The advancement in knowledge of lightning protection, as developed by the investigations in Chicago since the 1920 paper on the same subject, has been largely. due to an intensive study in the past five years made with the active cooperation and assistance of the manufacturers of lightning arresters. The development in recent years of methods of laboratory tests with the lightning generator and the cathode ray oscillograph render unnecessary a continuance of these statistical research investigations. The examination of 474 transformers burned out by lightning indicates the desirability of line transformers with non-aging insulation which will be lightning-proof against induced discharges when protected by the modern types of arresters. A comparison of the troubles on the distribution system due to lightning in Chicago with similar results in other cities indicates that considerable advantage is obtained by the use of the four-wire three-phase system with grounded neutral. Induced discharges of 400 kv. crest value have been noted. Direct strokes account for only 5 per cent of transformer troubles. No evidence could be found of shielding effect against induced discharges due to adjacent high trees, buildings, or other structures. Traveling waves are not a factor in the transformer troubles on the distribution system in Chicago. Modern types of lightning arresters excel the best type of 20 years ago in reduced annual charges, but not in efficiency as a protective device. Extra expense to reduce resistance of lightning arrester grounds below 50 ohms is not warranted.

Studies in Lightning Protection on 4,000-Volt Circuits-III

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.

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

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