Research On Some Key Techniques On High Performance Modular Shunt Active Power Filter

With the large-scale use of power electronics and power automation equipment in industrial field,the level of electrical automation and power production efficiency have increased,and the resulting power quality problems have become increasingly serious.The Shunt Active Power Filter(SAPF)is one of the main devices for power quality migitation.In recent years,it has been put into use in a large number of industrial fields.However,with the complexity of power quality issues found at the industrial field,the higher requirements on SAPF performance.At the same time,the modular SAPF has been widely used,due to its reliable and flexible capacity expansion,in the industrial scene where harmonics are greatly increased.Therefore,how to develop higher performance and higher reliability modularity SAPF system has become the research hotspot.In this thesis,several key technologies on SAPF higher performance and modular technologies are researched in depth.As the first step of SAPF control,the harmonics detection technology determines the performance of SAPF.In this paper,the sliding window Discrete Fourier Transform(SDFT)harmonic detection algorithm often used by SAPF is studied.The discrete transfer function is analyzed in depth,it is found that comb filters and the resonators included in the SDFT algorithm are the fundamental reasons for the long delay and poor stability of the SDFT algorithm.For the comb filter,this paper reconfigures the comb filter method and removes the useless zeros,which improves the dynamic performance of the SDFT.At the same time,the zeros of the comb filter are moved to the specified frequency by rotating the zeros.Aiming at the stability problem of SDFT algorithm,the pole of the eigenfrequency resonator containing the twiddle factor is moved to the real number 1 through a pre-rotation method.This eliminates the SDFT stability issues when quantizing the eigenfrequency resonator.Then for the three characteristic loads,the corresponding improved SDFT harmonic extraction algorithm is designed in detail.Finally,experiments verify the effectiveness of the improved SDFT algorithm proposed in this paper.For the harmonic tracking technology,the most important link of the SAPF control system,this paper introduces the repetitive control as the control strategy.Focusing on the intrinsic deficiencies of a cyclical delay in the internal-mode generator of the repetitive control,this thesis proposes a general improvement method for the internal-mode generator by reconfiguring the pole of the internal-mode generator,and then proposes a flexible and universal fast repetitive control design method.Afterwards,the stability,the ability of harmonic tracking,and the speed of error convergence of the improved fast repetitive controller are analyzed in depth.Based on the analysis results,a double-loop control structure with PI+ repetitive control is designed using the typical plug-in structure.A detailed design method of a universal fast repetitive control compensation link is given.Then,' according to the three typical harmonic loads,the corresponding fast repetitive controllers are designed in detail and the digital implementation methods are given.Finally,the experimental conditions for different load conditions are designed,and the experimental.verification of the universal fast repetitive controller is completed.The experimental results show the effectiveness and high perfomace of the proposed method.When SAPF works,there is unavoidable ripple on the DC link,which will affect the compensation performance of SAPF.Therefore,in this paper,a unified mathematical model is established and the DC-link ripple model of SAPF under all compensation conditions is deduced.In order to eliminate the effect of DC-link fluctuations on the compensation performance of SAPF,a DC-link ripple compensation circuit is designed in this paper.Through detailed hardware and controller design,the amount of fluctuations on the DC-link of the SAPF is absorbed into the DC-link ripple compensation circuit,thereby eliminating the DC-link fluctuations on the SAPF.For the problem of unbalanced upper and lower capacitances of three-phase four-wire three-level SAPF,this paper analyzes the mathematical model between the neutral current and the unbalanced voltage,and elicites the voltage balance factor based on the positive and negative level duty cycles.The modulating wave was used to calculate the voltage balance factor over the entire cycle,and the neutral current required for voltage balance was calculated.The calculated neutral current is added to the control loop,which eliminates the problem of uneven voltage distribution between the upper and lower capacitors.Finally,a corresponding experiment was designed for DC-link ripple absorption and DC-link balance control.The experimental results verify the effectiveness of the proposed method.After researched the above techniques for improving the performance of a single-module SAPF,this paper studied the modular parallel SAPF system.Firstly,this paper analyzes the Norton equivalent circuit of the modular parallel SAPF system,and finds that the various modules in the system have a complex coupling relationship with the grid.In order to reduce the effect of coupling,the hardware and software parameters of each module must be as consistent as possible.Further,when the hardware and software parameters of each module are the same,the analysis model finds that the grid impedance increases for each module in the system,and the increase multiplier is the same as the number of modules in the system.Therefore,this thesis gives the limitation of the number of modules in the system,and ensures that each module in the system can work normally under the effect of grid impedance amplification.According to the mathematical model of the modular parallel SAPF system,a centralized control strategy for the system is designed in this paper.Each module in the system is controlled by the upper controller.Based on the centralized control,the load current sampling method and the current-sharing control based on the average current-sharing coefficient is designed in detailed,which ensures the consistency of each module.At the same time,in order to ensure that each module of SAPF runs within its own capacity range and guarantees reliability,a compoud current limiting control method is proposed.By using the advantages of proportional limiting and truncation limiting flexibly,the risk and distortion brought by current limiting will be solved.Finally,in the development of a three-module parallel SAPF system,the verification experiment of the proposed method was passed.