Abstract: The three-winding transformer cannot construct a T-equivalent circuit with accurate physical meaning similar to the two-winding transformer. It was proved by differential equations that the two derivation processes of the traditional T-equivalent circuit for the three-winding transformer have the limitations of neglecting the magnetizing branch and the decoupling transformation. Then the equivalent circuit based on the controlled inductance and the accurate leakage inductance expressions were proposed. It pointed out that there are physical concept differences and theoretical numerical errors between the traditional equivalent leakage inductance and the true accurate leakage inductance. And the phenomenon that the traditional equivalent leakage inductance of the medium-voltage side is close to zero was revealed. Compared with the actual accurate leakage inductance based on the on-site recording, the traditional equivalent leakage inductance of the transformer with high-voltage winding built-in（T-Hin） has a larger error, and the deviation of the generated zero-sequence current is larger, which may affect the protection setting, accident analysis and improvement of measures, and increase the risk of disoperation. Therefore, the traditional equivalent parameters are not suitable for analyzing such inrush current issues. It is recommended to use the actual and accurate parameters mentioned for analysis and calculation.