Selectivity and sensitivity of overcurrent relay protections

Overcurrent protections are the first protections that have been used to protect the equipment in the event of faults and abnormal conditions in the power system. The principle of protections operation is based on detection of the increase of the value of the current at short circuit and abnormal operating modes of the equipment in comparison with their normal mode of operation. The issues related to the fulfillment of the requirements for selectivity and sensitivity of the overcurrent protections are still relevant today, because the timely disconnection of the damaged equipment depends on the correct setting of the protection and its operation. The paper discusses the conditions for setting the overcurrent protection and how they determine the sensitivity and selectivity of these protection in medium voltage power grids.