Investigation into the electrical properties of crusher stone used in substation earthing systems

摘要

One of the critical aspects in the design of a Substation is the Earthing System. The Earthing System provides a means of dissipating fault current without exceeding the operating limits of equipment and provides a safe environment for personnel to work in. With the increasing fault levels on Eskom's Transmission & Distribution networks it is essential to ensure that the Earthing System is properly implemented and conforms to the stated design parameters. The problems being experienced in ensuring the actual surface layer material procured complies with the technical requirements as specified in the design is a concern. Of particular concern is the determination of the wet resistivity of the surface layer material. Eskom specifies crushed stone as the surface layer material, and indicates that it should have a wet resistivity value of at least 3000 Ωm with aggregate size between 25 - 38 mm. However, there is no industry standard on how to measure the wet resistivity of the surface material. This paper aims to investigate if the high level methods described in IEEE 81 can be used to successfully measure the wet resistivity of surface layer material repetitively on site. Furthermore, the ultimate aim would be to develop a portable system that would allow the resistivity of surface layer material to be measured on site. If the development of a test method is successful, it could minimize the uncertainty regarding the electrical properties of the crusher stone used in Eskom's Substation, and support the design assumptions.