Measurement of charge distributions in coaxial structures-application to high voltage cables

The authors present direct measurements of charge distributions in coaxial cables conducted during the application of a high voltage, using the pressure wave propagation (PWP) method. It is shown that, as could be expected, the main effects take place near the inner electrode, where the applied electric field is the largest. Near this electrode, charges are transferred which lead to the creation of homocharges and to a decrease of the interfacial electric field. Heterocharges build up with time and compensate the effect of the homocharges on this electric field. A stabilized state is reached at 50°C after approximately 120 hours. Heterocharges of two types have been observed, some localized very close to the inner electrode, the others extending over a quarter of the insulator. The origin of the first type can be attributed to the diffusion in the insulator of impurities present in the inner semiconducting electrode impurities dissociated by the local electric field. These results demonstrate that the PWP method can be applied to the study of high-voltage cables