Research On APF Based On Unbalanced Load Of Three-phase Four-wire System

The increasing demand for electric power is due to the continuous development of quality of life and the rapid improvement of power electronics.This also makes the phenomenon of power supply system current quality decline more and more serious.Thus,this thesis did some related researches on solving grid harmonic pollution and current unbalance on line side,which was in-tended to deal with the problems like harmonic pollution and current unbalance on line side in the grid system.Researches mainly included harmonic detection,modulation algorithm as well as con-trol system,in which off-line test was made by Simulink and hardware-in-loop real-time test was performed by Star Sim HIL.First of all,the harmonic detection algorithm was studied.Because the detection of harmonic current in three wire four wire system needed the extraction of zero sequence harmonic current,the internal relationship between FBD algorithm and i_p-i_qalgorithm was found in this thesis through theoretical analysis,and the detection algorithm of zero sequence harmonic current based on single-phase circuit FBD and i_p-i_qcomposite algorithm was proposed accordingly.Then,the i_p-i_q algorithm and FBD algorithm based on three wire three wire system,which extracted the fundamental positive and negative sequence at the same time,were extended to the harmonic detection of load current in three-phase-four-wire system by getting rid of zero sequence current of each phase of load and extracting zero sequence harmonic current.Furthermore,for the treatment of unbalanced current,if only the i_p-i_q algorithm was used,the fourth-order matrix change was needed,and zero sequence current on each phase needed to be re-moved before the i_p-i_q algorithm can be used.Hence,with the purpose of lowering computation of system,FBD and i_p-i_q composite algorithm without zero sequence current preprocessing were ap-plied to the detection of harmonics in three-phase-four-line unbalances load system and fundamental negative sequence and fundamental zero sequence current.As for the next step,the mathematical model of main circuit was set up on the basis of APF structure;After theoretical calculation,the capacitance and voltage of DC side and the inductance of AC side converter of main circuit were selected;The parameters of DC side current regulator,voltage regulator and voltage sharing regulator based on PI control were adjusted on the basis of the deduced open-loop transfer function of current loop and voltage loop.The voltage utilization ratio of DC side of SVPWM was higher than that of SPWM,but the two-dimensional SVPWM couldn’t control the neutral current,and the three-dimensional SVPWM modulation algorithm based on four leg had a huge demand for computing,therefore the three-dimensional space voltage vector(3D-SVPWM)modulation algorithm of upper and lower capacitor split three leg was adopted in this thesis.This thesis gave a detailed analysis to the working principle and implementation algorithm of this algorithm,which realizing the governance of neutral current.At last,Simulink simulation test and hardware-in-the-loop real-time test were made succes-sively to test and verify harmonic current,fundamental sequence,current detection algorithm of fundamental negative sequence,3D-SVPWM modulation algorithm,current control algorithm,and voltage control method.The test results proved that the above algorithms were feasible for grid side harmonic pollution and unbalanced current control of unbalanced load system.