Page 52 - Practical Power System and Protective Relays Commissioning

P. 52

Chapter 5

Power Transformers Theory

Testing and Commissioning

5.1 INTRODUCTION
A power transformer is the device that transfers the power through power
system with different voltage levels. Both step-up power transformers and
step-down power transformers exist. This is a very expensive component in
a power system and should be maintained in good condition to perform at
the optimal level. A new device is introduced to monitor the insulation con-
dition by monitoring the partial discharge in transformers as live monitoring
and can help to predict when the transformer insulation will fail, which is
the most common cause of failure of power transformers.

5.2 TRANSFORMER CONSTRUCTION

Fig. 5.1 shows the different parts of a transformer.
A transformer consists of the tank and its accessories, core, and winding
assembly, and the insulating and cooling medium. The tank is made from
welded steel and painted, and is mechanically very strong in able to support
the transformer core and windings and the weight of the oil. Accessories
may include the following:
A breather to compensate for the expansion and contraction of the cool-
ing medium (oil)due to temperature changes. Small units have an air
space above the oil connected to the atmosphere by the breather. The
breather should protect the oil from changes due to humidity, snow, etc.
A conservator or expansion tank it is a small tank that keeps the main
tank full of oil and allows for the expansion and contraction of oil due to
temperature changes and has a breather connected to the atmosphere.
A pressure relief valve which relieves the high pressure that occurs inside
the transformer under any electrical fault condition.
Temperature indicators that include:
Oil temperature indicator set in two stages: alarm, and starting fans
for cooling
Winding temperature indicator set in two stages: alarm and trip.

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