Parallel Operation of Three Phase Transformer

Parallel operation of three phase transformer is very common in three phase power generation, transmission and distribution. It is advantageous to use two or more Transformer units in parallel instead of using a single large unit. This offers flexibility for maintenance as well as operation.

Advantage of Parallel Operation of Three Phase Transformers

• It increases the reliability of supply system. Let us try to understand how this happens.  Suppose a fault occurs in any one of the Transformer unit. In such case, the faulty transformer may be taken out of service while the remaining transformers will feed the power supply. If there were only one large transformer unit is installed for supplying the load, the supply to the entire load will be interrupted during breakdown of the transformer. Thus the reliability of supply system is increased by parallel operation of transformers.
• The size of transformer increases with the increase of its rating. Therefore, a larger transformer will be bigger in size. Therefore, its transportation form manufacturer to the Site will be difficult. Whereas, transportation and installation of small sized transformers are comparatively easy.
• The maintenance opportunity in case of parallel operation is increases. One or more transformers may be taken under maintenance while the remaining transformers will supply the load at reduced power.

Condition for Parallel Operation of Three Phase Transformers

Following are the necessary conditions for parallel operation of 3 phase transformers:

• The line voltage ratio of the transformers must be same.
• The transformers should have equal per unit leakage impedance. (You may read per unit system)
• The ratio of equivalent leakage reactance to equivalent resistance should be same for all the transformers.
• The transformers should have the same polarity.

The above four conditions are also applicable for parallel operation of single phase transformers. Apart from the above four condition, there exists two more conditions which should be fulfilled for parallel operation of three phase transformers:

• The relative phase displacement between the secondary line voltages of all transformers should be zero. This means that transformers to be connected in parallel must belong to same Group number like Yy0 and Dd0 belong to same group number viz. Group 1.
• The phase sequence of secondary line voltages of all the transformers should be same.

There may arise several questions in your mind at this point of time. Though I can’t guess all those but can definitely guess some of them. Some of the common questions are explained below.

Why relative phase displacement between the secondary line voltages should be zero? Or what would happen if transformers of different group numbers are connected in parallel?

Let us assume that two transformers of different group numbers are connected in parallel as shown in figure below.

The secondary line voltages of these transforms will not be in phase rather they will be displaced from each other as shown in figure below.

A voltage across the switch S1, S2 & S3 is developed due to this displacement in secondary voltages. The voltage across the switches S1, S2 and S3 will be equal to the length of phasor joining (a₂, a₂’), (b₂, b₂’) & (c₂,c₂’) respectively. Due to this voltage when these switches are closed, a large circulating current will start flowing through the secondary winding. This may damage the transformer. Therefore it is essential that transformers belong to the same group number so that the relative phase displacement between the secondary line voltages is zero. However, the transformers of group number 3 &4 can be successfully operated in parallel.

Why phase sequence should be equal for parallel operation of 3 phase transformers?

The phase sequence of secondary line voltages of all the transformers should be same.  If the phase sequence is same, there will not be any voltage across the switches S1, S2 and S3. However, an improper phase sequence may result in complete damage of the transformer. Let us consider the figure below.

In the left figure, the phase sequence of secondary line voltage of both the transformers are correct> therefore, zero voltage will be developed across the switch S1, S2 and S3. But in rightmost figure, the phase sequence is reversed for second transformer. Under this case, no voltage will be there across S1, but voltage equal to line voltage (c₂-b₂’ and b₂-c₂’) will be developed across the switches S2 and S3. These switches may not be designed to withstand this voltage. Therefore, parallel operation is not possible.