Sub-Module Capacitor Voltage Control for Alternate Arm Converter With Current Commutation Modulation

In this paper, a sub-module capacitor voltage control (SCVC) is proposed to alleviate the energy balance restriction in the alternate arm converter (AAC). Considering that the arm inductor has a great impact on the energy exchange in AAC, the current commutation modulation is demonstrated firstly in this paper to extend the operating range. Then, the current commutation process for the alternate arms is further explored and derived by analytical expressions. During the commutation process, the current through the turning-on arm is rising, while the contralateral arm current is reversely freewheeling through the diodes of director switches (DSs). Moreover, the mechanism of the capacitor voltage sag caused by commutation process is clarified, which provides the fundamental to design the controller. In this manner, the sub-module capacitor voltage control (SCVC) strategy is further provided to compensate the capacitor voltage sag and achieve the normal operating in the four quadrants. Finally, the simulation and experimental results are both performed to verify the effectiveness of the presented modulation and control strategy in the different conditions.