ENGINEERING PERSPECTIVES ON SOIL PROPERTIES IMPORTANT TO THE DESIGN AND PERFORMANCE OF EARTH ELECTRODES AT LOW AND HIGH FREQUENCY

Summary

Earth electrode design for high voltage substations and tower foot electrodes for overhead power line towers are driven from both safety and lightning performance points of view. Conventionally, data obtained from tower footing resistance measurements are used in the design of tower foot electrodes. This data is representative of the low frequency behaviour of the electrode typically encountered under 50 Hz fault conditions. More modern equipment, however, allows for the measurement of the tower foot impedance, representative of the high frequency behaviour of the electrode typically encountered under lightning conditions. With only tower footing resistance requirements historically embedded in line design earthing specifications, the line design engineer now faces the dilemma whether to accommodate only the tower footing resistance or both the tower foot resistance and tower foot impedance in the design. Quantifying the high frequency properties of soil is not a trivial task and interpretation of data brings its own challenges. This scaffold paper addresses some of the questions faced by the design engineer in the context of low versus high frequency behaviour of soil with suggestions to address these. Distinction is purposefully made between ionising and non-ionising conditions in soil with the focus of the discussion on the non-ionising behaviour of soil. The paper in particular, relies strongly on the universal impedance model for soils as developed by Longmire (1975) and its application to a substitute horizontal buried electrode. Application of this model is further discussed in the paper. The main contribution of the paper is on suggesting how the tower foot impedance can be applied where a design calls for tower foot electrode resistance values. The paper concludes with guidance and an example of how this can be achieved in the context of lightning performance. Finally, reference is made to research conducted on the topic at the University of the Witwatersrand.

Additional informations

Authors

Pretorius, Pieter H