Islanding Detection And Islanding Scheme Of The Micro-grid

The needs to sustainable development of electricity, improve energy efficiency and reduce environment pollution have favored the development of Distributed Generation (DG). But the problems come with increasing DG penetration in Distributed network. Micro-grid solves the problems, gives full play to DG, improves power supply reliability, and has no pollution, saves investment, because of its flexible and intelligent control. So Micro-grid which is the effective supplement of the modern tremendous power system represents the development trend of Distributed Network. In order to fully exert the advantages of Micro-grid, improve power supply reliability, it's better to realize the seamless transition form interconnected to islanding. The appearance of Non-intentional islanding affects personnel and equipment safety, so the study of islanding detection and islanding division has important significance.At present, islanding detection method resident in the DG unit can be categorized as internal passive, internal active and external switch state detection. Internal passive methods are simply and have no impact to power quality, but the Non-Detection Zone (NDZ) of which is large, and the threshold is hard to determine. Internal active methods have high sensitivity, can reduce Non-Detection Zone (NDZ), but affect electrical quality, and the detection time is long. Switch state detection, which is based on communication channels, with high cost, have a good development prospect with the development of intelligent distributed network.The paper analyzes the advantages and disadvantages of existing islanding detection methods,and the reason that islanding can not be easily made out with balance between active and reactive power generation and consumption. Then according to the characters of internal active detection methods, the paper brings forward an inverter islanding detection method based on frequency-reactive feedback. Through the study on the control and feedback principle of DG based on inverter, the paper combines frequency-reactive feedback and dq transformation mechanism, puts forward the islanding detection method. In fact, with feedback it increases the unbalance between reactive power of DG output and reactive load, which will result in frequency changing in the same direction, exceeding frequency limits finally. The theory analysis and experimental simulation shows this method is simply to realize, can reduce the NDZ effectively and also is suitable to multi-inverters, only have small effect to power quality. In all the islanding detection method can be used in Micro-grid.The thesis also introduces two types of islanding scheme at present: intentional islanding division and dynamic islanding division. Through summarizing the existing islanding algorithm, put forward existent lack and prospect the development of future islanding scheme.