Measurements on pressboard to understand the effect of solid insulation condition on monitoring of power transformer winding clamping pressure

Abstract

Pressboard is the main building material of a power transformer winding structure. Through-thickness compression behaviour of pressboard has a direct influence on the transformer winding clamping pressure. Being a cellulose based polymeric material with a porous structure, the through-thickness compression behaviour of pressboard is highly non-linear. Also, the mechanical properties of pressboard are moisture, temperature, and ageing status dependent. However, information on the exact behaviour of the through-thickness compressibility of pressboard under typical operating conditions of a power transformer has not been well documented in the literature. This has posed difficulties in developing reliable techniques to detect loose clamping conditions in power transformer windings. This study presents the results of laboratory measurements on the effect of moisture temperature and ageing on the through-thickness compression behaviour of oil-impregnated pressboard. From the measured results, it was found that the compressibility of pressboard could increase with increasing temperature, ageing and moisture ingress. It was also found that moisture and ageing tend to exaggerate the thermal softening of pressboard. Subsequent finite element analysis confirmed that transformer clamping pressure has significantly altered with the pressboard condition.