Possibilities to protect pole-mounted distribution transformers by means of externally gapped MO arresters

Currently Finnish standard SFS2646 requires a special steep front impulse test for pole-mounted distribution transformers protected by spark gaps or externally gapped metal oxide arresters (EGA). These requirements are quite stringent (2000 kV/μs) and it is thus necessary to use specially designed transformers with this kind of overvoltage protection. It might be possible to decrease the required test voltage steepness and EGA spark gap setting which makes it interesting to study whether the standard transformer designs could be used with EGA protection, reducing the amount of different transformer designs stored. In order to investigate the possibilities to protect standard transformers with EGAs, windings of two transformers were tested using different impulse steepnesses and spark gap settings. The results provide a general indication of withstand and protection characteristics of transformers and EGAs.