Three single-phase transformers are connected in a Y-Δ configuration to form a three-phase transformer bank supplying a 120 kVA, 230 V, 0.85 pf lag load. Calculations show the primary phase current is 17.39 A and since it is a Y connection, the primary line current is also 17.39 A.

1. Example 1
Three single phase, 50 kVA, 2300/230 V. 50 Hz
transformers are connected to form a three
phase 4000/230 V transformer bank. The three
phase transformer supplies a 3Φ 120 kVA, 230
V, 0.85 pf (lag) load.

2. Y-!
A1
B1
C1
N1
T1 T2 T3
A2
B2
C2
Figure 10.44: Phase wiring for “Y-∆” transformer.

3. I Phase = I Line
VLine = √3 x V Phase

4. P formula: P = VI I= P
V

5. P2
Is =
(Vphase × √3)
120 ×10 3
Is = = 301.23 A
(230 x √3)

6. VPhase = VLine
Iline
ILine= √3 x IPhase (or Iphase = )
√3

7. ILine = √3 × IPhase
ILine
∴IPhase =
√3
301.23
=
√3
= 173.91 A

8. a = 2300 ÷ 230 = 10
So, I1 = I2 ÷ a
I1 = 173.91 ÷ 10 = 17.39 A

9. In a star system, I Phase = I Line
Therefore, the line current and
phase current in the primary
windings
= 17.39 A.