Application of communication based distribution protection schemes in islanded systems

Present significant protective issue of microgrids is the absence of sufficient fault current, which is caused by the limitation of output of power electronic inverters. Most existing solutions are communication based methods. Two of these methods seem to have sound operation: one is using coordinated differential relays which have been prevalently installed in transmission system; the other one is based on coordinated Park transformed (abc-dq) voltage measurement which avoids using weak current signals. This paper will introduce the philosophy and calculation functions of these two protection schemes, and outline their major differences on factors of accuracy, calculation complexity, and obstacles towards practical operation. The conclusion will be drawn based on the simulations on RTDS (Real Time Digital Simulator), using the micro grid model developed from UKGDS-EHV1 standard network. In this paper, major functional elements will be shown in details, including: the inverter-based micro-generator, network and protection blocks. Finally, a new protection scheme will be proposed to illustrate the potential for improved operation.