Parallel operation of Transformer


We know that the transformers are used for isolation, stepping up and down of voltages, impedance matching, etc. We know the basic operations and mathematics for the transformer operations but what about their parallel operations. Let’s discuss it in detail.

Parallel Operation of Transformers:

When the primary windings of the transformers are connected to the common voltage supply like a bus and the common load is connected to the secondary windings of the transformers, then the operation going out is called the Parallel Operation of Transformers.


  1. Increases the efficiency of the system.
  2. The system gets flexible.
  3. System’s Reliability increases.


The only disadvantage of the parallel operation of the transformers is that the short-circuit current of the transformers starts to increase gradually.

Parallel Operation of Transformers: Why it is used?

It is used because of the following reasons:

  • In many applications, when the load has bypassed a certain limit then the parallel operation of transformers is used because it is not practical to use a very large transformer.
  • There will be the scope of expanding the beyond load capacity of transformers already installed by using the parallel operation of the transformers.
  • The breakdown inline transmission causes the shut-down of the entire system. The parallel operation of the transformers will not shut-down any system.
  • The maintenance of the transformers causes the problem to do the force shut-down or interruption in the line supply. The parallel operation of the transformers ensures that there will be no interruption if you take any transformer out for maintenance or repairing.


  1. The polarities of the transformers must be the same.
  2. Transformer’s turn ratios must be equal.
  3. Across the transformer’s internal impedance, the voltages at the full load must be equal.
  4. The ratio of the winding resistances to winding reactances for parallel transformers must be equal because this condition makes sure that the parallel transformers are being operated at the same power factor, thus sharing the same real and reactive power.


Ideally, we know that when we connect two transformers in parallel; there exists the whole system ideal. There is no leakage of fluxes, currents, losses of all the types.

In a practical manner, that does not happen. Let’s discuss what happens in the parallel operation of the transformer in detail.

Suppose there are two transformers connected in parallel in such a manner of having the common voltage bus on the primary side and the common load bus at the secondary side. We see that the voltages at the primary side of the transformers are the same as it is being supplied by the common voltage bus. But in practice, the voltages at the secondary sides of both of the transformers will not be the same due to internal leakage fluxes, current, hysteresis loops, etc.

Suppose one transformer is of the rating of 400V-11kV and the second one is of the rating of 400V-10kV because of the internal transformer losses. So, in this case, the voltages at both of the secondary sides are not the same on the common load bus. Both of the transformers have got the same impedances and the respective transmission lines from both the transformers have also some impedance.

Because of these reasons, a circulating current will flow in both the transformers and their respective transmission lines. Hence, a lag or lead in the currents will happen and the magnitudes will also get to change due to which the real and reactive power and the power factor of both the transformers get to change in that manner.

Keeping that scenario in mind, the full load demand will be reduced. Due to this, real and reactive power and the power factor controlling should be introduced.

Now discuss how the power factor and the real and reactive power is controlled.

We know that in AC systems, there are three current parameters which are magnitude, phase angle, and frequency which need to be controlled for making the AC systems efficient.

Reactive Power Control:- So take anyone transformer and connect a small variable transformer primary to the center tap input of the taken transformer and secondary windings to the center tap output of the taken transformer. Hence, reactive power gets controlled. That happens because output voltages of the transformer get to be added/subtracted to the voltages of the variable transformer. Hence, the magnitude of voltages at the common load bus becomes equal.

Real Power and Power Factor Control:- Take the second transformer and do the same process as done for the reactive power control except for the variable transformer, there will be a fixed transformer. In this case, the voltages of the fixed and the taken one transformer will lead or lag to each other by 900. The resultant voltage will be the one to define the power factor. If the output voltages of the fixed transformer are so much small that it does not affect the transmission line then, the Real power is controlled and the power factor will be approximately equal to one.

When it comes to the power factor, assume that the transformers used for the parallel operation are three-phase systems. The three-phase output of the transformers will be 1200 shifted to each other. When the real power is being controlled, the power factor will be 300 from line to line voltages to each line voltage respectively. So, each of the line to line voltages of the transformer is practically 1200 causing no change in the system.

Hence, the reactive and real and the power factor is controlled in parallel operation of the transformers in the result of the circulating currents.


We have seen that in parallel operation of the transformers, the circulating currents cause the problem because of the mismatching of the characteristics of the transformers. Due to which copper losses and overloading of one of the transformers occur. And keep in mind, that scenario is controlled by the reactive and real power and the power factor of the transformers. We could have used the online tap changing transformers but they are much expensive and it is the costly solution and the maintenance of those transformers is also expensive. That’s why the parallel operation of the transformers is used.

Related Topics;

  1. All About Transformer
  2. Equivalent circuit of the transformer
  3. Open Circuit and Short Circuit Test of Transformer
  4. Efficiency and losses of a transformer
  5. 3-phase transformer