1. Three-Phase Transformer
A three-phase system in used to generate
and transmit electric power.
Three-phase voltages are raised or lowered
by means of three-phase transformers.
A three-phase transformer can be built in
two ways
By suitably connecting a bank of three single-phase transformers
By constructing a three-phase transformer
on a common magnetic structure
(Y−Y , ∆ − ∆ , Y− ∆ or ∆ −Y)

2. Three-Phase Transformer Connections
The primary or secondary windings may be connected in either star (Y) or delta (∆)arrangement.
The four most common connections are
Y-Y
∆-∆
Y-∆
∆-Y.

3. Y-Y Connection

4. ∆-∆ Connection

5. Y-∆ Connection

6. ∆-Y Connection

7. Three-Phase Transformation with Two Single-Phase Transformers
connection of two identical single-phase
transformers in open delta(or V-V connection)
T-T connection (or Scott connection) of
two non identical single-phase transformers

8. Open-Delta or V-V Connection

9. The transformer windings ab and bc will deliver power given by
Pab = Vab Ia cos (30° + φ)
Pbc = Vcb Ic cos (30° − φ)
Let Vab = Vcb = V, the voltage rating of transformer secondary winding
Ia = Ic = I, current rating of the transformer secondary winding

10. p.f. = 1 i.e. φ= 0° ... For resistive load
Power delivered to the resistive load by V-V connection is
Pv = Pab + Pbc
= Vab Ia cos (30° + φ) + Vcb Ic cos (30° − φ)
= Vab Ia cos (30°) + Vcb Ic cos (30°)
= V I cos (30°) + V I cos (30°)
= 2 V I cos (30°)

11. With all the three transformers connected in delta, the power delivered to the resistive load is
P = 3 V I
Pv / P = 2 V I cos (30°) / 3 V I
= 0.577
Hence the power-handling capacity of a V-V circuit (without overheating the transformers) is 57.7% of the capacity of a complete ∆-∆
circuit of the same transformers

12. In a V-V circuit, only 86.6% of the rated capacity of the two transformers is available.

13. Power Factor of Transformers in V-V Circuit
When V-V circuit is delivering 3-phase power, the power factor of the two transformers is not the same
If the load power factor angle is φ, then
p.f. of transformer 1 = cos (30°− φ)
p.f. of transformer 2 = cos (30° + φ)

14. i) When load p.f. = 1 i.e. φ = 0°
In this case, each transformer will have a
power factor of
ii) When load p.f. = i.e. φ= 30°
In this case, one transformer will have a
p.f. of cos (30°−30°) = 1 and the other of
cos (30° + 30°) = 0.5
When load p.f = 0.5 i.e. φ= 60°
p.f. of cos (30°−60°) = and the other
of cos (30° + 60°) = 0.

15. Applications of Open Delta or V-V Connection
The circuit can be employed in an
emergency situation when one
transformer in a complete ∆-∆circuit must
be removed for repair and continuity of
service is required.
Upon failure of the primary or secondary
of one transformer of a complete ∆-∆
circuit, the system can be operated as V-V
circuit and can deliver 3-phase power
(with reduced capacity) to a 3-phase load.

16. Scott Connection or T-T Connection