Low frequency measurements on generator insulation

Abstract

This thesis describes an experimental arrangement which has proved successful in the measurement of one of the most significant quantities as regards the deterioration of insulation, namely discharge energy loss/cycle. The arrangement was used at very low frequency (0.1 Hz) and recorded discharge energy loss/cycle in thermally aged sections of 500 MW stator bars with micaceous insulation. The purpose of thermally ageing the stator bar sections was to create both large and small voids within the insulation, the intention being: (1) ' To measure the change of discharge energy loss/cycle with ageing time of the sections. (2) To differentiate between both the large and small voids created in the sections. Whilst it was possible to show the increase of discharge energy loss/cycle with ageing time, it was not possible, with the present experimental arrangement, to detect voids of differing dimensions. This was because of surface discharge across the guard electrode interspace, and lack of precision in the measurements. However, it is believed that a proposed system will be more successful. The present experimental arrangement responds to both large and small amplitude discharge pulses and is more sensitive than conventional integrating instruments used at power frequencies. Tan Ϭ due to discharge was measured on the stator bar sections at both 50 and 0.1 Hz, but on differing experimental arrangements. Apart from the initial ageing times of the specimens, varied approximately linearly over the range investigated, although they differed by a constant amount. Some of the difference was accounted for by only measuring half the discharge energy loss/cycle because of commutation noise from the VLF generator, the other being unaccountable and due to the measurements of similar, but not necessarily equal quantities on different experimental arrangements. During the experimental work, discharge inception voltage was found to be invariant with frequency.