Fault Diagnosis And Islanding Detection Of Photovoltaic(PV) Grid-connected Inverter

PV grid-connected power generation system is mainly made up by PV arrays,grid-connected inverter and grid which are likely to have faults. Single switch deviceopen-circuit fault diagnosis of grid-connected inverter and islanding detection of thesystem are the further research contents in this thesis.The reliability of grid-connected inverter is related to the safety of grid and itself.The main circuit of grid-connected inverter is made up by a number of switch devices,and switch device has short-circuit or open-circuit fault. An open circuit faultdiagnosis method of switch device for three-phase PV grid-connected inverter isproposed accord to the characteristics of PV grid-connected inverter. The voltagebetween the grid neutral point and the negative terminal of DC side is different fromthat of normal working, when the switch device open-circuit fault of grid-connectedinverter occurs. As a character, the voltage is taken, and the grid voltage vector anglebased on the unit power factor PV grid-connected inverter is combined for referenceto diagnose the open-circuit fault device. Simulation and experimental results showthat this method can locate open-circuit fault device accurately in one current cycle.Islanding is a problem that all distributed generation systems have to face.Islanding may be detrimental to grid equipments and people. Some common passiveand active islanding detection methods are introduced. And the P×Q coordinatesystem is adopted to analyse the non-detection zone of passive islanding detectionmethods. The Qf0×Cnormcoordinate system instead of the P×Q coordinate systemwhich has some shortages is adopted to analyse the non-detection zone of islandingdetection methods that based on frequency.Active frequency drift islanding detection method has been further researched.The implementations of single-phase and three-phase active frequency drift methodsare given in detail. A variable feedback coefficient active frequency drift method isproposed to solve the output power quality deterioration problem caused by traditionalmethod. Small feedback coefficient is used when grid frequency fluctuation is small,and big feedback coefficient is used when grid frequency fluctuation is big.Simulation and experimental results show that it has little influence on the outputpower quality under normal condition, and its detection capability is prominent whenislanding occurs, and its detection time meet the requirement of national standard.