Parallel Operation of Transformers

Definition of parallel opeartion of transformer

In several power transmission and distribution sector, parallel operation of transformers are effectively done. As the name implies, parallel operation of transformer is achieved by connecting two or more power transformers in parallel with each other. So, in case of parallel operation, the primary windings of the transformers are connected to a common voltage source and the secondary windings are connected to a common load. This is a simple definition of parallel operation of transformers.

Why parallel operation of transformer is needed

Actually, parallel operation of transformer is preferred because this arrangement has various advantages as well as it serves economical purposes too. In power sectors, parallel operation is done by installing several small rated transformer units in parallel, instead of installing a large rated one. The main reasons for parallel operation are discussed as follows:

  • Electrical power system deals with very high level of voltages and current. Sometimes, very large loads are connected with a distribution substation. So, a very high rated large tarnsformer is needed to perform the operation. But construction and operation of such huge transformer is not so economical. So, it is better to use several small rated transformers in parallel as this operation is easier and economical too.
  • Suppose, any transformer in parallel operation get damaged and needs some maintanance work. But, in this case the power distribution is unaffected because the other parallel transformers can easily handle the situation. In this way, the faulty transformer can be taken out for servicing purpose without hampering the continuity.
  • We know that, routine checkup and maintanance is very imporatant for important for the safety operation of a transformer. In parallel operation, any one of the transformer has a support with other transformers. So, in parallel operation, routine maintanance is done with each and every transformer by switching them on and off alternately. This method provides great ease of use for the operating personel.
  • In a substation, the load demand is always not fixed. In future, there may be some condition when the load demand may get incraese beyond the combine capacity of transformers already installed. Then installing a separate large transformer unit for serving the extra load is quite difficult as this will involve a massive change in design of the substation. But, in parallel operation of transformer, adding a transformer unit is not very difficult task and in this way, the additional load demand can be easily handled. So, it is understood that parallel operation of transformers is quite better than installing single transformer units.

In the following diagram, the parallel opeartion of transformer is elaborated. parallel operation of transformers

a1 = turns ratio of transformer A
a2 = turns ratio of transformer B
V1 = Common source voltage which is connected with the primaries
ZL = Common load which is connected across the secondaries
ZA = Equivalent impedance of transformer A reffered to secondary
ZB = Equivalent impedance of transformer B reffered to secondary
IA = Secondary current of transformer A
IB = Secondary current of transformer B
VL = Secondary load voltage

Conditions of parallel opeartion of transformers

In order to get proper parallel operation of transformers, we need to mainatain some conditions. These conditions are discussed as follows:
Must have conditions: These conditions are the necessary conditions for parallel opeartion of single phase transformers as well as three phase transformers . These conditions must have to satisfied to initiate the parallel opeartion.

  • Same transformer polarity: The polarities of all transformers in parallel operation should be the same. As the same input power is provided for all parallel transformers, so opposite or unequal polarity of any transformer will leads to alow a circulating current in the transformers. Obviously, no load current will be fed to the secondary load. Thsi circulating current also leads to draw a primary current from supply when all the transformers are on no-load condition.
  • Equal turns ratio: The turns ratio or voltage ratio of all parallel transformers should be equal to achieve a satisfactory parallel operation. We know that, same primary voltage is applied to all parallrl transformers. If the turns ratio of them are not identical, then different secondary voltages will appear in different transformer unit. But, all the secondaries are connected with a common busbar. So, different load voltages will leads to develop a considerable amount of circulating current.

Desirable conditions: The follwing conditions are called desired conditions as these conditions are not vital to initiate the proper parallel operation but if they are satisfied, then the economy and reliability of the system get incraesed.

  • Equal resistance / reactance ratio: It is desired that the ratio of winding resistance versus reactance is equal for all parallel transformers. If this condition is satisfied, then the operation become more satisfactory.
  • Same phase sequence: The phase sequence is an imporatant parameter. As AC is a series of sinusoidal waves so all the phases reach their maximum positive amplitude after a cycle. This is termed as phase sequence and the phase sequence should be identical for all the parallel transformers.