1. Transformer Current & Voltage Practice

2. Wye connected windings:
Have a common connection point
Have two voltages available “L-L & L-N”
Has only one current
Line current

3. Delta connected windings:
Have no common connection point
Have only one voltage available “L-L”
Delta can be closed “connected” in more than one way
Has two currents
Line current
Phase or winding current

4. Definitions
Delta: Phase voltage is line voltage but phase current is not line current.
Line current ÷ √3= phase current
Line voltage = Phase voltage
Wye: Phase voltage is not line voltage but phase current is line current.
Line voltage ÷ √3= phase voltage
Line current = phase current

5. 230 kV 13,280 23 kV HS Line 230 kV LS Line 23 kV
Transformer 3 Ø 230/13280/23000 Gnd. Wye
25 MVA
230 kV N H1 H2 H3 X1 X2 X3 C B A a b c W1 W2 W3 W4 W5 W6
13,280
23 kV

6. Voltage on HS 115 kV. Only one voltage because it is delta
HS Line 115 kV
LS Line 12 kV
Transformer:
3 3 Ø 115/12000/6928 Gnd. Wye 40MVA
Voltage on HS 115 kV. Only one voltage because it is delta N H1 H2 H3 X1 X2 X3 C B A a b c W2 W3 W5 W6
201
116
16.6/1
1925
Voltage on LS 12,000 & 6,928 because it is wye connected
Transformer Ratio 16.6/1

7. 1255 LS Line 23 kV Transformer ? MVA N H1 H2 H3 X1 X2 X3 C B A a b cW2 W3 W5 W6
1255

8. Typical GSU Transformer Connection SystemX3 X2 X1 H3 H2 H1 a b c C B A W1 W2 W3 W4 W5 W6
Generator

9. Transformer Solutions
Information: 230/23kV 25 MVA transformer
What is the voltage A-B, B-C, C-A ?
What is the voltage a-b, b-c, c-a?
What is the voltage a-n, b-n, c-n?
What is the voltage across W1, W2, W3?
What is the voltage across W4, W5, W6?
What is the current in line A, B, C ?
What is the current in W1, W2, W3?
What is the current in W4, W5, W6?
230 kV
23 kV
13,280 kV
230 kV
13,280 kV
25 MVA / 230 kV / √3 = amps
25 MVA / 3 = 8.33 MVA /230 kV = 36.3 amps
25 MVA / 23 kV / √3 = amps

10. Transformer Solutions
Information: Δ-Υ 230/23kV 25 MVA transformer
What is the voltage on High voltage line?
What is the voltage on low voltage line?
What is the voltage on low voltage phase?
What is the voltage across W1, W2, W3?
What is the voltage across W4, W5, W6?
What is the current in line A, B, C ?
What is the current in W1, W2, W3?
What is the current in W4, W5, W6?
230 kV (Given)
23 kV (Given)
13,280 kV ( V 𝐿−𝐿 ÷ √3)
230 kV (Same as the line for Δ winding)
13,280 kV ( L-N voltage)
25 𝑀𝑉𝐴 ÷230,000÷ 3 =62.75
25 𝑀𝑉𝐴 ÷230,000 ÷√3÷√3=36.23
25 MVA / 23 kV / √3 = amps

11. Transformer Solutions
What is the current in line a, b, c?
On the delta side of the transformer
Same as winding W4, W5, W6= amps
The line current ÷ √3 = Winding Current ;
62.76 ÷ √3 =
Calculating three phase transformer MVA
Voltage X Current X three windings
230,000 X 36.3 amps = 8,349,000 x 3 (windings)= 25,047,000 = 25 MVA
13,280 X amps = 8,334,129 X 3 (windings)= 25,002, 388 = 25MVA

12. Transformer Ratio 3 Phase 230/23 kV 25 MVA
Transformer is 230/ 23 kV
Ratio is NOT this ration but is the ration of the voltages across the windings.
W1 230 kV, W4 13,280 Ratio is 230,000 / 13,280 = 17.32/1 N H1 H2 H3 X1 X2 X3 C B A a b c W1 W2 W3 W4 W5 W6
Note:
W4 current/ W1 current = /36.3 = 17.3
amps X 13,280 volts = 8.3 MVA
36.3 amps X 230,000 volts = 8.3 MVA
8.3 MVA X 3 (windings) =25 MVA

13. How Are They Connected?
primary
secondary H

14. N H1 H2 H3 X1 X2 X3 C B A a b c W1 W2 W3 W4 W5 W6

15. Both transformers are Δ-Υ but notice the Δ windings are not closed the sameX1 X2 X3 C B A a b c W1 W2 W3 W4 W5 W6 N H1 H2 H3 X1 X2 X3 C B A a b c W1 W2 W3 W4 W5 W6

16. Single Phase Transformer information: HV 115 kV LV 7200 V
Capacity 16,670 KVA
primary
secondary
Primary amps
16,670,000 ÷ 115,000 = 145 amps
Secondary amps
16,670,000 ÷ 7,200 = 2315 amps
Turns Ratio
115,000 ÷7200 = 15.97/1
Calculating single phase transformer MVA
Voltage X Current
115,000 X 145 amps = 16,675,000= 16.67MVA
7,200 X 2315 amps = 16,668,000 = MVA
Current ratio Check
2315÷145 = 15.97/1

17. Transformer Information For Practice Calculations
3Ø, Δ-Υ 115/12 kV 25MVA
3Ø, Δ-Υ 115/23 kV 40 MVA
3Ø, Δ- Δ 23/ 13.8 kV 5 MVA
3Ø Υ- Υ 4160/ MVA
3Ø, Δ- Δ 115/ 13.8 kV 10 MVA
1Ø Υ- Δ 230/22 kV 350 MVA
Delta and Wye 3-phase circuits : Electronics Worksheet