Research On Bidirectional LLC Resonant Converter Applied To The High-power Power Electronic Transformer

With the rapid development of railway industry,the research work of electric locomotive in our country is changing rapidly.From the original Shaoshan electric locomotive to the Harmony electric locomotive,to the Revival high-speed EMU train.At present,the traditional power frequency transformers used in electric locomotives have problems such as large size,heavy equipment and low efficiency.Therefore,it is urgent to study the high efficiency converter.Based on the national key research and development plan,this paper has carried out research on the key technology of bidirectional LLC resonant converter in high-voltage and high-power power electronic transformer,laying a foundation for the development of next-generation rail transit.Based on the relevant research of existing power electronic transformers,in this paper,the full-bridge three-level bidirectional LLC resonant converter is taken as the research object,and the work principle and commutation path of the converter is analyzed,emphatically.The First Harmonic Approximations used to analyze its circuit characteristics,and the gain characteristics and impedance characteristics,and the realization conditions of soft switching engineering were derived.Secondly,based on the work principle and characteristic analysis,the parameter design of the LLC resonant converter main circuit,including resonance parameter design,transformer design,and switch tube selection.And then build a simulation model in Matlab/Simulink.The traditional unilateral control mode of the resonant converter is analyzed to work in three modes,forward traction,reverse braking,and traction brake switching.The multi-mode operation of resonant converter is realized.Theoretical analyses and simulation results find that when the LLC resonant converter is in reverse operation,the off-current of the secondary side switch is very large,resulting in large switching losses.For the NPC three-level topology,there is an imbalance between the internal and external component voltages during the reverse operation.Due to the difference between forward and reverse gain characteristics,the output voltage fluctuates greatly when traction and brake conditions are switched.In view of the above problems,this paper apply bilateral synchronization control and derives the forward and reverse gains in this control mode.By simulation and comparison,it is found that the bilateral synchronous control achieves zero current switch of the secondary side switch next time in the reverse operation state,reducing switch losses.However,it is not appropriate to use bilateral synchronization control in forward traction conditions,and the loss increases at this time.Therefore,this paper proposes the switch logic of the two control methods,using unilateral control in high-power traction condition,in low-power traction and reverse brake conditions,bilateral synchronization control is employed.Finally,this paper verifies the designed main circuit parameters and the applied bilateral synchronous control strategy of the resonant converter through the experimental platform.Comparing the experimental results of the two control methods,the advantages of the bilateral synchronous control can be clearly found.This control method achieves the equalization of the internal and external components stress of the NPC full bridge in the reverse operation,which significantly reduces the off-current of the secondary side switch next time in brake condition.The switch loss is reduced,the efficiency of the circuit is improved during braking,smooth switching is achieved while the traction and brake conditions are switched,and the fluctuation range of the output voltage is reduced.The output voltage stability and the switch smoothness of work condition have been completely and effectively verified.