Differential protection of microgrids with central protection unit support

Abstract

Deployments of Distributed Generators (DGs) have large impacts on the structure of electrical networks. In order to tackle these issues, the `Microgrid' concept has been proposed. However, due to their unprecedented structure, these smaller grids experience very significant protection issues. Conventional fault current protection schemes cannot be used and should be modified due to the existence of generators at all levels of the distribution system. Furthermore, two distinct operating modes (grid connected and islanded modes) exist in microgrids causing the fault currents in a system to vary substantially. It is also a challenge to operate Inverter Interfaced DGs (IIDGs) and estimate their fault currents. Consequently, differential current protection gains more attention for the microgrid protection. Differential protection is very versatile since it does not require prior knowledge of fault currents. The dynamic structure of the microgrids, ever-expanding networks and new deployments would require such a versatile protection. This paper presents a conceptual design of a protection system which employs a microgrid central protection unit (MCPU) to coordinate differential current protection. This system is also used to accommodate multi-terminal differential protection and propose a roll-back strategy, in case of communication failure. The proposed system is also compared with the differential protection from a reliability perspective.