Abstract: Differential protection is one of the primary protections for power transformer, which has good reliability. Because of the unique structure of Scott transformer, its differential protection is different with that of traditional power transformer. To improve the practical performance of differential protection, a new protection scheme is proposed in the paper. With two differential protection modes, the whole performance of differential protection is greatly improved by combining phase constraint theory of adaptive constraint coefficient of the second harmonic. The proposed scheme is better than traditional scheme evidently through theoretical analysis and RTDS simulation. And the setting values of differential protection for transformer can be set by phase, which increases the flexibility of the protection, and meets the requirements of differential protection of Scott transformer.