Transformer is one of the most important electrical device in power system. But sometimes, the high-voltage transformers in transmission system are forced to work in the half-cycle saturation by the geomagnetically induced currents (GICs) [1]–[4]. In other words, during the geomagnetically disturbances (GMDs), the solar wind introduces a time-variant magnetic field to the earth, then an additional electric field is induced. Though in most GMDs, the induced electric field is less than 1V/km with 10–4–1Hz frequency, it can draw over 100A per phase GICs on windings of some transformers. The comparison of ?–I (magnetic flux to magnetizing current) curve of transformer core in normal condition and half-cycle saturation mode is demonstrated in Figure 1.

In order to model the GIC saturation of transformers, finite elements analysis (FEA) is an effective approach to solve the electromagnetic field distribution. Generally, for GIC saturation of transformers, FEA uses the following steps to solve the problem:

Step1: Input the geometry and material property of the transformer, if geometry is symmetric, then simplify the geometric structure to 1/2 or 1/4 model, and configure the boundary of interest.

Step2: Generate fine meshes of the model, then impose the external circuital model to simulate the GICs and the operating condition of ac system.

Step3: Solve the model in time domain, generate the results.

For example, a finite element model of a three-phase seven-limb shell transformer is built in Figure 2(a), and with FEA tool ANSYS MAXWELL 16.0, the harmonic components in magnetizing current under various GICs is shown in Figure 2(b).

References

[1] S. Lu, Y. Liu, and J. De La Ree, “Harmonics generated from a dc biased transformer,” IEEE Transactions on Power Delivery, vol. 8, no. 2, pp. 725–731, Apr 1993.

[2] S. Lu and Y. Liu, “FEM analysis of dc saturation to assess transformer susceptibility to geomagnetically induced currents,” IEEE Transactions on Power Delivery, vol. 8, no. 3, pp. 1367–1376, July 1993.

[3] X. Li, X. Wen, P. Markham, and Y. Liu, “Analysis of nonlinear characteristics for a three-phase, five-limb transformer under dc bias,” IEEE Transactions on Power Delivery, vol. 25, no. 4, pp. 2504–2510, Oct 2010.

[4] Zhuohong PAN and Yilu Liu, “Harmonics current and reactive power consumption of three-phase seven-limb transformer during geomagnetically disturbance”, technical report, 2014.