Page 257 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Series (Secondary only) operation of transformers

Objectives: At the end of this lesson you shall be able to:
• state the necessity of series operations
• state the conditions to be fulfilled for series operation.

Series operation:
The connection diagram for series operation (secondary only) of two identical transformers is given below (Fig 1)

Fig 1

Necessity for series operations:
In general, the transformers are available with some standard input (primary) and output (secondary) voltages. In
order to get some intermediate voltage for example ,
36V, 48 V for special purpose, the series operation of transformers (secondary only) are necessary.
In series operation, individual secondary voltages of both transformers are added if they are connected with
proper polarity, but the current ratings are remains same.

Condition for series operation:
Both transformers should be identical i.e,
a Voltage ratio/turns ratio must be same
b Polarities must be same

c Type of core of both transformers (core or shell type) must be same.
d Input voltages of both transformers must be same.
e KVA ratings of both transformers must be same.
f Percentage impedance or per unit impedance of both the transfers must be same.
Precautions:

• The polarities of secondary of both transformers should be connected in
proper way, same as series connection, to get the voltage added, otherwise the
output voltage will be zero.
• As the output voltage is double that the individual secondary voltages, care to be
given to ascertain the insulation level of the secondary windings.

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