The Research On Key Issues Of Three-phase Four-wire APF

The birth of fully-controlled switching devices has enabled the rapid development of power electronics technology.However,with the extensive use of nonlinear power electronic devices,a large amount of harmonic currents are injected into the power grid.In addition,reactive current and three-phase unbalanced current also make the power quality of the power grid face severe challenges.Active Power Filter(APF)can effectively reduce the harmonic content,reactive content and three-phase current unbalance of the power grid,and has a good and obvious effect on improving power quality.The theoretical research of APF has attracted extensive attention from scholars at home and abroad.The three-phase four-wire three-level APF capacitor neutral point is connected to the grid neutral line,which can additionally compensate for zero-sequence harmonics,which is more in line with the actual working conditions.The three-phase four-wire APF topology introduced in this article is a Neutral Point Clamp(NPC)three-level converter.First,construct a three-phase four-wire three-level converter mathematical model and analyze the operating principle,introduce the neutral point voltage balance balance mechanism and several commonly used modulation strategies.In this paper,the command current extraction method based on the traditional instantaneous reactive power theory and Fourier transform theory can be used to effectively extract the harmonic current,reactive current,negative sequence current and zero sequence current of the three-phase load,and analyze the APF compensation principle.Based on the synchronous rotating coordinate system to separate the positive and negative sequence of the grid voltage,this paper adopts a grid phase correction strategy based on forgetting iterative filtering.The inner loop command current tracking uses Deadbeat Control(DBC).By analyzing the three-phase four-wire DBC control principle and control model,the DBC control strategy based on digital chip calculation is given.Aiming at the problem of traditional DBC tracking current,this paper adopts an improved DBC algorithm for current prediction correction and current error compensation,which can effectively improve the compensation accuracy.Finally,by constructing a three-phase four-wire APF experiment platform based on LCL filter,the effectiveness and feasibility of the control strategy proposed in this paper are verified.