Generalized PWM-Based Method for Multiphase Neutral-Point-Clamped Converters with Capacitor Voltage Balance Capability

Abstract

This paper presents a generalized PWM-based control algorithm for multiphase neutral-point-clamped (NPC) converters. The proposed algorithm provides a zero-sequence to be added to the reference voltages that contributes to improve the performance of the converter by: i) regulating the neutralpoint (NP) current to eliminate/attenuate the low-frequency NP voltage ripples, ii) reducing the switching losses of the power semiconductors and iii) maximizing the range of modulation indices for linear operation mode. The control method is formulated following a carrier-based PWM approach. Hence, dealing with complex space-vector diagrams to solve the modulation problem for multiphase converters is avoided. The recursive approach means that it can be easily extended to n-phase converters without increasing the complexity and computational burden, making it especially attractive for digital implementation. The proposed method allows regulating the NP voltage without the need for external controllers; therefore, no parameter tuning is required. The algorithm has been tested in a four-leg NPC converter prototype performing as a three- and four-phase system and operating with balanced and unbalanced loads.