Handbook of Electrical Installation Practice - BeKnowledge

Distribution Transformers 325
Fig. 13.4 Silicone-cooled distribution transformer; 2200kVA complete with output
circuit-breakers (Goodyear Transformers Ltd).
fittings for synthetic fluid cooled transformers, as materials and components used
with mineral based oils may not be compatible.
Transformer manufacturers are looking at the future designs of synthetic fluidfilled
transformers in order to take advantage of the high temperature capabilities
of these transformer coolants. Such transformers may well result in savings in
weight, size and cost, as it will be possible to make better economic use of these
expensive materials.
Dry-type transformers
Dry-type transformers are defined by IEC 726 as designs where the core and windings
are not immersed in an insulating liquid. Cooling is usually by natural air circulation
through the windings. Permissible winding temperature rise depends on the
type of insulation used on the winding conductors and between the windings.
Transformer windings with Class A insulation are limited to a temperature rise
of 60°C. When Class C insulation is used 150°C is the limit. All dry-type transformers
with Class B insulation and above are considered to be fire resistant to