Dielectric properties of fibrous insulation as affected by repeated voltage application

It is believed by many that the most plausible explanation of fibrous insulation failure is the pyroelectric theory. Under this theory as elaborated by Steinmetz, Wagner and others, insulation under stress is heated by the transformation of electric energy to thermal energy. Insulations are considered as of the nature of poor conductors and subject to the same characteristics. The transformation of electric to thermal emergy is therefore dependent upon the inherent electrical and thermal conducting properties of the material. This transformation proceeds at a rate proportional to the stress applied, until such a voltage value is reached where the heat is generated in the insulation at a rate faster than it is dissipated to the surrounding medium. Further increase in voltage leads to a rapidly mounting temperature with ultimate insulation failure. A strict interpretation of this theory would indicate that insulation failure is a matter of the insulation resistance — temperature relation. Therefore, as the heat stored in a dielectric during stress is allowed to dissipate, care being taken to prevent injury from the testing electrodes, etc, the orignial properties of the material should be restored. This has been found to be true only to a limited extent. The present paper deals largely with the effect of relatively high-voltage applications on sheet insulation tested between parallel plate electrodes. It is shown that the question of the mechanism of insulation failure can be separated into two parts. First, failure caused by short-time voltage applications as determined by rapidly applied tests, and secondly, failure of insulation under longer periods of stress as determined by the minute or endurance test methods. Peek has found that voltages greatly in excess of the “rapidly applied” 60-cycle puncture voltage may be applied to the insulation without rupture, if the time of application be sufficiently short. All such over voltages injure the insulation, “probably by a mechanical tearing and the effect is cumulative.” In these experiments of short duration, the problem of heat storage in the insulation is eliminated. Raynor has found that the rapidly applied strength of insulation is greatly lowered by the previous application of a high voltage. This decrease in strength, however, is lost, given sufficient rest period between the initial and final voltage application. These results are substantiated in the work of the present paper. The length of rest period is shown to be proportional to the initial test voltage applied. For tests of long duration, the work of this paper has been divided into two parts, — those voltages producing failure with an arbitarry time limit (16 minutes) and those voltages which are able to be applied for an indefinite time without a puncture. According to the pyroelectric theory, the first class deals with those voltages of such value as to produce a slowly mounting temperature rise in the insulation which ultimately reaches a value leading to rapidly decreasing insulation resistance and total loss of dielectric strength. The second class includes those voltages of such value that the rate of transformation of electric to thermal energy is equaled by the rate of dissipation of the heat so formed; thus preventing heat storage in the insulation. According to the thermal theory of breakdown, neither of these voltages should produce permanent injury to the dielectric, — the first, if removed before the stage indicating rapid loss of insulation resistance is reached, and the second, even if applied indefinitely. The present paper shows that the application of voltages of either of the above types leads to deterioration in dielectric strength of fibrous insulation, even aside from such effects that might be traced to corona or mechanical injury to the material. The effect for voltages of the first class is cumulative and expressed by the formula R × T = K where R = number of repetitions of voltages, T the time of each voltage application, and K the time needed to puncture for the same voltage continuously applied. The application of this formula to various insulations, its limitations, etc., are discussed. The work detailed herein concerns the application of a-c. voltages at room temperature. The investigation is being extended to cover both low and high a-c. and d-c. voltages at and above room temperatures.