An Improved Transient Traveling-Wave Based Direction Criterion for Multi-Terminal HVDC Grid

Fast and reliable direction criterion is important for the protection of multi-terminal HVDC grid, not only for direction pilot protection, but also for excluding backward fault in single-ended protection. The direction criteria based on dc current variation (Δi) or dc current change rate (di/dt) have been used in the dc transmission system. However, the acting reliability is influenced by the high-transition resistance, line distributed capacitance, and so on. Although the direction criteria based on the boundary effect of the dc reactor have excellent operation performance in dc grid, they are only feasibility when the dc reactors are installed on both ends of each dc line. In this paper, the traveling-wave (TW) based direction criterion, which is not based on the boundary components, is proposed for use in multi-terminal HVDC grid, and its applicability is analyzed. To improve the acting sensitivity and reliability of the TW based direction criterion, an improved transient TW based direction criterion is proposed. Finally, simulations based on the Real Time Digital Simulator (RTDS) are carried out to verify the feasibility and superiority of the proposed improved direction criterion.