Research On Control Strategy Of The Isolated Bidirectional AC/DC Converter In Hybrid Microgrid

The research content of this thesis is from Shanxi Province Key Research and Development Program(Internantional Cooperation Program),"Research on Control Strategy for Bidirectional Power Converter in Hybrid AC/DC Micro-grid(201603D112001)".Due to the rapid development of renewable energy power generation and microgrids,the boundary between source and load is no longer as obvious as in traditional power system.The consumers of electric energy in traditional system may also become providers of electric energy.The fuzzification of the source-load boundary of the power system puts forward new requirements for power equipment.The bidirectional power converter based on the dual active bridge structure has the advantage of electrical isolation while achieving bidirectional energy transmission,which is a hot research topic today.However,the two-stage conversion control method for the converter has the disadvantage that the conversion efficiency is not easy to be improved,and the existing single-stage conversion control method can realize efficient transmission,but most of the control is complicated and linear control of the output power cannot be realized.How to balance converter efficiency and control complexity is a problem that must be solved.Aiming at the control and optimization of isolated bidirectional power converter based on dual active bridge structure,a single-stage power control strategy based on dual-phase-shift plus variable frequency control is proposed in this paper.Firstly,the two-stage power conversion control strategy for isolated bidirectional power converter based on dual active bridge structure is studied and improved.The isolated bidirectional power converter is divided into two parts: the front-stage dual active bridge structure and the rear-stage three-phase full bridge structure.The dual active bridge control model based on single-phase-shift control and the three-phase full bridge mathematical model are established.The traditional double closed-loop control of the rear-stage three-phase full-bridge is improved based on the model.And the improved double closed-loop control is applied to the control strategy of the isolated bidirectional power converter in the hybrid microgrid.The designed control strategy was verified on the MATLAB\Simulink platform.Compared with traditional control,the improved control strategy has a better dynamic response.Then,because the isolated bidirectional power converter with two-stage power conversion has the disadvantages of low power conversion efficiency,large intermediate capacitance and high switching loss,a dual-phase-shift plus variable frequency single-stage power control strategy is proposed to avoid these shortcoming.According to the single-phase and three-phase isolated bidirectional power converter topologies,the control strategies for single-phase and three-phase converters are designed respectively.And the zero-voltage turn-on range of the switches in front-stage dual-active bridge is analyzed.For the single-phase isolated bidirectional power converter,the dual-phase-shift plus variable frequency control is adopted for the front-stage dual-active bridge,and synchronous-rectify control is adopted for the rear-stage synchronous rectifier bridge.For the three-phase isolated bidirectional power converter,the dual-phase-shift plus variable frequency control is adopted for the front-stage dual-active bridge,and 33% PWM control is adopted for the rear-stage three-phase full bridge.By applying variable frequency control,output power can be controlled linearly by the shifting-phase ratio in dual-phase-shift control;the rear-stage single-phase synchronous rectifier bridge and three-phase full bridge all work in low switching loss mode in the designed control strategy which can improve the converter efficiency;the intermediate capacitance of the converter is small which can effectively reduce the converter volume.In order to verify the correctness of the proposed control strategy,a single-phase and a three-phase isolated bidirectional power converter simulation control model was built on the MATLAB\Simulink platform.The simulation results confirmed the effectiveness of the control strategy.Finally,a single-phase isolated bidirectional power converter prototype using the dual-phase-shift plus variable frequency control strategy was developed based on the existing devices of the laboratory.The bidirectional power transmission of the prototype was tested to verify the actual feasibility of the proposed control strategy.