Stability Of AC Three-Phase Voltage Source PWM Rectifier Cascade System

With the depletion of fossil fuels such as oil, coal and natural gas, the utilization ofrenewable energy has gained more and more attentions. The three-phase pulse-widthmodulation (PWM) rectifier with a LC filter at the input side is a common arrangement indistributed power system, forming a cascaded ac system. Even the three-phase PWMrectifier is in stable operation independently, when the LC filter is incorporated, the cascadesystem will be instable due to the interaction.This thesis analyzes the basic operating principle of the three-phase PWM rectifier. Thesmall signal model is established under the dq0rotating coordinate. The current loop,voltage loop, phase-loop lock (PLL) parameters are designed under the dual close-loopcontrol strategy. Impedance matching cascade criterion is deduced based on the generalizedNyquist stability criterion, which is commonly used to judge the stability of MIMO system,and the impedance characteristics of the output impedance of the LC filter and the inputimpedance of the rectifier. The impedance matching cascade criterion is more easilyobtained and can judge the cascaded stability more directly. Using the impedance matchingcriterion, the stability of the system can be judged by the magnitude and phase of theeigenvalues of the output impedance and the element on the main diagonal of d-axis ofinput impedance matrix in Bode diagram. Based on the impedance matching criterion, theimpedance regulator is designed to change the input impedance of the rectifier to improvethe stability of the system.A20kW three-phase PWM rectifier cascade system with220V/50Hz ac input and700Vdc output is built. The experimental results are presented and verify the correctness of theimproved stability criterion and the design of impedance regulator.