369 / 2022-03-15 17:03:21

Influence of DC bias on three-leg transformer zero sequence characteristics

DC bias,FEM model,Eddy loss,zero-sequence impedance

The half-wave saturation or deep saturation of transformer are generally caused by the DC bias. The zero-sequence flux of the three-leg transformer will increase with the increase of DC current. The large DC current will result in the saturation of transformer tank. This phenomenon will change the zero sequence characteristics of transformer, and cause the local overheating of transformers.


The 3D FEM model of three-leg transformer is built in Ansys.  The flux leakage and eddy loss through the transformer tank under normal operation and DC bias are compared and studied.


The zero-sequence test of the YN/D connection transformer is simulated. As shown in Fig.1, the magnetic induction in the tank will increase under DC bias. The flux leakage in tank is also higher. The reluctance of zero-sequence flux path will decrease with the increase of flux leakage. In the T-type equivalent circuit, the zero-sequence impedance is inversely proportional to the reluctance. The zero-sequence impedance under DC bias is larger than that of normal operation as shown in TABLE 1.  With the peak value of the magnetic field on the tank surface increases under DC magnetic bias, the eddy loss in the tank will increases according to the nonlinear power loss formula as shown in Fig.2.


The DC bias will cause the increase of the flux leakage in the tank. The zero-sequence impedance and eddy loss of the three-leg transformer increases significantly due to the increase of the flux leakage. This finding indicates the zero sequence characteristics of three-leg transformer are significantly affected by the DC bias.