Page 156 - Handbook of Electrical Engineering

TRANSFORMERS 139

• AF AIR-FORCED:- Air forced cooling is arranged by using fans and trunking on the outside of
the transformer. These can be applied to dry-type or liquid insulated transformers.

Transformers rated up to approximately 2.5 MVA are usually fitted with cooling tubes or tank
mounted radiators. These units would typically feed low voltage switchboards. Between 2.5 MVA and
15 MVA the use of tubes would be inadequate and tank mounted radiators would be necessary. Above
about 10 MVA the radiators would be separately mounted from the tank and coupled by pipework.
The overall construction of oil and liquid filled transformers would be IP55 as defined
in IEC60529.
Oil industry sites are often located in hostile environments which also have aggressive transport
routes for the delivery of their equipment. It is therefore necessary to construct the windings and
core components in such a manner that they can withstand impacts and rough handling during
transportation to site. The windings should be robustly braced to ensure that they do not move during
transportation.
Other variations including using water-cooling are possible but these are not commonly
encountered.
The amount of heat typically dissipated from a liquid-immersed transformer is about 12.5 watts
per square metre of surface area per degree C. If such a transformer is inside a room or module then
this heat must be removed by changing the air regularly, or by the HVAC cooling system. The heat
dissipation can also be calculated directly from the known efficiency of the transformer at full load.

Transformers are usually fitted with devices to indicate the temperature of liquid, windings
and the core. These may be direct-reading thermometers, indirect resistance temperature detectors
(RTDs) or thermocouples. Signals from these devices are used to trip the feeder switchgear in the
event of excessive temperature.
Liquid-immersed transformers are fitted with special safety relays and devices to safeguard
the unit from internal faults and explosions. Slowly generated faults tend to produce gas from the oil
or liquid.

The gas accumulates in a special chamber which is fitted with two float switches, and these
operate alarms and trips when the gas accumulates slowly or rushes in during internal explosions. This
system is called a Buchholz relay, and is normally used only on transformers fitted with conservator
tanks. Transformers below about 1600 kVA are often sealed type liquid-immersed units. Internal
explosions are released by using a special blow-off valve. Often the space above the liquid level in
this type of unit is filled with an inert gas such as nitrogen.
Reference 3 gives an excellent description of all the aspects of transformer design, testing
and operation.

6.5.1 Conservator and Sealed Type Tanks

Conservator type transformers are fitted with an overhead tank which is approximately half full of
the oil or liquid insulant. The overhead tank is allowed to breath to atmosphere as the liquid level
varies with the average temperature inside the transformer. It breathes through a small vessel filled
with silica gel which absorbs the water vapour that may pass into or out of the transformer.