Research On Three-phase Back-to-back Converter

High efficiency and energy saving becomes a development trend of electric energy conversion owing to the shortage of energy and the diversification of industrial load.During the industrial production process,power converter plays an important role.However,some problems still need to be solved to enhance the performance of traditional power converter.In the traditional power converter,e.g.,the non-controlled rectifier and phase controlled rectifier,the input current contains large harmonic content,which results the serious power quality problem of power grid.Furthermore,the power flowing path in these converters is not bidirectional,which may waste some power energy.Therefore,how to solve these problems becomes a hot issue in power electronics.To solve above mentioned problems,several solutions have been proposed and analyzed,and the back-to-back PWM converter is the most promising solution due to some obvious advantages,such as bi-directional power transfer,sinusoidal input current,and unity power factor.However,some problems still exist,such as high frequency harmonic current and poor dynamic performance of DC link voltage during load changing.And these drawbacks may limit the power rating of the back-to-back PWM converter.Thus,how to solve these problems has significant engineering meaning.In this thesis,the operational principle and mathematical model of the three-phase back-to-back PWM converter is first analyzed.Then,the control system of the three-phase back-to-back PWM converter is designed.Finally,simulation and experimental results are provided to prove the feasibility of the design scheme.The main contents of this thesis are concluded as follows:(1)The sate-of-art of the three-phase back-to-back PWM converter is discussed,and then the main existing problems,such as slow dynamic response are abstracted.(2)Mathematical model of the back-to-back converter is built,which includes models of three-phase static coordinate system,two-phase stationary coordinate system and two-phase rotating coordinate.And the load model isdevelopment based on three-phase induction motor.(3)Based on the mathematical models of converter,the control strategy has been studied,and some common control algorithms and control parameters of PWM rectifier and inverter have been discussed.To restrain the harmonic content and improve the utilization ratio of the electric energy and the response speed,this thesis analyzes the current deadbeat control and the active power feed-forward method.In addition,the double generalized two order integral software phase locked loop technology and SVPWM technology are also discussed.Finally,the feasibility of the control method is verified by Matlab.(4)A prototype is built to verify the discussed controlling strategies,which is composed of DSP based control circuit,IGBTs based power circuit and corresponding driving and signal conditioning circuit.The control code is generated by modular design method,which reduce the hardness of programming.After analyzing experimental results,some conclusions can be obtained.The discussed control strategies meet the requirements of the normal industrial application.In addition,the current deadbeat control and active power feedback can improve harmonic suppression ratio,the energy utilization rate and response speed.