| Due to the energy crisis and environmental pollution caused by the overexploitation of fossil energy,the distributed generation system has been widely applied because of its advantages of sustainable development and high energy utilization.However,new energy power generation is susceptible to the environment,which shows randomness and instability.The research on microgrids has attracted more and more attention to improve the stability and efficiency of distributed generation.The AC/DC hybrid microgrid combines AC microgrid and DC microgrid,which has the advantages of few power conversion stages and high reliability.As the vital interface between AC bus and DC bus,the three-phase bidirectional AC/DC converter plays an essential role in bidirectional power flow and maintaining bus voltage stability,which is the main research focus in this paper.Firstly,this paper proposes a nine-bridge single-stage isolated three-phase bidirectional AC/DC converter.By adopting Y-Δ connected transformers,the converter can realize high-frequency electrical isolation and achieve an extra 2:1 step-down ratio.The symmetrical operation mode of three transformers(3T mode)is proposed when all the components of the converter operate normally.The detailed analysis of the operation mode is presented under 3T mode.By proposing the modified SVPWM algorithm,the voltage-second balance of three transformers is guaranteed.Three transformers operate in a symmetrical mode that results in the equal distribution of total power.Secondly,when an open-circuit fault occurs in a certain phase transformer or the bridge leg connected to it,the dual transformer operation mode(2T mode)is proposed.When an open-circuit fault occurs in the DC-side switches,the asymmetrical operation mode of three transformers(A3T mode)is proposed.The detailed analysis of the operation mode and modified SVPWM algorithm are presented under 2T mode and A3 T mode.The converter can operate under fault-tolerant mode only by adjusting the driving signals without changing the connection of the circuit when the fault occurs.Thirdly,the main circuit parameter design,power loss analysis,and hardware circuit design of the converter are presented in detail.Compared with 3T mode,2T mode shows advantage in efficiency under light load.In order to improve the converter efficiency,the converter can operate under 2T mode for the light load and 3T mode for the heavy load consequently.Finally,a 2.8k W prototype was built in the laboratory,and the experimental results verify the theoretical analysis well. |
