Handbook of Electrical Installation Practice - BeKnowledge

320 Handbook of Electrical Installation Practice
distribution transformers are normally provided with bolted link arrangements for
primary tapping selection. It is not recommended that tapping switches are used for
the adjustment of high voltage tapping on dry-type transformers, as they are subject
to contamination, which has led to failures in service.
Winding conductors are usually copper strip, or round wire. Some designs of distribution
transformer use copper sheet, sometimes called foil windings, for the l.v.
coils. Aluminium sheet is used for both the l.v. and h.v. windings of some designs of
resin encapsulated transformer. Conductor insulation materials are chosen for the
particular application, ranging from cellulose papers for liquid-immersed types, to
synthetic materials, capable of withstanding continuous high temperatures, for drytype
designs.
Mineral oil-filled transformers
This is the most common type of distribution transformer. It can be found on electrical
supply systems in every country of the world. Although the insulating oil is a
flammable liquid, the reliability of the oil-immersed transformer has been proven
over many years on supply systems where security of supply is of the utmost importance.
However, mineral oil is flammable and although most fault conditions that
occur within the windings of a transformer result in no more than a discharge of
oil, it is possible, particularly when an electrical arc occurs just below the surface of
the oil, for it to be ignited. For this reason oil-immersed distribution transformers
are usually positioned outside buildings within a suitable fence enclosure or in separate
brick-built buildings away from personnel.
Where transformers filled with mineral oil are sited within an occupied area, it is
usual to find some type of safety provision. This may take the form of bund walls,
soak-away pits, or automatic fire-extinguishing equipment. However, the additional
costs of the civil engineering work required can make the installation of a mineral
oil-filled transformer uneconomic. In these instances, a dry-type transformer, or a
transformer filled with a high fire-point liquid, is often selected for the installation.
The integrity of the insulation system of an oil-immersed transformer relies partly
on the condition of the oil. It has been common practice on most established supply
networks in the world to let transformers breath naturally as the insulating liquid
expands and contracts with load. However, it has also been recognised that some
form of protection system that prevents the contamination of the liquid by air-borne
pollutants has the advantage of a longer insulation life, particularly when load
factors are high.
The simplest form of oil protection system, which is perfectly adequate for most
installations of distribution transformers below 500kVA installed in the temperate
zones of the world, is a silica-gel dehydrating breather. Here, the air drawn into the
transformer tank during reduced load conditions is passed through an oil bath, to
reduce solid contaminants, and then through the dehydrating crystals of silica-gel
to remove the moisture. It is essential, however, that the silica-gel crystals are maintained
dry and replaced as soon as the colour changes from blue to pink.
Probably the most common form of oil protection system is found with the conservator
or expansion vessel; this has a sump which traps most air-borne pollutants
(Fig. 13.1).