Impulse Voltage Tests on a 4,800-Volt Distribution Substation

Lightning voltage flashovers have been experienced in substations feeding a 4,800-volt ungrounded distribution system. Lines enter the substations through three-phase, underground cables varying in length from a few hundred to about nine hundred feet. Lightning arresters are connected at the substation bus and at the junctions of overhead lines and entrance cables. The arcovers have consistently occurred on lines equipped with voltage regulators and at points between the line terminal of the regulator and the station end of the entrance cable. Impulse voltage tests were made on one of the substations which had been taken out of commission, to determine the cause of the flashovers and to find a remedy. Surge voltages from a lightning generator were sent into the station through the entrance cable of a circuit equipped with a voltage regulator and volt-time oscillograms were taken at the principal points of interest. Oscillograms are presented which show that the impedances of the regulator series windings to surges were high enough to cause reflections of the incident wave and a consequent building up of voltage at the station end of the entrance cable. It is also shown that the energy drawn from the cable through the regulator series windings is so small for the early part of the wave that the lightning arresters on the bus cannot protect the cable against high-voltage surges which may enter the substation.