Research On Three-level Grid-connected Inverter At Low Switching Frequency

With the deepening of the concept of green development,new energy power generation has also developed rapidly in recent years.New energy power generation has gradually developed toward the increase of single-unit capacity and grid-connected capacity,making the research of medium-voltage high-power grid-connected inverters a hot spot..In the megawatt-level high-power grid-connected inverter,in order to reduce the loss of the switching device,prevent the temperature rise of the switching device from being too high,and ensure the safe operation of the device,the switching frequency of the megawatt-level medium-voltage high-power grid-connected inverter is usually The setting is low(below 1000Hz).The lower switching frequency will make the output voltage and current of the grid-connected inverter contain more low-order harmonics,and the low-order harmonic content will make the filtering difficult and seriously affect the output performance of the inverter.In order to solve the problem of high output low harmonic content of the grid-connected inverter at low switching frequency,this article will conduct the following research:First,around the medium-voltage high-power grid-connected inverter,the working principle and mathematical model of the three-level inverter are analyzed,and different PWM modulation methods are studied.For the low switching frequency of this paper,SHEPWM is selected as the The modulation method is intended to eliminate the higher harmonics output by the inverter at low switching frequencies.Secondly,the model of SHEPWM is studied.By studying the working principle of SHEPWM and the characteristics of the switching angle,the nonlinear equations of SHEPWM are listed,and the traditional numerical solutions and intelligent algorithms of the equations are analyzed.The particle swarm optimization algorithm is selected to optimize to solve SHEPWM Calculate the appropriate switching angle of the nonlinear equation,and build the Simulink simulation platform in Matlab to verify that the SHEPWM method eliminates low-order harmonics at low switching frequency.Then,in practical applications,in order to prevent the switching device on the same bridge arm of the inverter from short-circuiting,it is usually necessary to set a dead time for the switching device,which will affect the output of the inverter.Therefore,the dead zone effect of the three-level grid-connected inverter is analyzed,and the dead zone compensation is carried out in conjunction with the SHEPWM method used in this article;during the actual operation of the grid-connected inverter,the modulation degree of the inverter is in a fluctuating state.In SHEPWM,the difference is different.The degree of modulation corresponds to different switching angles,so the conversion of the degree of modulation will also make the output quality of the inverter poor,and even the switch angle at the next moment is less than the switch angle at the previous moment,so the dynamic characteristics of the SHEPWM method are analyzed,and The method of keeping the modulation degree for a certain period of time solves the drawbacks caused by the fluctuation of the modulation degree,and the feasibility of the proposed method is verified by simulation.Finally,the harmonic effects generated by the three-level SHEPWM grid-connected inverter are analyzed,and the filter selection and calculation are carried out.The inductance value of the single-inductance filter is too large,which occupies a large area,which affects the efficiency of the inverter.Therefore,the derivative filter of the LCL filter is studied,and the damping resistance parallel inductance LCL filter is used to improve the efficiency of the filter.And,through simulation,the feasibility of the proposed improved filter is verified.