| Electric locomotive auxiliary converter provides protection for locomotive traction and braking system, and its performance directly affects the normal operation of locomotive. Because of the large capacity of this kind of auxiliary converter, the early auxiliary converters usually exist a series of defects, such as bulky, noisy, big loss, and so on. In order to realize miniaturization, low loss and high efficiency, increasing the frequency of the converter has been applied widely and it increases system power density, improves transient response speed, and suppresses radio noise. Some problems such as switching power loss, diode reverse recover current, and electromagnetic interference caused by high-frequency operation of switches should be considered for the high-frequency design.This dissertation studies a high-frequency and high power auxiliary converter based on the passive soft-switching technology, and the power and operating frequency of the converter is 260kVA and 15kHz, respectively. Firstly, a new topology of the converter is proposed and the working principles of key circuits are analyzed. Then, the state space averaging method and intelligent control are used for the modeling and control of the converter. Finally, based on the given qualifications, the parameters of main circuits are calculated and a model machine is developed, and then experiment tests of key models are implemented to verify the correctness and practicality of auxiliary converter.(1) Topology of auxiliary converter based on the passive soft-switching technologyConsidering the oriented object and working environment of the auxiliary converter, a new topology of a high-power and high-frequency auxiliary converter is proposed by applying the passive soft-switching and DC-(AC)-DC-AC converting technique. The front-end of the auxiliary converter is composed of three DC/DC converters connected with each other with the input-series-output-parallel style, and such modular architecture reduces switching voltage level and guarantees system stability. The back-end of the auxiliary converter is composed of a DC/AC inverter which transforms DC into AC, and the usage of the passive soft-switching clamp circuit technology reduces the weight and volume of the converter. Moreover, the operation principles of main circuits, such as passive soft-switching circuit, energy recovery circuit, and clamp circuit, are descried, and an improved RCD clamp circuit is proposed to improve the efficiency of the auxiliary converter.(2) Modeling and analysis of auxiliary converter based on state space averaging methodFor the high-frequencyDC/DC converter in the auxiliary converter, modeling and analysis of the auxiliary converter is presented applying the state space averaging method. Considering the strong nonlinearity of the converter, in order to obtain satisfying steady state, transient and dynamic characteristics, equivalent circuit of the converter is established on the condition of considering the transformer leakage inductance, output filter inductor and equivalent short-circuit resistance of the capacitor. Then the key voltage and current waves in a cycle is analyzed, and the corresponding equivalent circuits and state space equations under different states are derived. Finally, throug the averaging process and linearization process, the small-signal transfer function and state space model of the converter are obtained in continuous operating mode.(3) Intelligent control of auxiliary converter based on genetic algorithmIn order to ensure the performance of the auxiliary converter against the negative effects caused by the uncertainties of electrical parameters, variability of load and any other factors, a genetic algorithm based control method for the auxiliary converter is proposed. For the DC/DC converter, an appropriate PWM control scheme is selected, and then the double closed-loop control structure of the DC/DC converter is built, where good dynamic response is obtained by current feedback, i.e. inner loop and desired crossover frequency and phase margin are achieved by voltage feedback, i.e. outer loop. Then, traditional PID technology is used, and the simulation results verify the control effect after selecting the PID parameters according to certain principles. However, since the traditional PID control method is difficult to obtain the optimal parameters of the controller, a PID tuning algorithm based on genetic algorithm is proposed to optimize the performance of DC/DC converter. Under the double closed-loop control structure, the genetic algorithm is employed to optimize the parameters of PID controller for the voltage loop, and PI controller is used to achieve current feedback control. The simulation results show that the PID controller based on genetic algorithm can get better dynamic response and stability margin. For the DC/AC inverter, the genetic algorithm is employed to control the space vector modulation, which achieve effective control of three-phase output voltage waveform.(4) Design, implementation and test of auxiliary converterBased on the theoretical study, the hardware and software of the 260kVA passive soft-switching auxiliary converter is designed, the model machine is developed, and some experiments for key models are carried out on the model machine. According to the working condition and various technical indicators, the parameters of the main circuit are determined, and the auxiliary converter is established. Moreover, experimental tests and analysis of the established device are performed, the results verify that the designed auxiliary converter is correct and practical under actual operating conditions. |
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