Islanding detection scheme for distributed generation systems using modified reactive power control strategy

The reactive power control (RPC)-based technique is a promising solution for islanding detection. The existing RPC-based islanding detection techniques have limitations of injecting unwanted reactive power during normal operation, almost same detection time for various percentage of unbalance in load and false detection of islanding for small disturbances when steady-state frequency is close to its threshold limit. To overcome the first limitation, the phase angle (theta) between the actual and nominal point of common coupling (PCC) voltage is added to the reference reactive power signal of the controller. The angle (theta) has zero value during normal system operation and increases prominently after islanding. To obtain proportionately faster detection for various percentage of unbalanced load conditions, the positive sequence PCC voltage is used in the detection scheme. Owing to major power imbalance between generation and load at steady state, the frequency may come close to its threshold. Under this situation, even a small disturbance will drag the frequency beyond threshold and false islanding detection will occur. To overcome this problem, discrete wavelet transform analysis of theta is considered in the decision-making criteria. The proposed scheme is tested for islanding conditions as well as under various non-islanding events.