How to Protect Hardened Facilities from Very High Level Transients Induced on Power Line by Lightning or EMP

Abstract

In 1 ightning or EMP protection problems it is sometimes necessary to take into account very high level trans­ ients collected by the commercial power line. These high transients may occur in cases of direct lightning strikes on the line, or in cases of EMP due to low altitude burst or ground burst close to the 1 ine. In these cases the classical protection devices are not adequate because in general arresters cannot withstand energies higher than a few tens kJ. The goal of the presentation is to show the limita­ tions of classical protection devices and to present different solutions to solve this problem. 1. LIMITATIONS OF CLASSICAL PROTECTION DEVICES Experience has shown that an effective way to deal with abnormal lightning or EMP overvoltages on power lines is to divert them to earth by the means of surge arresters. Surge arresters are constitued of sparkgaps in series with silicon carbide blocks which form the basis of a non linear resistor so that currents due to the n o r m a l voltage could be extinguished by series gaps while the overvoltages are limited by the much larger currents they can divert to earth. While gaps are able to divert to earth large amount of charge (in terms of coulombs), surge arresters have a lower limit for charge flowing because of energy deposited in the silicon carbide block. This limit in charge flowing capability is also dependent on the number of discharges and time beetween discharges. To increase this capability is not easy because the designer must deal with many parameters such as : functionnal service voltage, time response of the arrester, maximum current discharge withstanding, maximum voltage drop in the arrester when current is f1owing, mechanical and geometrical characteristics taking into account insulation levels, characteristics of the power network (neutral earthing, etc ...). This means that the order of magnitude of charge withstanding of 240/415 V arresters is in general tens of coulombs or in terms of energy tens (up to a few hundreds) of kJ. It is also necessary to take into account that spark gaps and arresters keep some memory of the previous charges that they have had to withstand.