Inverter disconnect events increasingly pose a reliability risk with the growth of solar generation.
On June 8, the North American Electric Reliability Corporation (NERC) released its report on the loss of 1,200 MW of solar generation in southern California during a system disturbance that unexpectedly caused inverters at solar generation facilities to trip or momentarily cease to operate. The report provides solar plant owners and engineers with recommendations to prevent future occurrences. According to NERC, inverter disconnect events pose an increasing reliability risk given the expansion of solar generation.
Growing solar penetration has made the response of solar generators to system disturbances more critical. If NERC and utility-scale solar generators adopt the report’s recommendations, the likelihood of both recurrences and government-imposed regulations will be reduced. The Federal Energy Regulatory Commission’s (FERC’s) recent orders requiring renewable generation to promote frequency response (Docket No. RM16-6), reactive power (Order No. 827), and ride-through capability (Order No. 828) indicate a willingness to impose regulatory requirements on renewable generation where FERC sees it as necessary to preserve system reliability. Separate and apart from NERC action and any voluntary industry response, the report may lead FERC to consider such action.
On August 16, 2016, the Blue Cut fire ignited in southern California and spread toward an important transmission corridor containing four high-voltage lines. Faults on the four lines resulted in the loss of nearly 1,200 MW of solar generation.
NERC’s subsequent investigation identified three contributors to the power loss.
The investigation also revealed that similar inverter disconnects had occurred during other disturbances in the recent past.
NERC’s investigation found that inverters that trip instantaneously based on near instantaneous frequency measurements may trip erroneously when faults distort waveform. While the PRC-024-2 Reliability Standard specifies a no-trip range for voltage and frequencies, solar generation owners and inverter manufactures do not generally consider the standard to prohibit momentary cessations. Further, owners and manufacturers generally believe that anything outside the no-trip range is in a must-trip range.
The investigation also found that most installed inverters will momentarily cease providing current immediately when voltages go outside their normal operating range. These inverters require major modifications or replacement to stop this inverter response, but some inverter manufacturers are no longer operating, suggesting a recognition that such modification or replacement might not be possible.
NERC’s investigation then found that inverter manufacturers are using the IEEE 1547-2003 standard, which many generator owners believe they must meet—even though the IEEE standard conflicts with PRC-024-2.
The investigators concluded that this type of disturbance presents a previously unknown reliability risk, which is expanding as the use of solar generation increases.
NERC’s investigation resulted in several recommendations to mitigate or prevent future occurrences.
“Knowing that [the fault causing inverters to erroneously trip or momentarily cease] was not an isolated incident and considering the rapid increase in solar installations . . . these types of inverter disconnect events could be a potential reliability risk that needs to be analyzed and mitigated,” NERC concluded.
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J. Daniel Skees