A synthesized capacitors voltage control for modular multilevel converter in HVDC application

The capacitors voltage balance is one of the major problems in modular multilevel converter control which has not been resolved perfectly. Based on energy transmission analysis through steady state equations, this paper proposed a synthesized balance control scheme that involves leg voltage, arm voltage, individual voltage balance control and circular current control. Comparing with other methods, the proposed scheme incorporates the leg balance control into circular current control, and employs the differential mode voltage to balance the upper and lower arm energy. The proposed method can maintain capacitors voltage balance under different power factor conditions with few harmonics in circular current and dc line voltage. The proposed strategy has been verified by detailed simulation results.