New magnetizing inrush restraining algorithm for power transformer protection

Large power transformers belong to a class of vital and very expensive components in electric power systems. Accordingly, high demands are imposed on power transformer protective relays. The operating conditions of transformer protection, however, do not make the relaying task easy. Protection of large power transformers is one of the most challenging problems in the area of power system relaying. Magnetizing inrush inhibit is one the issues. Traditional second harmonic restraining technique may face security problems as the level of the second harmonic may drop below the reasonable threshold setting (around 20%) permanently or for several power system cycles during magnetizing inrush conditions. This is particularly true for modern transformers with magnetic cores built with improved materials, but it has a bearing upon old designs as well. Numerical relays capable of performing sophisticated signal processing functions enable the relay designer to re-visit the classical protection principles and enhance the relay performance, facilitating faster, more secure and dependable protection for power transformers. A new magnetizing inrush restraining technique presented in this paper uses the angular relationship between the first and second harmonics of the differential current. Thus, the technique adds a new dimension to the traditional approach that measures the magnitude ratio only between the fundamental frequency component and the second harmonic.