Development Of Bidirectional Converter For Microgrid And Research On The Compound Control Method

In recent years, microgrid has become the focus of electric power development strategic of many countries for its environmental friendly performance and flexible intelligent control. The bidirectional converter becomes the research hotspot as the bridge of microgrid energy storage systems and the AC bus. This paper researches on the half bridge structure of three-phase voltage PWM converter by analyzing the detailed main circuit topology structure, the working principle and the mathematical model of different coordinates. The design process of the experimental prototype is presented and the control method is studied. In this paper, the main work and achievements are as follows:1. The half bridge structure of three-phase voltage PWM converter is selected as the research object, according to the requirement of bidirectional converter performances in the microgrid system. The working principle and the working process of rectifier and inverter is studyed by analyzing the topology structure of the main circuit, and the mathematical models of the microgrid bidirectional converter in the three phase static coordinate system(a, b, c), the two phase static coordinate system(alpha, beta) and the two phase rotating coordinate system(d, q) are established.2. The power main circuit parameters of bidirectional converter for microgrid and the hardwares and softwares of control circuit are designed. The hardwares design includes the DSP minimum system, AC and the DC detection circuit, the corresponding signal conditioning circuit, the zero crossing detection circuit, drivers, the protection circuit and the power supply module. The softwares design includes the main program, interrupt service routines and subroutines. And a texperimental prototype of bidirectional converter for microgrid is built.3. The mathematical model of bidirectional converter for microgrid in the two phase rotating coordinate system(d, q) is used for analyzing the voltage and current double closed-loop control structure. According to the characteristics of time-varying nonlinear, strong coupling of converter system, a compound control method is presented based on the theory of modern nonlinear control. The inner loop of current uses the control method of feedback linearization. The nonlinear system of the converter is linearized by using the conversion of nonlinear state and feedback, and the active current and reactive current of the system is decoupled completely. The voltage outer loop uses sliding mode control and makes full use of the advantages of good robustness for system parameters change to solve the problem of the converter system time-varying parameters. This paper presents the design process of the double closed loop with PI control and analyzes the SVPWM modulation process.4. A simulation model of the bidirectional converter for microgrid are bulit. Compared with the traditional PI control method, the results show that the bidirectional converter for microgrid using the compound control method that the current inner loop using feedback linearization control and the voltage outer loop using sliding mode control can work with unit power factors and has a superior dynamic and static performance. The AC side current has a high dynamic response speed, high tracking precision, and small harmonic current. The DC side voltage has a high dynamic response speed, good performance of anti-interference and robustness. The bidirectional converter using the compound control method can reduce the "harmonic pollution" of the microgrid AC bus, and improve the charging efficiency of battery energy storage system, it achieves the target of green energy transformation.