AC MACHINES-1 (66761) Theory

7. Realize the construction and Principle of operation of three phase transformer.

7.6. Explain the application of V-V and T-T connection.

This reduces the project cost. An open-delta connection may be used to provide three phase power up to 57.7 % of the actual capacity of the three phase bank. If the three phase load is not reduced while using open-delta, there is high chance that one transformer gets overloaded; which can be more dangerous

The open-delta, also known as the V-V connection, is a 3-phase arrangement that makes use of only two, instead of three, single-phase transformers, as shown in Fig. ... In the case of three transformers, the line current is times the current in the transformers windings when connected delta and when the load is balanced.

The main advantage of a V-V is a reduction of cost of equipment and ... In V-V connection the transformer primary and secondary winding are ..

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.

Applications of Scott Connection

It is used to supply the single phase loads such as electric train which are so scheduled as to keep the load on the three phase system as nearly as possible. The Scott-T connection is used to link a 3-phase system with a two–phase system with the flow of power in either direction.

A teaser transformer is the second transformer of a two transformer set in a Scott-T or T-T transformer bank. The teaser transformer is tapped at 86.6% of the main transformer on the HV set of windings. The two transformers convert a 3-phase input to a 2-phase,

The two transformers convert a 3-phase input to a 2-phase, 5-wire, output.