Research On The Active Power Filter Based On Cascaded Multilevel Converter

The large-scale use of nonlinear loads in power system produces a large number of harmonics,which causes serious pollution to power quality and affects the safe and effective operation of power system.The active power filter can realize the power quality control by injecting harmonic current with equal amplitude and opposite phase to the grid.Therefore,the research of active power filter is of great significance.However,in high power occasions,the switching frequency of power devices has inherent contradiction with high power.It can effectively solve the problem by using cascaded topology to improve the switching frequency and multi-level output.Therefore,cascaded APF as the research direction of APF is effective and promising.First of all,this paper analyzes the cascade topology of APF.The output effect of the high voltage is verified through the research on the working mode of the cascaded H bridge.The three H cascade structure is used to output the seven level,which has a good effect on the harmonic current.Excellent modulation strategy is the key factor to exert the potential of cascaded APF.Through the comparison of several modulation strategies for cascaded converter structures,using 5th and 7th harmonics as modulation wave,we will select the suitable modulation strategy for the cascaded APF.Secondly,the harmonic detection methods are studied.By comparing several methods,the two methods of instantaneous reactive power theory are compared.The simulation and design of the ip-iq method with stronger anti-interference ability are carried out.Proposing the improved harmonic current detection method and the advantage of the improved harmonic current detection is derived by mathematical analysis,and the low-pass filter is designed.By comparing the simulation results with the ip-iq method,the results show that the improved harmonic current detection method has good detection performance.Thirdly,the cascade APF control strategy is studied.The key parameters of the suitable cascaded APF main circuit are the basis of the effective operation of the APF,so design the grid connected inductor and the DC side suspended capacitor.Excellent harmonic current tracking control has great influence on the compensation effect of cascaded APF.Compared with the control analysis of hysteresis current control and triangular wave current control,this paper will be used as the simulation design and verification of the current waveform control.The stability of the DC voltage and the voltage balance of the H bridge are the key to ensure the stable and effective operation of the cascaded APF.This paper proposed an improved DC side suspended capacitor control.The simulation and design of the overall voltage control and the suspendedcapacitor voltage balancing are carried out respectively.The results show that overall voltage control is effective and the improved DC side suspended capacitor control is feasible and superior.Finally,based on the above research,this paper carries on the overall simulation design and verification of the cascade APF.The effectiveness of the system is demonstrated by FFT analysis of the compensated harmonics.On the basis of steady state,second harmonic sources are introduced to verify the good dynamic compensation performance of the system.