Development Of Three-phase Four-leg SVG

In recent years,nonlinear load and power electronics devices were widely usedin the low-voltage power supply system,so load balancing and harmonic problemsappear gradually in three-phase four-wire system,and zero sequence current to theharm of power grid is particularly prominent.According to these problems such asload imbalance,zero sequence,negative sequence,harmonic elimination,reactivepower compensation and other issues in the three-phase four-wire grid,we haveresearched and designed reactive power compensation control system of three-phasefour-leg SVG.Firstly,this paper introduced the background of reactive power compensation inthe power system,the research significance,the harm and the developmentstatus;Then expounded the working principle of SVG,and introduced three commonmain circuit topology of three-phase four-wire SVG.By comparing the advantagesand disadvantages of each structure,we chose the three-phase four-leg topologicalstructure which is more simple and practical;Then compared and analysed severalcommonly used reactive current detection methods in the three-phase four-wiresystem,and took zero sequence current separation based on the theory ofinstantaneous reactive power which is improved from ip-iq current detection.Thismethod can well detect the reactive current in three-phase four-leg SVG system.Thenthe control strategy of three-phase four-leg SVG main circuit was studied,and chosethe method of3D-SVPWM to compensate current,and briefly introduced the spacevector modulation in the α-β-γ coordinate system.This paper focused on the principleof three-dimensional space vector modulation in a-b-c space coordinates,and switchvector selection,duty ratio calculation,as well as arrangement of this method.Thecalculation formula of duty ratio which is deduced indicated that compared with thespace vector modulation method in the α-β-γ coordinate system,the algorithm in a-b-c space coordinates is simpler and easier to digitize.At the same time,the controlstrategy of main circuit adopted PI regulator to stabilize the DC voltage.Using MATLAB to simulate and analyze the reactive current detection method and control strategy of the main circuit which was adopted by this paper,and thesimulation results were satisfactory.Finally, with TMS320F2812as the corecontroller,we designed hardware circuits and software programs of the three phasefour-leg SVG control system,and built the experimental platform.The test resultsshowed that the design method chosen in reactive current had a good compensationeffect,as well as three-phase unbalanced current,thus eliminating impacts of zero-sequence current.