A New Research into the Voltage Endurance of Solid Dielectrics

A new method of investigation into the voltage endurance of solid dielectrics is described. This method is founded upon measurements of the short-time electric strength of specimens prestressed by the same magnitude of ac voltage but for different times. By plotting electric strength against the aging time and comparing the experimental points with theoretical patterns, obtained by a theory previously put forward, determination of the coefficient of voltage endurance n is possible in a short time and with good precision. To check the results, other means are also used. The experimental data are plotted on the Weibull paper in order to determine the dimensional coefficient b which is closely related to n. Measurements of nondestructive properties are carried out, too; according to the aging theory remarked above it seems possible, in fact, to determine the coefficent n by means of such measurements. After showing the theoretical background of the new research, experimental data, pertinent to solid insulating materials for medium-and high-voltage cables, are reported. For polyethylene (PE) the data agree very well with the theory, for a value of n equal to 13. Ethylene-propylene rubber (EPR) shows an uneven period in the first hours of stressing, after which the material seems to settle with a high value of n, larger than 20. In both cases the new method has shown its great usefulness in arriving at the result in a very short time and with high reliability.