| Dual-three-level converters using IGBT as the switching device are widely used in high-power variable-frequency speed control systems because of their advantages such as small ripple.However,the problem of electromagnetic interference(EMI)generated by the fast action of switching devices is becoming more and more severe,which not only affects the reliability of the system itself,but also causes high-frequency pollution to the power grid,making it an emergency to study the subject.Compared with the EMI problem of the two-level variable frequency speed control system,it is more difficult to conduct the theoretical EMI analysis of the three-level system,due to the more complicated mathematical model establishment,the more challenging deduction of the interference source,and the more complex and changeable parameters in the propagation path.Besides,it is necessary to design a special experimental verification platform and to extract the parasitic parameters of the high power variable frequency speed control system.Literature review also reveals a shortage of researches in this field both home and abroad.This paper takes a MW-class dual three-level variable frequency speed control system as the object to conduct the research.Firstly,the mechanism and propagation path of EMI in high-power three-level power converters is analyzed.Secondly,the conducted EMI of the variable frequency speed control system is modeled and simulated from two aspects in the time domain and frequency domain respectively.The simulation method of both Matlab and Saber combined is adopted in the time domain modeling for the first time,which not only takes advantage of the exactness in the physical model of IGBT in Saber,but also makes full use of the ease of the Matlab control strategy implementation,thus enabling a simulation closer to the actual situation.The accuracy of the frequency domain model is decided by the accurate mathematical model of the interference source.Based on the process analysis of two-level SPWM control strategy,the switching function f(x,y)and the voltage contour line of SVPWM under three-level PD and POD modulation are obtained,after which,the mathematic model of three-level SVPWM interference source under these two modulation modes is computed respectively,and the frequency domain modeling of the three-level variable frequency speed control system is realized under different modulation modes.The prediction model is simplified with the idea of seismic sensitivity,with which some invalid branches are deleted,so that the prediction model can directly reflect the communication path of interference.The paper provides a reference for the optimization design of the electromagnetic compatibility structure and the diagnosis of electromagnetic interference fault.The paper also studies the electromagnetic interference generated by different modulation methods in the SVPWM control algorithm of three-level variable frequency speed control system.The problems covered includes: the EMI generated by the system when the positive phase carrier,the inverted phase carrier,and the random modulation wave are adopted respectively;the EMI produced by the system when different modulation ratios and modulation degrees are adopted under the same phase carrier modulation.Moreover,the paper outlines some proposals to suppress the EMI from the aspect of control strategy.Finally,the conducted EMI filter has been designed to suppress EMI in case that the measured system conducted EMI exceeds the national standard.The EMI suppression ability of designed EMI filter is verified by the experimental results within a specified frequency band.The simulation results and experimental results show that the research results in this paper can provide a good reference for the study of electromagnetic interference in the three-level or even multi-level high-power variable frequency speed regulation system. |
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