Current Dependent Characteristics of a 500kV Transmission Tower Footing Base

This paper presents experimental results of lightning current characteristics of a 500kV transmission tower footing base with various grounding electrodes using a 3000kV impulse generator. Further, general prediction curves for the lightning performance of the grounding electrodes under various soil resistivity are compared with the experimental results. The impulse tests on them were carried out with currents up to 30kA. The tests under such a large current have not been achieved before, therefore the results of these tests provide a lot of significant knowledge about current dependent characteristics of various grounding electrodes as well as a tower footing base, such that the reduction of tower footing resistances can not be expected under the soil of low resistivity which is below 1000Ωm, and the effect of various grounding electrodes connecting to the tower base for high impulse currents can not function well, mainly due to the interference between the tower base and various grounding electrodes. The critical soil ionization gradient, however, is obtained by alternative impulse tests using a driven hemispherical and rod electrode in uniform soils. Then general prediction curves based on Korsuntcev's idea are derived. The availability of the curve is confirmed by comparison with the experimental results, and the possibility of predicting the lightning performance of tower footing base under the soil of higher resistivity is discussed.