Performance of differential protection for three-winding power transformers during transient CT´s saturation

The authors present the results and conclusions from extended digital simulation studies concerned with the performance of three-winding transformer differential protection under severe transient saturation of conventional current transformers (CT). The main goal of these studies was the verification of the above discrimination criterion under all possible power system conditions, having also regard to the harmonics produced by saturated CT. As a result of the investigations, some countermeasures are proposed which assure the correct fault discriminations even for extremely distorted current waveforms