Conditions for parallel operation of transformers

Parallel Operation of transformers

Two transformers are connected in parallel means that the two primary windings are connected to supply bus and the two secondary windings are connected to load bus-bars as shown in the figure.

Parallel Operation of Transformer

Parallel Operation of Transformer

Need of parallel Operation of Transformers

  • To supply a load in excess of the ratings of an existing transformer, Two or more transformers may be connected in parallel with the existing transformer. This is more economical connecting an extra small transformer in parallel instead of keeping an another large capacity transformer. The cost is also less for purchasing extra small rating transformer.
  • Parallel operation of transformers provides more reliability i.e. even in the failure or out off service of one transformer half of the bus load can be driven using signal transformer in emergency cases.


Conditions for parallel operation of Transformers

There are various conditions that must fulfilled for the successful operation of transformers as follows.

  1. The line voltage ratio of two transformers must be equal.
  2. The per unit impedance of each transformer should be equal and they should have same ratio of equivalent leakage reactance to the equal resistance(X/R).
  3. The transformers should have same secondary winding polarity.
  4. The Transformers should have same phase sequence (Three phase transformer)
  5. The transformers should have the zero relative phase replacement between the secondary line voltages.(Three phase transformers)

1. The line voltage ratios of the two transformers must be equal

This condition is used to avoid the inequality EMF induction at the two secondary windings. If the two transformers connected in parallel have slightly different voltage ratios, then due the inequality of induced emfs in the secondary voltages, a circulating current will flow in a loop format in the secondary windings. This current is greater than the no load current and will be quite high due to less leakage impedance during load. When the secondary windings are loaded, this circulating current will tend to unequal loading on two transformers and one transformer may be over loaded and another may be less loaded.

2) Equal per unit leakage impedance

If the ratings or line voltages are equal their per unit leakage impedance’s should be equal in order to have equal load sharing of the both transformers. If the ratings are unequal then the transformer which has less rating will draw more current and it leads to unequal load sharing. It may also lead to mismatch in line voltages due to voltage drops. In other words, for unequal ratings, the numerical values of their impedance’s should be in inverse proportional to their ratings to have current in them inline with their ratings.

A difference in the ratio of the reactance value to the resistance value of the impedance results in different phase angles of the currents carried by the two parallel transformers. Due to this phase angle difference between voltage and current, one transformer may be working on high power factor and another transformer may be working on lower factor. Hence real power sharing is not proportional between the two transformers.

3) The transformers should have same secondary winding polarity

The transformers should be properly connected with regards to their polarity. If they are connected with in correct polarities then the two emf’s induced in the secondary winding which are in parallel, will act together and produce a short circuit between the two of them. Total loss of power supply and high damage to the transformers.

4) The Transformers should have same phase sequence

In case of three winding transformers in-addition to the above conditions the phase sequence of line voltages of the both transformers must be identical for parallel operation. If the phase sequence is not correct in every voltage cycle each pair of phases will get shorted

5) The transformers should have zero relative phase displacement between the secondary line voltages

This condition indicates that the two secondary line voltages should have zero phase displacement which avoids UN-intended short circuit between the phases of two windings.

There are four groups which in to which the three phase windings connections are classified:

  • Group 1: Zero phase displacement (Yy0,Dd0,Dz0)
  • Group2: 1800 Phase displacement (Yy6,Dd6,Dz6)
  • Group3: -300 Phase displacement (Ys1,Dy1,Yz1)
  • Group4: +300 Phase displacement(Yd11,Dy11,Yz11)

The letters Y,D and z represents the Star, Delta and zigzag type winding connections. In order to have zero phase displacement of secondary side line voltages, the transformers belonging to the same group can be paralleled. For example with Yd1 and Dy1 can be paralleled. The transformers  of groups 1 and 2 can be paralleled with their own group where as the transformers of group 3 and 4 can be paralleled by revering the phase sequence of one of them. For example a transformer with Yd11 connection of group 4 can be paralleled with that having Dy1 connection by reversing the phase sequence of both primary and secondary terminals of the Dy1 transformer.

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