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Vector Group Test of Transformer

The vector group of transformer is an essential propertyfor successful parallel operation of transformers. Henceevery electrical power transformer must undergo throughvector group test of transformer at factory site forensuring the customer specified vector group oftransformer.

The phase sequence or the order in which the phases reachtheir maximum positive voltages, must be identical for twoparalleled transformers. Otherwise, during the cycle, eachpair of phases will be short circuited.

The several secondary connections are available in respectof various primary three phase connection in a the threephase transformer. So for same primary applied three phasevoltage there may be different three phase secondaryvoltages with various magnitudes and phases for differentinternal connection of the transformer. Let's have adiscussion in detail by example for better understanding.

We know that, the primary and secondary coils on any onelimb have induced emfs that are in time - phase. Let'sconsider two transformers of same number primary turnsand the primary windings are connected in star. Thesecondary number of turns per phase in both transformersare also same. But the first transformer has star connectedsecondary and other transformer has delta connectedsecondary. If same voltages are applied in primary of bothtransformers, the secondary induced emf in each phase will

be in same time-phase with that of respective primaryphase, as because the the primary and secondary coils ofsame phase are wound on the same limb in the core oftransformer. In first transformer, as the secondary is starconnected, the secondary line voltage is 3 times of inducedvoltage per secondary phase coil. But in case of secondtransformer, where secondary is delta connected, the linevoltage is equal to induced voltage per secondary phase coil.If we go through the vector diagram of secondary linevoltages of both transformer, we will easily find that therewill be a clear 30o angular difference between the line

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voltages of these transformers. Now, if we try to run thesetransformers in parallel then there will be a circulatingcurrent flows between the transformers as because there is aphase angle difference between their secondary line

voltages. This phase difference can not be compensated.Thus two sets of connections giving secondary voltages witha phase displacement can not be intended for paralleloperation of transformers.

The following table gives the connections for which from theview point of phase sequence and angular divergences,transformer can be operated parallel. According to theirvector relation, all three phase transformers are divided into

different vector group of transformer. All electrical powertransformers of a particular vector group can easily beoperated in parallel if they fulfill other condition for paralleloperation of transformers.

GROUP Connection Connection

0(0o)

Yy0 Dd0

1( 30o)

Yd1 Dy1

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6

( 180o)

Yy6 Dd6

11( - 30o)

Yd11 Dy11

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Procedure of vector group test of transformer

Lets have a YNd11 transformer.

1) Connect neutral point of star connected winding withearth.

2) Join 1U of HV and 2W of LV together.

3) Apply 415V, three phase supply to HV terminals.

4) Measure voltages between terminals 2U 1N, 2V 1N, 2W 1N, that means voltages between each LV terminal and HVneutral.

5) Also measure voltages between terminals 2V 1V, 2W 1W and 2V 1W.

For YNd11 transformer, we will find,2U 1N > 2V 1N > 2W 1N

2V 1W > 2V 1V or 2W 1W

The vector group test of transformer for other group canalso be done in similar way.