By Juliane Hübert (British Geological Survey, Edinburgh)
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 with extensive consequences for the national power supply. The UK government has listed severe space weather events as one of the highest priority natural hazard. Therefore, it is important to fully understand GICs to enable the mitigation of this hazard. 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 UK 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 metres away. By examining the differences between the magnetometers, we work out the additional current flowing in the HV line.
Figure 1: Recorded times series during the G3 geomagnetic storm on 25-26 August 2018. Panels a-d) Horizontal magnetic field components at DMM site Whiteadder (WHI), East Scotland. Panel e) Line GICs at WHI; Panel f) GIC data from a Hall probe at Torness substation.
In the study, we present the design and initial deployment of the first differential magnetometer method (DMM) systems in the UK and measurements from the first site installed at Whiteadder in eastern Scotland. At this site we have successfully detected geomagnetically induced currents in a 400 kV high voltage power network. The Figure compares line GIC data recorded at Whiteadder (panels a-e) to data from a Hall probe at the nearby substation at Torness (panel f) during the 26 August 2018 storm. The measured GICs from the line and the Hall probe show excellent temporal correlation, though with significant differences in amplitude, illustrating that line measurements with DMM and Hall probes at grounding points capture different but complementary views of GIC flow in a network. Using the latest model of the HV network and electric field variations estimated from a magnetotelluric survey, we show that the measured line and earthing GICs match the expected modelled values during the geomagnetic storm. This is the first study to validate such a complex network model using direct and indirect measurements of GICs.
The full article can be found here:
Hübert, J., Beggan, C. D., Richardson, G. S., Martyn, T., & Thomson, A. W. P. (2020). Differential Magnetometer Measurements of Geomagnetically Induced Currents in a Complex High Voltage Network. Space Weather, 18, https://doi.org/10.1029/2019SW002421