Nonlinear dielectric response of vented water treed XLPE insulation

This paper describes experiments performed on laboratory aged Rogowski test objects. Results from measurements of residual AC breakdown strength, degree of water treeing and the dielectric response are presented. The main reason for performing laboratory ageing was to facilitate diagnostic testing as a function of ageing time of samples containing mainly vented water trees. A new method, based upon inserting NaCl particles at the interface between the upper semiconductive screen and the insulation, was found to successfully enhance initiation and growth of vented water trees. The results show that vented water treeing was found to cause both the time and frequency domain dielectric responses to become nonlinear at a low test voltage, and to reduce the breakdown strength of the test objects. However, the dielectric response in the frequency domain was found to be more nonlinear than in the time domain.