Page 260 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Parallel operation of 3-phase transformer:

Fig 1 shows the connection diagram for parallel operation of two numbers of 3-phase transformers. In this case,
the connection of both of transformer 1 and 2 are (delta - star)same.
However to operate the 2 transformers of having Y/D and connection, their primary and secondary line voltage
D/Y must be same. In this case,the turns ratio may not be equal, but the voltage ratio between the terminal voltage
of primary and secondary must be same.

If two transformers having different ratings, are connected in parallel the their percentage impedance must be
same, where as the numerical impedance of transformer 1 will have half the impedance of transformer 2. In this
case both the transformers will share the common load in propertional to their KVA ratings.(Fig 1)
For best performance of the parallel operation, the regulation of both the transformers must be same . If the
percentage impedance of both the transformers are different. Than one transformer will be operating at a higer
power factor and other will be operating at a lower power factor.

Fig 1

Cooling of transformer - Transformer oil and testing

Objectives: At the end of this lesson you shall be able to:
• explain the necessity of cooling
• state the methods of cooling.

Necessity of cooling
Transformer is heated up when current flows through its, winding. This causes the liberation of heat. In large
size transformer, where power rating is high, large amount of heat is liberated. This will affect the insulation of the
windings as well as reduction of transformer efficiency. This heat should be transformed from transformer winding
and dissipated in the atmosphere .

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CITS : Power - Electrician & Wireman - Lesson 41-49 CITS : Power - Electrician & Wireman - Lesson 41-49