NORTH AMERICAN ELECTRIC RELIABILITY CORPORATION
Solar Loss Disturbance Report Uncovers Reliability Gap in Frequency Measurement Errors

An August 16, 2016 occurrence in the Western Interconnection resulted in the loss of approximately 1,200 MW of photovoltaic (solar) generation. A joint NERC and Western Electricity Coordinating Council task force investigated this event and found the loss of inverter power injection was due to a perceived low-frequency condition and low-voltage blocking of the inverters.

The task force produced the 1,200 MW Fault-Induced Solar Photovoltaic Resource Interruption Disturbance Report, which found that inverters are susceptible to tripping during transients generated by faults on the power system. The report found many of the resources tripped as they used near-instantaneous frequency measurements, which erroneously calculated the frequency. The task force recommends a minimum delay for frequency tripping is needed to ensure an accurate system frequency measurement has been captured.

Additionally, during voltage excursions inverters may cease to supply power. The report recommends that these resources need to quickly return to support bulk power system reliability. The task force report also includes several key recommendations for industry going forward.

"The growth of inverter-based technologies that operate in micro-seconds is rapidly changing the characteristics of the bulk power system and can present new challenges," said James Merlo, vice president and director of Reliability Risk Management at NERC. "It is important that generator owners and operators work with the inverter manufacturers to ensure that resources have sufficient ride-through' capability to support reliability."

The loss of significant amounts of asynchronous generation with inverter technology highlights a previously unknown risk to reliability. NERC and industry will begin taking steps to mitigate the risk, while conducting additional studies and reviewing existing NERC Reliability Standards.

Grid reliability remains a key priority for the Electric Reliability Organization. NERC and industry will continue to work with inverter manufacturers to better understand capabilities and requirements for reliable integration and employment of inverter technologies on the bulk power system.

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