[5] [5] Buticchi G, Lorenzani E. Detection method of the DC bias in distribution power transformers［J］. IEEE Trans Industrial Electronics, 2013, 60(8): 3539-3549.

[7] [7] Picher P, Bolduc L, Dutil A, et al. Study of the acceptable DC current limit in core-form power transformers［J］. IEEE Trans Power Delivery, 1997, 12(1): 257-265.

[8] [8] He J, Yu Z, Zeng R, et al. Vibration and audible noise characteristics of AC transformer caused by HVDC system under monopole operation［J］. IEEE Trans Power Delivery, 2012, 27(4): 1835-1842.

[9] [9] Monteiro T C, Martinz F O, Matakas L, et al. Transformer operation at deep saturation: model and parameter determination［J］. IEEE Trans Industry Applications, 2012, 48(3): 1054-1063.

[10] [10] Dickmander D L, Lee S Y, et al. AC/DC harmonic interactions in the presence of GIC for the Quebec-New England Phase II HVDC transmission［J］. IEEE Trans Power Delivery, 1994, 9(1): 68-78.

[11] [11] Fuchs E F, You Y, Roesler D J. Modeling and simulation and their of three-phase transformers with three legs under DC bias［J］. IEEE Trans Power Delivery, 1999, 14(2): 443-449.

[12] [12] Nobuo Takasu, Tetsuo Oshi, Fumihiko Miyawaki, et al. An experimental analysis of excitation of transformers by geomagnetically induced currents［J］. IEEE Trans Power Delivery, 1994, 9(2): 1173-1182.

[15] [15] Pfutzner H, Shilyashki G, Bengtsson C, et al. Effects of DC bias on regional flux and magnetostriction of a single-phase transformer core modeled by 3-D MACC［J］. IEEE Trans Magnetics, 2018, 54(5): 1-6.

[16] [16] Biro O, Koczka G, Leber G, et al. Finite element analysis of three-phase three-limb power transformers under DC bias［J］. IEEE Trans Magnetics, 2014, 50(2): 565-568.

[17] [17] Plasser R, Koczka G, Biro O. Improvement of the finite-element analysis of 3-D, nonlinear, periodic eddy current problems involving voltage-driven coils under DC bias［J］. IEEE Trans Magnetics, 2018, 54(3): 1-4. )