GB833790A - Current transformer - Google Patents

Abstract

A current transformer (see Group XXXV) is encapsulated in an epoxy resin. One example comprises 29.2% by weight of epoxy resin (e.g. the reaction product of epichlorhydrin and bisphenol A), 6% of a plasticizer such as a long chain carboxylic acid, 7.8% of phthalic anhydride as cross-linking agent and 57% of electrical grade flint (98% through a 200 mesh screen, 99.8% pure silicon dioxide). The epoxy resin and plasticizer are separately heated to 300 DEG F. and mixed, whereupon the predried flint and phthalic anhydride are added and mixed for 5 minutes at 300 DEG F. Other mouldings in the transformers are formed from a resin comprising 34% by weight epoxy resin, 10% phthalic anhydride and 66% flint. Other fillers, for example slate, quartz and clay, may be used.ALSO:A current transformer (see Group XXXV) is encapsulated in a casting resin comprising epoxy resin together with a plasticizer such as a carboxylic acid, phthalic anhydride, and a filler such as slate, quartz, flint or clay. The mould may be polished aluminium or brass and provided with a silicone releasing agent or silver plated. The transformer is dried for two hours at 320 DEG F. in a forced circulation oven and assembled with the resin in the mould. It is then placed in a heated vacuum tank and evaporated to 29-25,5 inches of mercury at a temperature 300-320 DEG F. and then cured at atmospheric pressure for 16 hours at 200-260 DEG F. After release from the mould, the cast unit is further cured for 4 hours at between 250 DEG F. and 260 DEG F. Rubber covered cushions of crumpled aluminium foil which are disposed about the transformer are compressed as the resin contracts on cooling. One example of resin comprises 29,2% by weight epoxy resin 6% of carboxylic acid, 7,8% phthalic anhydride and 57% flint. The epoxy resin and plasticizer are separately heated to 300 DEG F. mixed, added to the flint and phthalic anhydride and mixed for 5 minutes at 300 DEG F. Preformed bosses which support the core of the transformer are moulded from a resin comprising 34% epoxy resin, 10% phthalic anhydride and 66% flint.