A Reduced-Scale Experimental Study of Tower Impulse Grounding Impedance

Lightning withstand level of the transmission line is directly affected by the value of the tower impulse grounding impedance. While it's difficult to obtain the tower impulse grounding impedance accurately as its nonlinearity under impulse current. Therefore the study of the tower impulse grounding impedance characteristics is significant to lightning protection. Reduced-scale experiment was used in this paper to research the tower impulse grounding impedance characteristics. Reduced-scale experiment is carried on the grounding electrode model reduced from the real grounding electrode with a certain scale n. Compared with full-scale experiment, reduced-scale experiment has advantages in cost and convenience. According to the reduced-scale theory, not only the spatial scale, but also the time scale reduces. Therefore the rise time of applied impulse current should be fast enough. This paper used three-electrode field-distortion gas switch and low inductance circuit to realize fast current rise time and low jitter. After reduced-scaled experiment platform set up, a large number of experiments were done to research the effect of impulse current amplitude, grounding electrode size and soil resistivity on the impulse grounding impedance characteristics. The experiment result shows these conclusions: (1) the impulse grounding impedance decreases with the increasing amplitude of current and the spark effect shows saturation effect as the current increases. (2) the impedance decreases significantly with the increasing grounding size and it also shows saturation effect when grounding size is large enough. (3) the impulse grounding impedance has approximate linear relationship with soil resistivity when the soil resistivity is less than 580 Omega center dot m.