Research On Influence Factors And Modelling Methods Of Geomagnetically Induced Currents In Large Power Grid

Geomagnetically induced currents (GIC) may affect the safe operation of power systems. With the construction of the ultra-high voltage (UHV) power grid, China becomes the country with the largest scale and the most voltage-levels power system. Considering that China is geographically a large country and that the earth conductivity structure is very complex, there may be a high GIC disaster risk for the Chinese power system. Thus, it is necessary and significant to ascertain all the factors influencing GIC in mid-low latitude areas, and build an accurate GIC calculating model, which can meet the practical needs of the Chinese power system. In view of this, the influence factors of GIC including the space, the earth and the power system, and the modelling method of GIC in a complex large power grid are studied in this dissertation. The main work and the results are summarized as follows.Considering the different effects of the ionospheric space currents to the geoelectric fields induced by the geomagnetic disturbances (GMD), two space current source models are defined representing two extremes, i.e."small-scale source current"'and "large-scale source current". As the surface impedance can be regarded as a tool to determine the horizontal electric field with a given magnetic variation, the numerical simulation and mathematical analysis of the surface impedance of "small-scale and large-scale source currents" are considered as a function of the horizontal distance, the height and the frequency. The results prove that the idea presented previously that with respect to a given magnetic variation "the geoelectric field induced by the line current model is always smaller than that induced by the plane wave" is not completely correct. Anyway, it is shown that the "large-scale source current" model is appropriate for calculating the geoelectric fields experienced by the Chinese power gridWith the example of the "Halloween GMD event", the influence of the different latitudes and different earth conductivities are studied quantitatively in the frequency domain and time domain. It is found that although the intensity of the geomagnetic variations are low relatively in mid-low latitude areas, the electric field due to the different earth conductivity could differ by factors from4to10. Also, the latest standard GIC calculating model "GIC-Benchmark" network is used as a probe to investigate the influence of the different latitudes and different earth conductivities to the GIC. This approach avoids the dependence of the results on the specific network, and the results show that GIC in mid-low latitude areas can be higher than that in high latitude areas due to the effects of the earth conductivity.With the parameters and the characteristics of a real power grid, the influence of the length of the transmission lines, the topology of the network and the structure of the transformers on GIC are studied in detail. It is concluded that: longer transmission lines lead to larger GIC, but there is not a simple relationship between GIC and the line length, for short lines, GIC are linearly related to the line length; for long lines, GIC are almost independent of the line length and approach a limiting value. In practice, for the calculation of GIC at a substation, we need to consider the effects of all the transmission lines not only the lines connected to this substation, and the largest substation GIC typically occur at the edges of the network due to the "edge effect". Also, considering the effect of the different transformer structures, the calculating method of the effective GIC is given for both conventional transformers and autotransformers.In view of the scenarios with the different transformer windings connections and the different transformers combination, the modelling method of GIC in multiple-voltage-levels power system is proposed based on the Nodal Admittance Matrix method. Consequently, the problem of calculating the GIC within a substation which is connected to different voltage-level transmission lines is solved. Based on this modelling method, the full-node model of the Sanhua UHV Grid is established, and the effects of GIC in the500kV system on GIC in the1000kV UHV substation is proved. The effects are most significant when the500kV lines which are connected to the1000kV substation are asymmetrical to the electric field.In this dissertation, based on all the effects of the space, the earth and the power system factors on GIC, the modelling method of GIC in large multiple voltage-level power grid is proposed, and the characteristics of the GIC affecting each other in different voltage-level grids are demonstrated, also the modelling theory and the method are proved with the case study of the parameters of the Sanhua Planning Grid. The calculated results of GIC in the Sanhua Grid under the real GMD impact show that there may be a high GIC risk in the1000kV power system with the influence of the500kV system. Therefore, the technical measures are proposed for the UHV power system in China to prevent the GMD disaster, that is significant for the construction and safe operation of China's power system in the future.