The subcycle overvoltage condition describes the electrical generation fault mode that is associated with the inverter-based resources (IBR, like solar photovoltaics and wind turbines) and can cause a massive and instantaneous loss of electricity generation. When the overvoltage condition is detected, the IBR devices self-protect by disconnecting from the grid and can only come back online once the voltage returns to the design limits. [1] In the meantime, a cascading failure can be triggered due to lack of generation capacity that remains online.[ citation needed ]
The typical fault scenario is two-stage: [1]
The problem was accidentally exacerbated by a requirement to have a shunt capacitor across the connection terminals of an IBR. This feature was expected to provide the reactive power support to the grid (the typical inverter is designed for a unity power factor), yet after a short remote line fault it increased the overvoltage at the generator connection and contributed to the problem (the later designs followed the IEEE 1547-2018 standard and did not use the shunt capacitors in the MC mode). As a result, solar and wind farms with older inverter modules had exhibited multiple farm-wide disconnections that were almost-instantaneous (quarter of the AC cycle). [1] For example, in 2017 the Canyon Fire 2 incident took out 900 MW of solar capacity. [2]