The electrical characteristics of impregnated cable papers

IN CONNECTION with the investigation of ionization in impregnated paper insulated cables conducted at The Harvard Engineering School under auspices of the impregnated paper cable research committee of the National Electric Light Association, it was found necessary to determine over a considerable range of voltage gradient, temperature, and frequency, the electrical characteristics of cable paper impregnated with different cable oils and compounds. The impregnation was conducted under almost ideal laboratory conditions so that little if any gas was occluded. When the results of the measurements were rationalized and analyzed, it was found that to a remarkable degree they all conformed to the same general laws. These laws and the analyses of the electrical characteristics into components may appear to be only empirical. However, they are results of some fundamental causes, involving probably molecular and intramolecular reactions. Therefore, aside from any value which the results presented may have as engineering data, they may at some time be of value in confirming or in disproving some more fundamental theories of dielectrics. Moreover, it is found that actual cables when impregnated so thoroughly that they manifest no appreciable ionization, have electrical characteristics comparable to those obtained with these samples impregnated under almost ideal conditions. The conclusions presented in this paper are as follows: 1. An abrupt change in the slope of the cooling curves of some cable compounds occurs in the neighborhood of 50 deg C; this may indicate changes in the molecular structure. 2. It appears to be a general law that the power loss in cable compounds varies as a constant exponential power of the voltage gradient, the exponent usually being 2 at room temperature; at higher temperatures it may be greater or less than 2. 3. The power factor characteristics of cable compounds are exponential functions of the voltage gradient, the exponent being related to that for the power characteristics. 4. The power-frequency characteristics are essentially linear, having a positive intercept at zero frequency. 5. The power factor-frequency characteristics are essentially rectangular hyperbolas. 6. The equivalent series resistivity-voltage gradient characteristic is the same type of function as the power factor-voltage gradient characteristic. 7. The equivalent parallel conductivity-voltage gradient characteristic is the same type of function as the power factor-voltage gradient characteristic. 8. The power-temperature characteristics for constant frequency and constant voltage gradient appear to consist of 2 terms, each of which is a constantexponential function of the temperature. (A.I.E.E. paper No. 33–58)