Research On Islangding Detection Of Muiti-inverters Parallel Operation Of Micro-grid

In recent years,the amount of primary energy use is deteriorating increasingly,energy depletion and environmental issues are prominent,more and more clean and green renewable energy is gradually emerged which is widely used and access to the grid.Among the numerous emerging green energy,photovoltaic power generation accounts for a large proportion,and it is more and more important for the security and reliability of the system for a grid connected power generation system with grid connected photovoltaic system.Then,it is very important for the islanding detection of photovoltaic power generation system,which is the basic guarantee of the interconnection work between the grid and distributed generation system.At the same time,with the access of photovoltaic power generation system of micro grid increasingly large scale,the power capacity of single inverter has not meet the actual needs,multiple inverters in parallel and simultaneously work has become more and more.Therefore,it has become a practical application value for islanding detection in multi machine systemFirst of all,this paper introduce the concept,occurrence principle and harm of islanding effects,and have a discuss on conditions of islanding effect and islanding detection standard using at home and abroad,then lead out islanding detection methods.There are two main classification,communication side and local side.The local detection method can be divided into passive detection and active detection,the advantages,disadvantages and application scope of these methods are compared in detail.At the same time,this paper introduces four kinds of coordinate model to describe the islanding non-detection zone.This paper chooses the active frequency shift(AFD)detection method and the active phase shift(APS)detection method as the focus of this paper.Secondly,the two methods of active frequency shift and active phase shift are discussed respectively with expounding each detection principle,the respective parameters of the design are verified by simulation.The relationship between the choice of chopping fraction and the degree of current distortion of active frequency shift is investigated.Phase expression of auto phase shift is simplified,which is extended by slip-mode frequency shift(SMS).Then,the feedback coefficient is optimized and improved by introduced fuzzy control,and the model is built afresh to verify the feasibility of the improved algorithm.Finally,as more and more photovoltaic systems are connected to the power grid,the parallel operation of multiple inverters has become a trend.In this paper,it chooses two inverters as the discussion object and designed four inverters combination methods: Passive and active frequencyshift,active frequency shift and with positive feedback,active frequency shift and active phase shift,parallel operation of two active phase shift detection method with frequency detection error.Their mutual relation and influence,their non-detection zone are discussed respectively,and the simulation results are verified in various parameter settings and blind areas which explore the influence of various factors on islanding detection performance of multiple inverters in parallel operation.