Transformer is an expensive device and one of the most important parts in a powerudsystem. Internal faults can cause a transformer to fail and thus, it is necessary for itudto be protected from these faults. Protection doesn’t mean that it prevents damage toudthe protected transformer but it is to minimize the damage to the transformer asudmuch as possible, which consequently minimizes the subsequent outage time andudrepair cost. Therefore, fast and reliable protection system should be used for limitinguddamages to the transformer by rapidly disconnecting the faulty transformer from theudnetwork, which also leads to the elimination of the stresses on the system itself andudpreventing damage to adjacent equipment.udThe main aim of this thesis is to propose transformer protection technique that isudfast and highly sensitive to internal faults that occur inside the transformer, toudovercome the problems of current transformer saturation and inrush current, and toudmake it immune to the external faults (through faults) that occur outside of theudtransformer protection zone.udThe current transformer saturation and inrush current are significant problems sinceudthey cause malfunction of the protection system, which consequently will disconnectudthe transformer because they are considered faults. This improper disconnection ofudtransformer is not desirable as it shortens its life time. So the proposed protectionudtechnique was designed to be fast and to avoid maloperation caused by saturationudand inrush current.udThe proposed protection technique was based on current signal processing. Threeudmethods, namely the application of correlation coefficients, current change ratioud(CCR) and percentage area difference (PAD) were proposed based on practical andudsimulation tests. These techniques were successfully proved by carrying out tests onudSimulink models using MATLAB/SIMULINK program and on a practicaludlaboratory model.udIn transformer transient state, the response time for the methods that were used toudaddress the problem of inrush condition, was 10 ms for CCR when transformer wasudon no-load and 5 ms for PAD when the transformer was on-load. This response timeudCurrent Signal Processing-Based Techniques for Transformer Protectionudvudis faster than the most popular method relying on second harmonic, which needs atudleast one cycle (20ms in 50 Hz systems) to recognize the condition. In transformerudsteady state, it was proved that the proposed correlation method was capable ofuddetecting the internal faults successfully within a very short time, ranging from 0.8 toud2.5 ms according to the type and severity of the fault and in addition was able toudovercome the problem of current transformer (CT) saturation.udThe contribution of this research is the development of a transformer protectionudtechnique, which is simple in design, fast and reliable in fault detection and at theudsame time capable of overcoming the problems of current transformer saturation andudinrush current.
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