Method and device for power system protection

The present invention relates to a method for protecting a zone in a power system, which zone comprises a number of transmission lines connected to power sources and a number of transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone, wherein the method comprises the steps of: continuously measuring all the incoming currents (Iin) to the zone, continuously measuring all the outgoing currents (Iout) from the zone, and continuously calculating the differential current (Id) according to Id = Iin - Iout. The method is characterised in continuously integrating Iin, Iout and Id according to Formula (I), where T is the fundamental frequency cycle, whereby changes of the continuously integrated values IOUT< and I>D< constitute indications of whether faults on the power system occur within or outside the zone. The present invention also relates to a device and computer program product for performing the method.