A repetitive control scheme for neutral-point low-frequency fluctuation suppression of three-level neutral-point-clamped converter

The Neutral-Point Balancing is always a problem for Three-Level Neutral-Point-Clamped Converter. The problem can be solved by the zero-sequence voltage injected in the modulation-reference signal for Carrier-Based Pulse Width Modulation (CB-PWM) strategy or by allocating the time of the small vector for the Nearest Three Space Vector Pulse Width Modulation (NTSV PWM). This paper firstly deduces the precise model of the Neutral-Point (NP) voltage in frequency domain based on CB-PWM. At the same time, a novel plug-in repetitive controller and PI controller are used to calculate the zero-sequence voltage injected. The proposed method can suppress low-frequency NP fluctuation which contains mainly third order harmonic in steady state and gain a smooth regulation in dynamic process compared with the conventional method. In addition, a Dual Sampling and Dual PWM duty ratio Update (DSDPU) scheme is used to expand the bandwidth of Neutral-Point (NP) balancing controller. The proposed concept will help extend in the NTSV PWM. Performance of the presented strategy for neutral-point balancing is also experimentally verified.