Research On Modulation And Control Methods Based On CPS-TPWM Cascade Of Multilevel APF

With the rapid development of electrified railway in our country,the non-linear load in the system increases greatly,which brings about serious harmonic pollution and other problems.Active Power Filter(APF)is a research hotspot of harmonic compensation,which can dynamically compensate harmonics in power grid and improve power quality.The APF with conventional topology can only be applied to low voltage field because of its limited voltage level and switching frequency,which can not meet the practical requirements.In view of the above problems,this thesis studies the single-phase cascaded H-bridge parallel APF.This topology can be applied to high-voltage and high-power applications,and easy to achieve modular design and packaging,which has good engineering application value.Firstly,an improved Carrier Phase Shift Trapezoidal PWM(CPS-TPWM)is introduced based on Carrier Phase-shifted Sine Wave Pulse Width Modulation(CPS-SPWM).CPS-TPWM modulation technology combines the advantages of cascaded multilevel technology.It can achieve higher equivalent switching frequency at lower switching frequency and improve the utilization ratio of DC voltage of APF.It will have wide application prospects in APF and high power converter fields.Secondly,basing on single-phase cascaded H-bridge APF,there are differences in losses among the single H-bridge modules due to their own differences.Finally,the DC side voltage of the series H-bridge module appears unbalanced during the operation process,which will have a greater impact on the operation of APF.In order to effectively control the stability and balance of capacitor voltage on DC side of single-phase cascaded H-bridge APF,this thesis introduces the inner and outer control strategy,that is,the outer control strategy realizes the stability control of total voltage on DC side of cascaded H-bridge APF through traditional voltage and current inner loop control,and the inner control strategy equalizes the voltage of each H-bridge module by adding active voltage vector to make APF able.Enough safe and effective operation.At the end of this thesis,the APF simulation model of single-phase cascaded H-bridge is built under the environment of Matlab/Simulink,and the simulation experiments of each functional module and the whole function of the system are carried out.The effectiveness of the modulation strategy and control strategy is verified by the simulation of the system.