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

the limited power allocation strategy is studied.