Research On Harmonic Detection And Control Method In Shunt Active Power Filter

The non-linear loads which are increasingly applied in power grid lead to serious harmonic pollution recently.A common solution to this problem is utilizing the shunt active power filter(SAPF)in power grid.In the distorted grid with the complex loads,it is difficult for the traditional phase locked loops(PLLs)to accurately detect grid phase,with the result that the output current of SAPF can not be synchronized with the load current,and then SAPF can not achieve satisfactory harmonic compensation effect.On the other hand,the traditional harmonic detection technologies and current control strategies are difficult to make SAPF obtain fast dynamic response speed while ensuring good steady-state accuracy.Therefore,it is necessary to study new phase-locked loop,harmonic detection technology and current control method to improve the harmonic suppression performance of SAPF.This dissertation makes deep researches on above key technologies in the SAPF,and the main research contents are as follows:First,a method of trigonometric function delay signal cancellation(TFDSC)is proposed,it can not only filter out arbitrarily specified single harmonic sequence,but also cancel out multiple harmonic sequences by cascading multiple operators.This method serves as a fundamental basis for following researches,such as PLL,harmonic detector and vector repetitive controller in this dissertation.When the grid voltage contains a lot of harmonic components,the conventional PLL faces three difficulties: high accuracy,fast tracking performance and low computational complexity.To this end,a PLL based on extended trigonometric function delay signal cancellation(ETFDSC-PLL)is propsoed in this dissertation,and this method can detect grid phase by completely filtering other harmonic components.Moreover,the proposed PLL can be applied to different grid conditions through flexibly designing its parameters.Finally,the proposed PLL's effectiveness is verified by simulation and experiment.In complex non-linear load environment,the traditional selective harmonic detection faces the difficulties similar to those in PLL.To o vercome these difficulties,a new method of single harmonic detection is proposed,this method can extract the desired single harmonic component by completely filtering other harmonic components,and it can be used in different load conditions by flexibly designing its parameters.However,when the desired signal contains a large number of harmonic components,the calculation of the single harmonic detector is very heavy.For this reason,a method of harmonic sequence detection is proposed for the first time,it can extract the desired harmonic sequence by completely filtering other harmonic sequences with low computational complexity and fast response speed,and it can also extract multiple harmonic sequences step by step.In addition,in order to significantly reduce the computational complexity of harmonic detector,a method of indirection harmonic detection is proposed,it can separate all the harmonic components by directly filter out the positive-sequence fundamental components.These three proposed methods are suitable for different applications according to their respective advantages.Finally,the proposed harmonic detection methods' effectiveness is verified by simulation and experiment.Combined with the proposed trigonometric function delay signal cancellation and the traditional repetitive control,a method of vector repetitive control(VRC)is proposed in this dissertation.On the one hand,the window of VRC can be designed flexibly according to different harmonic reference signals.On the other hand,harmonic reference signal can be divided into different harmonic sequences by the harmonic sequence detector.Then a method of the harmonic sequence-type VRC can be derived by the combination of them.This method can markedly reduce the window of VRC,then its tracking performance can be promoted fast,which overcomes the problem of slow dynamic response of traditional RC.Finally,a vector compound control is proposed by paralleling PI control.By comparing with traditional compound control,simulation and experimental results have shown that the proposed control scheme can not only make SAPF system achieve good steady-state accuracy,but also further improve its dynamic performance.In addition,the proposed PLL,harmonic detector and vector repetitive control can also be applicable to other grid-connected converters.