Streamer propagation study of surface creepage in transformer insulation materials

Ester fluid has potential for being used as an environmentally friendly alternative to mineral oil in transformers. However the breakdown strength and surface creepage properties of ester fluids are not entirely understood. A good test method is needed in qualifying the different liquids and the creepage properties of thin solid insulation materials. In this paper a test method designed for studying the streamer propagation along the interface between solid insulation material and oil is evaluated. A simple method with cylindrical geometry is introduced. The interface between polyaramide solid insulation and fluid insulation has been compared using lightning impulse voltage (1.2 / 50 mu s). Both ester fluid and mineral oil have been studied. Weibull distribution of the result data showed higher shape parameter than e.g. U-shape electrode test set-up. Furthermore, both 2nd and 3rd streamer modes were detected. All the streamers were initiated in the oil wedge between electrode and solid material surface.