Algorithm for a digital transformer differential protection based on a least-squares curve-fitting

An inrush-current detection algorithm is described. The algorithm is based on a least-squares curve fitting of the sampled differential current by a decaying DC and five harmonic components. Inrush is detected on the ratio of the second harmonic and the fundamental component of the differential current. The sampling frequency and data window are chosen in close relation to noise amplification and frequency characteristics of the digital filters developed. In spite of the rather detailed curve fitting the calculation burden is kept to a minimum by a suitable choice of the time reference. The algorithm is tested offline with simulated fault data