Ultra-high-speed non-unit non-differential protection scheme for buses of MMC-HVDC grids

摘要

Half-bridge modular multilevel converter-based high-voltage direct-current (MMC-HVDC) grids are technically and economically viable solutions to transmit bulk electrical energy generated from geographically dispersed and remote renewable resources. However, due to the half-bridge converter's inability to control fault currents, these grids need specialised protection schemes to increase reliability and equipment safety. This study proposes a non-unit non-differential protection scheme founded on a comparative orientation measure of fault components for buses of half-bridge MMC-HVDC grids. Considering the selected protection strategy, the protection zone of this scheme can also cover a part of the converter station's ac side; in which severe short-circuit faults should be quickly detected and isolated as well since they may cause the entire grid outage. This scheme can be used as the main protection or a backup of the conventional differential current protection, and subsequently, increase the protection system dependability by increasing the functional diversity. The test results disclose that the proposed scheme has superior sensitivity to identify single-pole and double-pole faults with resistances up to 1500 and 2000 omega, respectively, at the protected dc bus with the maximum delay of 1 ms, while it is robust against all ac and dc faults outside the designated protection zone.