Parallel Operation of Three Phase Transformers


Three phase transformers are the heart of an electrical power distribution system which is used to set voltage up or down. It consists of primary and secondary windings. When it comes to three-phase power generation, transmission and distribution, the parallel operation of three phase transformers are common. Using two or more transformer units in parallel is more beneficial than using a single large unit due to its adjustability in maintenance and operation.

Other benefits of the parallel operation of three phase transformers include

Reliability– The parallel operation is known to boost the credibility of a supply system. For instance, in case of a fault, the faulty transformer might be discarded and the rest of the transformers will continue the power supply. On the other hand, if an entire load of power supply was given only on a large transformer unit then a breakdown will disrupt the supply to the entire load. This is how parallel operation increases the reliability of the supply system.

Size– The size of the transformer and its rating are directly proportionate to each other which mean the size increases with the ratings. This often makes transportation from manufacturers to the site complicated. In contrast, both the transportation and installation of small-sized transformers are easy.

Maintenance– It has already been mentioned that parallel operation offers easy maintenance which is also a major advantage of it. If a particular transformer is kept aside for maintenance the other transformers will carry on supplying the load at low power.

An ideal parallel operation of 3 phase transformers demands certain conditions that include the following

* The line voltage ratio of the transformers has to be equal
* The transformers must possess the same per unit leakage impedance
* Again the ratio of equivalent leakage reactance to equivalent resistance need to be the same for each and every transformer
* Polarity should also be the same for all the transformers.
* The phase displacement between primary and secondary voltages also must be equal for the transformers which are to be attached for the parallel operation.
* The phase sequence should also be the same

Here are the explanations of some commonly asked questions that will be helpful for you to understand the topic more clearly.

Why relative phase displacements between the secondary line voltages have to be zero? What will be the consequences of connecting transformers of different group numbers in parallel?

Let us just assume a situation in which two transformers of different group numbers are joined in parallel as drawn in the following figure. The secondary line voltages of these transformers will not be in phase instead they will be shifted from each other as revealed in figure 1.

A voltage across the switch P1, P2 & P3 is developed because of this displacement in secondary voltages. The voltage across the switches P1, P2 and P3 will be the same to the length of phasor connecting (x2, x2’), (y2, y2’) & (z2,z2’) respectively.

Due to this voltage when these switches are closed, a huge circulating current will start passing through the secondary winding which can even damage the transformer. Therefore it is crucial for transformers to belong to the same group number so that the relative phase displacement between the secondary line voltages remain zero. The parallel operation of group 3 and 4 transformers is an exception to this case because they can be successfully operated in parallel. Figure 2

Why the phase sequence is required to be equal in three phase transformers

We have already mentioned in the list of conditions for successful parallel operation of the three-phase transformers, that the phase sequence of secondary line voltages of all the transformers should be the same.  If the phase sequence remains the same, there will be no voltage across the switches P1, P2, and P3. However, an inappropriate phase chain may result in complete damage to the transformer.

Now, look at this picture.

In the first figure, the phase sequence of the secondary line voltage of both the transformers are correct, therefore, zero voltage will be developed across the switch P1, P2, and P3. On the other hand, in the next figure, the phase sequence is inverted for the second transformer. In this case, no voltage will be there across P1, but voltage equal to line voltage (z2-y2’ and y2-z2’) will be developed across the switches P2 and P3. These switches may not be built to endure this voltage. Therefore, the parallel operation is not possible.