Research On The Suppression Of Common-mode Conduction EMI Based On Three-phase Four-leg Inverter

Three-phase inverter with pulse width modulation(PWM) has been widely used in various occasions, but the output midpoint voltage relative to the reference of inverter changes fast due to the high frequency on-off of switching device, which results in common-mode current through the parasitic capacitance of switching device. High dv/dt and di/dt will bring serious electromagnetic interference(EMI), which will not only affect the normal operation of other devices, but also bring great harm to the inverter itself. This paper mainly studies the following topics: Modeling three-phase inverter, the selection of modulation scheme and the causes analysis of common-mode interference; suppressing the common-mode voltage of three-phase inverter based on an improved SVM modulation scheme; eliminating the common-mode interference based on a novel three-dimensional space vector modulation scheme of three-phase four-leg inverter.The common-mode conduction path of PWM inverter is analyzed, the common-mode circuit model of three-phase inverter is established, and a detailed analysis and comparison of two commonly used three-phase inverter modulation scheme is carried out, which notes SVM is more suitable for three-phase inverter. Then, we analysis the common-mode voltage of each switching cycle with the traditional SVM modulation scheme in detail, and point out the reason for the existence of common-mode interference.An improved scheme is proposed based on the analysis of several no zero vector SVM modulation schemes, and simulation shows this improved scheme decreases common-mode voltage to the original one third without changing the characteristics of output voltage, the common-mode interference is reduced as a result. We note that if only a change of the modulation scheme, the structure of three-phase three-leg inverter can't completely eliminate the common-mode voltage no matter what kind of modulation scheme.We turn the three-phase three-leg inverter to the three-phase four-leg inverter from the perspective of the circuit configuration in order to eliminate the common-mode interference of three-phase inverter, then analyze the common-mode circuit model of three-phase four-leg inverter, get the equation of common-mode voltage and the condition in which the common-mode voltage is zero. Finally, a novel three-dimensional space vector modulation scheme is proposed, both the theoretical analysis and simulation show this novel modulation scheme can make the common-mode voltage always zero, and thus eliminate common-mode interference.A series of experiments are carried out in a 25 kW three-phase four-leg inverter, the results show that the common-mode voltage can be eliminated to zero effectively with the finally proposed novel three-dimensional space vector modulation scheme based on three-phase four-leg inverter, and the common-mode interference can be suppressed even eliminated effectively as a result. And the waveform quality of three-phase inverter' output voltage is still good, THD is still low, which indicate that we achieve suppressing common-mode interference without affecting the output waveform quality.