As a very important characteristic for DC grid

the active and reactive power of a hybrid multilevel convertor (HMC) can be controlled independently

so the AC current is still controllable when there is a fault on the DC side. The AC voltage synchronous method and the energy balance principle of the static synchronous compensator (STATCOM) model and high voltage direct current (HVDC) model are studied. Based on the energy balance principle

the capacitor voltage stabilizing principle of the arm in the STATCOM model and HVDC model is put forward

and the capacitor voltage balance method is realized in detail. In order to verify the principle and the method of the capacitor voltage balance

a PSCAD model is developed according to the parameter of the simulation example. The processes each of charging

deblocking

active power changing and DC fault are simulated. Results of the simulation show that the active current and reactive current of the HMC can be controlled independently of each other with the DC side fault of the HMC isolated. The capacitor voltage balance principle and method proposed are also proved effective. This work is supported by National Basic Research Program of China (973 Program) (No. 2012CB215200).