A novel travelling-wave direction criterion for hybrid multi-terminal HVDC system

Fast and accurate fault direction criteria are crucial for efficient protection in hybrid multi-terminal HVDC transmission systems. The current variation method is commonly used to identify fault direction, but it is affected by fault resistance and distributed capacitance. This paper proposes a new direction criterion based on the backward fault travelling wave, which has stronger resistance to interference from transition resistance and operates at a faster speed than other methods. Additionally, this paper provides the time-domain expression of the travelling wave after multiple reflections between the fault point and the converter station port, enabling the calculation of the threshold setting for the criterion. The new direction criterion is tested in PSCAD/EMTDC, and simulation results demonstrate that it can accurately identify fault direction and has a strong ability to withstand transition resistance. The time-domain expressions of travelling wave with multiple reflections and refractions are deduced emphatically when the fault occurs near the head or end of the dc overhead line. A new direction criterion based on the backward fault travelling wave is proposed.image