Patent ID: 7002784

Claim:
A method for current compensation of a protection system for protecting a zone in a power system having a number of incoming transmission lines connected to power sources and a number of outgoing transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone and a number of current transformers (CT) responsively coupled to the transmission lines, comprises the steps of: continuously measuring all incoming currents (I in ) to the zone, continuously measuring all outgoing currents (I out ) from the zone, continuously calculating a differential current (I d ) according to I d =I in −I out , continuously calculating q=I out /I in , and continuously comparing q with set values, continuously comparing I out with a set value Set 1 , setting I out =I in at an instant when q changes and I out exceeds a value Set 1 , which is an indication of the occurrence of a fault external to the protection zone due to saturation of the CT of the faulted outgoing transmission line, and using the value I out as a compensated current I comp in the protection system in order to prevent tripping of the protection system due to external faults.