An observer-based protection system for microgrids

Microgrids work in two modes, grid connected and islanded. Short circuit levels in islanded mode tend to be small compared to those in grid connected mode. Moreover, power flows in microgrids are not always unidirectional. For these reasons, it is difficult to protect microgrids with using relaying strategies traditionally used in distribution systems. In this paper, a new technique is presented which uses a state observer as a fault detector and a filter to identify faults that occur within the zone of protection. This system can be centralized or decentralized. It needs only one current measurement and two ends voltage measurements for each zone. Using suitable voltage sensors, and, where available, smart meters to measure voltages, it is possible to devise a low-cost protection system using the proposed approach. The performance of the proposed method in protecting a small test system is demonstrated on a Real Time Digital Simulator (RTDS). The proposed approach is shown to be effective in grid-connected and islanded modes.