With the development and widespread application of ultra-high voltage direct current(UHVDC)transmission technology
the multi-infeed HVDC system in the load centers of China will become a prominent problem in the development of China's power grids. UHVDC with hierarchical connection can improve the alternating voltage support capacity in the multi-infeed HVDC system and has been included in the national power grid planning. The circuit structure has changed for UHVDC with hierarchical connection. Compared to the traditional control strategies
those applied to UHVDC with hierarchical connection mode in voltage balance control
reactive power control
power control for valve exit should be improved. With respect to voltage balance control
the alphamax inverter control(AMAX)is used in each valve
while the voltage balance control is achieved by the converter transformer tap control. For the reactive power control
the valves connected to AC grid 1 and AC grid 2 control reactive power
respectively. With respect to the power control for valve exit