Dr. Lizana-Fuentes, Ricardo

Arm voltage and submodule (SM) capacitor voltage balancing is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). The arm voltage balancing is achieved through a zero-sequence voltage controller in carrier pulse-width modulation (CPWM). In this study, a dual space-vector pulse-width modulation (SVPWM) technique is proposed for an MMC, which eliminates the external controller for arm voltage balancing. In this approach, the three-phase top and bottom arms are independently controlled using SVPWM. In addition, the capacitor voltage balancing can be achieved using redundant switching vectors. However, this will increase the computational load on the space-vector modulator. Therefore, an external capacitor voltage-balancing approach is proposed to minimize the computational complexity. The proposed approach uses the direction of load current instead of the arm current in SM selection process. As such, the required number of current sensors is reduced to 50% in a three-phase system. The proposed modulation and voltage-balancing approach are simulated and experimentally verified on the MMC system with three-level flying capacitor (3L-FC) SMs. Simulation and experimental results show the successful balancing of the arm voltage and SM capacitors voltage.

In a modular multilevel converter (MMC), the voltage balance among the submodules is mandatory to generate the multilevel stepped waveform across the load and to ensure the equal voltage stress on the semiconductor devices. In addition, the output power quality (voltage and current waveforms) and the converter reliability greatly depend on the design methodology of a voltage-balancing approach. The improper design of the balancing approach causes higher voltage and current harmonic distortion and device power losses, which further affects the efficiency of the MMC. In this letter, an improved voltage-balancing approach is proposed to reduce the output voltage harmonic distortion and device power losses. The performance of the proposed approach is verified through MATLAB simulations and experimentally on a three-level-flying-capacitor-based MMC system. Also, the performance of the proposed approach is compared with the existing methodology to prove its superiority.