Transformer Quick Reference
Voltage Rise in Transformers due to Capacitor Bank:
- The voltage drop and rise on the power line and drop in the transformers. Every transformer will also experience a voltage rise from generating source to the capacitors. This rise is independent of load or power factor and may be determined as follows:
% Voltage Rise in Transformer=(Kvar / Kva)x Z
- Kvar =Applied Kvar
- Kva = Kva of the transformer
- z = Transformer Reactance in %
- Example: 300 Kvar bank given to 1200 KVA transformer with 5.75% reactance.
- % Voltage Rise in Transformer=(300/1200)x 5.75 =1.43%
Standard Size of Transformer (IEEE/ANSI 57.120):
Single Phase Transformer | Three Phase Transformer |
| 5KVA,10 KVA,15 KVA,25 KVA,37.5 KVA,50 KVA,75 KVA,100 KVA,167 KVA,250 KVA, 333 KVA,500 KVA,833 KVA,1.25 KVA,1.66 KVA,2.5 KVA,3.33 KVA,5.0 KVA,6.6 KVA,8.3 KVA,10.0 KVA,12.5 KVA,16.6 KVA,20.8 KVA,25.0 KVA,33.33 KVA | 3 KVA,5 KVA,9 KVA,15 KVA,30 KVA,45 KVA,75 KVA,112.5 KVA,150 KVA,225 KVA,300 KVA,500 KVA,750 KVA,1MVA,1.5 MVA,2 MVA,2.5 MVA,3.7 MVA,5 MVA,7.5MVA, 10MVA ,12MVA,15MVA,20MVA ,25MVA, 30MVA,37.5MVA ,50MVA ,60MVA,75MVA,100MVA |
Standard Size of Transformer:
Standard Size of Transformer | KVA |
| Power Transformer (Urban) | 3,6,8,10,16 |
| Power Transformer (Rural) | 1,1.6,3.15,5 |
| Distribution Transformer | 25,50,63,100,250,315,400,500,630 |
|
Impedance of Transformer (As per IS 2026):
MVA | % Impedance |
| < 1 MVA | 5% |
| 1 MVA to 2.5 MVA | 6% |
| 2.5 MVA to 5 MVA | 7% |
| 5 MVA to 7 MVA | 8% |
| 7 MVA to 12 MVA | 9% |
| 12 MVA to 30 MVA | 10% |
| > 30 MVA | 12.5% |
Size of Cable on Secondary Side of Transformer (11KV/433V) | ||||
Ref: KSEI Handbook | ||||
| Rating of T/C (KVA) | Primary current (Amp) | Secondary Current (Amp) | Min. Size of Neutral Earthing Conductor (mm2) | Minimum Size of Cable (mm2) |
| 63 | 3.3 | 84 | 25X3 | 50mm2 |
| 100 | 5.25 | 133.3 | 25X3 | 95mm2 or (2×50 mm2) |
| 160 | 8.4 | 213.3 | 25X3 | 185mm2 or (2×95 mm2) |
| 200 | 10.49 | 266.6 | 25X3 | 300mm2 or (2×120 mm2) |
| 250 | 13.12 | 333 | 25X3 | 2×185 mm2 |
| 315 | 16.53 | 420 | 31X3 or 25X4 | (2×300 mm2) or (3×185 mm2) |
| 400 | 21.80 | 533 | 38X3 | (3×300 mm2) or (2×400 mm2) |
| 500 | 26.20 | 666.5 | 25X6 | (3×400 mm2) or (4×240 mm2) |
| 630 | 33 | 840 | 31X6 | 4×400 mm2 |
| 750 | 39.36 | 1000 | 50X4 | Bus Bar Trucking (min. Isc 50KA) |
| 1000 | 52.50 | 1333 | 210mm2 | Bus Bar Trucking (min. Isc 50KA) |
| 1250 | 65.50 | 1667 | 290mm2 | Bus Bar Trucking (min. Isc 50KA) |
| 1600 | 83.98 | 2133 | 380mm2 | Bus Bar Trucking (min. Isc 50KA) |
| 2000 | 105.00 | 2666 | 450mm2 | Bus Bar Trucking (min. Isc 50KA) |
HT Fuse on Primary Side of Transformer (11KV/433V) | ||||
| Rating of T/C (KVA) | Primary current (Amp) | Secondary Current (Amp) | HT Fuse | |
| Min (Amp) | Max(Amp) | |||
| 63 | 3.3 | 84 | 10 | 16 |
| 100 | 5.25 | 133.3 | 16 | 25 |
| 160 | 8.4 | 213.3 | 16 | 40 |
| 200 | 10.49 | 266.6 | 25 | 40 |
| 250 | 13.12 | 333 | 32 | 40 |
| 315 | 16.53 | 420 | 40 | 63 |
| 400 | 21.80 | 533 | 40 | 63 |
| 500 | 26.20 | 666.5 | 50 | 100 |
| 630 | 33 | 840 | 63 | 100 |
| 750 | 39.36 | 1000 | 75 | 160 |
| 1000 | 52.50 | 1333 | 100 | 160 |
| 1250 | 65.50 | 1667 | 100 | 200 |
| 1600 | 83.98 | 2133 | 160 | 250 |
| 2000 | 105.00 | 2666 | 200 | 250 |
Accuracy Class Letter of CT:
Metering Class CT | |
| Accuracy Class | Applications |
B | Metering Purpose |
Protection Class CT | |
C | CT has low leakage flux. |
T | CT can have significant leakage flux. |
H | CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.) |
L | CT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.) |
Accuracy Class of Metering CT:
Metering Class CT | |
| Class | Applications |
| 0.1 To 0.2 | Precision measurements |
| 0.5 | High grade kilowatt hour meters for commercial grade kilowatt hour meters |
| 3 | General industrial measurements |
| 3 OR 5 | Approximate measurements |
Accuracy Class of Protection CT:
| Class | Applications |
| 10P5 | Instantaneous over current relays & trip coils: 2.5VA |
| 10P10 | Thermal inverse time relays: 7.5VA |
| 10P10 | Low consumption Relay: 2.5VA |
| 10P10/5 | Inverse definite min. time relays (IDMT) over current |
| 10P10 | IDMT Earth fault relays with approximate time grading:15VA |
| 5P10 | IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA |
Size of Capacitor for P.F Correction:
For Motor | |
| Size of Capacitor = 1/3 Hp of Motor ( 0.12x KW of Motor) | |
For Transformer | |
| < 315 KVA | 5% of KVA Rating |
| 315 KVA to 1000 KVA | 6% of KVA Rating |
| >1000 KVA | 8% of KVA Rating |