This thesis presents a simple, yet a clever way of using the current phase angle to develop low bandwidth communication-assisted line protection strategies for medium and low voltage AC microgrids, particularly those with multi-inverter interfaced distributed generators. It is now a trend in both AC transmission and distribution segments of power network that inverters interface renewable energy to the system. Unlike synchronous generators the fault feeding, and control characteristic of these generators are different and mostly influenced by the topology, switching, control deployed in the power electronics interface. The limited and controlled fault current challenges the existing conventional protection schemes. Offering higher power supply reliability and system resilience than conventional radial distribution systems, multi-inverter based microgrids, particularly those with loop and mesh typologies, are characterised by bidirectional power flow. This further constrains traditional protections such that communication-less protection schemes become ineffective for such systems. So unit protection types, such as differential protection, become more technically suitable for such microgrids despite the necessity for a communication system.
In this thesis, two current direction based protection schemes for medium voltage islanded microgrids have been developed. The change in current flow direction in a line is detected using the cosine of the positive sequence current phase angle. Expressing the change and no-change of the flow directions as binary states, a low bandwidth communication based protection scheme is proposed comparing the binary states from local and remote ends of the line. To further enhance the scope and reliability of this scheme, a second protection scheme is proposed in Chapter 7 whereby the cosine function is combined with the rate of change of the slope of the phase angle (ROCOSP). This combination allows the detection and isolation of a fault even with the failure of the communication channel between relays protecting a faulted line. Furthermore, these scheme can work together and share the communication infrastructure as primary and backup protections. The performance of these schemes was assessed through simulations of microgrid models developed in Matlab/Simulink.