The arm of alternate arm multilevel converter(AAMC)is composed of full-bridge sub-modules in series and insulated gate bipolar transistors(IGBTs)with inversed parallel diodes in series. AAMC conducts the upper arm and the lower arm in turn to output the multilevel waveform to approximate the positive half and the negative half of the sine wave. How to maintain the voltage of the capacitors of sub-modules at a preset value is the key problem for the control of AAMC. Based on the energy balance principle of the bridge arm

an energy balance equation for bridge arm is established by commutation the conduction of the upper and down arms at a particular angle

and the calculation method of the commutation angle to balance the capacitor voltage of the sub modules is proposed. By study of the maximum conduction voltage of an arm

complementary property between the solution of two commutation angles and the reactive power is put forward. The optimization selection of commutation angle based on the direction of the reactive power effectively reduces the maximum conduction voltage of the arms and the number of sub-modules needed by the arm. Through the research of the coupling mechanism of commutation angle and capacitor voltage

a local linear first-order model of the capacitor voltage is proposed. Based on this

a capacitor voltage control strategy combined with feed-forward control and proportional-integral(PI)control is proposed for improving the stability and adaptability of the voltage. By the PSCAD/EMTDC simulation

the effectiveness of sub-module capacitor voltage control method based on the commutation angle is verified. This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2015AA050101).