Local measurement based ultra-fast directional ROCOV scheme for protecting Bi-pole HVDC grids with a metallic return conductor

This paper demonstrates that the direction of a fault in an HVDC grid can be determined by comparing the rate of change of voltage (ROCOV) values at either side of the di/dt limiting inductors at the line terminals. For bi-pole HVDC grids with a metallic return and single earthing point, it is shown that directional information can be preserved by adding a small inductor in series with return conductor at each converter stations. Utilizing the estimated direction of the fault, a more secure ROCOV based transmission line and bus protection schemes are developed, relying on only local measurements. Through detailed time domain simulations, |dv/dt| characteristic under different fault conditions are analysed and the performance of the proposed fault detection schemes are evaluated for a test grid consisting of both cables and overhead lines. The ability to detect very low to significantly high resistance faults and the robustness of proposed scheme under changing grid configurations and operating conditions are demonstrated.