What is the Scott-T connection in the transformer?
Three-phase to three-phase and three-phase to two-phase conversions are carried out by connecting three single-phase transformers and vice versa. The Scott connection also is applied to get 2 phase supply from the available three-phase supply which is mainly required in electric furnaces.
There are two transformers used for three-phase to two-phase conversion. The one transformer is called the main transformer having centre tapping(50%) on both the primary and secondary windings. The second transformer is called teaser or auxiliary transformer which has 86.6% tapping.
The fig. shows the Scott-T connection diagram.
- The CB is the main transformer primary.
- The AD is the teaser transformer primary.
- The cb is the main transformer primary.
- The ad is the teaser transformer secondary.
- The assumption line voltage is 100 volts.
- The transformation ratio is taken unity.
- The point D and d of the teaser transformer are connected to the centre tapping of the main transformer.
- 100 Volts(line) three-phase supply voltage connected to an A, C, B.
- This forms so two transformers are connected to Scott. This connection seems to be geometrical as the English letter T and hence Scott connection is also known as the T-T connection.
Phasor Diagram of Scott Connection Transformer
The vector Diagram of Scott's connection for the primary is shown in fig.
- The 100 volts are divided into exact 1/2, 1/2 across CD and DB. i.e EDC
= EDB = 100/2 = 50 volts each but having phase difference of 180
degree. The two sections of coils are on the same magnetic core but connected
in opposition.
- Three sides of an equilateral triangle represent 3 voltages each of 100 volts. This is a voltage triangle of the primary of the main transformer.
- AD is the altitude of an equilateral triangle. This is the teaser transformer primary and its magnitude is equal to √3/2 x 100 = 86.6 volts. Hence EDC = 86.6 volts and lagging by 90 degrees to the main voltage.
A symmetrical equilateral triangle abc is for secondaries shown in fig.
- For a load of unity power factor, secondary current Idb lags behind the voltage Edc by an angle 30 degrees. Similarly, current Idc leads voltage Ecd by an angle of 30 degrees.
- The above phaser relation shows that the teaser and each half of the main transformer operate at different power factors and hence full ratings of transformers are not utilised fully.
The teaser transformer operates at 86.6 % of rated voltage and the primary and secondary coils of the main transformer operate at power factor = Cos30 = 0.866. This is equivalent to the main transformer's primary and secondary coils working at 86.6 % of their KVA ratings. Hence capacity/rating = 0.866 just the same as in the open triangle condition.
If instead of 100 volts, the primary and secondary teaser transformers are designed for 86.6 volts, then we say they will operate at their full ratings. if so then total rating will be = (86.6 + 86.6)/(100 + 86.6) = 0.928 of the total rating. This gives us KVA utilized/KVA available = 0.928. This ratio was 0.866 in the open triangle.
The Scott-T connection is more economical than the open delta(V-V) connection, as the ratio is 0.928>0.866.
Advantages of Scott-T connection
1. With help of the Scott-T connection, three-phase, two-phase and single-phase loads are supplied.
2. In the Scott-T connection Neutral point is available for grounding purposes.
Disadvantages of Scott-T connection
1. In the Scott-T connection Three-phase supply is created from the two winding. This causes an unbalanced load on both windings and an unbalanced load is supplied to the load.
2. In Scott-T connection design of the neutral is solidly grounded. If neutral is not solidly grounded then it causes the secondary voltage to be unstable.
Applications of Scott connections
1. Scott-T connections are used in electric furnaces installations. In the Scott-T connection three-phase supply is transformed into the two-phase supply and which draws the balanced load from the supply.
2. The Scott-T connection is used in electrical trains to supply a single-phase load.
3. The Scott-T connection is mostly used to link the two-phase to three-phase system