Control Techniques for CRM-Based High-Frequency Soft-Switching Three-Phase Inverter Under Unbalanced Grid Conditions

This article presents control techniques for the critical conduction mode(CRM) based high-frequency soft-switching three-phase inverter under unbalanced grid conditions. The soft-switching technique with silicon carbide (SiC) power devices allows the inverter to operate at hundreds of kHz with high efficiency and high power density. To extend the soft-switching technique to more practical cases, behaviors of the inverter under unbalanced grid conditions, specifically voltage sag, are investigated. During the voltage sag, the ac voltage at the grid becomes unbalanced. This requires unbalanced ac current reference to avoid output active power oscillation. With the unbalanced grid voltage and the ac current reference, the inverter loses the soft-switching capability. Continuous conduction mode (CCM) operation occurs where discontinuous conduction mode (DCM) operation is intended leading to large turn-on loss. To eliminate the CCM issue, two improved modulation techniques are proposed by using off-time extension in the CRM phase and by skipping pulses in the DCM phase. This article discusses how the CCM operation arises and details the proposed control methods. The performance of the proposed control is validated experimentally under a variety of grid imbalance cases with an SiC-based three-phase inverter prototype.