A Capacitor Voltage Balancing Control Method for Five-Level Full-Bridge Grid-Tied Inverters Without Split-Capacitor Voltage Sampling

Abstract

Neutral-point (NP) potential imbalance is a crucial issue in five-level NP clamped (5L-NPC) full-bridge inverters. The basic idea of the conventional closed-loop NP potential balancing control methods is to sample the voltages of split dc-link capacitors first and then regulate the modulation waveform according to different voltages of split dc-link capactiors. In this paper, an open-loop NP potential balancing control for 5L-NPC full-bridge grid-tied inverters (FB-GCIs) is proposed. A balancing factor $k$ is introduced into the modulation signals and regulated according to the calculation results of the modulation index. Therefore, although the voltages of split dc-link capacitors have not been sampled and controlled, the dc-link capacitor voltages can be balanced over a wide modulation index range. Both the switching states and the NP potential balancing region of the 5L-NPC FB-GCI are analyzed in detail. The design guidelines of the balancing factor $k$ are also presented. A 5L-NPC FB-GCI prototype is built for the experimental tests, and experimental results validate the feasibility and effectiveness of the proposed control method.