Page 258 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Three single phase transformers for three phase operation

Objectives: At the end of this lesson you shall be able to:
• list and interpret the four types of connections of primary and secondary windings
• state the phase and line values of current and voltage.

There are various methods available for transforming 3-phase voltages, that is for handling a considerable
amount of power. There are four possible ways in which the primary and secondary windings of a group of three
transformers may be connected together to transfer energy from one 3-phase circuit to another. They are:
Primaries in U, Secondaries in U
Primaries in U, Secondaries in D
Primaries in D, Secondaries in D
Primaries in D, Secondaries in U.
Star/Star or U/U connection: Fig 1 shows the connection of a bank of 3 trans-formers in a star-star. This
connection is most economical for small, high voltage transformers because the number of turns per phase and
the amount of insulation required is minimum. This connection works satisfactorily only if the load is balanced.
For a given voltage V between lines, the voltage across the terminals of a U connected transformer is ; the coil
current is equal to the line current I.

of a U connected transformer is V 3 ; the coil current is equal to the line current I.
Star - Delta or U/D connection: In primary side 3 transformers are connected in star and the secondary consist
of their secondary connected in delta as shown in Fig 2. The ratio between the secondary and primary line voltage
is 1/ times the transformation ratio of each transformer. There is a 30o shift between the primary and secondary
line voltages. The main use of this connection is at the substation end of the transmission line.

Fig 1 Fig 2

Delta - Delta or D/D connection: Fig 3 shows three transformers, connected in D on both primary and secondary
sides. There is no angular displacement between the primary and secondary line voltages. An added advantage
of this connection is that if one transformer becomes disabled, the system can continue to operate in open-delta
or in V-V. In V-V it can be operated with a reduced capacity of 58% and not 66.6% of the normal value.
Delta - Star or D/U connection: (Fig 4) This connection is generally employed where it is necessary to step up
the voltage, as for example, at the beginning of high tension transmission system.

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