Adaptive setting of protective relays in microgrids in grid-connected and autonomous operation

In this paper, the capability of the numerical relays within the microgrid is sophisticatedly used to adapt the settings of the relays to the status of the microgrid. Different settings for the overcurrent relays are calculated off-line and saved in the relays. Whenever the microgrid is disconnected from the grid, the settings are automatically changed to the associated group of settings. This would guarantee that the faults with much smaller short circuit levels are detected by the overcurrent relays in the microgrid. The status of the microgrid is transferred by a communication link that is normally available in microgrids to the relays. The status could be identified by the operation of the main switch that connects the utility-grid to the microgrid. A sample microgrid is presented to show the details of the proposed method. Firstly, the overcurrent relays within the microgrid are set in grid-connected mode, and then the settings are calculated in autonomous operation. It is shown that for a fixed setting of the relays, the coordination is not achieved, while the transfer of the settings to a new group would solve the issue efficiently.