Research On Control Strategy Of Modular Multilevel Converter In Solid State Transformer

With the rapid development of social economy,people pay more and more attention to energy shortage and environmental pollution.For this reason,distributed generation represented by wind energy and solar energy has been promoted for its advantages of green,environmental protection and sustainable utilization.However,a large number of distributed generation grid-connected and the variety of load types will seriously affect the power quality of the grid,while the traditional transformer does not have the ability to actively regulate the power quality.Solid-state transformer not only can realize the basic functions of voltage conversion,electrical isolation and energy transmission,but also has the advantages of controlling bidirectional energy flow,regulating power quality,facilitating distributed generation access and harmonic control,which can solve the shortcomings of traditional transformer.Modular Multi-level Converter(MMC)provides a good solution for the application of solid-state transformer in high-voltage and high-power applications because of its highly modular structure,easy redundancy design,high-voltage DC bus,unbalanced load operation ability and high output waveform quality.Therefore,in this thesis,modular multi-level converter is used as the input rectifier stage of solid-state transformer to study its operation characteristics and control.Firstly,this thesis analyses the topology and working principle of modular multi-level converter,and further clarifies the relationship between internal and external electrical quantities of MMC by establishing a mathematical model,which lays a theoretical foundation for designing the control strategy of MMC.The recent level approximation modulation and carrier phase-shifted pulse width modulation are analyzed and compared.The carrier phase-shifted pulse width modulation is used as the modulation mode of this topic.Aiming at the surge current problem caused by MMC start-up,a pre-charging control method for MMC AC side start-up is designed.In order to stabilize the DC bus voltage and realize the unit power factor of AC side,double closed-loop of voltage and current control strategies applied in three-phase MMC topology and single-phase half-bridgeMMC topology are designed respectively.To solve the problem of unbalanced capacitor voltage of sub-modules and inter-phase circulating current,the causes of unbalanced capacitor voltage of sub-modules are analyzed,and the internal components of the circulating current are revealed.On the basis of the explicit internal power flow direction of MMC,an energy equalization control strategy is designed to achieve the balance of capacitor voltage of all sub-modules and achieve the purpose of restraining inter-phase circulating current.Finally,in order to verify the validity and feasibility of carrier phase-shifted pulse width modulation strategy,double closed-loop of voltage and current control strategy and energy equalization control strategy,three-phase five-level MMC and single-phase half-bridge five-level MMC are built in MATLAB/Simulink platform for simulation and verification,and a single-phase half-bridge three-level MMC experimental platform is built for experimental verification.