Applicability analysis for traditional T-equivalent leakage inductance of three-winding transformer during energization

The zero-sequence overcurrent protection always misoperates when the Yg/Yg/D-connected three-winding high impedance transformer with high-voltage winding built-in (T-Hin) is energized. During the energization, it works as a two-winding transformer (W2T). In the past, the traditional T-equivalent leakage inductances calculated through three short-circuit inductances are regarded as the leakage inductances of the "W2T" during energization. Whether the equivalent parameters of three-winding transformer (W3T) can be used to analyze the energization process is demonstrated in this paper. The original derivation process of the traditional T-equivalent parameters of W3T is analyzed first. It is proved that a planar physically correct equivalent circuit can't be constructed for W3T and the traditional T-equivalent parameters have theoretical deviation during energization. An identification method for accurate equivalent parameter of "W2T" is proposed. The accurate equivalent parameters, identified by the recorded waveforms of the on-site transformers and model transformers, verify the deviation analysis. Simulations show that the accurate equivalent parameter based zero-sequence current of THin has larger deviation, which may affect the protection setting, accident analysis and increase the risk of misoperation. So, the accurate equivalent parameters are recommended for energization process rather than the traditional T-equivalent parameters.