When a transformer as to supply an excess load (increased load demand) than the existing load, there are two alternatives,- To achieve the power requirement for the increased load the old transformer is replaced by another transformer of a higher rating or,
- By operating another transformer together with the old transformer by connecting parallelly.
In practice, to supply excess load it is difficult the replace the old transformer with a new high-rating transformer and it is uneconomical. Hence, we connect transformers in parallel to supply the increased load demand. There are certain conditions that need to be fulfilled for the satisfactory parallel operation of transformers.
Necessity of Parallel Operation of Transformers :
The operation of transformers in parallel becomes necessary owing to the following reasons,- 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.
Conditions for Parallel Operation of Transformers :
The system should meet the following conditions for the operation of the transformers parallelly,- The voltage and frequency ratings of the transformers should be for the incoming bus-bars voltage and frequency.
- The terminals of the transformers should properly be marked and connected with regard to polarity otherwise the windings get short-circuited or lead to flow large circulating current.
- The transformation or turn ratios of the transformers should be the same. Otherwise, the circulating current will produce in the secondaries of the transformers to equalize the voltage. This circulating current will reduce the capacity of the transformer.
- The percentage (per-unit) impedance of the transformers operating parallelly should be the same in order to share the load on the transformers according to their kVA ratings.
- The transformer's windings resistance to reactance ratio should same or else the power factors of the transformers supplying the load will be different with respect to their ratings.
Therefore, the above conditions (1) and (2) are essential with an incorrect connection of polarity of transformers operating parallelly will result in short-circuit because the short-circuit current is high than the rated and it can damage the windings.
If condition (3) is not satisfied, there is a possibility of parallel operation of transformers. But due to the inequality of induced EMFs in the transformers, there will be some circulating current between the windings even on no-load.
When secondaries are loaded, the circulating current produced will tend to produce unequal loading conditions. Hence, it is impossible to operate a transformer at its maximum kVA output rating without overloading anyone from the parallelly operating transformers supplying a common load.
If conditions (4) and (5) are not satisfied, a parallel operation is possible, but the division of load according to the kVA ratings of the transformers will not be proportional. Hence the capacities of the transformers cannot be utilized to a full extent.
