Scott Connection of two 1-phase transformer


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


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