A pilot protection based on Zero-Sequence voltage ratio for AC lines in hybrid Multi-Terminal HVDC transmission system

The reliability of protection is the precondition for the security of hybrid multi-terminal high-voltage direct-current (HVDC) transmission system. For AC system at the receiving terminal, due to the infeed of DC system, the conventional AC line protection may malfunction. In this context, a new protection utilizing zero-sequence voltage ratio is proposed in this article. Based on the fault analysis of zero-sequence voltage distribution, the comparative voltage is defined, and find that under an internal fault, for two sides of line, the ratios of zero-sequence voltages to their corresponding comparative voltages are less than 1. Under an external fault, for the distance faulty end, the voltage ratio is less than 1, however, near faulty side, the voltage ratio is larger than 1. Thereby, this difference is utilized to construct the fault identification criterion. Simulations show that the proposed protection can correctly identify internal and external faults. Meanwhile, it is immune to DC commutation failure, and has a good tolerance to high-resistance faults.