Research Of Dual-PWM Converter Rectifier Control Strategy And System Realization

The AC-DC-AC variable-frequency speed control system using uncontrolled rectifier bridge as rectifier has some shortcomings,such as big ac-side input current harmonic,low power factor and impossibility of quadrant operation etc. In order to overcome the shortcomings, the PWM converter takes place of uncontrolled rectifier link in conventional dual-PWM variable-frequency speed control system. This dual-PWM variable-frequency speed control system using dual-PWM technique to drive independently rectifier and inverter has many advantages,such as similar sine input current waveform of the ac side,unit power factor,adjustable output voltage and current frequency,sine output current waveform,quadrant operation, bi-directional power flow etc.But, when asynchronism motor changes speed or load suddenly, as a result of the response of the dc side capacitor voltages is comparatively slow, the dc side capacitor voltage produces big fluctuations.Although increasing the capacity of the dc side capacitance can avoid this kind of circumstance, but the large capacity of dc let is huge, expensive,and affects the whole system dynamic performance because of the slow response. A dual-PWM control method based on active power directly feedback can get smaller dc voltage fluctuation, reduce the capacity of dc capacitor and improve the system dynamic response speed.A double PWM control converter based on active power directly feedback was studied,and the PWM rectifier was the research focus of this paper. Main circuit structure,working principle and mathematical model of different reference frames associated with the three-phase voltage model PWM rectifier were analysed, fixed switching frequency PWM current control of the direct current control strategy was introduced,the double closed loop control system of the voltage outer loop and current inner loop were designed.The experimental results met the design requirement.The mathematical model of three-phase voltage source PWM rectifier,positive and negative sequence detection methods of the ac side voltage and current and two different control strategies under condition of unbalance network voltage were analysed.Three-phase voltage source PWM rectifier simulation models under unbalanced condition were designed using MATLAB,the simulation result shows that:The positive and negative sequence components of voltage and current could be well separated using positive and negative sequence detection methods; The ac side negative sequence current could be restrained to make three-phase ac current basically symmetrical;The Dc side voltage second harmonic could be restrained to make dc side voltage values reach a set value and remain stable without the second harmonic.In the study of PWM inverter part, the mathematical model and coordinate transformation of asynchronism motor were analysed;the calculation method of rotor magnetic field position was emphatically expounded;the double closed loop speed regulation system,the outer loop of the speed and current inner loop,was used in the rotor field-oriented vector control system.The double PWM control method based on active power feedback was analysed,which basic principle was that:the active power of the inverter was directly feedback to the current loop given of the rectifier through the balance control of the rectifier and inverter. In addition,the calculation formula of rectifier current loop given value was deduced in the research. The simulation model of the conventional dual-PWM system and the PWM system based on active power feedback were set up using MATLAB, the simulation results showed that:compared with the conventional method, using active power feedback method can be effectively reduced the fluctuation of dc side voltage.In experimental system,the control system heart of the rectifier and inverter was respectively TMS320F2812 DSP and TMS320LF2407A DSP, the calculated active power feedback of TMS320LF2407A DSP was transfered to TMS320F2812 DSP in the method based on active power feedback.The CAN modules of two DSP were used to carry power transfer through in this article.The hardware circuit and the software realization method of the communication system were designed,the transmission results of experimental data showed that this method is feasible.The software design and experiment of the dual-PWM variable-frequency speed control system based on active power feedback were done, the experimental results comparison between this method and the conventional method showed that the method could prevent dc side voltage fluctuation effectively in dynamic process,such as motor start,stop,increasing and reducing load or speed,improving the entire anti-disturbance performance,achieving the desired control effect.