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Transformers are one of the most important and strategic component in generation, transmission and distribution systems. Therefore, it is essential to avoid any scenario in which a risk may threaten this important network equipment. Differential protection as a main protection scheme of transformers has typically not been sensitive enough to detect winding minor turn-to-turn faults (less than 10%). Therefore, this paper after surveying existing algorithm in literature and discuss their defects, tries to develop new algorithm for detecting minor turn-to-turn faults of transformers. To that end, root mean square of negative sequence current of primary side of transformers is used and new concept of modified differential current is introduced. The proposed algorithm is tested on a typical transformer and the results are discussed.