Online Optimization of Zero-Sequence Voltage Injection of PWM Strategy for 3L-NPC converters

Various pulse-width modulation (PWM) methods have been proposed to improve the control performance of three-phase three-level neutral-point clamped (3L-NPC) converters. However, the state-of-the-art PWM methods usually have a trade-off between switching losses and neutral-point (NP) voltage balancing performance. In practical applications, different con-trol objectives should be prioritized according to the operating conditions of the power converter. In that case, the existing PWM methods cannot ensure optimum control performance of the power converter when the operating conditions (e.g. modulation index, power factor and output frequency) vary in a wide range. In this paper, an algorithm for online optimization of the zero-sequence voltage injection (ZVI) is proposed. The proposed method can prioritize different control objectives and select the most suitable PWM method based on the operating point without any parameter adjustment. The obtained results have validated that a combination of different ZVI methods can provide an optimum trade-off between the switching losses and NP voltage fluctuations for different converter operating conditions.