up a greater portion of the power on the various grids in the U.S.
The culprit in the collapse of a significant portion of the Texas grid was the performance of inverters, equipment that converts to direct current from the solar panels to alternating current on the grid. Wind and hydro resources that generate DC power also require inverters, and some wind was affected by the Odessa Disturbance, according to NERC. The event says 192 MW of reduced output from the gas plant, 1,112 MW of solar output lost, and 36 MW of wind.
In typically flat engineering language, NERC said, “None of the affected inverter-based resources were tripped consequentially by the fault itself. Rather, all reductions were due to inverter-level or feeder-level tripping or control system behavior within the resources.” When the AC line has a specific disturbance, the inverter shuts down the DC feed. At least one brand of inverters, which NERC intentionally did not name, has a built-in 5-minute delay before resetting.
The inverter issue came up during NERC’s recent summer reliability assessment. As Utility Dive reported, NERC’s John Moura told reporters on a conference call, “The inverter tripping challenge is really one of the most risky issues we have to deal with as an industry in order to ensure we can reliably interconnect the nearly 500 GW of solar we see coming online in the next 10 years.”
Moura noted that the issue came up in 2016 in California when the Blue Cut Fire tripped several transmission lines and causing almost 1,200 MW of solar energy capacity to go offline unexpectedly.
Moura said that “these types of risks have the potential to have widespread impacts across the entire interconnection.”
–Kennedy Maize
Twitter (kennedymaize)