Backward travelling wave-variation-based protection for the transmission line of a hybrid HVDC system

The hybrid high-voltage direct current (HVDC) transmission system comprising a line-commutated converter (LCC) on the rectifier side and a voltage source converter (VSC) on the inverter side combines the advantages of both LCC and VSC. It has broad prospects in many application fields. However, at the VSC-based inverter side of the hybrid system, there is no dc filters, and the inductance of the smoothing reactor is much smaller than that of the LCC station. Therefore, it is hard to distinguish line internal and external faults at the VSC side for the conventional protections based on the attenuation characteristic of the line boundary. In order to solve this problem, this study proposes a non-unit protection for hybrid HVDC transmission lines. The protection judges the fault section based on the differences of the backward travelling waveforms under internal and external fault conditions. Simulation results show that the proposed protection is able to distinguish internal faults from external ones, identify the faulty pole exactly, and has enough sensitivity under large resistance faults