Lightning Surge Response of Ground Electrodes

Experimental and analytical evidence shows that lighning surge currents ionize the air in the soil. This ionization process tends to increase the contact area of the electrode with surrounding material. The larger area results in lower resistance and less voltage stress per unit current. Most realistic models for non-linear changes in footing resistance are difficult to derive. Recently, a similarity approach to footing modeling was proposed [1]. We describe the approach and derive a new model for low-current response, based on electrode size and surface area. The simple model agrees well with formulas derived by Rudenberg [2] for a wide range of electrode shapes. When models for impulse footing impedance are used in the prediction of lightning outage rates, the currents computed for backflashovers on EHV transmission lines are often over 400 kA. These currents are well beyond the values estimated from observations of lightning radiated fields for negative flashes. In spite of that, lightning flashovers are observed on 500-kV lines. To resolve the discrepancy, in addition to the work on surge-reduced footing resistance, the surge response of a conducting ground plane was studied. Any ground electrode, even a perfectly conducting plane, will exhibit a transient surge impedance as the front of the lightning current spreads out. This impedance is fairly high (60 Ω) at the base of the tower but quickly falls off to low values. In effect, the ground plane near the tower looks like an extension of the tower, bent outward.