Research On Active Suppression Method Of Conducted Electromagnetic Interference For Three-phase PWM Inverters

Voltage source inverter using pulse-width modulation(PWM)technology is an important carrier for industrial and civil electric energy conversion.However,electromagnetic Interference(EMI)emitted by the continuous and high-frequency switching actions of power electronic devices will affect the safe and reliable operation of the system,and with the promotion of wide-band-gap semiconductor power devices and the application of high-voltage and large-capacity equipment,the EMI issues will become more serious.The traditional method of suppressing conducted EMI is to insert EMI filters into the system to block the propagation path,but the use of passive components obviously increases the volume and weight of the system.Active suppression technology represented by spread-spectrum modulation,active common-mode voltage suppression and active gate drive can reduce EMI without using EMI filters.On this basis,combined with the necessary EMI filter can easily meet standard requirements and reduce the volume and weight of passive components,increasing the power density of the system,which have broad prospects in applications.However,the current research on the active suppression technologies of conducted EMI fails to fully exploit the suppression capability and consider the comprehensive performance of the PWM inverter.Therefore,this dissertation proposes several active suppression methods to reduce the emission level of conducted EMI for the three-phase PWM inverter.This dissertation analyzes the key factors that determine the noise source of conducted EMI in the PWM inverter: topology,switch combination and switching behavior.For the selected topology,the switching frequency and pulse position included in the switch combination determine the conducted EMI between k Hz and MHz,and the dv/dt and di/dt included in the switching behavior affect the conducted EMI in the MHz band and above.At the same time,a mathematical model of the relatively independent noise source—common-mode voltage is established,pointing out that the pulse position of the switch combination determines the common-mode voltage.Therefore,the degree of freedom of the switching frequency can be used to suppress the EMI between k Hz and MHz;the degree of freedom of pulse position can suppress common-mode interference;and the optimization of the switching behavior can be used to suppress EMI in the MHz band and above.The dissertation also briefly introduced domestic and foreign EMI testing standards,testing methods of conducted EMI and common-mode voltage/current,testing equipment and testing principles,which guides the research on active suppression methods of conducted EMI for PWM inverters.In order to suppress the conducted EMI between k Hz and MHz,a spread-spectrum modulation method based on the distribution characteristics of switching frequency is proposed.The dissertation analyzes the influence of the base value,changing range and changing characteristics of switching frequency on the spectrum,and studies how to make full use of the freedom of the switching frequency to suppress conducted EMI.The concept of distribution density is introduced to feature the change characteristics of the switching frequency,and it reveals that distribution density plays an important role in reducing EMI for spread-spectrum modulation.The differential equation and its solution of switching frequency satisfying arbitrary distribution characteristics are deduced,and the boundary conditions are used to optimize the performance of PWM inverter,such as reducing switching loss and fixing the range of switching frequency.The experiment tested the proposed spread-spectrum modulation scheme based on uniform,normal and exponential distribution,and the conducted EMI is attenuated 20 d B at the key frequency points that determine the design of EMI filter.The experiment further compared the five-dimensional performance of the PWM inverter for nine modulations to comprehensively evaluate the proposed spread-spectrum modulation method,proving the advantages in comprehensive performance for the proposed modulation strategies.In order to suppress the common-mode noise of inverters,the general common-mode voltage suppression technologies suitable for multilevel inverters are proposed.According to the number of voltage levels,modulations of multilevel inverters are divided into two types:nearest level modulation(NLM)and volt-second balanced modulation such as carrier comparison or voltage vector synthesis.Therefore,the common-mode voltage of the inverter under different modulations is derived.Based on that,modulation methods that suppress common-mode voltage are researched.For high-voltage multilevel inverters with a large number of levels,optimization is based on the NLM method,and a modulation method based on the nearest zero common-mode voltage vector approximation is proposed.As the number of levels increases,the reduction in DC voltage utilization and power quality brought about by this method is negligible,and the common-mode voltage can theoretically be reduced to zero.In view of the fewer number of levels,the existing vector synthesis based method to eliminate common-mode voltage is rather complicated,so this dissertation proposes a simple common-mode suppression method based on carrier stacking comparison.The method has strong versatility and is suitable for a variety of multilevel typologies with odd levels.Simulation and experimental results verify the effectiveness of the two methods in suppressing common-mode voltage and current.The frequency of common-mode noise caused by modulation is eliminated,only leaving the common-mode noise caused by the nonideal factors such as delay,dead-time,and switching process.In order to suppress conducted EMI in the MHz band and above,an active gate drive scheme based on collector-emitter voltage trajectory tracking control is designed.First,the switching characteristics of power devices are analyzed,and the influence of dv/dt and di/dt on high-frequency EMI is pointed out.Secondly,the spectrum of the pulse edge traced as a sine-wave is derived.Compared with the conventional slope-type edge pulse,that switch trace can improve the attenuation speed of high-frequency EMI.Based on that,an active gate drive scheme is designed to make the collector-emitter voltage track the switching trajectory of the sinusoidal edge pulse,and an active drive hardware circuit is developed.Based on that,a realization scheme of active drive is developed,and a high-bandwidth operational amplifier was used to realize closed-loop control so that the collector-emitter voltage could track the sine-edge pulse width switching trajectory.Finally,through simulation and double-pulse test experiments,the switching characteristics and trajectory tracking effects of the designed scheme are verified,and continuous pulse experiments are conducted to test the conducted EMI.Compared with the traditional gate drive,the designed active drive can suppress the emission level of conducted EMI in high-frequency.In this dissertation,several active suppression methods based on spread-spectrum modulation,common-mode noise suppression and active gate drive suitable for PWM inverters are proposed to suppress conducted EMI in the full frequency range,which can reduce the design requirement of EMI filters,thereby increasing the power density of the system.