Large-scale experimental field tests of practical earthing systems under transient conditions

Understanding the behaviour of earthing systems under impulse currents is fundamental for the design of new systems and for improving the performance of existing systems under transient conditions. Several analytical approaches and powerful computation tools for the analysis and design of earthing systems have been developed; Analytical calculation methods deal with uniform or layered soil structures only, and there is very little experimental evidence to validate the accuracy of these methods. As can be found in the literature, there is a significant set of test results on laboratory scaled-models of earth electrodes as well as some limited data on earthing systems of operational electrical substations and transmission lines. The shortfall of laboratory scaled-models is related to the boundary conditions, which makes them less suitable for the validation of calculation/computation methods. On the other hand, operational electrical installations are located in areas characterised by non-uniform soil structures with both lateral and vertical variations. Since experimental investigations on full-scale earthing systems are very limited, it is now timely that further work is required to improve the understanding of the conduction mechanisms that control the performance of large earthing systems and, in particular, their performance under transient conditions. This paper describes experimental investigations carried out at two outdoor earthing test facilities with detailed experimental arrangements presented: a) At the university test facility in Llanrumney, Cardiff, experimental results were obtained on a purpose-built 275kV transmission tower base using a novel earth return electrode system. The impulse resistance was measured and its non-linear current dependence was verified experimentally; b) The second test site is located at the pumped-storage power station in Dinorwig, North Wales, where a test rig has been built on a large water reservoir in order to achieve close to uniform resistivity conditions. The large test area and the uniformity of the medium on site has allowed more accurate testing and simulation of electrodes and earthing grids of realistic size. Furthermore, different impulse voltages of variable magnitudes were applied. The experimental results are compared with computed values obtained from different numerical models.