The protection of solid insulation by lightning arresters

Introduction THE protection of distribution transformers or other apparatus against lightning involves the basic problem of limiting the voltage applied to the insulation to a value adequately below the insulation strength. The effect on insulation of chopped waves and full waves of standard impulse wave shapes such as 1.5×40 microseconds has been thoroughly investigated in the past. However, there has been a lack of the information necessary to compare the effect of chopped waves with the effect of voltages of other than standard wave shape resulting from arrester breakdown followed by the impedance drop that occurs during the impulse current discharge. Also there have been little data available on the failure of low voltage insulation. For these reasons an investigation has been made using oil immersed solid insulation of a type used in distribution transformers. The puncture voltage and number of shots to failure were determined when the insulation was in parallel with lightning arresters or gaps which controlled the wave shape of the voltage applied to the insulation. These results have been correlated with the single shot puncture voltage determined from tests in which failure occurred on the wave front. It was found that repeated applications of voltages in excess of approximately 70 per cent of the single shot puncture voltage may damage the insulation.