A new total frequency deviation algorithm for anti-islanding protection in inverter-based DG systems

Islanding situation is a very serious problem in distributed generation (DG) system. For inverter-based DG systems, phase shift anti-islanding techniques such as slide mode shift (SMS), automatic phase shift (APS), active frequency drift (AFD), and active frequency drift with positive feedback (AFDPF) have been proposed because of their effectiveness in preventing most of the islanding cases. However, existing phase shift techniques are all based on the assumption that at a grid failure, the voltage frequency of the inverter can be driven by its output current in a desired direction, up or down, until the inverter's frequency is drifted into the over frequency relay and under frequency relay (OFR/UFR) window. However, when the quality factor of the local loads is high, traditional phase shift mechanisms may not work as desired, instead, the frequency of the inverter could oscillate around a certain frequency point after islanding occurs. This is due to the high ratio of the energy stored in and the energy consumed in the local load. To solve this problem, a total frequency deviation (TFD) in a moving time frame is introduced as the islanding index while using adaptive logic phase shift (ALPS) as the basic phase-shift motivation. The TFD value becomes significantly large after the islanding happens. The TFD value can interact with ALPS algorithm to move the frequency of the inverter continuously until it reaches the OFR/UFR tripping window. Both simulation and lab tests have demonstrated the highly effectiveness of this anti-islanding algorithm