الفهرس | Only 14 pages are availabe for public view |
Abstract Transformers are major elements of any power systems. Normally they must be properly protected by differential relays. This protection system should be precise and reliable via implementation of strong algorithms that able to differentiate between faulted and unfaulted conditions to fully guarantee power continuity. The used protection type should minimize the time of disconnection for faults within the transformer and to reduce the risk of any failure to simplify eventual repair and to increase the system reliability. The operation of the transformer under any abnormal condition such as faults or overloads compromises the life of the transformer, which means adequate protection should be provided for quicker isolation of the transformer under such conditions. a false operating current in differential relays may exist due to mismatch between the CTs ratios and the power transformer ratio, Variable ratio of the power transformer caused by a tap changer, Phase shift between the power transformer primary and secondary currents for delta–wye connections and Magnetizing inrush currents created by transformer transients where this false current may cause the relay mis- operation. The relay percentage-restraint characteristic typically solves the first two problems. A proper connection of the CTs solves the phase-shift problem. A very complex problem is that of discriminating internal fault currents from the false differential currents caused by magnetizing inrush current which will be more investigated in the proposed work study. ii Most of the early methods of differential protection of transformers are based on harmonic content of differential current. These methods are based on the ratio of the second harmonic to the fundamental component of the differential current. As for the inrush current condition this ratio is greater than that in the internal fault conditions. But the second harmonic can also be generated during internal faults on the transformers or due to saturation of CTs or due to long transmission lines so this situation may cause greater ratio of the second harmonic as in inrush current or more. The wavelet transform is a relatively new and powerful tool in the analysis in the transient phenomena because its ability to extract information from the transient signals simultaneously in time and frequency domain, rather than the conventional Fourier Transform which can only give the information in the frequency domain. This thesis presents an approach for the differential protection of power transformers. This uses three different techniques (current wave shape, time, wavelet transform (WT)) based discrimination techniques, in combination with adaptive network-based fuzzy inference system (ANFIS) to discriminate internal faults from inrush currents. The performance of the simulated model is demonstrated by simulation of different faults and switching conditions on a power transformer using Matlab/Simulink software Package. The wavelet transform technique is first applied to decompose the differential current signals through the CTs secondary side into a series of wavelet components, each of which is a time-domain signal that covers a specific frequency band. Thus, more distinctive signal features that iii represent internal faults and inrush currents are extracted. As a result, by quantifying the extracted features, a decision for distinguishing an internal fault from an inrush current in different power transformer systems can be accurately made by ANFIS in terms of the differences in the quantified features. The extensive simulation results presented show that the proposed technique needs very simple input signals , but can accurately discriminate between an internal fault and an inrush current in different power transformer systems. |