Research On Key Techniques And Reliability Of Multi-modular Active Power Filter Parallel System

With the rapid development of power electronic technology and wide application of electric power automation equipment,industrial productivity has been promoted and the level of electrical automation has been improved noticeably.However,harmonic pollution in power grid is increasingly serious.Active Power Filter(APF)is one of the main effective devices for harmonic suppression and has been popularized in recent years.Meanwhile,industrial applications have put forward more requirements such as large capacity,high power density and high reliability for APF.Thus,modular APF become an important development direction and research focus for its advantages of expansion flexibility,high reliability,etc.In this thesis,several key technologies and reliability problem of multi-modular APF parallel system are researched in depth.To achieve high compensation precision,repetitive control is adopted as the control strategy of current loop.Then its stability,harmonic tracking ability and error convergence speed are discussed.In view of the poor dynamic response of traditional repetitive control,this thesis analyzed two methods for realizing the rapidness,which are adding the reference current feed-forward channel and reducing the repetitive control cycle.To solve the adaptability problems of repetitive control in the condition with power grid frequency deviation or the non-integer value of cycle delay link N when realizing its rapidness,and the design difficulty of phase compensation link under low sample frequency,fractional thought are introduced into this thesis.A double fractional repetitive controller with frequency adaptability is proposed.By remoulding the traditional internal model into a finite number of adjacent integer-order internal model,new type of internal model based on fractional-order cycle delay link is proposed.Similarly,making a finite number of adjacent integer-order phase-lead link instead of the traditional fixed integer-order phase-lead link,fractional-order phase compensation link is achieved.Then the controller is designed in details.The classical Lagrangian interpolation algorithm is used to realize the fractionization of corresponding links.Performance comparison with traditional method is done and the stability problem of phase-lead link under low sample frequency is analyzed.Finally,experiments have verified the validity of the proposed methods.Based on above current control strategy,configuration of multi-modular APF parallel system is discussed and each components as well as the basic working principle of whole system is introduced.To improve the filtering performance of LCL filter,a novel multi-objective 4-D visualization algorithm is proposed.Then the control strategy of multi-modular APF parallel system is analyzed.Two-level sampling mode of load current is used to achieve the synchronism of each module's modulation signal.Compensation coefficient is assigned averagely to guarantee the performance consistency among modules.Besides,a current sharing control scheme based on average value adjustment is proposed to correct the current difference.To ensure the reliability,a compound reference current limit control based on compensation frequency band option is put forward.Considering the defect that every module needs a fixed physical address before bus communication,an intelligent module numbering method based on Modbus protocol is proposed and the adaptability is implemented.Finally,corresponding experiments are carried out in the three-module parallel system.Due to the complex impedance coupling among modules,single-phase Thevenin equivalent circuit is derived for analyzing the resonance problem of multi-modular parallel system easily.Then based on the deduced mathematical model,resonance characteristics in the same hardware and software parameters condition or different parameters condition are analyzed.Then kinds of resonance damping methods are discussed.Considering the defect that the conventional active damping method need extra sensors to sample the current or voltage of filter capacitors,a novel active damping method based on the high-frequency component feedback of grid-side inductor current is proposed.Then the key feedback function is designed and the resonance damping performance with high-pass filter feedback and quasi resonant controller feedback are compared.Finally,the mathematical model of multi-modular APF parallel system with novel active damping is established and corresponding characteristics are analyzed similarly.The experimental results have validated the effectiveness of the proposed method.To improve the system reliability further,this thesis studied the fault-tolerant control strategy of multi-modular APF parallel system.A switch redundant fault-tolerant scheme is designed inside module and the controllability of post-fault topology has been verified.Then four operation mode of switch redundant APF and the characteristics of post-fault voltage vectors are analyzed.The SVPWM control algorithm and its implementation procedure are derived.To solve the capacitor voltage unbalance problem of post-fault topology,a vector control method which can compensate the DC-link neutral-point potential shift is put forward.Combing with the operating characteristics of multi-modular APF parallel system,a fault-tolerant scheme among modules based on split-phase control and bus communication is proposed.The transfer mechanism of compensation capacity is discussed and the post-fault unbalanced circuits as well as its current loop stability is also analyzed.Finally,the feasibility of two fault-tolerant schemes is verified through the experiments.