Citation:Matandirotya, E., P. J. Cilliers, and R. R. Van Zyl (2015), Modeling geomagnetically induced currents in the South African power transmission network using the finite element method, Space Weather, 13, 185-195, doi:10.1002 Abstract Geomagnetically induced currents (GIC) are a result of time variations of the geomagnetic field, which induce a geoelectric field at the Earth's surface. Geomagnetic perturbations are enhanced during adverse space weather events called geomagnetic storms. All ground-based conductor networks can be affected by GIC during such events. As a way of assessing the magnitude of GIC expected in a particular technological system, models are developed, in which the computation of the induced geoelectric field is a key step. Computation of GIC in the South African power transmission network has so far been done using a uniform Earth model and improved using a layered Earth conductivity profile. In this work we present geoelectric field results obtained by using the finite element method (FEM) and improved GIC estimates using a realistic conductivity profile, magnetic field data interpolated from two South African observatories, and a new method for estimating the network coefficients, a and b, which map the north-south and east-west electric fields to their respective GIC components. The performance of the chosen FEM model demonstrates that it is an effective tool for GIC modeling. Unlike previous engineering techniques, our method for estimating the a and b coefficients from GIC and measured magnetic field data gives results that are independent of prior knowledge of the network configuration. The GIC estimated using the a and b coefficients obtained from the proposed method compares well with the measured GIC during the late October 2003 geomagnetic storm.

Geomagnetically induced currents (GICs) in conductors connected to the Earth are driven by an electric field produced by a time-varying magnetic field linked to magnetospheric-ionospheric current perturbations during geomagnetic storms. The GIC measurements are traditionally done on the neutral-to-ground connections of power transformers. A method of inferring the characteristics of GIC in power lines using differential magnetic field measurements is presented. Measurements of the GIC in the power lines connected to a particular power transformer are valuable in the verification of the modeling of GIC in the power transmission network. The differential magnetometer method (DMM) is an indirect method used to estimate the GIC in a power line. With the DMM, low-frequency GIC in the power line is estimated from the difference between magnetic field recordings made directly underneath the power line and at some distance away, where the magnetic field of the GIC in the transmission line has negligible effect. Results of the first application of the DMM to two selected sites of the Southern African power transmission network are presented. The results show that good quality GIC measurements are achieved through the DMM using Commercially-Off-The-Shelf magnetometers.

High voltage transmission line design requires careful insulator selection to ensure good operational performance. This paper reports on the in-situ measurements of leakage current (LC) on composite and glass insulators of the Cahora Bassa high voltage direct current (HVDC) transmission line in South Africa over a 6month period. The influence of temperature, humidity, dew, rain and the HVDC line's voltage and current on LC are investigated. The results show that the composite and glass insulator LC behaviour is similar, except in cases of high humidity or rain. At the commencement of rainfall and humidity (>90%), elevated LC levels are observed on glass insulators, while composite insulators demonstrate lower LC levels under these conditions. Under nominal weather conditions of no rain and low humidity, the LC measurements exhibit an almost square-wave behaviour with LC switching between lower (≈20 µA) and higher values (≈60 µA) with relatively short transitions on a daily basis. This phenomenon can be ascribed to condensation on the insulators, which is a primary determinant of the LC levels on contaminated insulators. The line current and voltage fluctuations do not influence the LC level.

Power transmission using HVDC transmission technology has existed for several years. As part of the design specification for such schemes, transmission line and substation insulator selection is crucial to ensure good operational performance. Leakage current may occur on insulators due to contamination, ageing of insulation materials and aggressive weather conditions, amongst others. If not mitigated, these currents can lead to line faults which occur as flashover of the insulator and the degradation of the insulator material. Monitoring leakage current on alternating current transmission lines may be accomplished using current transformer devices. However, this cannot be used in direct current transmission lines due to static magnetic fields. Toughened glass insulators are generally preferred for HVDC schemes, but composite insulators are proving to be cost-effective alternatives. However, because of its relatively new application in HVDC schemes, little has been published relating to operational data on typical leakage current magnitudes, waveforms and consequential effects thereof. ESKOM currently operates and maintains the South African sections of the Cahora Bassa HVDC scheme. At present, the utility does not have an in-situ, non-intrusive solution for monitoring the leakage currents on this scheme. This paper presents an overview of leakage current and the options available for measuring it. In-situ and non-intrusive sensors have the benefit of being used for real time monitoring as well as for live line work.

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