Common Mode EMI Modeling And Suppression Of Three-Phase Inverter System

Nowadays,three-phase inverters have been widely used in many applications.Among them,three-phase two-level has been widely used in aerospace,ship and other fields,and modular multilevel converter?MMC?also has been widely used in high-power applications such as transmission and transformation equipment in power system.However,the voltage is hopped with the high-speed switching of the switching devices in the three-phase converter,along with the parasitic capacitance to the ground,causing severe common mode?CM?electromagnetic interference?EMI?,which will not only endanger the safe operation of the equipment itself,but also endanger the safe and reliable operation of the surrounding susceptible equipment.This paper mainly studies the modeling and suppression technology of CM EMI of three-phase inverter system.First,CM EMI modeling is performed on a conventional three-phase two-level inverter.Impedance analyzer is used to measure the frequency characteristics of the inductors,capacitors and so on in the system.Based on Matlab,the RLC circuit network is used to fit the frequency characteristic curves and to establish the high frequency equivalent models of passive devices.Characterized model of IGBT is set up to simulate their actual switching process.The self-parasitic parameters and the parasitic parameters to the ground of IGBT is obtained through analytical methods and finite element analysis Q3D Extractor respectively.On this basis,the system's CM EMI model can be established and the correctness of the CM EMI model is verified through comparison between CM EMI spectrums predicted by the simulation and measured by the experiment measurement.Then,for the CM EMI of the three-phase two-level inverter,a suppression method based on an improvement H8 topology is proposed.That the CM voltage in the transition from non-zero-switching state to zero-switching state is decided by capacitors C₁-C₈ in the circuit topology is analyzed.And the relationship of them is deducted to achieve zero CM voltage jump.Aiming at the defects of the original modulation strategy,the improved modulation strategy is applied so that the system is not affected by the dead time,that is,the CM voltage does not jump.The proposed topology and modulation strategy for suppression CM EMI in this chapter is verified through simulation and experiment methods.Finally,for the CM EMI of MMC three-phase grid-connected inverter with complex topology,a EMI modeling method is proposed,which is based on frequency domain.Through theoretical analysis,the CM sources existing in the system are separately modeled.Moreover,the dead time effect is expressed to point out the importance of taking the dead time into consideration in the high frequency CM EMI model.Taking the skin effect into consideration,a 4^th order RL circuit is used to model the equivalent model of the cables.Further,the high frequency CM EMI model of the MMC system is established.The comparison results of two CM EMI spectrums obtained through simulation are used to verify the correctness of the proposed EMI modeling method.