Connection of many distributed generations (DG) has a major impact to the setting of numerical overcurrent relays since the power produced by wind power plants or photovoltaic power plants varies considerably. Therefore, in order to not exceed the limits of transportable power in certain sections, in the case of unavailability of a 110 kV line, territorial power dispatcher imposes operating regimes that have consequences to numerical overcurrent protections. Since power system is a dynamic system, changing the source impedance ratio (SIR), affects sensitivity, selectivity or in some cases can lead to the impossibility of starting the numerical overcurrent relay. Hence, in order to improve the functionality of a numerical overcurrent protection, under constant modification of the source behind the relay, the paper is proposing an adaptive overcurrent algorithm based on the phase to phase differential currents (PDC). As a result, the relay is sensitive to all types of phase to phase short circuits and the protection area is improved through the recognition of equivalent impedance source.

Keywords: Network topology, Relays, Fault currents