Research On Control Strategy Of Parallel Three-level Power Quality Control Device

With the advance of the country’s energy structure transformation,the issues of power quality are more complex and changeable,which should be addressed by power system department.In the thesis,a parallel three-level power quality control device is used to control power quality problems effectively,including current distortion and three-phase unbalance.This thesis mainly analyzes the harmonic detection algorithm,modulation algorithm and control strategy in the system.Firstly,the mathematical model of a three-phase three-wire parallel three-level power quality control device is established;a harmonic detection algorithm based on sliding window iterative discrete Fourier transform(Sliding Window Discrete Fourier Transform,SDFT)is introduced.However,the SDFT algorithm has a delay of the fundamental wave period,which slows down the dynamic response of the system.By analyzing the transfer function of SDFT algorithm,it is presented that the N zeros introduced by the comb filter are the main reason of long delay in the SDFT algorithm.Therefore,a harmonic detection algorithm based on Generalized Discrete Fourier Transform(General Discrete Fourier Transform,GDFT)is adopted.According to the characteristics of harmonics,the GDFT algorithm can reconstruct the comb filter in different application places,which can detect harmonic flexibly and quickly and improve the dynamic performance of the system.Secondly,through the analysis of the conventional SVPWM modulation algorithm,the three-phase action time is composed of differential mode and common mode components.However,the conventional modulation algorithm regards the two as a whole to calculate the action time,which makes the solution process complicated.A simplified SVPWM modulation algorithm is introduced to obtain the reference value of the three-phase action time from the differential mode component,which makes the calculation simple and convenient.At the same time,considering the expansion of modulation degree,balance of midpoint potential and narrow pulse,corresponding solutions are proposed.Different problems are transformed into three-phase action time adjustment problems,and corresponding correction amount is added to the reference value of the three-phase action time.Thirdly,the parameter design of the traditional PI + repetitive control series compound control system is deduced in detail;the mechanism of the repetitive controller is analyzed.Considering that the repeating controller has a fundamental period delay problem,a fast repeating controller is introduced to improve the dynamic performance of the controller.The fast repeat controller can introduce different poles according to the distribution of the frequency spectrum in the reference current signal,flexibly realizing the fast and accurate tracking of the reference signal,and improving the rapidity of the system.In view of the influence of DC bus voltage pulsation on the current inner loop,a voltage outer loop control strategy based on a notch filter is adopted.A notch filter is added to the DC bus voltage feedback loop to filter out the voltage ripple signal,leaving only the DC component.Then,the influence of the DC bus voltage pulsating signal on the current inner loop is suppressed,and the performance of the treatment device is improvedFinally,the hardware and software design of the parallel three-level power quality control device experimental platform is introduced in detail,and the effectiveness of the harmonic detection algorithm,modulation algorithm and control strategy used in this article is verified through experiments.This dissertation includes 104 figures,7 tables and 82 references.