Impact of Momentary Cessation on Sub-cycle Dynamics in Grid-Connected Solar PV Systems

Abstract

Recently, the North American Electric Reliability Corporation (NERC) has reported that the unintended loss of solar generation occurred in Southern California over a large geographic area. One of the major reasons for this generation loss is the tripping of solar generators due to the overvoltage in a less than one cycle time frame (i.e., sub-cycle overvoltage) experienced by solar PV inverters when the grid suffers voltage drop following the transmission-level disturbances. Moreover, the sub-cycle dynamics become more complicated when solar PV inverters enter the momentary cessation operation mode in response to abnormal grid conditions. In this paper, solar PV systems with detailed inverter models that consider inverter switching dynamics and momentary cessation function are constructed for RTDS-based real-time electromagnetic simulations to explore the impact of momentary cessation on sub-cycle dynamics. It is found that undesired sub-cycle transient events not only occur when momentary cessation starts to cease energy injection but also happen when momentary cessation restart to inject energy into the system during the restoration process. Furthermore, the undesired sub-cycle transient events become more significant when increasing the number of solar PV inverters or under weak grid operating conditions. On the other hand, the severity of sub-cycle transient events can be reduced by increasing the diversity in the recovery time of solar PV inverters during the restoration process after momentary cessation.