Comparison of two simple high-frequency earthing electrodes

Abstract

Lightning strikes on high-voltage transmission lines may create hazardous touch potentials on adjacent substation equipment and damage to control equipment. When designing suitable substation earthing electrodes to overcome this problem and safely dissipate the transient fault currents to ground, it is essential to consider the behaviour of the current flow. The steady-state frequency equivalent to a lightning strike is at least 0.25 MHz, which corresponds to a current skin depth ? of about 10m in homogeneous soil of conductivity 0.01 Sm-1. This surface effect causes the impedance of an electrode system to be considerably larger than the power frequency resistance. Thus, a dedicated electrode is normally placed in parallel with the low-frequency earthing system, usually taking the form of a simple vertical rod of copper-coated steel about 5m long. However, the work presented in this paper suggests that a long rod is not ideal for the purpose. Using a relatively simple numerical finite-difference procedure it has been found that a flat disc electrode parallel to the surface of the ground achieves a significant improvement over the performance of a vertical rod. Both rod and disc have been solved in the frequency domain, but the rod has also been analysed in time-stepping form so that the peak voltage for a given imposed current can be compared with that deduced from the equivalent steady-state complex impedance.