Study On Gird-connected Photovoltaic Generation System And Method For Islanding Detection

Since that solar energy is an inexhaustible and reliable green energy, thevigorous development and utilization of solar energy is the only road to build a cleanand sustainable energy system, and photovoltaic power is the most convenient waywhich has received extensive attention and made great progress in recent years.Therefore, the in-depth study of grid-connected photovoltaic system has extremelyimportant practical and historical significance in saving conventional energy,protecting environment and promoting economic development. However, theconnection of photovoltaic system to the grid will bring negative influence to theoperation of grid, among which the greatest technical challenge and potential dangeris islanding effect. Hence, islanding detection and protection is a high-profile topic atpresent.This paper mainly studies the islanding detection methods for grid-connectedphotovoltaic system and grid-connected inverter. It first sets up a simplifiedsimulation model of grid-connected photovoltaic system to analyze its main parts.Based on the analyses, this paper chooses the basic DC/AC full-bridge inverter wherevoltage source inputs and current source outputs, and adopts the PWM current controlmethod for harmonic compensation. The simulation lays a good foundation for thestudy of islanding detection methods.This paper then introduces various passive and active islanding detectionmethods and analyzes their detection blind area, reliability, and whether they aresuitable for grid-connected inverter in detail. Through the comprehensive analysis ofthe methods, the Active Frequency Drift method (AFD) is chosen. The author buildsthe model of grid-connected PV system and uses the AFD method to conduct thesimulating calculation and testing. On this basis, the paper analyzes the detectionblind area existing in AFD, and the influences of chopping fraction (cf)’s value to thetesting time, the output current harmonic distortion (THD), and the blind zone (NDZ)size, etc.In view of the shortcomings of AFD, this paper puts forward an islandingdetection method based on q-axis disturbance signal injection. The theoreticalanalysis and simulation results have verified the feasibility of this method. Thismethod has very small blind zone and when photovoltaic system is operated ingrid-connected mode, it has the advantage of tiny harmonic distortion and good power quality. Moreover, as soon as the islanding phenomenon occurs, the q-axis disturbancesignal will promptly make the frequency shift out of the detection blind area untiltrigger the islanding protection. The stimulation results show that the detection time issmaller than2s and has little influence on the electric energy quality, which provesthe feasibility, rapidity and validity of the method.