Dielectric loss-angle measurement of multi-core high-tension cables, with special reference to the Schering bridge

Abstract

In section (1), the special difficulties of dielectric loss-angle (D.L.A.) measurement on multi-core cables are examined. The conclusion is reached that an iner-admittance, say between one core and another, cannot be considered to have associated with it a definite D.L.A., but will have a varying D. L. A. with different stress distributions. Measurements mede with one stress distribution cannot be used to foretell the D. L. A. with any other; and 3-phase D. L. A. cannot be precisely estimated from measurements made with single phase currents.In section (2), a new method of obtaining the inter-D. L.A. in conformity with the conclusions of section (1) is described. The Schering bridge equations are fully worked out and the corrections for various stray capacitances are given.In section (3), it is shown that the wavelength effect appreciably affects the apparent D.L.A. when the cable exceeds in length a small fraction of the wavelength. Corrections on the apparent D.L.A. are given.