Negative Sequence Current Injection Based Active Islanding Detection Method

This article presents a fast and accurate active islanding detection scheme for a grid-tied voltage source inverter (VSI). In the proposed scheme, the VSI generates a small negative sequence voltage to continuously inject a small negative sequence current into the system. The voltage at the point of common coupling (PCC) is measured and the negative sequence component of PCC voltage is extracted. The negative sequence voltage generated by the VSI and the negative sequence voltage at PCC are combined to yield a time-invariant coefficient, which is proposed here to be used for islanding detection. In case detection is done solely based on the magnitude of negative sequence voltage at PCC, higher threshold has to be set as a small magnitude of negative sequence voltage at PCC may also be present due to grid unbalance and other transient events. In the proposed scheme, the islanding detection coefficient depends on the similarity between the injected and measured negative sequence voltage and hence is not dependent on the absolute magnitude of the negative sequence voltage. Thus a very low magnitude of negative sequence current can be injected to effectively detect islanding conditions without a significant impact on power quality. Simulation results show that the proposed scheme is effective for unbalanced grid voltage, or unbalanced load, even in the presence of noise in acquired data. Experimental studies on a laboratory prototype validate the performance of the proposed islanding detection scheme.