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This paper is exploring requirements for the line current differential function (87L) with regards to the tolerance to current transformer (CT) saturation. Typically, requirements provided by the manufacturers or standards are to eliminate CT saturation completely by proper sizing of CTs, which is not always practical.First, the general knowledge of CT fundamental and saturation is introduced, including CT circuit model, simulation model, AC saturation, DC saturation and factors driving CT into saturation. Then the impact of CT saturation on the line current differential function is explained in terms of mathematical deduction and simulation, where the effects of saturation caused by internal and external faults on 87L are examined.Percentage differential elements have algorithms designed to tolerate CT errors including CT saturation. How to use these algorithms properly and how to estimate reliability of the differential during CT saturation conditions is not an easy task. Therefore, techniques that have been used or can be used in 87L to reduce CT requirement and improve relay security are discussed.Most important, a practical analysis tool is presented for different applications, including breaker-and-ahalf or ring configurations, to analyze reliability of 87L during CT saturation, evaluate the differential relay security, investigate the effect of adjusting 87L settings, choose the proper size of CT and examine the possibility of reducing CT requirement.

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