Detection And Control Technology Study Of Shunt Active Power Filter

Along with development and improvement of the industry and society,more and more power electronics devices are used,which inject many harmonics into the power system.As an efficient device to deal with this problem,Shunt Active Power Filter(SAPF) has received widely attention.Normally,a SAPF control system can be divided into four main parts: PhaseLocked Loop(PLL),Harmonic Detection,output current controller and dc-side voltage controller.The conventional Synchronous Reference Frame PLL(SRF-PLL) suffers from a very poor performance when grid under unbalanced and harmonically distorted conditions.To overcome this problem,this paper proposes two PLL structures,the first one is called Cascaded Least-Error Squares Filter-based PLL(CLES-PLL),and the second one is a openloop LES Filter-based PLL(OL-LES-PLL).The proposed two methods focus on improving the ability of disturbance rejection and dynamic performance,respectively.The harmonic detection method based on Generalized DFT(GDFT) has the advantages of high detection precision and fast dynamic performance,but it needs big computation resource when it is used to detect many harmonics.To solve this problem,this paper proposed a new harmonic detection method called Quasi-GDFT(QGDFT),which reduces the calculations while ensuring harmonic detection accuracy.By extending QGDFT to Decoupled-QGDFT,this paper proposed single-phase harmonic detection method based on Decoupled-QGDFT,which also has advantages of high detection precision,small calculations and fast dynamic performance.The conventional compound control strategy based on PI and Repetitive Control(RC) has complex digital implementation and slow dynamic performance.To solve these problems,firstly,this paper extends CRC to Vector RC(VRC),then proposes a compound control strategy based on PR and VRC.The proposed control method reduces the complexity of the control algorithm and improves the flexibility and dynamic performance of the controller.Aiming at the problem that the DC-side second harmonic voltage affects the compensation ability of SAPF for unbalanced current when the load highly unbalanced,this paper presents a DC side voltage control method based on Moving Average Filter(MAF).The proposed control method can completely suppress the second harmonic without reducing the bandwidth of DC voltage loop.The system ensures the unbalanced current compensation ability of SAPF and the dynamic performance of DC voltage loop at the same time.