A Novel Pilot Directional Protection Scheme for HVDC Transmission Line Based on Specific Frequency Current

HVDC lines play an important role in transmission power over distance, while faults frequently occur on DC lines due to complex terrain and terrible weather conditions, which is a major cause of HVDC outages. Traveling-wave-based protection and voltage derivate theory are used as the primary protection for HVDC transmission lines to detect a ground fault, and differential protection is employed as backup protection. However, traveling-wave-based methods and voltage derivate theory still have shortcomings and current differential protection operates with long time delay. Thus, protection with high reliability is very important and necessary to ensure the security of HVDC transmission lines. By analyzing the direction characteristics of specific frequency current at DC line ends, a novel pilot protection principle for HVDC transmission lines is proposed. The specific frequency current is adopted through spectrum analysis of DC-filter and their directions are analyzed during internal and external fault, and it is found that the directions of specific current detected at DC line ends are the same during internal fault, while their directions are opposite during fault outside of DC lines. Therefore, the direction difference of specific frequency current can be used to identify external fault from internal fault. An improved backup protection scheme for HVDC transmission line is also proposed by resetting two voltage derivative criterions in traveling-wave-based protection. Simulation results show that the proposed protection scheme is valid and feasible, and is of capability to distinguish fault with high transition resistance.