AC MACHINES-1 (66761) Theory

Open-Delta or V – V connection

If one of the transformers of a D – D is removed and 3-phase supply is connected to the primaries as shown in Fig. 33.11, then three equal 3-phase voltages will be available at the secondary terminals on no- load. This method of transforming 3-phase power by means of only two transformers is called the open – D or V – V connection.

It is employed :

1. when the three-phase load is too small to warrant the installation of full three-phase transformer bank.

2. when one of the transformers in a D – D bank is disabled, so that service is continued although at reduced capacity, till the faulty transformer is repaired or a new one is substituted.

3. when it is anticipated that in future the load will increase necessitating the closing of open delta.

One important point to note is that the total load that can be carried by a V – V bank is not two-third of the capacity of a D – D bank but it is only 57.7% of it. That is a reduction of 15% (strictly, 15.5%) from its normal rating.

Suppose there is D – D bank of three 10-kVA transformers. When one transformer is removed, then it runs in V – V. The total rating of the two transformers is 20 kVA. But the capacity of the V – V bank is not the sum of the transformer kVA ratings but only 0.866 of it i.e. 20 ´ 0.866 = 17.32 (or 30 ´ 0.57 = 17.3 kVA). The fact that the ratio of V-capacity to D-capacity is 1/ 3 = 57.7% (or nearly 58%) instead of 66 2 per cent can be proved as follows :

It is obvious from above that when one transformer is removed from a D – D bank.