Distance Protection of AC Grid With HVDC-Connected Offshore Wind Generators

When a transmission line close to points of common coupling (PCCs) experiences a short-circuit (SC) fault, the fast reactive power control of voltage-source converter-HVDC (VSC-HVDC) is likely to affect the protective relay operation of transmission lines. To study the performance of distance relays on an ac grid with an offshore wind HVDC network, this paper presents an apparent impedance calculation method, which utilizes the bus impedance matrix (Zbus) to calculate the impedances viewed by distance relays during a three-phase SC fault. The proposed method is used to identify the potential miscoordinated Zone 2 relays in the proposed combined ac/dc system. The analysis is verified by software simulation results. It is shown that the proposed method results in accurate impedances viewed by distance relays. It also identifies the protective device settings on the ac grid that need to be adjusted due to HVDC control of offshore wind generators.