Research On Symmetry Of EMI Filter Used In Three-phase PWM Inverter

With the rapid development of the power electronics industry,three-phase PWM inverters are more and more widely used in manufacturing and transportation systems.Although the PWM modulation strategy has excellent performance in power control,it will cause additional electromagnetic interference problems.In systems that use more inverters,such as photovoltaic inverters or motor drive systems,the problem of EMI affecting the normal working state of the system is particularly prominent.In these systems,the conducted electromagnetic noise caused by the PWM modulation strategy needs to be suppressed by EMI filters to ensure the normal operation of the inverter.The traditional EMI filter design method is to decompose noise into common-mode and differential-mode noise,and design the required common-mode and differential-mode filter component parameters separately when the differential mode and common mode are completely decoupled.However,this completely decoupled analysis method is based on the completely symmetrical circuit structure of the filter.However,due to the limitation of the component production process,the actual filter cannot be completely symmetrical.The asymmetry of the filter components will cause the mutual conversion of common mode noise and differential mode noise.When the noise conversion situation is serious,it is very likely to affect the filtering performance of the filter,which is very likely to cause the system to fail to meet the corresponding electromagnetic compatibility standards.Therefore,it is very necessary to study the attenuation mechanism of EMI filter performance and the method of component selection due to asymmetric factors.In the published literature,most of the research on the symmetry of filters is qualitative analysis,and there is a lack of instructive theoretical calculations.At the same time,there is very little basis for the selection of filter component tolerance parameters.Ignoring component tolerances may lead to a decrease in filter performance.Blindly pursuing highprecision components will greatly increase manufacturing costs while improving filter performance.With little success.In order to find the critical value of the filter element tolerance,this article models and analyzes the common filter elements in the three-phase system,that is,capacitors and inductors,and proposes corresponding selection suggestions.Aiming at the three-phase conduction noise problem under SPWM modulation,this paper first analyzes the performance attenuation mechanism of the three-phase asymmetric filter,uses the symmetrical component method to redefine the differential mode noise and common mode noise in the three-phase system,and analyzes three The harmonic distribution law of each phase sequence under phase SPWM modulation,the formula of actual insertion loss when the filter is asymmetrical is studied.Then explore the noise conversion mechanism and behavior model of asymmetric components,take the three-phase LC filter as an example,establish the noise conversion model when the capacitor is asymmetry and the inductor are asymmetry,and verify the correctness of the model through simulation.Secondly,based on the noise conversion model of the LC filter,the relationship between the capacitance and inductance asymmetry and the noise conversion gain is analyzed separately,and the conditions that the component asymmetry should meet without affecting the filter performance are calculated,and Based on the actual situation,the corresponding selection suggestions and method procedures are put forward.Finally,the experiment was verified through a three-phase SPWM inverter and multiple sets of LC filters.