Three-phase Transformer Connections

Virtually all power distributions is by poly-phase system of voltages. Three-phase transformations may be made with the use of properly connected single-phase transformers. These connections are in extensive commercial use. The most frequently used connections are the following :

(i) Primary X – secondary Y,

(ii)Primary Δ – secondary Δ.

(iii)Primary Δ– secondary Y, or vice versa

(iv)Primary and secondary open Δ.

(v)Primary T secondary T (Scott connection).

Thus the most common connections are Y-Y, Δ-Δ, Y-Δ, Δ-Y, open delta or V-V and Scott connection on T-T connection.

10.1.1. The Y-Y connection. Fig. 65 shows a bank of three transformers connected in Y on both the primary and secondary sides. If the ratio of transformation of each transformer is K, the same ratio will exist between the line voltages on the primary and secondary sides. This connection will give satisfactory service only if the three-phase load is balanced ; when the load is unbalanced, the electrical neutral will shift from its exact centre to a point that will make the line to neutral voltages unequal.

The above advantages are enumerated with the understanding that, other things being equal, its rival is the delta-delta connection.

1. In case the load on the secondary side is unbalanced, as in the case of distributing network, the potential of the star-point will assume any value if the star-point is not earthed. This may impose full-line voltage on secondary windings. The shifting of the neutral point must be prevented by connecting the primary star-point to the star-point of the alternator winding.
2. In spite of grounding the star-point, if there is a third harmonic in the form of the alternator voltage, the third harmonic will appear in the voltages of the secondary side. This will cause triple frequency currents in the three-phase circuits. These currents when they flow in the neutral wire are additive and do not cancel out. Hence they will cause interference to telephone lines located along the same route.
3. The magnetising current in a transformer has third harmonic components. These currents will find a return path via the connection between the primary star-point of transformer and the neutral point of the alternator. However, if this connection is missing, these components will distort the flux wave which will produce a voltage having a third harmonic in each of the transformer, both on the primary and secondary sides, And, as before, if the star point on the secondary is earthed, or grounded, triple harmonic currents will appear in the secondary circuit, and they will flow through the neutral wire causing interference to telephone lines in the vicinity.
4. If the star-points of both the primary and the secondary sides are not earthed, the regulation of the phases will be very poor if the load happed to be unbalanced as the case of distribution network.

10.1.2. The Δ-Δ connection. Fig. 66 shows a bank of transformers connected in Δ on both the primary and secondary sides. This arrangement is generally used in systems in which the voltages are not very high and especially when continuity of service must be maintained even though one of the transformers should fail.

Fig. 66. The Δ-Δconnection of transformers.

1. The system voltages are more stable in relation to an unbalanced load.
2. If one transformer fails it may switched out of the line and operation continued at a reduced power level. This is known as open-delta or V-V operation.
3. There is no distortion of flux, because the third harmonic component of magnetising current can flow in the delta connected primary windings without flowing in the line wires.
4. No difficulty is experienced due to unbalancing of loads on secondary side.

1. In comparison in Y-Y connections it requires it more insulation.
2. The absence of star-point may be disadvantageous. If one line gets earthed due to fault, maximum voltage between windings and core will be full line voltage.

10.1.3 The Y-Δ connection. The Y-Δconnection as shown in Fig. 67

Fig. 67. The Y-Δ connection.

If is principally used where the voltage is to stepped down, as for example, at the end of a transmission line. It is also employed in moderately low voltage distribution circuits for stepping down from transmission voltages of 4000 or 8000 V to 230 (and 115 V).

• The Y connection takes advantage of the fact that one that leg of a Y, or the line-to-neutral voltage, is less than the line-to-line voltage by a  factor. This is especially important when the primary voltage is a few hundred thousand volts.
• The Y-Δ does have a phase shift between the primary and secondary voltages. This 300 phase shift means that a Y-Δ transformer bank cannot be paralleled with either a Y-Y or a Δ-Δ. The phasor voltage differences between the two systems would be around sin 300 = 0.5 times the secondary voltages. This would cause an excessive circulating current between transformer banks.

10.1.4. The Δ-Y connection

• The three-phase Δ-Y connections are shown in Fig. 68

This type of connections is employed where it is necessary to step up the voltage, as for example, at the beginning of a high-tension transmission system.

• The ratio of secondary to primary voltage is  times the transformation ratio of each transformer.
• The neutral of the secondary is grounded for providing 3-phsae 4-wire service. This connection is popular since it can be used to serve both the 3-phse power equipment and single-phase lighting circuit.

This connection is not open to the objection of a floating neutral and voltage distortion because the existence of a Δ-connection allows a path for the third-harmonic currents. It would be observed that the primary and secondary line voltages and line currents are out of phase with each other by 300. Because of this 300 shift, it is impossible to parallel such a bank with a Δ-Δ or Y-Y bank of transformers even though the voltage ratio are correctly adjusted.

Fig. 69. The V-V (open Δ) connection of transformers.

If one of the transformers of a Δ-Δ bank is removed and a three-phase source is connected to the primaries (as shown in Fig. 66), three equal three-phase voltages will be measured at the secondary terminals at no-load. This method of transforming three-phase power, using two transformers, is called the open-delta, or V-V connection.

This type of connection is used in the following cases :

(i)               When the three-phase load is comparatively small so that the installing does not warrant a three transformer bank.

(ii)            When one of the transformers in a Δ-Δ bank fails, so that the service may be continued until the faulty transformer is repaired or a good one is substituted.

(iii)          When it is anticipated that the future load will increase to warrant the chosing of the open Δ at the some time later.

V-V connection has a number of features that are advantageous:

• Upon failure of the primary or secondary of one transformer of a complete Δ-Δ transformer circuit, the system reverts to a V-V circuit, so this is an automatic standby. The power-handling capacity of a V-V circuit is  times the capacity of a full Δ-Δ of the same transformers. This feature works both ways, so a circuit is sometimes installed as V-V with the understanding that its power handling may be multiplied by  by adding one more transformer.
• Open delta or V-V circuits do introduce some voltage unbalance due to the non-symmetry of the voltage regulation effects under load. However, the small degree of unbalance is not normally noticed by a motor load or other types of commercial load.

Disadvantages of V-V connection :

1.  The secondary terminal voltages tend to become unbalanced to a great extent when the load is increase, this happens when the load is perfectly unbalanced.
2. The average factor at which the V-bank operates is less than that of the load. This power factor is actually 86.6 per cent of the balanced load factor. Another important point to note is that except, for a balanced unity power factor load, the two transformers in the V-V bank operate at different power factors.

Uses of V-V connection:

(i)                           The V-V circuit is frequently used for two auto-transformers. Here advantage is taken of power handling of auto-transformers and their superior voltage regulation and efficiency.

(ii)                        Another major use of V-V transformer banks is in A.C. motor starting.

10.1.6. Scott or T-T connection. The connection of one polyphase system into another polyphase system is possible by suitably connecting the windings of transformers. One of the early types that was used in Scott or T-T connection, by which a 2-phase system is available from a 3-phase system or vice versa.

In Fig. 70 two single-phase transformer M and T, the primaries of which are connected to a 3-phase supply. The secondary of M forms one phase and the secondary of T the other phase of a true 2-phase system. M is called the main transformer and T is called the teaser. One end of the teaser primary is connected to the mid-point of the main primary. The two ends of the main primary are connected to two lines wires of a 3-phase, 3-wire system, and the third line wire is connected to a tapping X on the teaser primary.