Research On Harmonic Detection And Waveform Control Technology Of Shunt Active Power Filter

With the development of science and technology,more and more nonlinear loads have been put into use,which has caused increasingly serious harmonic pollution problem in the power grid.Shunt Active Power Filter(SAPF),as a common harmonic control solution,can be equivalent to a precisely controlled harmonic current source.Theoretically,it can fully compensate the load harmonic current and maintain the grid current sinusoidal.At present,SAPF has been widely used in power system harmonic control,and how to further improve the compensation performance of SAPF has become a research hotspot.Aiming at the shortcomings of SAPF in harmonic detection,current control and DC voltage control,this paper carries out in-depth research.The Traditional Sliding Discrete Fourier Transform(SDFT)can directly extract any single harmonic,but the grid load current contains many harmonic components.Using traditional SDFT to detect load harmonic current,the calculation complexity is high.In addition,the detection delay of traditional SDFT is long,which is a fundamental period.In order to solve the above problems,the sequence extraction SDFT harmonic detection method and fractional comb filter harmonic detection method are proposed.Generally speaking,the load current harmonics are mainly concentrated below the 25 th order,and present in the form of characteristic sequence.Through the analysis of traditional SDFT,it is found that the detection delay of SDFT is jointly determined by the comb filter part and comb cancellation part,and the harmonic oeder extracted by SDFT is determined by the zero gain point of comb cancellation part.The sequence extraction SDFT is improved from the traditional SDFT by redesigning comb filter and comb cancellation part.The sequence extraction SDFT realizes the extraction of harmonic sequence,and has less harmonic detection delay and computational complexity.The fractional comb filter harmonic detection method further optimizes the sequence extraction SDFT harmonic detection method,removes the denominator,directly filters and decomposes the load current to obtain the required harmonic sequence.The fractional comb filter harmonic detection method avoids the zero gain point cancellation of sequence extraction SDFT.The fractional comb filter harmonic detection method has good stability and is conducive to digital realization.Simulation and experimental results verify the effectiveness of the proposed method in SAPF load current detection.The traditional repetitive signal generator has infinite gain at all integer frequencies,which affects the dynamic performance.This is directly reflected in the fact that the convergence period of the traditional repetitive controller is one fundamental period which is long.In order to improve the above shortcomings,this paper proposes a cascaded vector repetitive controller which only compensates specific sequences.Compared with the traditional repetitive controller,the dynamic performance is improved.In addition,in view of the problem of introducing poles located on the unit circle in the cascade part and the design problem of the repeated control delay part,this paper presents a general design method for the cascaded vector repetitive controller,which makes the controller more stable and easier to realize in engineering.This paper also analyzes the performance of the parallel structure repetitive controller proposed in the literature in recent years,and demonstrates the advantages of the cascade structure proposed in this paper.Simulation and experiment results verify the effectiveness of the proposed cascade vector repetitive controller.When SAPF compensates unbalanced load current,the unbalanced load current will couple a second harmonic voltage with a larger amplitude to the SAPF DC-side capacitor voltage.This second harmonic voltage will pass through the voltage control loop and become the d-axis reference command of the current control loop.This makes the current control loop reference command additional d-axis second harmonics,in addition to load harmonic currents and DC active compensation currents.The second harmonic will cause the additional third harmonic current and fundamental negative sequence current of SAPF,which makes the current of power grid distorted and unbalanced.In order to solve the above problems,this paper puts the Moving Average Filter(MAF)and its compensator in the traditional DC voltage proportional integral(PI)control loop,which can completely eliminate the influence of the second harmonic in the DC voltage.So that SAPF can accurately compensate the non-linear unbalanced load.In addition,the proposed method has good dynamic performance and steadystate performance.The simulation results verify the effectiveness of the proposed method.