Two transformers are connected in parallel means that the two primary windings are connected to the supply bus and the two secondary windings are connected to load busbars.
Need of Parallel Operation:
Increased Load Demand -
When the load demand increased behind the full-load capacity of the transformer. To meet the additional demand on a power system it becomes necessary either to replace the existing unit with a large capacity unit or to install an additional unit to carry the increased portion of the load.
No Transportation Problems -
It is easy to transport small transformers when compared to a single large unit transformer of the same rating.
Continuity of Supply -
In case of a fault in the transformer, it is difficult to maintain the continuity of supply to the load when a single large transformer. In parallel operation, the faulty transformer gets isolated from the load, and the remaining transformers still serve the load. This increases the reliability of the system.
Expansion of Substation -
It is easy to expand the substation with parallel operation of transformers by connecting the new transformer in parallel with the existing transformer. Thereby increasing the overall capacity of the substation.
Limitation of Transformer Ratings -
It is impossible to build each and every transformer according to the load requirement. Hence, two or more transformers are connected to meet the large load.
To maximize electrical power system efficiency:
Generally, an electrical power transformer gives maximum efficiency at full load. If we run numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time. When the load increases, we can switch one by one transformer connected in parallel to fulfill the total demand. In this way, we can run the system with maximum efficiency.
To maximize electrical power system availability:
If numbers of transformers run in parallel, we can shut down any one of them for maintenance purposes. Other parallel transformers in the system will serve the load without total interruption of power.
To maximize power system reliability:
If any one of the transformers runs in parallel, is tripped due to a fault of other parallel transformers. the system will share the load; hence power supply may not be interrupted if the shared loads do not make other transformers overloaded.
To maximize electrical power system flexibility:
There is always a chance of increasing or decreasing future demand for power systems. If it is predicted that power demand will be increased in the future, there must be a provision of connecting transformers in the system in parallel to fulfill the extra demand because, it is not economical from a business point of view to installing a bigger rated single transformer by forecasting the increased future demand as it is an unnecessary investment of money. Again, if future demand is decreased, transformers running in parallel can be removed from the system to balance the capital investment and its return.