Islanding detection method with negative-sequence current injection under unbalanced grid voltage

This paper proposes an active islanding detection method for the BESS (Battery Energy Storage System) with a three-phase inverter which is connected to the unbalanced AC grid. The proposed method adopts the DDSRF (Decoupled Double Synchronous Reference Frame) PLL (Phase-Locked Loop) so that the independent control of positive-sequence and negative-sequence current is successfully carried out using the detected phase angle information. The islanding state can be detected by sensing the variation of negative-sequence voltage due to the injection of 2-3% negative-sequence current from the BESS. The proposed method provides secure and rapid detection under the sag and swell of AC grid voltage. The feasibility of the proposed method was verified by computer simulations with PSCAD/EMTDC and experiments with a 5kW hardware set-up, considering the test circuit of islanding detection suggested by IEEE 1547 and UL1741.