Intra-phase SOC Balancing Control Strategy with THVI

Due to the lack of a common DC bus in cascade H-bridge energy storage converters, the DC-side currents to battery modules contain significant second-order harmonic components. These harmonics can cause battery heating and accelerate aging. Effective measures are needed to suppress these harmonic currents. This study introduces the cascade H-bridge energy storage system's topology and control strategy, analyzes the generation mechanism of second-order harmonic currents using power conservation principles, and examines their impact under traditional intra-phase SOC balancing control. It proposes a modified intra-phase SOC balancing control strategy that injects a third-order harmonic voltage. This method adds a third-order harmonic voltage to the modulation voltages of each submodule, converting second-order harmonic currents into fourth-order harmonics that are easier to filter out, thereby reducing ripple current in the battery modules. Additionally, this approach ensures that second-order ripple currents across battery modules in phase are consistent in amplitude and phase, aiding in centralized filtering. Simulation results validate the effectiveness of intra-phase SOC balancing control strategy with THVI(Third Harmonic Voltage Injection).