Abstract: In order to improve the reliability of fault detection and fault line selection for modular multilevel converter based high voltage direct current（MMC-HVDC）transmission line,a novel pilot protection principle is proposed.Based on the characteristics of the MMC-HVDC system,custom difference current（named CDC-BPN）between the positive pole at one terminal and the negative pole at the other is constructed and used for voltage and current protection.The analysis shows that the absolute value of CDC-BPN for DC line faults is obviously higher than that of normal operation condition and AC system faults.In addition,the sign（positive or negative）of CDC-BPN for positive pole-to-ground fault,negative pole-to-ground fault and pole-to-pole fault is obviously different.According to these characteristics,apilot protection criterion is constructed to recognize DC line faults and fault line.Simulation results show that the proposed protection is able to detect DC line fault under different fault conditions and select the fault line quickly and reliably.