Comparison of different geoelectric field methods to calculate geomagnetically induced currents in North China

Geomagnetically induced currents (GICs) are caused by the interaction between the geomagnetic storm and the Earth's conductivity structure, which can enter the power grid and cause blackouts in extreme cases. To avoid significant economic losses, it is essential to correctly estimate the intensity of GICs first and then take corresponding protection measures to the power grid. There are many methods to carry out this estimation, of which the results can be very different. To evaluate these differences, four estimates of GICs for the 1000-kV grid in North China were compared during the geomagnetic storms that occurred from November 7 to 9, 2004, based on the conductivity models and magnetotelluric (MT) data from previous studies. These four estimations include using field MT data (data-space method), a one-dimensional (1D) conductivity model (1D model-space method), and two 3D conductivity models (3D model-space method) in the same area from different research teams. In consideration of the complex topology of the power system and the interactions between different power lines, the estimation using field MT data obtains the highest accuracy. Those using the 3D conductivity models rank the second and the one using the 1D conductivity model ranks the third. Among all, the data-space method is sensitive to the so-called corner effect in some transformers, which infers that this method provides the most correct ground current path information. The crustal structures of the two 3D conductivity models are slightly different. This slight difference does not bring any troubles to the final geophysical purpose but leads to a butterfly effect in the estimation of GICs, causing the difference between the estimation results based on the two models to an extreme value of 267.2 A. Based on the statistical analysis of the maximum intensity and duration of GICs in 20 substations, we believe that the GICs of the North China 1000-kV power grid have reached a very high-risk level, and the assessment results are 2 similar to 3 times higher than other Kp = 8 geomagnetic storm events. Previous studies have shown that there are four conductors in the North China region, namely C1, C2, C3, and C4. After analyzing these conductors, we have concluded that C3 and C4 have a greater impact on the geoelectric field and GICs. Ultimately, it has furnished advantageous guidance for the installation of GICs monitoring apparatus and the discernment of optimal substation siting.

Automated summary

The intensity estimation of geomagnetically induced currents (GICs) varies depending on the method used. The estimation using field magnetotelluric (MT) data provides the highest accuracy, followed by the estimation using 3D conductivity models and the estimation using a 1D conductivity model. The GICs in the North China 1000-kV power grid have reached a very high-risk level, with C3 and C4 having a significant impact on the geoelectric field and GICs.