Page 255 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Parallel operation of two single phase transformers

Objectives: At the end of this lesson you shall be able to:
• state the necessity of parallel operation of transformers
• state the conditions to be full filled for the parallel operation of transformers
• explain how to determine the polarity terminals of transformer.

Necessity of parallel operation of transformers
1 When the power demand of the load increases, two or more transformer may be operated in parallel.
2 When the power demand decreases, only required numbers of transformer may be operated with their full load
capacity. Where as the remaining transformers may be switched "OFF" and taken for general maintenance/
service.

3 Thus the efficiencies and life of the transformers increases and the losses are reduced.
4 It provides more reliability of power i.e., even one transformer fails or become out of service, other transformers
will supply to the certain amount of load.
5 It is not economical to manufacture a single very large capacity transformer. Thus operationg two or more
numbers of optimal capacity transformers in parallel is more economical.
6 It is easy to plan the maintenance schedule of the transforemers, hence the cost of maintenance and spares
are reduced.
Conditions
1 the same voltage ratio
2 Input voltage must be same
3 the same per unit (or percentage) impedance

4 the same polarity
5 the same phase sequence and zero relative phase displacement, for 3 phase transformers.
Of these (4) and (5) are absolutely essential (1) and (2) must be satisfied to a close degree.
There is more allowance for a wide extent with (3), but the more nearly it is true, the better will be the load division
between several transformers.
Parallel operation
Fig 1 shows two single phase transformers connected in parallel with their primary windings connected to the
same supply and their secondary windings supplying a common load.
When operating two or more transformers in parallel, to have satisfactory performance the following conditions
should be met

Voltage ratio: If voltage readings on the open secondaries of various transformers, to be run in parallel, do
not show identical values, there will be circulating currents between the secondaries (and therefore between
primaries also) when the secondary terminals are connected in parallel. The impedances of transformers is
small, so that a small percentage voltage difference may be sufficient to circulate considerable current and cause
additional I2R loss.
When secondaries are loaded, the circulating current will tend to produce unequal loading conditions. Thus it
may be impossible to take the full load output from the parallel connected group without one of the transformers
becoming excessively heated.
Impedance: The currents carried by the two transformers are proportional to their ratings:
• if their numerical or ohmic impedances are inversely proportional to those ratings, and
• their per unit impedances are identical.

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