observatory, differential magnetometer and magneto-telluric data for

modelling geomagnetically induced currents in Spain

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)

(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



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.