A Capacitor Voltage Pre-Charge Method for Back-to-Back Five-Level Active Neutral-Point-Clamped Converter

The five-level active neutral-point-clamped (5L-ANPC) converter has a great application potential in high power variable-frequency drives (VFDs) with high and medium voltage. However, the traditional method for capacitor voltage buildup is not applicable to the capacitors inside the back-to-back (B2B) 5L-ANPC converter. To solve the problem, the topological features of the B2B-5L-ANPC converter are fully analyzed, and a capacitor voltage pre-charge strategy for this converter is also proposed. With this strategy, all the capacitors in the converter can be charged by controlling the on and off states of switch tubes in the bridge arm, no additional hardware is needed. Moreover, the structural features of the voltage booster circuit were examined in four stages, and used to derive the capacitor voltage buildup formula in each stage. In addition, the relationship between voltage buildup rate, equivalent pre-charge resistor and capacity of capacitor is identified. Finally, Simulation results from a B2B-5L-ANPC simulation model which is constructed according to the actual system parameters and experimental results from a scaled-down prototype are provided to support the proposed control strategy.