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Owing to current transformer (CT) saturation caused by a decaying direct current component in the magnetizing inrush current, the restricted earth fault (REF) protection of a power transformer could maloperate during magnetizing inrush. This paper presents a new transformer restricted earth fault relay algorithm that can prevent false relay tripping and provide proper action during the energization of a faulty transformer. The integral of the product of the signals that are compared is calculated over 1 half-period of the signals. The performance of the proposed algorithm is tested and verified using a large number of transformer energization trials and fault cases. The signals required for testing of the algorithm were obtained by laboratory measurements, and then the algorithm was tested off-line. The influence of different levels of CT saturation due to different remanent fluxes in CT cores during power transformer energization was investigated. By applying computer simulations, the signals required for algorithm testing were generated. The investigation results presented in this paper show that the new algorithm provided sensitive protection for the power transformer for phase to ground faults and high security and stability during magnetizing inrush conditions in conjunction with CT saturation.

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