| Auxiliary converter is an important device of the metro auxiliary power supply system.It provides stable three-phase AC power source for various equipment in metro vehicles.The existing auxiliary converter has the problems of complex structure and low power density,because series or parallel structure is used in the DC-DC circuit of auxiliary converter to achieve wide input ability.Therefore,the study of auxiliary converters with wide input DCDC circuit can realize equipment lightweight,bring benefits for energy saving and cost savings.The study has important research significance.This thesis takes the design of auxiliary converters with advantage of high power density,light weight and high efficiency as research goal.A new type of auxiliary converter structure is studied.The main features of this structure are: the first stage circuit is variable modal threelevel half-bridge LLC converter,the second stage circuit is three-phase inverter.The first stage circuit has the advantages of wide gain range,high efficiency and large power density.This structure can effectively solve the problem that the first stage DC-DC circuit of the auxiliary converter is complicated in structure and low in power density.In this thesis,the circuit structure,working principle and control strategy of the auxiliary converter are studied from the aspects of theoretical research,simulation analysis and experimental verification.First,according to the performance requirements of the auxiliary converter,the circuit structure of the auxiliary converter is determined.Then the working principle of the threelevel half-bridge LLC converter is described.The time-domain analysis method is used to analyze the output gain characteristics of LLC converter under Pluse Frequency Modulation(PFM)and Pluse Width Modulation(PWM).According to the output gain characteristics of the LLC converter under PFM and PWM,the working modes of converter is divided into PFM mode and PWM mode.Switching the operating mode based on the input voltage can obtain wider output gain range and achieve soft switching in both prime and second sides.Secondly,the working principle of the three-phase inverter is introduced and the threephase inverter mathematical model is established.Based on this,the three-phase inverter voltage closed-loop control strategy is studied.The auxiliary converter simulation model was built by Matlab/Simulink software to verify the correctness and feasibility of the circuit structure,control strategy and parameter design of auxiliary converter.Finally,a low-power experimental platform based on FPGA control system was built.Separately verified the PFM,PWM,and variable modal control of LLC converter.Comparing the operation of the LLC converter under variable modal control and PFM by experiment under wide input voltage conditions,proved that variable modal LLC converter has narrower range of frequency changes and higher efficiency when output gain is less than the resonance point gain.The voltage control strategy of the three-phase inverter was verified.And the lowpower experimental verification of the auxiliary converter was completed,which further verified the correctness and feasibility of the auxiliary converter circuit topology and control strategy. |
PDF Full Text Request