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Home The Transformer 3Phase Transformer Connections OpenDelta or VV Connection  
See also: WyeWye Connection, DeltaDelta Connection, WyeDelta Connection  
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OpenDelta or VV Connection
The opendelta, also known as the VV connection, is a 3phase arrangement that makes use of only two, instead of three, singlephase transformers, as shown in Fig. 627. The opendelta connection is sometimes used in the case of instrument potential transformers for economy. It is also used initially in load centers, the full growth of which may require several years, at which time a third transformer is added for the conventional deltadelta operation.
The rating of two identical transformers operating deltadelta is 1/ of the rating of three similar transformers, but connected deltadelta. It would seem that this rating should be 2/3 instead of 1/. However, the line current is also the current in the transformer winding and, if I is the rated current of each of the two transformers and V the rated voltage, then the rated 3phase power is VI. 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 3phase rating of the three transformers is, therefore, 3VI, where I is the rated current of the transformer winding. If it were not for the leakage impedance of the transformers, the secondary voltages of an opendelta connection would be balanced when balanced 3phase voltage is applied to the primary. This is evident when we let V_{AB} V_{BC} and V_{CA} be the balanced 3phase primary applied voltages. Then
and
If the transformer leakage impedances are neglected, and their ratio is a, then the secondary voltages are
but from Kirchhoff's law
from which
showing that the secondary voltages V_{ab}, V_{bc}, and V_{ca} are equal in magnitude and displaced from each other by an angle of 120°. This is characteristic of balanced 3phase systems, and is true only for ideal transformers. In the case of actual transformers, the secondary voltages will not be balanced exactly, even when balanced voltages are applied to the primary in an opendelta arrangement because there are only two transformers and, therefore, only two, instead of three, leakage impedance voltage drops.


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