By parallel operation we mean two or more transformers are connected to the same supply bus bars on the primary side and to a common bus bar/load on the secondary side. Such requirement is frequently encountered in practice.
Two transformers A and B connected in parallel are shown in the figure given below. While connecting two or more than two transformers in parallel, it is essential that their terminals of similar polarities are joined to the same busbars as shown. 
The wrong connections may result in a dead short-circuit and primary transformers may be damaged unless protected by fuses or circuit breakers.
There are three principal reasons for connecting transformers in parallel.
- Firstly, if one transformer fails, the continuity of supply can be maintained through other transformers.
- Secondly, when the load on the substation becomes more than the capacity of the existing transformers, another transformer can be added in parallel.
- Thirdly, any transformer can be taken out of the circuit for repair/routine maintenance without interrupting supply to the consumers.
Some more reasons that necessitate parallel operation of transformers are as follows.
- Non-availability of a single large transformer to meet the total load requirement.
- The power demand might have increased over a time necessitating augmentation of the capacity. More transformers connected in parallel will then be pressed into service.
- To ensure improved reliability. Even if one of the transformers gets into a fault or is taken out for maintenance/repair the load can continued to be serviced.
- To reduce the spare capacity. If many smaller size transformers are used one machine can be used as spare. If only one large machine is feeding the load, a spare of similar rating has to be available. The problem of spares becomes more acute with fewer machines in service at a location.
- When transportation problems limit installation of large transformers at site, it may be easier to transport smaller ones to site and work them in parallel.
Conditions for satisfactory parallel operation
In order that the transformers work satisfactorily in parallel, the following conditions should be satisfied:
- Transformers should be properly connected with regard to their polarities.
- The voltage ratings and voltage ratios of the transformers should be the same.
- The per unit or percentage impedances of the transformers should be equal.
- The reactance/resistance ratios of the transformers should be the same.
Condition (1) – Connnection with regard to Polarity
Condition (1) is absolutely essential because wrong connections may result in dead short-circuit. Figure (i) shows the correct method of connecting two single-phase transformers in parallel. It will be seen that round the loop formed by the secondaries, the two secondary e.m.f.s EA and EB oppose and there will be no circulating current.
Figure (ii) shows the wrong method of connecting two single-phase transformers is parallel. Here the two secondaries are so connected that their e.m.f.s Ea and Eb are additive.
- This may lead to short-circuit conditions and a very large circulating current will flow in the loop formed by the two secondaries.
- Such a condition may damage the transformers unless they are protected by fuses and circuit breakers.
Condition (2) – Same Voltage Rating and Voltage Ratio
This condition is desirable for the satisfactory parallel operation of transformers. If this condition is not met, the secondary e.m.f.s will not be equal and there will be circulating current in the loop formed by the secondaries. This will result in the unsatisfactory parallel operation of transformers.
Let us illustrate this point. Consider two single-phase transformer A and B operating in parallel as shown in Figure below. Let Ea and Eb be their no-load secondary voltages and Za and Zb be their impedances referred to the secondary. Then at no-load, the circulating current in the loop formed by the secondaries is
Let us illustrate this point. Consider two single-phase transformer A and B operating in parallel as shown in Figure below. Let Ea and Eb be their no-load secondary voltages and Za and Zb be their impedances referred to the secondary. Then at no-load, the circulating current in the loop formed by the secondaries is
Even a small difference in the induced secondary voltages can cause a large circulating
current in the secondary loop because impedances of the transformers are small.
- This secondary circulating current will cause current to be drawn from the supply by the primary of each transformer.
- These currents will cause copper losses in both primary and secondary.
This creates heating with no useful output. When load is connected to the system, this circulating current will tend to produce unequal loading conditions i.e., the transformers will not share the load according to their kVA ratings. It is because the circulating current will tend to make the terminal voltages of the same value for both transformers.
- Therefore, transformer with smaller voltage ratio will tend to carry more than its proper share of load.
- Thus, one transformer would tend to become overloaded than the other and the system could not be loaded to the summation of transformer ratings without overloading one transformer.
Condition (3) – Equal Percentage Impedence
This condition is also desirable for proper parallel operation of single phase transformers. If this condition is not met, the transformers will not share the load according to their kVA ratings.
Sometimes this condition is not fulfilled by the design of the transformers. In that case, it can be corrected by inserting proper amount of resistance or reactance or both in series with either primary or secondary circuits of the transformers where the impedance is below the value required to fulfil this condition.
Condition (4) – Same Reactance/Resistance Ratio
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.
- Condition (3) is much more important than condition (4). Considerable deviation from condition (4) will result in only a small reduction in the satisfactory degree of operation.
- When desired, condition (4) also may be improved by inserting external impedance of proper value.
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