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Parallel Operation of Transformers

Parallel operations of transformer: Definition-Connection of primary winding of two transformers to a common supply voltage and connection of  the secondary winding of both the transformers to a common load is called parallel operation of transformers. The two transformer share loading.

Why Parallel Operation of Transformers is required?

It is economical to install numbers of smaller rated transformers in parallel than installing a bigger rated electrical power transformer. When we have two sources to be run in synchronization, the parallel operation of transformer is required.

Advantages of Parallel operation of Transformers

The parallel operation has mainly the following advantages.

1.Maximize electrical power system efficiency

The transformer operates at almost its full efficiency when it operates 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 load increases, we can switch on one by one other transformer  connected in parallel to fulfill the total demand. This way we can run the system with maximum efficiency.

2.Maximize electrical power system Stability

Any one of the transformers can be taken for maintenance if all the transformers run in parallel and, thus the other parallel transformers in system will deliver the load without total interruption of power.

3.Maximize electrical power system Flexibility

The total connected load can be increased or decreased according to the dynamic load scenario. If the load increases in future, one more transformer can be installed and it can be run in parallel with the existing transformers to cater the total load requirement.

Conditions of parallel Operation of Transformers

The following conditions must be satisfied for satisfactory performance when two or more  transformers run in parallel.These are the conditions for parallel operation of transformers.

  1. Same voltage ratio of transformer
  2. Identical percentage impedance
  3. Same polarity
  4. Identical phase sequence

Same Voltage Ratio

If two transformers of different voltage ratio are fed the same primary supply voltage, then there will be difference in the secondary voltage. Now, if these transformers secondaries are connected to the same bus for  delivering power to the common load, there will be a circulating current between secondaries and it will lead to circulating current between primaries.

The small voltage difference can cause large circulating current to flow in the winding. Thus circulating current cause I2R losses in the winding and cause heating of the winding.  Therefore, the secondary voltage of both the transformer must be the same for parallel operation.

Same Percentage Impedance

The both the transformer operating in parallel must have the same percentage impedance. The current shared by the parallel transformer should be proportional to the MVA rating of the transformers.

This is possible if the both transformers have the same percentage impedance. The current delivered  by these transformers are inversely proportional to their internal impedance. The internal impedance of the transformer is inversely proportional to its MVA rating.  In other words, percentage impedance or per unit values of impedance should be identical for all the transformers operating in parallel.

Same Polarity

The instantaneous direction of the induced emf in secondary or primary is the polarity of the transformer. If the instantaneous direction of induced secondary emf in two transformers are opposite to each other, the large circulating current flows from secondary of the one transformer to other. In this condition we can not run the transformers in parallel.
We can find the polarity of the transformer by the vector group of the transformer. The transformers with the same vector group is the necessary condition for parallel operation.

Same Phase Sequence

The  order in which the phases attains its maximum value is called the phase sequence. For parallel operation, the phase sequence must be identical. Otherwise, during the cycle, each pair of phases will short circuit.
The above said conditions must be strictly followed for parallel operation of transformers but totally identical percentage impedance of two different transformers is difficult to achieve practically, that is why the transformers run in parallel may not have exactly same percentage impedance but the values would be as nearer as possible.