Load Rejection Overvoltage Issues on Distributed Generation Projects

Client: Confidential

Increased penetration of solar photovoltaic (“PV”) generation in distribution circuits raises a potential issue with load rejection overvoltages. The condition occurs with the lightly loaded feeders. If the feeder load is much smaller than the total isolated generation then the load rejection overvoltage could pose a threat of damage to equipment insulation and surge arresters. In addition, the inverter characteristics used for PV installations present their own unique impacts. Different inverter designs respond differently to the phenomenon. Pterra used a time domain simulation tool to quantify the impact of load rejection on several different circuit configurations and several different types of inverters.

The study system is shown in Figure 1.

Figure 1.

The maximum magnitude of transient overvoltage (“TOV”) could reach 240% of the nominal voltage if the generation is 6 times the load on the islanded feeder. For a criterion setting, 120% of minimum load as the limit for PV penetration is conservative. In some utilities, this criterion is set at 150%. However, when in doubt, it is always preferable to run a simulation using the feeder and load electrical characteristics and the manufacturer model for their inverters.

Mitigation options to allow for increased PV penetration include implementation of a direct transfer-trip scheme, usually an expensive and complicated choice.