Distribution of real lightning strokes and their influence on test procedures for isolated LPS

In early 2010 IEC TC81 decided to set up PWI 62561-8 in order to extend the IEC 62561 family by a new part 8 ("Requirements for components for isolated LPS"). Currently two main insulation technologies are discussed in the standardization process. The first technology uses "Insulating stand-offs" and conventional down conductors, the second one uses "Insulating down conductors". One of the main test procedures that have to be defined in the new standard is the test of dielectric strength of the different technologies. Comprehensive laboratory tests show that the isolation capability in both technologies depends on the dielectric strength of the insulator material, on the surface discharge behaviour but also very much on the steepness and duration of impulse voltages [1]-[4]. The results show a very significant influence of the impulse duration which is in the relevant range for lightning strokes that depends strongly on the technology used. The calculation of separation distances for isolated LPS, being based only on one single pulse duration of 250 ns which is related to the negative subsequent strokes (NSS) according to the current IEC standard 151, is not sufficient. The negative first stroke (NFS) with a pulse duration of 1 μs has been included recently in the new IEC 62305 series 161. But this NFS has not been considered for the calculation of separation distances up to now and has got a significant impact on this calculation [1][7]. As a new approach the total probability of failure will be classified for typical insulation technologies by means of correlation of their v-t-curves. These curves describe the dielectric strength of the specific insulation technology used for LPS compared to the distributions in [6] covering the occurrence of real lightning. Finally a correction factor is introduced as a simplified attempt to classify isolated LPS in a way that they have the same efficiency compared to conventional LPS concerning their equivalent separation distance. This factor is to be discussed as a basis that enables scaling tests of the dielectric strength of components for insulated LPS with standard impulse voltages.