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Using observatory, differential magnetometer and magneto-telluric data for modelling geomagnetically induced currents in Spain |
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J. M. Torta (1), S. Marsal (1), J.J. Curto (1), O. Cid (1), J. Ledo (2), A. Marcuello (2), P. Queralt (2), A. Martí (2), J. Campanyà (3) |
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(1) Observatori de l’Ebre, CSIC - Univ. Ramon Llull, Roquetes, Spain (2) Institut Geomodels, Dept. Dinàmica de la Terra i de l’Oceà, Universitat de Barcelona, Barcelona, Spain (3) School of Physics, Trinity College Dublin, Ireland |
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Vulnerability assessments of the risk posed by geomagnetically induced currents (GICs) to power transmission grids benefit from accurate knowledge of the geomagnetic field variations at each node of the grid, the Earth’s geoelectrical structures beneath them, and the topology and relative resistances of the grid elements in the precise instant of a storm. The results of previous analyses on the threat posed by GICs to the Spanish 400 kV grid have been improved by resorting to different strategies to progress in the three aspects identified above. Firstly, although at mid-latitude regions the source fields are rather uniform, we have investigated the effect of their spatial changes by interpolating the field from the records of several closest observatories with different techniques. Secondly, we have performed a magnetotelluric (MT) sounding in the vicinity of one of the transformers where GICs are measured to determine the geoelectrical structure of the earth, and we have identified the importance of estimating the MT impedance tensor when predicting GIC, specially where the effect of lateral heterogeneities is important. Finally, a sensitivity analysis to network changes has allowed us to assess the reliability of both the information about the network topology and resistances, and the assumptions made when all the details or the network status are not available. In our case, the most essential issue to improve the coincidence between model predictions and actual observations came from the use of realistic geoelectric information involving local MT measurements. Lessons learnt from this and our previous GIC assessments are used in a new plan for establishing a series of local MT surveys to properly map the non-homogeneous geoelectric field, enabling the matching between the model predictions and actual GIC measurements across the entire Spanish territory. The number of GIC measurements are also planned to be increased by indirectly obtaining them with the deployment of magnetometers under some selected power lines, in contrast with the more usual way of measuring the current in the neutrals of the transformers at substations, and thus avoiding the necessity of interfering with the power companies. |
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