Experimental determination of the effects of steep front-short duration surges on 25 kVA pole mounted distribution transformers

Nineteen standard commercial 7200 V 25 kVA distribution transformers were tested to determine vulnerability of their insulation systems to steep-front short-duration surges. These transformers were tested both with and without surge arresters using a high-voltage pulser. The open-circuit pulser test voltages were 400 kV, 500 kV, 800 kV, and 1000 kV. These voltages exhibited an approximately 60 ns risetime and approximately 2000 ns to half value. Standard lightning impulse tests for this voltage class of distribution transformers were conducted prior to the steep-front surge tests to verify insulation integrity, and were repeated following the steep-front surge tests to ascertain if an insulation failure had occurred in the tested transformer due to the steep-front impulse. Failed transformers were disassembled to evaluate failure modes. It was determined that no insulation failures occurred on any transformers tested that were protected by surge arresters mounted directly on the transformer. Unprotected transformers failed at voltages around 250 kV to 300 kV peak. The failure mode was usually an internal flashover/puncture between the first few turns and the low-voltage winding or, sometimes, an inner layer of the high-voltage winding