This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps (

The a p p l i c a b i l i t y of the ANSI loading guide f o r power transformers is l i m i t e d t o ambient temp e r a t u r e s above O ' C because t h e thermal model does not account f o r v a r i a t i o n s in t h e o i l v i s c o s i t y and winding r e s i s t a n c e . A p r a c t i c a l c a l c u l a t i o n method has been developed based on t h e bottom-oil temperature and the s u r f a c e temperature a t t h e top of t h e cooling duct. R e s u l t s i n d i c a t e t h a t t h e proposed thermal model could provide a sound b a s i s f o r c a l c u l a t i n g t h e overload c a p a c i t y f o r ambient temperatures below O°C OK f o r s e v e r e overloads of s h o r t duration.

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A relatively high failure rate of the internal insulation of EHV power transformers has been reported by public utilities. This prompted a study of the safety margin of EHV transformers in service and under test. In this project, a number of transient overvoltages were digitally recorded in EHV substations and the transient voltage distribution along the HV winding was measured digitally on many untanked transformers.With this information, a transient induced at (and across) every disc of the winding by a system-generated overvoltage can be calculated and compared to a transient appearing at the same winding section during the impulse test. The pertinence and severity of the standard impulse test can be assessed from this comparison.Subsequently, a dielectric stress in the winding insulation was calculated with the aid of an electric field plotting program. A critically stressed path was then selected in the winding insulation, and a cumulative stress computed along this path. The insulation safety margin was subsequently determined by comparing the cumulative stress to a withstand reference curve for paper-oil insulation. The reference curve for the bulk oil insulation, and for the creep stress along a spacer or barrier was derived from published works.