Research On Novel Topology And Control Strategy Of Power Electronic Traction Transformer

As China's high-speed trains are widely put into operation,the contradiction between the high-speed railway vehicles,energy consumption,and environmental protection has brought huge economic pressure to operating enterprises and severe challenges to the scarce energy and environmental protection work.The power electronic traction transformer(PETT)has high power density,high efficiency,good control performance,and flexibility,which is in line with the development direction of China's new generation of high-efficiency traction components and has important research value.This paper mainly focuses on the problems of PETT in circuit topology,design of DC/DC medium-frequency link,system control,and auxiliary power supply.By combining theoretical analysis and experimental verification,the topology and control of PETT is studied.The specific research contents are as follows.For the research on circuit topology,this dissertation proposes a PETT circuit topology based on voltage self-balancing resonant(VSBR)converters.The working principle,parameter design,and circuit characteristics of the circuit topology are studied and compared.The circuit topology has the characteristics of low voltage stress of switches and good soft switching characteristics,which can further reduce the switching loss,improve the conversion efficiency and switching frequency of medium-frequency DC/DC converter.Moreover,the number of PETT modules and medium-frequency transformers will not increase with decreasing voltage stress of the switches and the control structures can be simplified.By comparing the performance of this topology with the other common PETT topologies,it can be concluded that this topology can realize the comprehensive optimization of switching frequency,efficiency,transformer quantity,and cost for PETT.For the study of DC/DC medium-frequency link parameters design,a gain modeling method for LLC RC is firstly presented in this dissertation.By modifying the resonant factor and load factor of the traditional fundamental harmonic approximate modeling method,a modified gain model of LLC RC is obtained,which can greatly improve accuracy of the proposed gain curve.Based on the modified gain model,the parameter design method of LLC converter is further proposed to realize the optimal design of the resonant parameters.The influence law of large dead time on the soft-switching characteristic of LLC RC is analyzed.The soft switching range with large dead time is derived quantitatively from three constraints,including freewheel diodes,junction capacitor discharge and device switching characteristics.And then the dead time optimization design method under high voltage and high power environment is proposed.The effect of secondary power ripple generated by single-phase PWM rectifier on resonant current of LLC RC is studied.The quantitative relationship of the voltage ripple and the current ripple is derived.The influence law of the LLC RC's output capacitance,output resistance,and excitation inductance on the resonant current ripple is analyzed,which can provide a new indicator for the output capacitance design of PETT.For the research on control strategy of PETT,according to the condition that traction mode and regenerative braking mode of trains switch frequently,an LLC-LC type bidirectional control strategy for LLC RC is presented.This control strategy can achieve automatic bidirectional power flow and keep the output voltage constant without any energy flow detection.At the same time,the soft switching can be always achieved.Then based on the equivalent model method,a simplified small-signal modeling method is proposed for LLC RC,which can help deduce the small-signal transfer function between input voltage,output voltage and switching frequency.Furthermore,a stability analysis method of DC/DC link based on single-stage power balance control strategy is proposed to guide the controller design of LLC RC.For the study on auxiliary power supply,this dissertation firstly studies the influence mechanism of starting strategy,fault redundancy strategy,and manufacturing or maintenance convenience of PETT on Centralized and distributed auxiliary power supply schemes.The characteristics of the two schemes are analyzed and the centralized auxiliary power supply is finally selected as the main research object.Then a centralized auxiliary power supply topology based on Buck-constant-current-source structure is proposed.By using the current source control mode and single-turn winding of the primary side of transformer,the problem of multi-module voltage sharing for centralized auxiliary power supply is finally solved.