PWM Techniques And Current Ripple Analysis Of Three-phase Voltage Source Converters

Recently, three-phase voltage source inverters(VSI) are widely used in the applications such as motor drives and grid connection. Current ripple produced by SVPWM and SPWM modulation has to be constrained in certain range so that motor rotor loss heating and grid-connected inverter harmonics could be restricted. Current ripple magnitude could be reduced by increasing switching frequency, while this could introduce extra switching loss. Larger filter inductance could reduce current ripple in gridconnected inverter, but it also reduces system power density. Many researches focus on the method to reduce PWM current ripple while keeping low switching loss at the same time. On one hand, the current most effective method is the hybrid modulation method, which uses different voltage sequence in various areas based on minimum RMS principle. However, this hybrid modulation method is complicated in terms of implementation, and could introduce extra switching loss with low carrier ratio application. Furthermore, a modulation method to reduce current ripple in low carrier ratio increases switching frequency. On the other hand, current study only focus on the application with same Dand Q-axis inductance. The applications with different D- and Q-axis inductance, such as Interior Permanent Magnet(IPM) motors in renewable energy vehicles, have not been studied yet.Toward these problems, this thesis proposes current ripple calculation method and proposes a new Advanced Modulation method. Compared with hybrid modulation methods, the proposed method could reduce current ripple in low carrier ratio without increasing switching frequency and loss. Moreover, the proposed method is easily implemented in DSP and could be extended to the applications with different D- and Qaxis inductance.First of all, this thesis researched the studies in current ripple calculation, proposes current ripple calculation method for applications with same D-/Q-axis inductance, such as Surface Mounted PMSM(SPM), induction motor, and grid. This method is then extended to 3-level converters. The accuracy of current ripple calculation method has been verified by simulation and experimentation.Based on quantitative analysis of current ripple, this thesis proposes a new modulation method to reduce current ripple magnitude and total harmonics distortion without increase of switching frequency in a low sample rate condition. The effectiveness of proposed method has been verified by simulation and experimentation.For applications with different D-/Q-axis inductance, this thesis proposes a novel method to calculate the current ripple. The mathematical model is built based on Interior Mounted PMSM(IPM). The proposed Advanced Modulation method are also applied to the applications with different D-/Q-axis inductance. The accuracy of current ripple calculation and effectiveness of proposed method are verified by simulation and experimentation.