Page 259 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

The primary and secondary line voltages and line currents are out of phase with each other by 30o. The ratio of
secondary to primary voltage 3 is times the transformation ratio of each transformer.

Fig 3 Fig 4

Parallel operation of 3-phase transformer

Objectives: At the end of this lesson you shall be able to:
• explain parallel opration
• states the conditions for parallel operation of 3 phase transformer
• states the necessity of parallel opertion.

Parallel operation
Operating two or more transformers by connecting their primaries in parallel to a common supply line and
connecting their respective secondaries in parallel with a common load-busbars is called as parallel operaion of
transformers.
Conditions for pararllel opertion of transformers:

When operating two or more transformer in parallel, the following conditions have to be satisfied for the best
performance of the transformer.
1 The voltage ratio must be same.

2 The per unit impedance or percentage impedance should be same i.e., the ratio between the equivalent
leakage reactance and the equivalent resistance(X/R)should be same.
3 The polarities must be same.
4 For three phase transformers

i The phase sequence must be same
ii The vector group must be same (i.e., The relative phase displacement between the secondary line voltages
must be zero)

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