The existing distribution system is experiencing radical changes with the rapid proliferation of Distributed Energy Resources (DERs)[1]. Microgrids have been proposed as a way of integrating large numbers of distributed renewable energy sources with distribution systems. One of the potential advantages of a microgrid is that it could provide more reliable supply to customers by islanding from the system in the event of a major disturbance.

Despite the several advantages, large-scale implementation of microgrids presents several technical challenges and barriers which have to be addressed. Deployment of microgrids powered by inverter based distributed generators such as grid-tied photovoltaics poses a threat to maintaining the security and dependability of the distribution network. Protective device coordination is considered as a critical challenge for a microgrid with high deployment of PV power. This is mainly due to the dynamically changing configuration of the microgrid, addition of fault current due to presence of DERs, bidirectional power flow, islanding and grid connected operation of the microgrid. The bidirectional power flows imposed by DERs together with continual fault-level variation caused by different operating modes (i.e., grid connected or islanded) would jeopardize the current-based protection schemes.

In view of the above issues related to microgrid protection, this thesis discusses the inadequacy of the conventional protection schemes, fault behavior of inverter interfaced distributed sources and impact on the coordination of relays. The impact of different levels of photovoltaic (PV) penetration on the protective device coordination in a microgrid has been studied in detail. A medium voltage distribution network is modelled and simulated using ETAP and several case studies have been implemented by varying the penetration levels of PV at various locations. Some of the identified impacts due to the high deployment of PV power are false tripping of feeders, nuisance tripping's, blinding of protection, and unwanted islanding. Based on the findings from the case studies potential solutions have been investigated. Most of the issues were mitigated by resetting the relays, using directional protection and current differential protection schemes. Finally, the thesis also proposes an adaptive protection algorithm to be applied with extensive communication between microgrid Central Protection Unit and the protective relays.