Validating GIC modelling in the Spanish power grid by differential magnetometry

A series of experiences and recommendations are presented concerning the measurement of geomagnetically induced currents (GIC) in the Spanish power transmission grid by use of the method of differential magnetometry under power lines, by which differential observations are made (one below the line and another at a few hundred meters away) using vector magnetometers to capture the magnetic effect of the GIC flowing through them. This indirect technique, aimed at obtaining observations to validate GIC computational models, is an alternative to the more common way of measuring the current flow in the transformer neutrals, as it does not rely on the involved power grid operators. In contrast, the selection of a suitable site devoid of human interferences, the need of power for the magnetometer/acquisition system, and the election of the appropriate instrumentation are difficulties that often require costly solutions. Our methodology includes the settlement of appropriate magnetometers with the correct levelling and orientation placed inside buried water-proof containers. The magnetometers are fed by solar panel-battery systems, and we have also developed low-consumption data-transmission models using Raspberry-Pi with GPRS connection technology. According to our experience, only induced currents above about 1 A give magnetic signatures that exceed the noise threshold. As we started measuring during the solar minimum and Spain is a mid-latitude country, the latter fact limited the significance of available recorded data, but we can already report and analyse the results for a number of minor geomagnetic storms.