Capacitor Voltage Balancing in Neutral Point Clamped and Hybrid Clamped Multilevel Converters with Redundant Level Modulation

Abstract

In multilevel converters, multiple capacitors are used to divide the dc-link voltage or to store energy for voltage steps. Keeping these capacitor voltages balanced is critical to proper operation. When capacitor voltages become unbalanced, several serious issues, such as the overvoltage stress on power devices and distortion in output voltage waveform, arise that can severely hinder the application of multilevel converters, especially in medium/high voltage systems.

Although the hardware auxiliary circuits can help balance the capacitor voltages, they bring the extra weight and volume, software methods can realize the voltage balance without these hardware circuits. Redundant level modulation (RLM), as the newly proposed control method, is combined with two traditional control methods, switching state selection (SSS) and zero-sequence signal injection (ZSI), to realize the capacitor voltage balance when these two traditional control methods alone cannot control the converters under balanced conditions. This proposed method is applied in many topologies, e.g., the four-level neutral point clamped (NPC) converters, the five-level NPC converters, two five-level hybrid clamped (HC) converters and the six-level HC converter. Considering RLM can bring some side effects, such as higher amplitudes of the common-mode voltage (CMV) and higher switching loss, some improvements are made to the proposed control method. A carrier-based method with flipped carriers can mitigate the CMV and the method which chooses the dominant phase firstly and only implements RLM in this phase can help reduce the extra switching loss. The single-phase systems are further considered, where the ZSI cannot be applied, and RLM should therefore be fully employed to balance the capacitor voltages. Simulation and experiments are performed the proposed control methods.

The proposed control method combining these three control methods can realize the capacitor voltage balance in the provided converters within the whole range of operation conditions. This method can also be applied to other topologies, such as multilevel converters with the higher voltage level, and new topologies with simple structures may be proposed with the help of this control method.

Date of Award	9 Dec 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Xibo Yuan (Supervisor), Jun Wang (Supervisor) & Nick Simpson (Supervisor)