Anti-Islanding Protection in Distribution Grids

Abstract: The growing number of distributed generation in the electrical system increasesthe risk of unintentional islanding. Power system islanding is the eventthat a part of the grid is continuously powered by the distributed generation,while it is already electrically separated from the power grid[1]. Several consequencescan be caused by unintentional islanding, such as endangerment ofpersonnel, uncontrolled voltage and frequency operation and equipment damagedue to unsynchronised reclosing. Therefore, it’s important to detect islandingevents quickly and reliably.The main objective of this thesis was to investigate the performance ofthree passive islanding detection methods for Ellevio’s 50kV distribution network.These detection methodswere under/over voltage, under/over frequencyand rate of change of frequency (ROCOF). Aiming at the objective, a dynamicmodel was built in MATLAB/Simulink for Ellevio’s distribution network,where the three methods were implemented and applied to the DGs.Moreover, the performance of the methods was evaluated by the non detectionzone (NDZ) and detection time, and it was studied under several simulations.Firstly, a NDZ of each method was found for a Single Machine Infinite Bus(SMIB) system and compared with the results obtained in Ellevio’s network.Besides, the detection time was measured under various islanding conditionsto evaluate the response speed of the detection methods. Apart from the sensitivityand response speed, the reliability of the detection methods was alsoinvestigated because the detection methods should not mistakenly operate inthe case of non-islanding events. Thus, the load switchings and short-circuitfaults were simulated as the disturbance events to test the reliability of detectionmethods. In the end, all three methods were compared, and their advantagesand disadvantages were summarized.