Experimental Determination of the Effects of Steep Front-Short Duration Surges on 25 KVA Pole Mounted Distribution Transformers

Steep front-short duration (SFSD) surges have been implicated in the failure of distribution transformers. Researchers, through calculations and measurements, have determined that surges with rise times of less than 100 nanoseconds may occur from such causes as lightning, SF6 switching, and nuclear electromagnetic pulse. This paper documents a series of directly-injected, SFSD surge tests performed on standard, commercial distribution transformers. The tests were conducted at Maxwell Laboratories, San Diego, California in October, 1986. The objectives of the tests were to: 1) verify the insulation integrity of a small sample of pole-mounted distribution transformers of types commonly used by electric utilities, 2) subject these transformers to directly injected SFSD electrical surges, 3) determine the dielectric withstand capability of the transformers to such surges, and 4) complete the evaluation of transformer failure modes through disassembly of failed units at a transformer manufacturing facility. A series of SFSD impulse tests were conducted on 19, 25-kVA, 12470Y/7200-volt distribution transformers. The impulse test voltages at the terminals of the high voltage pulser were 400 kV, 500 kV, 800 kV, and 1000 kV open circuit with rise times of approximately 60 ns and decay times to half-crest value of approximately 2000 ns. The voltage was applied to the transformer under test through a 400-Ohm series resistor. This series resistor represented the distribution line surge impedance. These tests were conducted both with and without commercially available gapped silicon carbide surge arresters installed on the transformers.