Protection of Power Transformers Against Lighting Surges

This report presents a brief history of the subject of protection of power transformers against lightning overvoltages. The nature and magnitude of these overvoltages are described pointing out which ones are the most dangerous to apparatus. The impulse voltage characteristics of transformers are given including volt-time curves, effects of (a) steepness of applied wave, (b) polarity of wave, and (c) mechanical deterioration of insulation in old transformers. Similarly, impulse voltage characteristics of lightning arresters, rod gaps, and protector tubes are shown, thereby enabling the user to intelligently apply these devices for the protection of transformers. The question of how to obtain various degrees of protection is fully discussed, ranging from the most effective protection to partial protection by using one or more of the following schemes: (a). Direct-stroke protection and lightning arresters installed. (b). Lightning arresters installed but direct-stroke protection omitted. (c). Direct-stroke protection and station gaps installed, but lightning arresters omitted. (d). Station gaps installed with neither direct-stroke protection nor lightning arresters. Finally the proper selection of surge protective devices is discussed for the purpose of enabling the user to select the protective scheme that best suits his requirements.