Capacitor Voltage Fluctuation Suppression Strategy of MMC Sub-module Based on Double-layer Circulating Current Control

• Published in: 2024 4th International Conference on Electrical Engineering and Mechatronics Technology (ICEEMT) pp. 87 - 92
• Main Authors: Zheng, Youzhuo, Luo, Zhaoyi, Hua, Long, Li, Chun
• Format: Conference Proceeding
• Language: English
• Published: IEEE 05.07.2024
• Subjects: Bridge circuits; Capacitors; circulating current suppression controller; Common-mode voltage of bridge arm; Current control; Fluctuations; Harmonic analysis; Mathematical models; modular multilevel converter; Multilevel converters; submodule capacitor voltage; Switches; Voltage control; Voltage fluctuations
• DOI: 10.1109/ICEEMT63201.2024.10692805

The circulating current of modular multilevel converter (MMC) bridge arm increases the current distortion rate and current stress of MMC bridge arm, and boosts converter losses, and increases capacitor voltage fluctuations. Considering the switching function of the Modular Multilevel Converter (MMC), a mathematical link has been established between the second harmonic of the common-mode voltage across the bridge arm and the Circulating Current Suppressing Controller (CCSC), within the d q rotating coordinate system. Consequently, a dual-layer control scheme has been crafted with the objective of diminishing the capacitor voltage fluctuations. The voltage control is implemented in the outer layer, while the current control is executed in the inner layer. The process involves creating a sub-module capacitor voltage model that accounts for second harmonic circulation. Following this, the reference value for the circulating second harmonic current is established via the outer loop control mechanism. The outer loop controller zeroes the second harmonic capacitor voltage, ensuring no steady-state error between the output and its setpoint. Then, the second harmonic voltage reference signal of the bridge arm is generated by the inner loop controller, and finally the pulse signal required for each sub-module is generated by modulation for input or removal. Finally, the circulating current injection strategy is verified by simulation and experiment. The findings show a decrease in both the fundamental and second harmonic of the sub-module capacitor's voltage, particularly in the suppression of the second harmonic.