An earthing design guide for single wire earth return (SWER) systems in the Northern Cape region

Abstract

The success of the Reconstruction and Development Programme (RDP) with reference to the electrification of rural areas will be enhanced if cheaper technologies 1 techniques are applied than the three-phase and single-phase (2 wire) networks now in use. Single-wire earth return (SWER) technology has been successfully implemented in other countries as part of their rural electrification plan, and already partially in Eskom. The SWER technology consists of a single overhead high voltage (HV) conductor and the earth is used as the current return path. Savings are possible due to the fact that less material and labour are required to construct the network. Existing SWER lines in Eskom indicated that savings were achieved. Further studies during this thesis, however, indicated that more savings and efficiency would have been possible if the earth electrodes installed had been designed according to the conditions on site (soil resistivities, soil type etc. were not previously rigorously taken into consideration). The success and safe implementation of SWER as a technology by Eskom in the Northern Cape region to electrify rural areas, are mostly dependent on the design of the earth electrodes to be installed at the isolating and distribution transformers, as well as the costs involved. Earthing of SWER systems is different from conventional earthing due to the continuous flow of current in the earth electrode compared with the conventional single-phase (2 wire) and three-phase networks where current will only flow in the earth electrode under fault conditions. This major difference highlights the importance of investigating the earthing practice related to the SWER technology in detail. The completed study addresses the need for a SWER earthing guide for application in the Northern Cape region. Design factors such as earthing materials, costs, thermal resistivity of the soil, ground potential rise as well as the special conditions related to the Northern Cape region, namely high soil resistivities, different soil types and seasonal variations, have for the first time been taken into consideration. As previously mentioned, the feasibility of a SWER scheme is dependent on the earthing costs involved, and this is mainly determined by the previous ground potential rise (GPR) limit of 20V. The ground potential rise limit is a safety limit directly adopted from Australian SWER schemes. SWER as a technology in the Northern Cape region with its special conditions is dependent on the possibility to increase the GPR limit of 20V. The feasibility of increasing the GPR limit formed the rationale behind this research. Studies indicate that the GPR limit can be increased from 20V to 35V in the Northern Cape region without sacrificing safety. The studies include simulations done by using a specialised software package called CDEGS. The success of this research is supported by the feasible, cost-effective safe earth electrodes designed and installed on the Rooiwal SWER scheme. Savings of 87% (R 167 352-00) were achieved by the installation of the SWER earth electrodes designed as part of this thesis. This excludes the additional savings of R 30 000-00 by not using conductive concrete such as mitronite as a standard on all SWER earth electrodes in the Northern Cape region.