Control Strategy Of Modular Multilevel Converter Applied In Power Electronic Transformers

With the continuous development of society,the power system has changed greatly in the system scale,power supply and load types,so the traditional power system cannot meet these requirements,the power electronic transformer arises at the historic moment.Among those,the ternary structure of power electronic transformer is widely used because of its advantages such as DC terminal,easy access to new energy,high feasibility,relatively complete functions,relatively simple control and so on.Among the variety of topologies used in power electronic transformer,The modular multi-level converter is considered as the state of the art converter among multi-level topologies and thus,is currently the subject of greatest amount of attention and research in high-voltage application,mainly due to its advantages of modular structure,inherent redundancy,low switching frequency,less switch loss,absence of DC-link capacitors,improved power quality,freedom from multiple isolated DC sources,ease of expandability,etc.And its application in power electronic transformer offers a good solution for high voltage and large capacity transmission system.However,it also has many inherent problems needed to be solved,such as modulation,voltage balancing control,harmonic suppression of circulating current.This paper is based on the power electronic transformer,and the modular multilevel converter is applied to the input stage,in this paper,the working principle,modulation,voltage balancing strategy and the circulating current suppression control are studied.The validity is proved by simulation and experimental results.The details are as follows:Firstly,the working principles of sub module and the three phase are introduced,and the mathematical model is established.Based on the mathematical model,the relationship among the three-phase voltage and current,the voltage of the capacitor,the voltage and current of the bridge arm and the voltage and current on the DC side are listed,providing powerful guarantee for the design of the controller.This paper analyzes and compares several commonly used modulation strategies of modular multi-level converter,and selects the carrier phase-shifted SPWM modulation mode as the modulation method.Secondly,the mathematical model of the three-phase voltage in the d/q coordinate system is established.Based on the model,controller of modular multi-level converter is designed,and the external voltage loop is added to stabilize the DC output voltage.This paper analyses the key factors of the DC capacitor voltage imbalance phenomenon.Several control methods used to solve the DC capacitor voltage initialization issue are compared,and the control of voltage closed loop is selected.In addition,the PIR controller is applied to the control method to improve the waveform quality on the basis of the original controller strategy.On the basis of theoretical analysis,a three-phase five level simulation model is built by using MATLAB/SIMULINK to verify the effectiveness of the proposed control strategy.Thirdly,the causes of the circulating current are analyzed,and the harmonic components of the circulating current are derived by means of formula derivation.The advantages and disadvantages of several circulating current suppression controllers are analyzed and compared.A new control strategy is proposed which based on the second fundamental frequency,negative sequence rotating coordinate transformation and the four fundamental frequency,positive sequence rotating coordinate transformation,besides,the PIR controller is used to improve the harmonic characteristic.The new control strategy can eliminate the double-and the fourth-order harmonic at the same time.The simulation results verify the validity of the proposed control strategy.Finally,the experiment platform of modular multi-level converter is built.The control system is the master slave of DSP+FPGA.In this paper,the parameters of the experiment platform and the design of peripheral auxiliary circuit are also stated.And experimental is made to verify the effectiveness of the used control strategy.