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Abstract Power transformer is amongst the most important components in a power system. Avoiding damage to power transformer is vital, other wise continuity in power delivery may be seriously disrupted. Furthermore, repairing or replacement is expensive and time consuming. So, the protection system of a power transformer must be robust, discriminative and fast. The discrimination between an internal fault and a magnetizing inrush current has long been recognized as a challenging power transformer protection problem. Conventional protection systems are designed to restrain during inrush transient phenomenon by sensing a large second harmonic component. However, this component may also be generated during internal faults due to current transformer saturation or the presence of shunt capacitor or distributive capacitance of a long EHV transmission line to which the transformer can be connected. in modern large power transformers, this harmonic component in the magnetizing inrush current tends to be relatively small because of the improvements of core materiaL Consequently, the commonly employed conventional differential protection based on second harmonic restraint will thus have difficulty in discrimination between inrush current and internal faults, thereby threatening transformer stability. Alternatively, improved protection techniques for accurately and efficiently discrimination between internal faults and inrush current have to be thus found. The wavelet transform is a relatively new and powerful tool in the analysis in transient phenomenon because of its ability to extract information from the transient signals simultaneously in both time and frequency domain, rather than conventional Fourier transform which can only give the information in frequency domain. This work presents a novel technique based on wavelet analysis to design a differential relay for power transformers. The designed relay is able to distinguish between magnetizing inrush current, and internal fault currents. The (ATP) transient program is used to simulate study cases for power transformer in different faulty conditions and normal operation. Daubichies Wavelet is. applied to analyze the three phase differential current signals output from simulation program. The outputs from wavelet analysis effectively trace the time evolution of various frequency components of all signals, so it becomes easy to identify inrush and other transformer faults. The extensive simulation studies have verified that the proposed technique is more reliable, simpler, than conventional methods and is suitable for different power transformers of various sizes and designs. |