Research on Output Voltage Waveform Control of Three Level Inverter under Unbalanced Load

Under the condition of unbalanced load, the three-phase output voltage of three-level inverter is asymmetric. The traditional double closed-loop PI control has a good control effect on the output voltage waveform when the load is balanced, but in the case of unbalanced load, the output voltage waveform contains a negative sequence component, and the control effect on the negative sequence component is limited. In order to improve the output voltage waveform under unbalanced conditions, the 90° symmetric component method is used to separate the positive and negative sequence components, and then PI control is applied to the positive and negative sequence components separately in the dual synchronous rotating coordinate system. Simulations and experiments have verified the correctness of the control strategy, which has a strong harmonic suppression capability and high output voltage quality under unbalanced load conditions.