The Closed-Loop Sideband Harmonic Suppression for CHB Inverter With Unbalanced Operation

For N-cells cascaded H-bridge (CHB) inverters based on unipolar double frequency phase-shifted pulsewidth modulation (PS-PWM), the output voltage is distorted and its harmonics are difficult to be suppressed under unbalanced conditions (different dc voltages and/or modulation indices). This article proposes a closed-loop harmonic suppression scheme based on the PS-PWM, and the scheme is easily implemented in the dq frame to adaptively adjust the displacement angles of the triangular carriers. The harmonic content can be suppressed to its possible minimum value without the limitation of the operational point by the proposed method. The adjustment rules of displacement angles and the harmonic suppression principle are presented in detail. This method can effectively restrain harmonic magnitudes without the limitation of the cells number and does not require complicated trigonometric function calculations or optimization algorithms. Moreover, the principle of symmetrical sideband harmonic suppression is also discussed. The effectiveness of the proposed method is verified by a downscale test platform of the single-phase CHB inverter.