A Study On Power Transformer Protection Based On Wavelet Analysis

Power transformers are most important equipment in power systems; they have a great influence on the secure and reliable operation of power system. With the development of power system and the application of modern materials, the power capacity of transformers becomes larger and larger, and the performance improvement of the transformer differential current protective relaying scheme is strongly required. This performance improvement depends on the transformer operating mechanism during the flow of magnetizing inrush currents and fault currents. The research work of this dissertation is devoted to a new differential current protective relying algorithm. This research work contains two stages; the first stage is concerning the transformer transient analysis, which is based on simulating the transformer model operating in one machine power system in order to obtain all the required transformer data concerning all its operation conditions. Simulating the transformer hysteresis loop is one of the key problems during the transient simulation of the transformer. The transformer simulation model had been built on the basis of the special quality of ferromagnetic material. Based on this simulation process, the influence of such factors as the core material, the setup of magnetic path parameters, the structure of magnetic path, and so, on the hysteresis loop can be clearly realized. Out of all transformers fault the kinds, the turn-to-turn fault and turn-to-ground faults which are most probability to occur, in addition to all other kinds of faults have been simulated. The second stage is to use the data files of this simulation work as the data source to the wavelet transform modulus maxima based protective relaying algorithm. The results of the wavelet maximum modulus shows that this algorithm as another application of the wavelet analysis in power system protective relaying field area, and for all types of faults and under various conditions of different fault resistances values source voltage phase angles, can efficiently detect fault currents and discriminate the internal fault currents from the external fault currents and also from the magnetizing inrush currents. This algorithm can efficiently discriminate the inrush currents flow experienced at the instant when an external fault is removed and also when the transformer is switched on to the supply source at no-load. Internal faults during the transformer switching on to the supply with or without load can also be distinguished. The theoretical analysis and simulation results demonstrate that this...