Handbook of Electrical Installation Practice - BeKnowledge

Distribution Transformers 319
Table 13.1 Preferred values of
rated power for three-phase
transformers.
There are two basic types of distribution transformer in common use today: one
in which the core and windings are enclosed in a liquid-filled tank which provides
cooling and insulation, and the other where the core and windings are cooled
directly by air. Liquid-filled transformers can again be sub-divided into units using
flammable substance (mineral oil) and those using various types of fire-resistant
liquid (silicone liquid or synthetic hydrocarbon).
Dry-type transformers are also available in two distinct types: those with the insulated
turns of the winding directly in contact with the cooling air, and those with
the complete windings encapsulated within a moisture-resistant cladding of epoxy
resin. Preferred ratings are listed in Table 13.1, but dry-type distribution transformers
are rarely used in ratings much below 500kVA. The ratings of individual
transformers within installations are determined by load requirements, the physical
space available, the distribution system voltage regulation, and the high voltage
switchgear fault rating.
The heart of the conventional three-phase distribution transformer is a laminated
electrical sheet steel core of three limbs. Each limb carries two concentrically wound
coils: the secondary (low voltage) inside nearest the core and the primary (high
voltage) on the outside, the whole being contained within an enclosure of steel. In
the case of the liquid-filled transformer the enclosure consists of a leak-proof
tank while the dry-type transformer merely requires a ventilated casing to enclose
the live parts. Some designs of dry-type transformer are totally enclosed in nonventilated
sheet steel housings, and have an IP rating of 55 to BS EN 60529. These
transformers are used in installations where environmental contamination would
preclude the use of conventional dry-type designs, but their superior resistance to
fire is required.
Tappings are usually included on the h.v. windings of distribution transformers
to alter the turns ratio between the high- and low-voltage windings and thereby
to compensate for variations in primary supply voltage in order to maintain the consumer
voltage within the statutory limits. On distribution transformers taps are
selected by means of an off-circuit device; the transformer must be disconnected
from the supply before the taps can be changed. The normal variation in supply
voltage provided for by high voltage tappings is ± 2.5% and 5%. Dry-type