Abstract
The following paper describes the results of experiments carried out by the author as an application of the electroscope method introduced by Prof. A. Schwaiger for measuring electric field and elucidates the results as applied to a model of a three-phase cable to show that the method is quite practical and accurate for the purpose of measuring electric field in general and at the same time the author intends to illustrate the configuration of the lectric field in the three-phase cable and tries to find out simple mathematical expressions for maximum electric stress, especially for the tangential stress, possible to occur in the dielectric of a three-phase paper cable.
Important results obtained from the research are as follows:-
1. The Schwaiger's "Electroscope Method" is a suitable one for the experimental solution of determining the distribution of lines of force in a complicated electrostatic field and gives accurate results (the experimental error can generally be expected to be less than 1%).
2. From experiments it is established:-
The maximum normal stress in the paper insulation of a three-phase paper cable is given by the formula (A) while the maximum tangential stress will be given by the formula (B). Therefore, the ratio between the two stresses is expressed by the formula (C),
where Vmax=the maximum potential of a conductor against the earth.
r=the radius of the conductor
m=ratio thickness of insulation/diameter of the conductor.
Thus the smaller the ratio, thickness of insulation around the conductor to its diameter, the more necessary it is to take the tangential stress into account in designing the cable insulation. Further the value of k given by the formula (C) is such that it should always be taken into account, because the tangential dielectric strength is far inferior to the normal dielectric str ngth of the paper insulation of the cable.
