Contribution to the electrical characterisation of potential grading varnishes

Potential grading materials are often characterised by their current-voltag e curves. In this paper, other physical quantities such as the surface pote ntial and the variation of the stator bar impedance with applied voltage ha ve been used to characterise the varnishes. These measurements enable the e valuation of the tangential electric field along the end-windings of stator bars, The transmission line model has been employed to provide simple anal ytical relations between the different physical quantities which characteri se a grading varnish. The equivalence between the current-voltage, surface potential and stator impedance measurements in the characterisation of the varnishes has been deduced from the theoretical calculations and has been c onfirmed experimentally. We also discuss the possibility of using capacitan ce-voltage measurements on real rotating machines to evaluate the state of ageing of the varnishes. (C) 2000 Elsevier Science B.V. All rights reserved .