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In substations with an inner bridge connection, the two main transformers are generally connected in parallel. The differential protection of the transformer, under normal operating conditions, may maloperate when the other transformer is switching on through the bridge breaker after scheduled maintenance. This paper analyzes the factors that may lead to the maloperation of transformer differential protection and concludes that local transient saturation of the current transformer (CT) caused by the decaying dc component in the magnetizing inrush is the main cause of the maloperation of the transformer differential protection. By detecting the time difference between the instant of sudden changes occurring in the transformer wye-side current and the instant of the differential current increasing, a new algorithm based on the time differential method is proposed in this paper, which can prevent the maloperation of transformer differential protection in substations with an inner bridge connection. On the basis of detailed analysis and simulation work, the new algorithm is verified to be applicable under the conditions of sympathetic inrush and CT saturation. [ABSTRACT FROM AUTHOR]

This paper presents a novel morphological scheme for the identification of transformer magnetizing inrush. The scheme decomposes a current signal into multiresolution levels based on synthesis and analysis operators of mathematical morphology. It is able to discriminate between inrush and internal fault currents even in the case of an inrush with a low second harmonic component and an internal fault current with a high second harmonic component. Simulation studies have been undertaken to evaluate the scheme based on a three-phase high-voltage power transformer operating under different conditions, which is simulated with an Alternative Transients Program. The testing results show that the proposed scheme is able to identify inrush currents reliably and provides an alternative solution for transformer differential protection. [ABSTRACT FROM AUTHOR]