Tracking Control of Modular Multilevel Converter Based on Linear Matrix Inequality without Coordinate Transformation

Abstract

Modular multilevel converters (MMCs) play an important role in the power electronics industry due to their many advantages such as modularity and reliability. However, one of the challenges is to suppress fluctuations of circulating current and capacitor voltage and to ensure the quality of the output current. In this paper, the upper and lower arm voltages are employed as control inputs to control the output current and circulating current which are fed back to track the desired value. Based on linear matrix inequality (LMI), the control law of the MMC with multi-input system is designed to optimize the control value. The optimum arm voltage is divided by the SM nominal capacitor voltage to determine the number of SMs inserted into the upper/lower arm. The Voltage Sorting Algorithm (VSA) is then used to suppress the capacitor voltage fluctuation. The proposed tracking control strategy is implemented in MATLAB/Simulink. The results show that even under a small number of SMs (4 per arm), the output current can track the desired values and have better harmonic performance (current THD: 5.42%,voltage THD: 5.67%), and the fluctuations of the circulating current can be suppressed. Furthermore, it has better robustness and three-phase variable load fault tolerance.

Keywords

• modular multilevel converter (MMC)
• tracking control
• linear matrix inequality (LMI)
• circulating current
• multi-input system