Research Of Protection Methods To Lines Emanating From VSC-HVDC Interconnectors Considering Control Characteristic

Based on self-turn-off devices such as Insulated Gate Bipolar Transistors(IGBTs),voltage source converters(VSCs)are capable of independent control of active and reactive power in four quadrants of the P-Q plane.With strong controllability and high reliability,the VSC-based high voltage direct current(VSC-HVDC)technology has broad prospects in the integration of renewable sources,power supply of islands,power transmission and so on.Interconnection of high voltage networks via a VSC-HVDC interconnector can realize power supply among different AC networks without increasing short-circuit currents,which is helpful to improve the power supply capacity and the reliability of power systems.Under normal conditions,the interconnector can balance loads in different networks,thus improving the efficiency of the system.When an AC network is under fault conditions,the interconnector can provide dynamic reactive power and voltage support to enhance the robustness of the power system.After clearance of faults,fast and effective power support can be achieved among different networks to avoid cascading failures in power system.VSC-HVDC interconnectors bring exclusive fault characteristics to AC networks,thus influencing the performance of traditional protections.Since fault characteristics are related to the control schemes of interconnectors,a fault ride-through(FRT)strategy applicable to the interconnector operating in four quadrants and capable of eliminating negative-sequence currents under unbalanced conditions is proposed first.Then,the additional terms of measured impedances of distance relays caused by fault resistances are derived using a symmetrical component method.Theoretical analysis shows the output currents of interconnectors are controllable after faults,which may cause malfunctions in distance protections installed on lines emanating from interconnectors.Pilot protection is also inapplicable to lines emanating from interconnectors.Furthermore,a novel pilot protection principle based on the ratio between phase currents and the ratio between negative-sequence currents flowing through both sides is proposed for lines emanating from the interconnectors whose control scheme aims at eliminating negative-sequence currents.The validity of theoretical analysis and the protection principle is verified by PSCAD simulations.