Distribution transformer performance when subjected to steep front impulses

Arrester-protected pole-type distribution transformers were subjected to the combined stresses caused by the conducted steep front impulse and rated 60 Hz voltages. The results are compared with those of similar experiments on unenergized distribution transformers and with low-voltage impulse wave distribution results. It is concluded that (1) tank-mounted direct-connected arresters provide a high degree of protection to these transformers, (2) tank-mounted externally gapped or crossarm mounted arresters may not provide sufficient protection, and (3) internal transformer insulation system failures caused by the steep front impulse do not necessarily result in immediate catastrophic failures of the transformers