Research On Key Technologies Of Three-phase Grid-connected Photovoltaic System

With the advantages of high efficiency and central management, high-powerphotovoltaic power generation is becoming the important approach of solar energyutilization. As the increase of the power generated by the distributed generationsystems, its effect on the power grid could not be neglected. In order to meet therequirement of technical specification of grid-connected PV inverter, currentharmonics elimination, LVRT, three-phase voltage imbalance problem andanti-islanding are researched in this paper. On the problem of current harmonics,indirect voltage harmonic compensation control strategy and direct current harmoniccompensation control strategy are proposed. Because of the great performance ofcurrent harmonic elimination, the direct current harmonic compensation controlstrategy is adopted.Due to some uncertain situations, the short-time faults usually occur in powersystem. In general, the grid has the ability of overcome the faults and recover tonormal situation. The PV inverters are connected with grid directly and could beinfluenced by the faults. In severe cases, the PV inverters will be disconnected with thegrid. It is known that the frequent switch between the inverter and grid not only reducethe service life of the devices, but also influence the stability of network because of thesharp fluctuation in power. So during the short-time faults, the grid-connected invertershould have the ability of fault-tolerant. In this paper, according to the symmetric faultand unsymmetrical fault, the consequences caused by faults are explored and thesolutions are proposed. The simulation results show that the inverter own the LVRTcapability.Another important problem in distributed generation systems is the islandingphenomenon. When the grid fault occurs and stop work, many accidents may behappened if the distributed generations still supply power to the local loads. In order toavoid the grid operating in the islanding state, the active frequency drift detectionmethod is applied. To improve the detection speed, the disturbance direction isoptimized and acceleration factor is used. With the active detection method, the controlunit could judge the islanding phenomenon and then control the inverter disconnectwith the grid so that to protect the devices and peoples’ safety.This paper analyzed the main technologies in PV grid-connected systems andproposed the corresponding measures. For the validation, simulation and experimentalresults showed that the control strategy reached the requirements.