Page 78 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.7

stances. Therefore, they have been banned. Most industries are now in the process of
replacing transformers containing Askarel with dry-type transformers or transformers
containing mineral oil.
7. Thermometer. It measures the oil temperature and initiates an alarm when the temper-
ature exceeds the alarm set point.
8. Low/high-voltage bushing. This ceramic bushing carries the low/high-voltage conduc-
tor and insulates it from the tank. The high-voltage bushing is usually filled with oil to
enhance the heat removal capability.
9. Low/High-voltage connection. It connects the low/high-voltage conductor to the circuit.
10. Conservator tank. It contains oil and has the capability of absorbing the swell of the oil
when it becomes hot.
11. Gas detector relay. Electrical faults are characterized by the formation of gas. In the
early stages of the fault, small quantities of gas are liberated. The amount of gas formed
increases with time, and a violent explosion could occur. Most of the damage and
expense can be saved if the fault can be discovered and corrected in its early stages.
The gas detector relay detects gas buildup in the tank. Any gas generation inside the
transformer passes through it. It has two parts. One part detects large rate of gas pro-
duction which could be caused by a major fault (such as hydrogen, acetylene, and car-
bon monoxide). The second part detects the slow accumulation of air and gases which
are released from the oil when it gets warm or from minor arcs. When the gas level
reaches a predetermined amount, an alarm is annunciated. The relay has also the capa-
bility to trip the transformer in case of a serious fault which would result in a sudden
rush of oil or gas through it.
12. Explosion vent. It prevents the buildup of high pressure in the tank due to gas forma-
tion (caused by oil disintegration) when a fault occurs. A relief diaphragm at the end
of the explosion vent ruptures when the gas pressure reaches a certain predetermined
value, to relieve the pressure to atmosphere.
13. Oil level. The oil level changes with the temperature of the oil. The core and winding
must always be immersed in oil to ensure they are adequately cooled and insulated.
14. Sight glass. It indicates oil level.
15. Breather. This allows air movement to the conservator upon swell and shrinkage of the
oil. It has a silica gel (air dryer) to remove moisture from the air entering the conser-
vator tank. The transformer oil must be kept dry always. A water content of 8 parts per
million (ppm) in the oil will reduce the dielectric strength to a dangerous level. Oxygen
has also adverse consequences on the oil (it oxidizes it, resulting in the formation of
sludge). Some transformers have a seal of inert gas (nitrogen) between the oil and
atmosphere to prevent oxygen and moisture from reaching the oil.
16. Radiator. It is a heat exchanger which cools the oil by natural circulation.
17. Ground connection. The tank is connected to ground (earth) to ensure the safety of
maintenance personnel.

Transformer Core
The core is made from thousands of laminations of grain-oriented steel. The thickness of
a lamination is around 12 mils. Each lamination is coated with a thin layer of insulating
material. The windings have a circular cross section. They are normally concentric. The
lowest-voltage winding is placed next to the core to reduce the voltage stress. There is a
layer of insulation between the low-voltage winding and the core. There are also oil ducts

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