# Scott Connection of two 1-phase transformer

62

T-T/Scott Connection:

• This is a method of transformation of 3-phase power from one voltage to another by using two transformers.
• First it was proposed by C.F. Scott, so it is frequently called as Scott connection.
• This scheme of connection as shown in figure 10, is widely used for 3-ɸ to 2-ɸ and vice-versa transformation.
• In this arrangement two transformers are required. One of the two transformers, the so-called main transformers, must have at least two primary and two secondary coils so that a center tap may be brought out from each side.
• The other transformer, known as Teaser Transformer must have primary and secondary windings, the number of turns of which are 0.866 times of the respective turns on the main transformer.
• The current ratings of the two transformers must be same. The two ends of man transformer and 86.6% tapping point on teaser transformer is connected to the 3-d supply on the primary side and to the load on secondary side.
• If VAB, VBC and VCA are the applied voltage across the line terminals then

VCD= VCA + VAD = V ∠120˚ + v ∠0˚

= j 0.866 V = 0.866 V ∠90˚

• The voltage applied across the teaser is 0.866 times of that across the main and has a phase difference of 90˚.
• If V is the line voltage and I is the current rating of the winding, which is also line current the combined volt-ampere rating of the two transformers is VI + 0.866 VI.
• The volt-ampere supplied are  VI, therefore, utility factor is  = 0.928.
• Therefore, the T-T connection is more economical than open Δ connection with utility factor of 0.866.
• Each transformer in a T-T connection must have a rating 15% greater than each of the transformers in a transformer bank.
• For any balanced load of power factor cos ф (lag) though the teaser operates at a PF of cos φ (lag), one of the two halves of the main transformer operates at PF of cos (30° + ф) and the other at (30°- ф).
• Advantage of this connection is availability on the teaser transformer and it provides 3ɸ, 4-wire system.

Also Read- Back-To-Back test