Research On The Impendence Characteristics Of PWM Rectifier

Three-phase pulse-width modulation(PWM) rectifier can not only provide stable dc voltage to the loads, but also realize unit power factor and sinusoidal current control. For these reasons, PWM rectifier is widely used nowadays. When PWM rectifier connects to power electronic loads with negative impendence characteristics, it may bring instability to the DC system. At the same time, when PWM rectifier connects to the power grid as a load, there may be instability too. At present stage impedance matching cascade criterion is used for determining the stability of the distributed generation system. So it is necessary to analyses the impedance characteristic of the PWM rectifier.This thesis analyzes the basic operating principle of the three-phase PWM rectifier. The small signal model is established under the dq0 rotating coordinate. The current loop and the voltage loop are designed under the dual close-loop control strategy. Comparisons between the simplified design method of voltage controller and small signal model design method have been made.Based on the small signal model, the DC side output impedance expression of the PWM rectifier has been derived. The measuring methods of DC impedance are introduced, and the PWM rectifier DC output impedance is measured through simulation. The result verifies the DC side output impedance expression of PWM rectifier.Factors, which influence PWM rectifier DC output impedance, such as the circuit parameters, the controller parameters, the control delay, switching frequency and so on, have been analyzed. Point out that the low and middle frequency band impedance is mainly affected by the voltage controller parameters; the high frequency band impedance is mainly influenced by DC capacitance. Because the middle frequency band impedance is more crucial in system stability analysis, so the voltage controller parameters decides system stability. The current loop has little influence on the DC output impedance, so a simplified calculation method of the DC impedance is obtained. Then this thesis analyzes DC load current feed-forward control. This control method improves the DC output impedance characteristics. These conclusions are verified by simulationThe AC side input admittance expressions of the PWM rectifier have been derived. Factors, which influence PWM rectifier AC input admittance, such as the circuit parameters, the controller parameters, have been analyzed. Point out that the middle frequency band impedance is mainly affected by the current controller parameters; the high frequency band impedance is mainly influenced by AC inductance.A 5 kW three-phase PWM rectifier system is built. Experiments of dc voltage response and DC impedance measurement are conducted. The experimental results are presented and verify the correctness of the impedance characteristics theory.