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.
Highlights
With the development of power electronics technology, the modular multilevel converter (MMC)has received extensive attention [1,2,3]
Due to its modularity, scalability, high efficiency, good harmonic performance, fault blocking capability, etc., it has been used more and more in the industry [4,5,6,7,8], such as energy storage system, medium voltage and high power motor drive system, power distribution system, high voltage direct current transmission, and so on. It is more challenging than other multilevel converters in the control of an MMC to guarantee the quality of the output current and voltage, at the same time, to minimize/eliminate circulating current while the capacitor voltages were maintained balanced [9]
Where icj is the circulating currents flowing through the MMC phase-j, which can be obtained by the following formula
Summary
With the development of power electronics technology, the modular multilevel converter (MMC). Due to its modularity, scalability, high efficiency, good harmonic performance, fault blocking capability, etc., it has been used more and more in the industry [4,5,6,7,8], such as energy storage system, medium voltage and high power motor drive system, power distribution system, high voltage direct current transmission, and so on It is more challenging than other multilevel converters in the control of an MMC to guarantee the quality of the output current and voltage, at the same time, to minimize/eliminate circulating current while the capacitor voltages were maintained balanced [9]. Phase Shift Pulse Width Modulation (PS-PWM) [15,16] is suitable for a small number of SM It has good output voltage and current quality, but the switching frequency is high, which takes up a lot of hardware resources and consumes a lot of energy.
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