Mechanism of “meandering” breakdown of insulation in an electrical machine

The core of large turbo-type generators is made up of cold-reduced low-loss silicon steel laminations, insulated by several varnish layers and located within a frame by longitudinal brass keybars, and held in compression by means of core end plates. From time to time the insulation is subjected to appreciable voltage stresses, highest near the ends of the core where leakage fluxes are greatest. Localised breakdowns of insulation between two adjacent laminations may then occur and increase eddy current losses. Direct monitoring of interlaminar voltages is difficult (1) and hence detailed examination of individual laminations on machines is the main source of information for the understanding of breakdown mechanisms (2,3). During such an examination, examples of a form of interlaminar breakdown not previously reported were found, and described in a previous paper (3). Damage revealed by scanning electron microscopy (SEM) was compared with that caused by arc discharges, but there were features of wormlike metallic filaments which could not be explained.