Sensitive ground protection for radial distribution feeders

Abstract

the oil presents an impulse ratio of from 2 to 3, whereas the parts of the breaker which are in air have impulse ratios of the order of 1.2." In other words, what we have done is verify the fact that oil under impulse is not a limiting factor in design be cause oil strength under normal and re covery voltage has taken care of the impulse factor. E. A. Walker's discussion calls attention to the importance of giving careful attention in making these studies to the amount of moisture in the oil under test and the form in which the moisture is present. Very early in our experiments, as was stated in the paper, we found that breakdown values were very sensitive to the oil moisture con tent. So far we have simply avoided most of the difficulties of variability due to this moisture content by using only oil of high dielectric test; for example, oil which would stand 30,000 to 40,000 volts when tested with a standard 0.1-inch oil testing gap. In fact, as Mr. Walker suggests, there is no doubt a fertile field for investigation to de termine the exact quantitative influence of water content and also of temperature on surge-voltage-breakdown values for oil.I do not think, as Mr. Walker has stated, that we have at all overemphasized the ad vantages of using a test tank of material which has a dielectric constant approxi mately the same as that of oil, because after all the prime reason for the use of a tank made of insulation material rather than metal is the avoidance of the difficult lead-in bushing problem and the use of the large amount of oil required when steel tanks are used. The fact that the dielectric constant of the insulation material used is so nearly the same value as the constant for trans former oil and that the dielectric constants for both the oil and the insulation material is only two to three times the constant for air, is a happy coincidence which makes our plan practical; whereas it would not be so good if we had to use a barrel made of some material such as porcelain, which has a much higher dielectric constant than the material of which our test barrel is made.At present I have no explanation for the phenomena described in Mr. Harrison's discussion and can only confine my remarks to expressing appreciation of the large amount of careful, conscientious work which he has done in obtaining for me much of the data used in this paper.Mr. Lovoff also has added to the paper data which he has obtained and which ex tend the curves used in the paper to values beyond those available at the time the paper was written. To him and to other stu dents who have helped in this work, I am deeply indebted, for without their help the large amount of testing necessary to produce this report could not have been obtained. In fact, like most research work of today, the results presented in this paper are available only because of much co-operation by many persons and it would certainly be unfair to close our discussion without expressing ap preciation to the Kelman Electric and Manufacturing Company, which provided much of t...

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