Additional reactive power control and emergency control of the ±1100kV UHVDC system with a hierarchical connection mode

This paper proposes an additional reactive power control strategy and an emergency control strategy for +/- 1100kV ultra-high voltage direct current (UHVDC) system with a hierarchical connection mode, in which the inverter station connect to two AC systems with different voltage levels. Firstly, the relationship between reactive power and DC voltage is analysed. Then, an additional reactive power control strategy is proposed to solve the voltage fluctuation caused by reactive power disturbance. Next, the reactive power emergency control strategy is proposed when a short-circuit fault occurs. Reactive power compensation devices will be inputted into the fault-free AC system to provide reactive power support through coupling relationship between two AC systems in the inverter side. Simulation results show that the additional reactive power control strategy can suppress reactive power disturbances without affecting the active power. Moreover, the reactive power emergency control strategy can reduce the probability of commutation failure in converters connected to the fault AC system while a short-circuit fault occurs.