What Is Scott Transformer Connection? how Scott Transformer Connection Works?
In order to overcome the disadvantage of the T connection, the Scott connection uses two single-phase transformers of a special design to transform three phase voltages and currents into two-phase voltages and currents.
The first transformer, called the ‘‘main,’’ has a center-tapped primary winding connected to the three-phase circuit with the secondary winding connected to the two-phase circuit. It is vital that the two halves of the center-tapped primary winding are wound around the same core leg so that the ampere-turns of the two halves cancel out each other. The ends of the center-tapped main primary winding are connected to two of the phases of the three-phase circuit.
The second transformer, called the ‘‘teaser,’’ has one end of its primary winding connected to the third phase of the three-phase circuit and the other end connected to the center tap of the primary winding of the main. The Scott connection requires no primary neutral connection, so zero-sequence currents are blocked.
The secondary windings of both the main and teaser transformers are connected to the two-phase circuit. The Scott connection is shown in Figure 2.18 for a two-phase, five-wire circuit, where both secondary windings are center-tapped and the center taps are connected to the neutral of the five wire circuit. Three-wire and four-wire configurations are also possible.
If the main transformer has a turns ratio of 1: 1, then the teaser transformer requires a turns ratio of 0.866:1 for balanced operation. The principle of operation of the Scott connection can be most easily seen by first applying a current to the teaser secondary windings, and then applying a current to the main secondary winding, calculating the primary currents separately and superimposing the results.
Apply a 1.0 per unit load connected between phase 1 and phase 3 of the secondary:
Secondary current from the teaser winding into phase 1 1.0∠90°
Secondary current from the teaser winding into phase 3 1.0∠90°
Primary current from A phase into the teaser winding 1.1547∠90°
Primary current from B phase into the main winding 0.5774∠90°
Primary current from C phase into the main winding 0.5774∠90°
The reason that the primary current from A phase into the teaser winding is 1.1547 per unit is due to 0.866:1 turns ratio of the teaser, transforming 1/0.866 1.1547 times the secondary current. This current must split in half at the center tap of the main primary winding because both halves of the main primary winding are wound on the same core and the total ampere-turns of the main winding must equal zero.
Apply a 1.0 per unit load connected between phase 2 and phase 4 of the secondary:
Secondary current from the main winding into phase 2 1.0∠0°
Secondary current from the main winding into phase 4 1.0∠0°
Primary current from B phase into the main winding 1.0∠0°
Primary current from C phase into the main winding 1.0∠0°
Primary current from A phase into the teaser winding 0
Superimpose the two sets of primary currents:
I a 1.1547∠90° 0 1.1547∠90°
I b 0.5774∠90° 1.0∠0° 1.1547∠ 30°
I c 0.5774∠90° 1.0∠0° 1.1547∠210°
Notice that the primary three-phase currents are balanced; i.e., the phase currents have the same magnitude and their phase angles are 120° apart. The apparent power supplied by the main transformer is greater than the apparent power supplied by the teaser transformer.
This is easily verified by observing that the primary currents in both transformers have the same magnitude; however, the primary voltage of the teaser transformer is only 86.6% as great as the primary voltage of the main transformer.
Therefore, the teaser transforms only 86.6% of the apparent power transformed by the main. We also observe that while the total real power delivered to the two phase load is equal to the total real power supplied from the three-phase system, the total apparent power transformed by both transformers is greater than the total apparent power delivered to the two-phase load.
Using the numerical example above, the total load is 2.0 per unit. The apparent power transformed by the teaser is 0.866 I a 1.0 per unit, and the apparent power transformed by the main is 1.0 I b 1.1547 per unit for a total of 2.1547 per unit of apparent power transformed.
The additional 0.1547 per unit of apparent power is due to parasitic reactive power flowing between the two halves of the primary winding in the main transformer. Single-phase transformers used in the Scott connection are specialty items that are virtually impossible to buy ‘‘off the shelf ’’ nowadays.
