Two or more transformers can be connected in parallel to increase reliability and reduce costs. This allows the transformers to share the load if one transformer's capacity is exceeded. For parallel operation, the transformers must have identical voltage ratings, identical phase shift, and the same impedance/resistance ratios to ensure equal power factors and load sharing. Connecting transformers in parallel improves reliability over using a single larger transformer and lowers maintenance costs.

1. PARALLEL
OPERATION OF
SINGLE PHASE
TRANSFORMER
Lecture Notes by Dr.R.M.Larik 1

2. INTRODUCTION
➢ The transformers are connected in parallel when load on one of the
transformers is more than its capacity.
➢ Two transformers are said to be connected in parallel if the primary
windings are connected to supply busbars and secondary windings
are connected to load busbars.
➢ The reliability is increased with parallel operation than to have
single larger unit.
➢ The cost associated with maintaining the spares is less when two
transformers are connected in parallel.
Lecture Notes by Dr.R.M.Larik 2

3. Conditions for Parallel Operation
Lecture Notes by Dr.R.M.Larik 3

4. CONDITION (I)
Lecture Notes by Dr.R.M.Larik 4

5. CONDITION (II)
Lecture Notes by Dr.R.M.Larik 5

6. Lecture Notes by Dr.R.M.Larik 6

7. CONDITION (III)
Lecture Notes by Dr.R.M.Larik 7

8. CONDITION(IV)
➢If the reactance/resistance ratios of the two
transformers are not equal, the power factor of the load
supplied by the transformers will not be equal.
➢In other words, one transformer will be operating with
a higher and the other with a lower power factor than
that of the load.
Lecture Notes by Dr.R.M.Larik 8