Handbook of Electrical Installation Practice - BeKnowledge

The development of modern plastics such as polyvinylchloride and polyethylene,
etc. and their use in the cable industry has done much to simplify cable manufacture,
cabling systems and termination. Almost all l.v. cable boxes are now air-filled
with a simple compression gland securing the metal sheath of the cable. Low-voltage
transformer terminations are usually an ‘all-in-one’ epoxy resin bushing plate
assembly with bolt-on connections.
The compound filling of 6.6, 10 and 11kV cable boxes on distribution transformers
has remained in many areas of the UK because the most common 6.6 and 11
kV cable is still paper insulated. Developments in the field of plastics have produced
cross-linked polyethylene cables where air-filled boxes with heat shrinkable sleevings
eliminate the need for compound filling and greatly simplify the cable termination
to transformers.
Dry-type transformers supplied in their own ventilated enclosures are generally
treated as liquid-immersed transformers and have cable boxes mounted on the
outside of the ventilated enclosure. Where the dry-type transformer forms part of
a complete installation on h.v. and l.v. switchgear, it is often required for the transformer
to be supplied without an enclosure; h.v. winding connections are terminated
in bolted connections just above the core and windings and l.v. connections are generally
terminated on busbars.
Cable-end terminations for installations of this type can still follow the same form
as the tankside cable box but with the three-phase bushings arranged and spaced
for copper connections in air.
The resin-encapsulated transformer was developed originally in Germany where
open installations are more commonplace. Special plug-in epoxy or moulded
polymer connectors and bushings, incorporating electrical stress relief screens, have
been developed by some companies and the incoming h.v. cable is simply laid into
a moulded cable holder and the bared conductor bolted to metallic inserts.
COOLING
Distribution Transformers 343
Transformers are identified according to the cooling method employed and the
letter symbols used are indicated in Table 13.9.The simplest form is where the windings
are cooled by natural air flow over the heated surface of the windings and core.
These are heated by the load and no-load losses respectively and the heat is transferred
to the surrounding air by convection and radiation. This type of cooling is
described as air natural or AN by reference to Table 13.9.
The natural movement of air over heated coil surfaces is not particularly efficient.
Winding conductors, disc- and coil-separator winding shapes, and the roughness of
conductor insulation all help to create eddies in the air flow over the windings and
reduce the heat transfer from the winding to the air. Even a small amount of forcedirected
air flow over the windings improves the heat flow, and ratings are increased
significantly.
A dry-type transformer cooled by direct forced air is designated as AF. A drytype
transformer which has natural cooling and the facility for automatic fan operation
should the temperature of the windings increase beyond normal limits has two