A Simplified DC Capacitor Voltage Balancing Control for a Unidirectional Five-Level Converter Based on Carrier-Overlapped and Phase-Disposition Hybrid PWM Modulation

The issue of unbalanced neutral-point voltage hinders the widespread application of five-level neutral-point clamped (NPC) converters in industry. A unidirectional five-level converter for ac motor drives was recently proposed. This converter takes a modified five-level boost rectifier as the front-end for input power factor correction. The neutral-point voltage balance is achieved by a complex control coordination between the rectifier stage and inverter stage. In this article, a simplified voltage balancing control strategy is proposed, first, the voltage balancing capacity of the rectifier stage is analyzed; then, considering that the traditional phase-disposition pulse width modulation (PDPWM) has low-output voltage total harmonic distortion (THD) but will cause the drift of neutral-point voltage, while the carrier-overlapped PWM (COPWM) can balance the neutral-point voltage in the full operating range, but has a higher output voltage THD, a carrier-overlapped phase-disposition PWM (COPDPWM), and a hybrid modulation strategy based on segmented combination of PDPWM, COPDPWM, and COPWM for the inverter stage are proposed. The strategy can greatly reduce the unbalanced degree of the neutral points currents to the level that can be fully compensated by the rectifier stage and has little impact on output voltage THD. The experimental results verify the validity of the proposed hybrid modulation strategy.