It is difficult to apply traditional differential protection to low-frequency systems. To improve this, a transformer differential protection method based on steady-state negative sequence components is proposed. It achieves improvement by analyzing the sequence component suppression strategy of low-frequency systems and considering the characteristic of negative sequence injection on one side after low-frequency system faults enter steady state. By comparing and analyzing the braking performance of three braking current calculation methods, the braking equation and braking coefficient of differential protection are obtained to prevent differential protection misoperation caused by current transformer saturation during external faults. The method adopts the differential protection opening condition of phase voltage sudden change to cope with special working conditions such as excitation inrush current and current transformer disconnection. Simulation analysis shows that this method can sensitively reflect internal metallic faults and minor faults, and maintain reliable braking margin when external faults are accompanied by current transformer saturation.