Research On Frequency Drift Islanding Detection Of The Micro-grid

In recent years, with the rapid development of Distributed Generation (DG), more and more distributed power systems are connected to the power grid. Microgrid with its smart and flexible control characteristics, can not noly solve the problem of large scale DG coupling, improve power supply reliability and energy efficiency, but also meet the requirement of system load increasement and consistent with the electric power sustainable development. The islanding operation mode of Microgrid includes non-intentional islangding and intentional islangding, no matter which model Microgrid in, successfully detect the islanding is a prerequisite for researching Microgrid, So it has great practical significance to study the technology of islanding detection. With this background, this thesis tries to study the islanding detection methods around the following aspects.(1) On the basis of studying the islanding features of Microgrid and relative islanding detection technology, came to the conclusion that the most serious islanding is occurred under the condition that active power is balanced in the islanding system, the resonant frequency of load is equal to the grid frequency and Qf=2.5.(2) Based on reading lots of relevant literature, summarized the existing islanding detection methods, and researched some typical islanding detection methods, analyzed each merits and defects by contrast. This thesis thoroughly studied the detection principle of frequency drift islanding detection methods, and pointed out that the root cause of detection failure for it is that the frequency offset angle caused by detection algorithm has been offset by load impedance angle.(3) None-detection zone (NDZ) is an important index to evaluate the performance of islanding detection methods. Aiming at the disadvantage of existing description space of NDZ, this thesis refers to a new space coordinate Qfg×Cnorm and researched the NDZ of SMS、AFD and AFDPF. Taking the NDZ of AFD and AFDPF for example, the accuracy of NDZ in the space coordinate Qfg×Cnorm is verified by simulation. According to the defects of frequency drift islanding detection methods, this thesis proposes an optimization AFDPF islanding detection algorithm based, and the result of simulation is studied.(4) Introduced current harmonic is the other important index to evaluate the performance of islanding detection methods. This thesis studied the influence of AFD and optimized AFDPF brought to power quality, came the conclusion that the total harmonic distortion (THD) increment of frequency drift islanding detection methods brought to is approximately equal to the chopping fraction of detection algorithm, that is to say △THD≈cf. For normal frequency fluctuations of power grid caused harmonic distortion increasing, this thesis proposed a new improved and low △THD frequency drift islanding detection algorithm, and verified the superiority of it by simulation.(5) In order to research the interaction of the detection performance under the condition of multi-inverter in parallel operation, this thesis first studied the dilution effects of detection performance between passive islanding detection methods and frequency drift islanding detection methods in space coordinate Qfg×Cnorm, and studied the influence of detection performance caused by detection error of frequency deviation △f...