Impulse Effective Length of Horizontal Grounding Electrodes for 220 kV Transmission Towers Under Different Soil Conditions

This study used a CDEGS electromagnetic-transient model to examine the impulse effective length of a horizontal grounding electrode connected to a 220 kV cat-head transmission-tower system. The model included the tower, four foundations, two overhead ground wires, adjacent-tower paths, and a buried horizontal electrode. Effective length was defined using a 5° tangent-slope criterion applied to the impulse-grounding-resistance–length curve. For a negative-polarity impulse with a magnitude of 20 kA and an 8/20 μs waveform, the calculated effective lengths were 7.679, 16.5, and 24.985 m in homogeneous soils with resistivities of 10, 100, and 1000 Ω·m, respectively. Calculations over 20–150 kA indicated that, within the investigated cases, soil condition produced a larger variation in the shunt coefficient than current magnitude. Limited field measurements in layered soil showed a qualitatively similar diminishing-return trend, but the three tested electrode lengths were insufficient for a statistically robust three-parameter fit and were not used as a direct validation of the complete tower model. System-level calculations further indicated that increasing soil resistivity from 10 to 3000 Ω·m reduced the grounding-channel shunt coefficient and the calculated lightning withstand level for the specified line configuration. The reported effective lengths are criterion- and configuration-specific engineering estimates rather than universal design constants.