Transformer,three phase:Scott Connection or T – T Connection

Scott Connection or T – T Connection

This is a connection by which 3-phase to 3-phase transformation is accomplished with the help of two transformers as shown in Fig. 33.13. Since it was first proposed by Charles F. Scott, it is frequently referred to as Scott connection. This connection can also be used for 3-phase to 2-phase transformation as explained in Art. 33.10.

One of the transformers has centre taps both on the primary and secondary windings (Fig. 33.13) and is known as the main transformer. It forms the horizontal member of the connection (Fig. 33.14).

The other transformer has a 0.866 tap and is known as teaser transformer. One end of both the primary and secondary of the teaser transformer is joined to the centre taps on both primary and secondary of the main trans- former respectively as shown in Fig. 33.14 (a). The other end A of the teaser primary and the two ends B and C of the main transformer primary are connected to the 3-phase supply.

The voltage diagram is shown in Fig. 33.14 (a) where the 3-phase supply line voltage is assumed to be 100 V and a transformation ratio of unity. For understanding as to how 3-phase transformation results from this arrangement, it is desirable to think of the primary and secondary vector voltages as forming geometrical TS¢ (from which this connection gets its name).

In the primary voltage T of Fig. 33.14 (a), EDC and EDB are each 50 V and differ in phase by 180°, because both coils DB and DC are on the same magnetic circuit and are connected in opposition. Each side of the equilateral triangle represents 100 V. The voltage EDA being the altitude of the equilateral triangle is equal to ( 3 / 2) ´ 100 = 86.6 V and lags behind the voltage across the main by 90°. The same relation holds good in the secondary winding so that abc is a symmetrical 3-phase system.

With reference to the secondary voltage triangle of Fig. 33.14 (b), it should be noted that for a load of unity power factor, current Idb lags behind voltage Edbby 30° and Idc leads Edc by 30°. In other words, the teaser transformer and each half of the main transformer, all operate at different power factors.

Obviously, the full rating of the transformers is not being utilized. The teaser transformer operates at only 0.866 of its rated voltage and the main transformer coils operate at cos 30° = 0.866 power factor, which is equivalent to the main transformer’s coils working at 86.6 per cent of their kVA rating. Hence the capacity to rating ratio in a T–T. connection is 86.6% — the same as in V – V connection if two identical units are used, although heating in the two cases is not the same.

If, however, both the teaser primary and secondary windings are designed for 86.6 volts only, then they will be operating at full rating, hence the combined rating of the arrangement would become (86.6 + 86.6)/(100 + 86.6)

= 0.928 of its total rating.* In other words, ratio of kVA utilized to that available would be 0.928 which makes this connection more economical than open-D with its ratio of 0.866.

Fig. 33.15 shows the secondary of the T – T connection with its different voltages based on a nominal voltage of 100 V. As seen, the neutral point n is one third way up from point d. If secondary voltage and current vector diagram is drawn for load power factor of unity, it will be found that

1. current in teaser transformer is in phase with the voltage.

2. in the main transformer, current leads the voltage by 30° across one half but lags the voltage by 30° across the other half as shown in Fig. 33.14 (b).

Hence, when a balanced load of p.f. = cos f, is applied, the teaser current will lag or lead the voltage by F while in the two halves of the main transformer, the angle between current and voltage will be (30° – F) and (30° + F). The situation is similar to that existing in a V – V connection.

Example 33.17. Two T-connected transformers are used to supply a 440-V, 33–kVA balanced load from a balanced 3-phase supply of 3300 V. Calculate (a) voltage and current rating of each coil (b) kVA rating of the main and teaser transformer.

Solution. (a) Voltage across main primary is 3300 V whereas that across teaser primary is