To detect the fault of the main transformer windings for AC locomotive without changing the original structure of the locomotive system, a non-invasive on-line winding fault diagnosis method for the main transformer is proposed. Proved by the mathematical theory, the relationship between ΔV-i₁ is a diagonal ellipse whose center is located at the origin of rectangular coordinate system. The related parameters of the ellipse vary with the change of the winding faults. Then, the feasibility and effectiveness of the proposed method are verified by experiment through dSPACE simulations. The results show that with the increase of short circuit percentage, the area of ellipse increases and the centrifugal rate decreases. When the axial displacement is increased, the centrifugal rate increases, while the elliptical length, short axis and its area decrease. When short-disk occurs, the elliptical short axis decreases and the inclination angle changes obviously. By comparing the ΔV-i₁ trajectory under normal and fault condition, the fault type and fault level can be determined.