Pilot protection for flexible HVDC transmission lines based on transient voltage ratio characteristics

To address the problems of poor speed performance and the large influence of the distributed capacitance of traditional pilot differential protection, a pilot protection scheme based on the characteristics of the transient voltage waveform is proposed. First, the expression of the relationship between the fault voltage of the current-limiting reactor and the measuring point is derived. From the theoretical derivation, it can be inferred that for internal faults, the voltage ratio of the current-limiting reactor to the measuring point on both sides of the HVDC line is constant over a short period. For external faults, the voltage ratio of the current-limiting reactor to the measuring point varies exponentially on the fault side, while on the other side, it remains constant. Based on the above characteristics, the waveform characteristics of the voltage ratio of the current-limiting reactor to the measuring point are used to establish a protection criterion. A pilot protection scheme based on the standard deviation coefficient is proposed. The simulation results show that this method can reliably identify internal and external faults and has high sensitivity and selectivity. Moreover, this method is reliable for different types of faults and fault resistances. In addition, the proposed protection scheme can identify the type of fault in a short time window, requires a low sampling frequency, and does not require data synchronization.