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Abstract-The article aims to study the change of instantaneous values of differential current in power transformer protection circuits during magnetizing inrush when the unloaded transformer is connected to a power source and when a fault occurs in the protected area. Also, time behavior of the unbalance current in the generator, differential protection during remote faults is considered. Saturation of measuring current transformers during the fault transients leads to distortion of signals in their secondary windings, which can cause a long delay in the disconnection of the protected object or false operation of the protection if traditional protective algorithms are used. The peculiarities of the change in the instantaneous differential current should be considered when developing the protection algorithm because this allows recognizing faults before the first saturation of electromagnetic current transformers. For quick and correct recognition of internal faults of the power transformer, the authors proposed the control of the maximum value of the derivative of the differential current and the duration of its monotonous changing at the beginning of the transient process. As studies have shown, the parameters controlled can differ significantly under normal operation conditions and at the time when a fault occurs. The authors also showed that monitoring the time behavior of the unbalance current in the generator, differential protection during external remote faults is necessary to avoid false tripping. Mathematical simulation was used to study the magnetizing inrush, short-circuit faults, and transients in current transformer windings.

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