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

Nowadays, the fossil fuels are going to be exhausted, and the environment has beenseriously polluted, thus the distributed power generation system (DPGS) based on renewableenergy such as wind and solar has attracted more and more attentions. Owing to thecharacteristics such as controllable output dc voltage and power factor, three-phase pulse-widthmodulation voltage-source rectifier (VSR) is widely used in DPGS. However, thesingle-inductor filter contains high switching current ripple. The common solution is to add aninput LC filter, leading to a cascade system. The cascaded system consists of LC filter andthree-phase VSR may have unstable phenomenon. This thesis is mainly dedicated to thestability of the ac cascade system.This thesis analyzes the operating principle of three-phase VSR and its modulationstrategy; builds its small-signal model in rotation (dq) coordinate system and derives itsopen-loop transfer function. Then, the double closed-loop with inner current loop and outervoltage loop is designed. According to the ripple in inductor current and the harmonicrequirement of the electrical equipments, the design guidelines of the LC filter are presented.The multi-input-multi-output (MIMO) model of the ac cascade system is derived, and thestability is analyzed based on the generalized Nyquist criterion. To study the interaction of thetwo sub-systems, this thesis simplifies the output impedance matrix of the LC filter and theinput impedance matrix of the three-phase VSR; gives out the eigenvalues of the two simplifiedmatrixes. Then, by judging whether the amplitude-frequency curves of the eigenvalues areinteracted and the phase differences in the interaction area are greater than180degrees or not,one can know the stability of the ac cascade system. Finally the basic conditions which makethe ac cascade system stable are analyzed and the value range of the LC filter is divided into thestable and unstable region of the cascade system.The concept of input impedance regulator is proposed to improve the stability of the accascade system in this thesis, which regulates the input impedance of the three-phase VSR toavoid the situation that the phase differences in the interaction area are greater than180degrees.On the basis, the input impedance regulator is designed and its impact is analyzed.A6.5kW prototype of the cascade system consists of input LC filter and three-phase VSRhas been fabricated and tested in the lab. The experimental results are presented to verify theanalysis and design results.