Protection of micro-grid in a power system

Abstract

Nowadays because of increasing integration of distributed energy resources (DER) such as wind turbine generators, photovoltaics (PV), fuel cells, etc., the existing power supply system is undergoing radical changes. DERs with locally connected loads form subsystems within the main grid called microgrids. As the microgrid becomes an important part of the distribution system, the issues and challenges arising from its implementation must be addressed. Designing a comprehensive protection system for a microgrid is an important technological problem. The protection system must meet general specifications such as sensitivity, reliability, and selectivity for both grid-connected and islanded mode of operation. Conventional protection devices based on the overcurrent method cannot provide comprehensive protection for the microgrid, therefore an adaptive protection system is required. this adaptive protection system must be sensitive in order to detect and clear even the minor fault currents. In this thesis a protection solution for both operational modes (grid-connected and islanded mode) is presented. Given the problems related to the protection of microgrids, this thesis analyses the in-adequacy of the conventional protection scheme and the fault behaviour of microgrids assuming distributed generators are present in both grid-connected and islanded mode operation and discusses their impact on the relay’s coordination. The magnitude of the fault current in a microgrid is reduced dramatically during transition from grid-connected to island operation. The adaptive relay, which identifies the operating mode and changes its settings to match the predicted fault current levels, is one solution to this problem. This thesis illustrates the development of an adaptive relay mechanism for a microgrid. This technique is realized by intelligent relay models that can recognize the microgrid's operational mode. The relays and power system are modelled using MATLAB Simulink. The results reveal that the relay system adapts its configuration by detecting the microgrid operating mode with a minor delay.