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An Improved Neutral-Point Voltage Balancing Algorithm for the NPC Three-Level Inverter Based on Virtual Space Vector PWM

This paper explores the limitations of neutral-point voltage balancing problem for different loading conditions and index modulations of neutral-point-clamped three-level inverters. However, the modulation, which based on the virtual space vector concept, that only requires the addition of the output three-phase currents equals zero, guaranteed the balancing of the neutral-point potential for any load over the full range of converter output voltage and for all load power factors. It is the shortcomings of those methods that to estimate the sectors and calculating a lot of trigonometric functions, which results in the increase of the calculating quantity of micro-processor. To solve these problems, a novel method for the virtual space vector concept based on 60o coordinate system is presented in this paper. The method proposed can omit the judging process of the sectors, reduce the calculating burden, and maintain the balance of neutral-point voltage between capacitances within a certain index modulation. The results of simulation are given to validate the improved virtual space vector modulation.