# Two transformers are connected in parallel. These transformers have different percentage impedance. It is likely to result in

Two transformers are connected in parallel. These transformers have different percentage impedance. It is likely to result in

### Parallel operation of transformers

There are a number of requirements that must be satisfied before two or more single-phase transformers can be ‘paralleled -i.e. before they can be connected in parallel with each other, in order to supply the same load. These requirements are

1. Same voltage ratio (turns ratio)
2. Similar percentage impedance
3. Similar kVA rating
4. Same polarity

### Similar percentage impedance:-

• A transformer’s percentage impedance can be determined by shortcircuiting the secondary winding with an ammeter, and gradually increasing the primary voltage until rated current flows in the secondary.
• The percentage impedance is then simply the ratio of that particular primary voltage to the rated primary voltage, expressed as a percentage.
• In other words, the per-unit or percentage impedance of the two transformers should be equal. If this condition is fulfilled, load sharing of the total connected load between the two transformers will be proportional to their kVA ratings.

So, for example, if a particular transformer has a percentage impedance of, say, ‘5%’, then it would take just 5% of the rated primary voltage to cause 100% of the rated secondary current to flow through the short-circuited secondary winding.

• For unequal ratings, the numerical (ohmic) values of their impedances should be in inverse proportion to their ratings to have current in them in line with their ratings.
• If the transformer does not have equal percentage impedance then the loading of the transformer is not in proportion to their kVA rating.
• A difference in the ratio of the reactance value to the resistance value of the per-unit impedance results in a different phase angle of the currents carried by the two paralleled transformers; one transformer will be working with a higher power factor and the other with a lower power factor than that of the combined output.
• Hence, the real power will not be proportionally shared by the transformers.
Scroll to Top