1.State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Beijing 102206, China;2.Guodian Northeast New Energy Development Company, Shenyang 110000, China;3.North China Branch of State Grid Corporation of China, Beijing 100053, China

Renewable energy sources are directly or indirectly connected to the grid through power electronic devices. Their fault currents have limited amplitudes, controlled angles, rich harmonics and strong nonlinearity, which are significantly different from that of conventional synchronous generators. The performance of conventional percentage restrained differential protection is facing challenges. In order to solve this problem, this paper proposes a pilot protection method based on Spearman rank correlation coefficient for transmission lines. When the system operates normally or an external fault occurs, a traversing current is flowing through the transmission line, so the current waveforms on both sides are completely opposite. When an internal fault occurs, there are huge differences between the transient current waveforms on both ends. Therefore, the Spearman rank correlation coefficient can be used to measure the correlation degree between the current waveforms on two ends, to distinguish the faults inside and outside the zone. The method is suitable for all kinds of renewable energy power plants, and has good performance on tolerating transition resistance and noise. Compared with the existing protection based on fault characteristics of renewable energy sources, the proposed method also shows good operation performance when the output power of renewable energy sources is weak and the breaker recloses into a permanent fault. Both simulation results and field recording data verify the effectiveness of the proposed method.