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.
- 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.
- 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
- 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
- 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 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
Condition (4) – Same Reactance/Resistance Ratio
- 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.