الفهرس 
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Abstract
Electrical power network is a system that consists of several electrical components that are able to produce electrical power. The hybrid AC/DC microgrid adapts to the trend of distributed power supplies grid connection and has a very broad space for development. A three-phase grid-connected photovoltaic system can be used in order to help grid network in supplying sufficient power for load demand. The
modelled PV system using PSCAD software could give acceptable result for case
studies conducted in this thesis. The AC and DC buses can be linked through a three-phase transformer. In typical power system networks, one of the most important components is the power transformer whose protection plays a dominant role to overcome power system challenges. The stability and better performance of power transformers even when they face different transient signals, are the main purpose of all protection schemes. In fact, the differential protection approach is considered as the standard protection of all power transformers. It is well known that the initial protection applied to transformer should not take any action during inrush condition even including high level of second harmonic components. In addition to this, the transformer protection should not trip while external fault action near or far away from transformer neutral point even including CTs saturation. The trip action should only operate when internal fault occurrence through the transformer involving both turn to earth faults (TEF) and turn to turn faults (TTF). Investigation of differential current signals only support few features
of fault resulting in false trip to the relay. Moreover, the implementation of discrete wavelet transform (DWT) with its simplicity enhances the differential protection and gives high performance in fault detection involving all transient states that may occur. A practical implementation of DWT based on detail and approximate coefficients analysis is proposed and monitored to identify different internal fault conditions (turn to earth and turn to turn fault) in power transformers and discriminate between external faults and internal faults also, between inrush condition and internal faults. As they may affect the differential protection and the relay operation. Meanwhile, the proposed fault identification scheme is evaluated according to different fault specifications such as an internal turn to earth fault, an internal turn to turn fault. In addition to this, an external 3-phase fault and single line to ground fault are investigated considering the impact of current transformer (CT) saturation. Moreover, the proposed scheme is tested under a transformer inrush current with and without evolving internal fault. All the simulated results are done on ATP with different fault ratios ranges from (10% up to 90%) for internal turn to earth fault with range of fault resistances (0.01 Ω up to 300 Ω) and with different fault ratios (15% up to 65%)
in case of internal turn to turn fault. Also, external fault is analyzed under different fault resistances (0.001Ω to 150Ω). Considering the process of fault identification in AC grids is complicated and subjected to new features. After all the presented characteristics are verified for a new AC and DC hybrid system. A three-phase grid connected PV model is designed using PSCAD software suppling sufficient power for the AC grid connected. For the hybrid AC and DC grid network, several practical faults are implemented and applied to the system out of which the results and protection procures are proposed. While, the studied cases give acceptable results for fault identification and fault localization
process which improves the efficiency of overall protection system application.