A Minute Asymmetrical Current Injection Based Anti-islanding Method for Three-Phase Grid-Connected Photovoltaic Inverters

An active anti-islanding protection method based on the current control for a three-phase grid-connected photovoltaic inverter is proposed in this paper. The current of phase-a is synchronous with the corresponding phase voltage at the point of common coupling in its positive half cycle, and a zero-current zone is inserted at the end of the cycle in a negative half cycle. As for phase-c, the zero-current zone is inserted in the positive half cycle of the current, and the current in a negative half cycle of phase-c is in phase with the corresponding phase voltage. Therefore, the currents of phase-a and phase-c in one cycle become slightly asymmetrical. Before the islanding takes place, the positive and negative half cycles of three-phase voltages are symmetrical due to the operation of the grid voltage. While islanding takes place, a time difference between the positive and negative half cycles of the voltages of phase-a and phase-c will be generated and the islanding is detected in accordance with the successive cycles fulfilling the conditions of islanding identification. In order to measure zero-crossings of the voltages accurately, an finite impulse response filter is used to smooth out the voltage harmonics and noises. Simulations and experiments for three-phase three-wire power systems have been carried out. The results verify that the proposed method can detect the islanding for a parallel RLC load or induction motors when the voltage frequency is within the range of the non-detection zone. It is seen from the analysis of the detecting principle that there is no influence of temporal voltage frequency fluctuation on the proposed method even if a large load connected to a weak power system frequently starts up or stops. (C) 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.