Non-linear Dynamic Response of a Ground Rod in Resistive Soils

The increasing use of intelligent electronic devices (IEDs) in substations, for supervisory control and data acquisition in power systems, makes critical the understanding of transients in grounding electrodes under lightning surges. Thus, acquiring greater knowledge of the dynamic response of grounding systems becomes strategic. The flow of high currents in earthed electrodes can produce high electric fields and the consequent ionization of the soil. The non-linear conduction mechanism in the soil provokes a timedependent reduction of the impulse resistance. This reduction may be very pronounced for several tens of microseconds and the effect can be advantageous for the insulation coordination with soils of high resistivity, eventually limiting the harmful inductive behaviour depending on frequency. Previous experimental work and computer simulation in a laboratory scale model, with a single soil resistivity value, demonstrated the importance of the energy injected for the ionization of the soil around the earthed electrode. A simplified physical model of the discharge progression in soils was proposed in order to explain the relationship between energy and transient grounding resistance. The model included waveform, peak value and rise time of the injected current. Extending the proposed method, this contribution presents and analyzes new experimental results obtained for several values of soil resistivity, between 290 and 3800 Omega m. Finally, from the experimental results obtained with this approach, a simple and practical procedure is proposed to estimate transient resistance reduction in actual vertical ground rods buried in soils having a different resistivity.