Setting-Less Pilot Protection for MTDC Systems with Weak or No Line Boundary Based on Model Differences between Forward and Backward Faults

Some sceneries of multiterminal HVDC transmission (MTDC) system cannot provide enough boundary conditions for single-ended protections. Pilot protections can fast and sensitively identify faults in these sceneries, but existing methods have problems related to protection setting, such as demand of detailed parameters, dependence on simulations, and complex algorithms. In this article, a novel setting-less pilot protection is proposed. First, the characteristics of fault transient voltage and current for forward and backward faults are analyzed. It is revealed that the fault u-i characteristic is consistent with negative resistance-inductance parallel model when the forward fault occurs, and the fault voltage and current exhibit a relationship of positive resistance model when the backward fault occurs. The above conclusion holds for different fault types, resistances, and topologies. The concept of model error is introduced in order to ascertain which of the two models best represents the relationship between the fault voltage and current. Then, a novel directional pilot protection is proposed. The proposed method determines the direction of fault by comparison of two fault models, it is simple and does not need threshold setting and high sampling frequency. Simulations and experiments have demonstrated that the proposed pilot protection has fast operation speed and high transition resistance tolerance up to 1500Ω.