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The transformer percent differential protection is not reliable enough during external fault. if the differential restraint characteristic is the only one criteria providing security. The saturation of CTs during external faults could lead to cases of dangerously high differential restraint ratios crossing the characteristic into the operating region and causing unwanted trip. The help of early CT saturation detection. as well as directionality check are vital for the protection to provide additional security during external faults. Careful analysis of the system including the power transformer. and current transformers. is needed to set the protection for optimum performance.
