Study On Topological Structure Of Power Electronic Traction Transformer

In recent years,China's high-speed railway and bullet train are developing towards lightweight and high-speed.For locomotive,traction transformer,as the core component of locomotive,is playing an increasingly important role and will usher new changes.Nowadays,power electronic traction transformer is replacing the traditional autotransformer gradually.Power electronic traction transformer combines high-frequency transformer and power electronic device organically,which not only can lighten the weight of locomotive,but also can lower the energy loss,and reduce the space occupied on the locomotive.It can also realize the functions of bidirectional flow of energy,electrical isolation,harmonic control et al,which provides a favorable condition for the high-speed development of modern rail transit industry.The topological structure of power electronic traction transformer is divided into three parts: input stage,isolation stage and output stage.The high-voltage input stage mainly controls and transforms the electric energy.The intermediate isolation stage effectively isolates and transforms the voltage at both ends.The low-voltage output stage provides the voltage and power required for the electric locomotive.Through the analysis and study for the topological structures at all stages,the topological structure adopted in the thesis was selected.The main work is as follows:Firstly,the existing topological structures of several main power electronic transformers were explained.Based on the anyalysis of those working principles and characteristics,the thesis determined the topological structure of AC-DC-AC power electronic traction transformer consisted of high voltage input stage MMC rectifier,intermediate isolation stage DC-DC converter and low voltage output stage DC-AC inverter.Then,the thesis analyzed in detail and compared the input stage topological structure of traditional H bridge with MMC.It is concluded that the topological structure of MMC has its advantages compared with the traditional H bridge topological structure.By calculating instantaneous active and reactive power,the direct power controller of single-phase PLL was studied.Secondly,the thesis analyzed the working principle of the traditional two-way H bridge cascaded DC-DC converter topology,gave the simplified half-bridge cascaded DC-DC converter topology,established its mathematical model,and carried out the simulation.Thirdly,the traditional three-phase three-wire topology was selected for the low-voltage output stage,and itsmathematical model was established.The SVPWM control strategy was used to replace the traditional SPWM control strategy.Simulation results show that the output inverter has good control effect.Finally,on the basis of determination of the overall topological structures of power electronic traction transformers,the simulation model of single-phase MMC power electronic traction transformer applied to rail transit was established,and three working conditions of traction,load mutation and regenerative braking were simulated and verified.Simulation results show that the MMC-based power electronic traction transformer has good active and reactive power control ability and can realize unit power factor operation.The output voltage,current amplitude and waveform can meet the requirements of locomotive electricity.The output power and motor speed can reach the level of current locomotive.The input current distortion rate is small and the DC side voltage ripple coefficient is small,which is beneficial to the stable operation of the whole traction system.