s. gh. seifossadat

Over current relays that are used commonly in distribution system can't detect high impedance fault. Therefore for that purpose it is necessary to design special algorithms. In this paper, a method based on harmonic analysis is presented. In this technique, the proposed relay, after fault occurs, extracts the required harmonic and according designed algorithm, occurrence of fault is reported. Software of this relay for different scenarios of high impedance faults was tested; also its hardware of relay in laboratory tests could correctly identify events.

In this paper, a new method is presented for discrimination of internal fault other abnormal conditions in power transformers. Since the differential current signals are non-stationary, a powerful tool such as S transform is needed for time- frequency analysis of them. First, by considering the effective factors, different simulations for internal fault, inrush current, external fault, over-excitation and ultrasaturation conditions are performed via PSCAD/EMTDC software in different operating conditions of transformer.Then, the obtained differential current signals are analyzed by modified hyperbolic S transform. Important characteristics are extracted from modified hyperbolic S transform and according to them, different conditions are distinguished and classified. These features include: energy contour of first level, variance index, standard deviation of the fundamental frequency and second harmonic component and three special criteria. The process of modified hyperbolic S transform and performing the proposed algorithm are performed in MATLAB software. In this paper, all of possible transient conditions and effects of the saturation of current transformers are considered. Finally, the accuracy and precision of the proposed method are compared with different methods in conditions with noise and without noise. The results show good performance of the proposed method.