Measurement of the division of lightning return stroke current among the multiple arresters and grounds of a power distribution line

A triggered-lightning experiment was conducted during Summer 2000 at the International Center for Lightning Research and Testing (ICLRT) in north-central Florida for the purpose of studying the lightning current division in an 829-m-long, 18-pole, three-phase plus neutral, unenergized, overhead distribution line equipped with six arrester stations. Eight lightning flashes containing a total of 34 recorded return strokes, as well as low amplitude, long duration steady currents, were artificially initiated (triggered) from natural thunderclouds using the rocket-and-wire technique, and the flash currents were directed to phase C (one of the two outer conductors of the three-phase cross-arm-configured line). Six of the eight triggered lightning flashes caused damage to one of the two closest phase C arresters. In the case when no arrester was damaged or was not yet damaged by current in the flash, it is inferred that about 40% of the return stroke peak current and about 25% or more of the return stroke charge transferred in the first millisecond passed to the neutral conductor through each of the two closest arresters located about 70 m away on either side of the strike point. The bulk of the peak current then flowed from the neutral conductor to ground through the groundings of the two closest arresters. The charge transferred in the first millisecond from the neutral to the eight system groundings, six at arrester stations, and one at each of the two line-end poles, appears to be distributed inversely to the low-frequency, low-current grounding resistances. From our measurements of return stroke current division and in view of the available data on the currents of first strokes in natural lightning, we estimate that over half of natural first strokes would result in an arrester failure in our test distribution line, which is representative of some distribution lines in service, within about 450 /spl mu/s of the initiation of the first return stroke current flow, in the absence of flashovers and other alternative current paths that might be provided by transformers or underground cable connections to allow the stroke current to bypass the arrester. Additional first stroke current flow beyond about 450 /spl mu/s and currents associated with subsequent strokes and potentially other processes should further increase the likelihood of arrester damage.