Hierarchical connection mode is a new and attractive mode for ±1 100 kV ultra-high voltage direct current (UHVDC) system connected to different voltage level AC systems to reduce the scale of the converter transformer. An additional power-voltage controller is designed to enhance the advantages of hierarchical connection mode. The proposed additional power-voltage controller controls the transmitting power between the different AC systems by controlling the firing angle of the series converters on the inverter side. The optimal power ratio is defined

based on which

a power coordinated control strategy is proposed which is capable of reducing the possibility of commutation failure. A ±1 100 kV UHVDC system hierarchically connected to the 500 kV/1 000 kV AC system is built in PSCAD/EMTDC. The simulation results show that the additional power-voltage controller is valid and the power coordination controller is effective in making full use of the transmitting capacity of converters and reducing the possibility of commutation failure. This work is supported by National Natural Science Foundation of China (No. 51277068)

Program for New Century Excellent Talents in University (No. NCET-12-0846) and Program of Introducing Talents of Discipline to Universities (“111” Program) (No. B08013).