Spearman Correlation-Based Pilot Protection for Transmission Line Connected to PMSGs and DFIGs

The power output generated by wind farms (WFs) is integrated directly or indirectly into the grid through power electronic devices and the fault current is significantly different from that of conventional synchronous generators) and exhibits a limited amplitude, a controlled phase angle, rich harmonics, and strong nonlinearities. Therefore, the traditional ratio braking type differential protection may have low sensitivity or may even refuse to operate. In order to solve this problem, a pilot protection method based on Spearman's rank correlation coefficient is proposed in this article. When the system operates normally or an external fault occurs, a traversing current flows through the transmission line, so the current waveforms on both sides are the opposite. When an internal fault occurs, there are large differences in transient current waveforms on both sides. Spearman's rank correlation coefficient can be used to measure the degree of correlation between the waveforms on both sides to distinguish between faults inside and outside the zone. Compared with existing protection based on fault characteristics of WFs, the proposed method shows good performance when the power output of WFs is weak and the circuit breaker recloses with permanent failure. Both simulation results and field-testing data validate the proposed method.