Predicting islanding operation of grid connected PV inverters

Increasing numbers of photovoltaic arrays are being connected to the power utility through power electronic inverters. Ths has raised potential problems of network protection. If, due to the action of the inverter or inverters, the local network voltage and frequency remain within regulatory limits when the utility is disconnected, then islanding is said to occur. Many methods to prevent this have been proposed and a SPICE-based model and analysis of the system are presented, specifically aimed at predicting the effectiveness of two of these. These are: active frequency drift, which causes the inverter current to be generated slightly lower or higher in frequency than the frequency of the terminal voltage, and slip mode frequency shift, which controls the phase angle of the inverter current as a function of the frequency deviation of the terminal voltage from that of the normal network frequency. Experimental verification of the model is provided. Active frequency drift using frequency speed-up rather than speed-down is shown to be preferable for inductive loads, but conditions that resulted in islanding could be predicted. Slip mode frequency shift proved to be a more robust antiislanding protection technique.