Differential Magnetometer Measurements of Geomagnetically Induced Currents in a Complex High Voltage Network

Abstract

Space weather poses a hazard to grounded electrical infrastructure such as high voltage (HV) transformers, through the induction of geomagnetically induced currents (GICs). Modeling GICs requires knowledge of the source magnetic field and the Earth's electrical conductivity structure, in order to calculate the geoelectric fields generated during magnetic storms, as well as knowledge of the topology of the HV network. Direct measurement of GICs at the ground neutral in substations is possible with a Hall effect probe, but such data are not widely available. To validate our HV network model, we use the differential magnetometer method (DMM) to measure GICs in the 400 kV grid of Great Britain. We present DMM measurements for the 26 August 2018 storm at a site in eastern Scotland with up to 20 A recorded. The line GIC correlates well with Hall probe measurements at a local transformer, though they differ in amplitude by an order of magnitude (a maximum of ∼2 A). We deployed a long-period magnetotelluric (MT) instrument to derive the local impedance tensor which can be used to predict the geoelectric field from the recorded magnetic field. Using the MT-derived electric field estimates, we model GICs within the network, accounting for the difference in magnitude between the DMM-measured line currents and earth currents at the local substation. We find that the measured line and earth GICs match the expected GICs from our network model, confirming that detailed knowledge of the complex network topology and its resistance parameters is essential for accurately computing GICs.Plain Language Summary Large geomagnetic storms create time-varying magnetic fields, which induce secondary electric fields in the conductive Earth resulting in geomagnetically induced currents (GICs). The high voltage (HV) power transmission network is connected to the Earth at grounding points in substations. These offer a low-resistance path for GICs to flow into the power network, potentially causing the transformers to malfunction. It is possible to directly measure GICs at substations using Hall effect probes, but due to cost and operational reasons, at present only four substations in the United Kingdom are monitored. Therefore, we have developed a new instrument to measure GICs indirectly using two magnetometers, one placed under the HV line and another a few hundred meters away. By examining the differences between the magnetometers, we work out the additional current flowing in the HV line. We compare our measurements to observations made by a Hall probe at a nearby transformer during a geomagnetic storm in August 2018. The GICs from the line measurements match the shape of the Hall probe, but there is a difference in amplitude. Further analysis shows that the amplitude differences arise from the resistance and earthing properties of the individual HV lines and transformers within the power network.

Geolectric field measurement, modelling and validation during geomagnetic storms in the UK

“…Thomson et al (2005) show GIC data measured at Torness substation which imply the geoelectric field was between 3-4 V/km at the peak of the 2003 storm when currents of 20 A were recorded (at a one-second cadence). Recent work on validation of the high voltage network model at Torness in Hübert et al (2020) corroborates well with this analysis. Given the limited number of large storms in the period of 2012 to 2020 when geoelectric field data are available to create the impedance functions, the estimates shown here should be considered as a lower bound of the geoelectric field generated during extreme space weather events.…”

“…The storm was likely caused by a CME though no classic shock signature was observed. The storm coincided with the deployment of differential magnetometer method (DMM) system which allowed a detailed analysis of GICs in the 400 kV high voltage power grid in East Scotland (Hübert et al, 2020).…”

“…Using the MT impedance functions for each site, the one-minute-mean magnetic field measurements from the 13-14 March 1989 and29-31 October 2003 were used to estimate the geoelectric field (Campanya et al, 2019;Hübert et al, 2020). The main limitation of this approach is the planewave assumption of the magnetic field for the MT response functions, which is generally applicable only for geomagnetically quiet times at mid to low latitudes.…”

“…Again, it is clear that the short-period variations of the geoelectric field have the strongest influence on measured GIC.Finally, a storm on the 26 August 2018 is shown in Figure4. Though a relatively minor storm (G3 on the NOAA geomagnetic storm scale), it coincided with the deployment of the Differential Magnetometer Method (DMM) systems ofHübert et al (2020) which allowed a comparison of both line and substation GIC to be made. Unfortunately, only a single channel of electric field data at each observatory was in operation due to various technical issues.…”

Study of a Quasi‐27‐Day Wave in the MLT Region During Recurrent Geomagnetic Storms in Autumn 2018

“…The other three storms, characterized by the minimum Dst indices around −50 nT and the Kp indices over 4, appear with the commencement of the recovery phase on September 11, September 22, and October 8, respectively. The strongest storm on August 26 has been widely reported (e.g., Chen et al., 2019; Hübert et al., 2020; Zhang et al., 2020), while the other three storms are rarely discussed in previous studies. Note that those four storms are observed with a characteristic of periodicity.…”