Abstract

Three-level Neutral-Point-Clamped (NPC) converters with pulse width modulation (PWM) strategy are one of the most widely used multi-level converters. Totally, constant switching frequency PWM and variable switching frequency PWM are the two most commonly used PWM strategies. Among them, variable switching frequency PWM can achieve better current performance due to the freedom of switching frequency. In this paper, the current ripple of three-level NPC converters is analyzed based on equivalent source theorem and the analytical current ripple expressions of three-level NPC converters corresponding to 27 different space voltage vectors are obtained. Based on theoretical analysis, a novel variable switching frequency PWM strategy with THD optimization of output current is proposed by setting the current ripple peak within a certain value. At last, the correctness of the theory in this paper is verified by simulation results.

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