When we connect the primary windings of two transformers to a common supply voltage and the secondary windings of both the transformers to a common load, this type of connection of transformer is said to be the parallel operation of transformers.
Reasons for parallel operation
The reasons for operating the transformers in parallel are as follows:
Single-phase transformers in parallel:
The diagram drawn below shows the circuit diagram of two transformers A and B connected in parallel.
a1 = turns ratio of transformer A
a2 = turns ratio of transformer B
ZA = equivalent impedance of transformer A referred to the secondary side.
ZB = equivalent impedance of transformer B referred to the secondary side.
ZL = load impedance across the secondary side.
IA = current supplied to the load by the secondary of transformer A.
IB = current supplied to the load by the secondary of transformer B.
VL = load secondary voltage.
IL = load current
Fig: Two single-phase transformers in parallel.
IA + IB = IL
By solving the above two equations, we get
Each of these currents has two components; the first component represents the transformer's share of the load current and the second component is a circulating current in the secondary windings.
Circulating currents have the following undesirable effects:
Conditions for parallel operation of Single-Phase transformers:
Three-phase transformers in parallel
The conditions for proper parallel operation of single-phase transformers are as follows:
The condition for the parallel operation of single phase and three phase transformer is the same but with the following additions: