SOLUTION

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

Same voltage ratio:- If two single-phase transformers with different voltage ratios (or turns ratios) are connected in parallel across a common primary voltage, then their secondary voltages will obviously be different. Under ‘no-load’ conditions (i.e. with no load connected), this will result in a circulating current between the loop formed by the two secondary windings. As the impedance of a transformer’s windings is low, this circulating current can be quite high, resulting in unnecessarily high I²R losses.

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.

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. 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.

Similar kVA rating:- Transformers with different kVA ratings will share the load more-or-less in proportion to those ratings (i.e. with each transformer carrying roughly its own share of the load), providing their voltage ratios are identical and their percentage impedances are close. However, it is generally recommended that the kVA-rating of any two transformers should never differ by more than a ratio of 2:1.

Same Polarity:- The ‘polarity’ of a transformer describes the instantaneous direction of the potential difference induced across the secondary terminals of that transformer, relative to that across the primary terminals.

The transformers should have the same polarity: The transformers should be properly connected with regard to their polarity. If they are connected within correct polarities then the two EMFs, induced in the secondary windings that are in parallel, will act together in the local secondary circuit and produce a  dead short circuit.