Research On The Improvement Of Output Current And Voltage Of Modular Multilevel Converter

In recent years, with the application of power electronic technology in the field of wind power generation and HVDC light, high voltage and high power electronic devices get extensive attention of scholars at home and abroad. Modular multilevel converter because of its modular structure, the use of sub module cascade, has good scalability, low loss operation, good quality of output waveform, good redundant fault tolerance and high reliability, as well as many advantages is considered to be one of the future of the most typical high-pressure high-power electric power electronic converter topology.This paper studies the topology of multilevel converter module and basic working principle, establishes the mathematical model of MMC, analyses the circulation and the capacitor voltage fluctuation of MMC, describes the current common modulation strategy and compares the characteristics of this method, studies the nearest level modulation, NLM has merit such as simple implementation, small calculation of control system, which is an advantage for many-level converter. However when the number of step level is small, the NLM has drawback such as the poor quality waveform of output current, high harmonic content and poor control accuracy. Aiming at the application of MMC in the medium and low voltage domain of the nearest level modulation method, this paper discuss an improved NLM method, the hysteresis control is used to correct the differential current, the modulation strategy is easy to implement, the dynamic performance is good, the controller calculation is small, which can track the reference current, reduce the current harmonic content, and significantly improve the MMC output waveform.Aiming at the problem of the parameter selection of the sub module capacitor voltage fluctuation of the modular multilevel converter and the problem of reducing the output quality of the AC voltage, this paper discusses the relationship between the capacitor voltage's fluctuation and circular current, power transfer. A circular current close-loop controller is designed based on proportional resonant to suppress the capacitor voltage, the controller design is simple without the second-order negative-sequence coordination transformation and the phase decoupling control, this method can effectively suppressing the ripple of the capacitor voltage and enhance the quality of the output voltage without increasing the capacitance, laying a foundation for the application of MMC in electric traction system, high voltage inverter and other high power field.