Reserach On Three-level AC/AC Power Electronic Transformer And Its Control Technology

AC/AC power electronic transformer is based on high frequency electrical isolation transformer,which has the advantages of high output power quality,high power density and active participation in power grid quality optimization.It has become a new development direction in the field of power transformation.At present,the research of power electronic transformer is still in its infancy.In order to further improve its function and performance,to enhance its load and grid adaptability,this paper proposes a new three-level AC/AC power electronic transformer.The topology,working principle,modulation strategy and output voltage closed-loop control strategy are studied.In this paper,a novel three-level AC/AC power electronic converter based on bi-directional switching half bridge three-level AC/AC topology is proposed.In view of the potential voltage offset of the two voltage dividing capacitors on the AC input side of the topology,a hybrid PWM modulation strategy is proposed to solve the problem.The steady-state output characteristic model of the converter is derived by using the fundamental analysis method.Secondly,the extended description function method is used to model the small signal of the proposed converter,and the detailed modeling steps and process are given.The high-order state space model of the converter is solved and compared with the steady-state model based on the fundamental wave analysis method to verify the consistency of the two methods.Furthermore,the small signal dynamic linear model of the proposed converter is obtained by small signal disturbance and corresponding derivation.Then,in order to obtain the excellent control performance of output voltage,the closed-loop control strategy of linear controller combined with repetitive control is given.The linear controller is used to realize the fast dynamic control of the system,while the repetitive controller is used to improve the quality of the steady-state waveform,and at the same time to suppress the influence of voltage harmonics.Based on the small signal model,the parameters of the linear control part and the repetitive control part of the closed-loop controller are designed and analyzed in detail.In order to verify the proposed topology and its modulation and control strategy,a simulation model based on MATLAB/Simulink is built,and its modulation strategy,closed-loop control performance under ideal and non ideal grid conditions are simulated and analyzed in detail.The simulation results show that the proposed topology and its modulation strategy achieve the expected AC/AC power conversion,and the corresponding closed-loop control strategy achieves excellent dynamic and static performance,and has the ability to suppress the influence of power grid harmonics.Finally,the experimental platform of the new three-level AC/AC converter based on DSP and FPGA is built,the overall hardware scheme is determined,and the hardware circuit of each part is designed.The function partition of DSP and FPGA is analyzed,and the detailed software program flow is given.