100 kVA Distribution Transformer Running at 121% Every Evening
A 100 kVA, 11 kV/433 V distribution transformer serves 58 consumers through two LT feeders. The evening peak measured with a clamp meter is 175 A per phase at a bus voltage of 400 V with a power factor of 0.82, sustained for about three hours daily. At the end of the peak the top-oil temperature is 78 °C and the ambient is 38 °C. The transformer is protected by a 10 A drop-out fuse on the 11 kV side and a 200 A fuse on the LT side. The nearest alternative unit is a 63 kVA transformer 300 m away on the other LT feeder, loaded to about 60 per cent. A spare 250 kVA transformer is available, but the existing plinth is only 1.2 m × 1.2 m and the DO fuse, LT cable and LT fuse board would all have to be changed.
1. Calculate the transformer loading in kVA and as a percentage of rating, and assess whether the daily peak is acceptable, including the temperature position.
Read the worked answer
S = √3 × V × I = 1.732 × 400 × 175 / 1000 = 121.2 kVA, i.e. 121 % of 100 kVA (126 % if the bus was actually at 415 V, giving 125.8 kVA). Real power ≈ 121.2 × 0.82 = 99.4 kW. So the unit carries about 21 % overload for three hours every day.
Temperature: top-oil rise = 78 − 38 = 40 K. The rating plate of such a unit typically allows about 45 K oil rise, so the oil is inside the limit - but with only 5 K margin. At this load the winding hot-spot runs roughly 10–15 K above top oil, placing it near 90–93 °C, close to the 98 °C hot-spot limit on which normal insulation life is based. Every 6–8 K above that roughly halves life.
Judgement: the transformer is thermally stressed but not in immediate danger; the correct response is to relieve the peak within days, not to keep running it and wait for failure. Verify the clamp reading and the actual bus voltage first, because the kVA figure scales with voltage.
2. Check the adequacy of the 10 A DO fuse and the 200 A LT fuse, showing the basis of selection.
Read the worked answer
11 kV side: FLC = 100 000 / (√3 × 11 000) = 5.25 A. A DO fuse must carry rated current plus permissible overload and magnetising inrush, yet lie below the transformer's through-fault damage curve and coordinate with the upstream breaker relay: 1.5–2 × FLC ≈ 8–10.5 A. A 10 A fuse is at the top of the acceptable band - workable, but check it against the fuse curve and the relay setting so that an LT fault operates the LT fuse first and does not blow the DO fuse.
LT side: FLC = 100 000 / (√3 × 415) = 139 A. A 200 A fuse is 1.44 × FLC, normal for motor starting and short-circuit protection. However, at 175 A it is carrying 87.5 % of rating for hours, so the element ages and nuisance blows on motor starts become likely.
The key point: a fuse protects against short circuits, not against a sustained 121 % overload. Neither fuse will protect this transformer from the daily peak; only load reduction, overtemperature protection or augmentation will.
Judgement: both fuses are adequate for fault duty; both are irrelevant to the overload problem.
3. Recommend a course of action in priority order, and list the tests/checks before any replacement unit is energised.
Read the worked answer
Priority order:
- Relieve load this week. Transfer about 20–25 kVA (the headroom on the 63 kVA unit at 60 % load, 300 m away) by re-making links or shifting a feeder section; stagger mill and agricultural loads by time-of-day agreement; check single-phase balance and rebalance.
- Measure the next three evening peaks with a calibrated clamp meter, logging voltage, current and power factor. If the peak remains above about 110 %, plan augmentation.
- Augment to 250 kVA. New plinth (1.2 × 1.2 m is too small), DO fuse to 16–20 A (FLC 13.1 A), LT breaker/fuse about 400 A (FLC 348 A), matching LT cable and busbar, two independent earths (neutral and body) tested and recorded, and the 11 kV surge arrester and structure clearances checked.
Before energising the replacement: oil level and leaks; oil BDV against the IS 335 in-service limit; IR with a 5 kV megger on HV–E, LV–E and HV–LV compared with the commissioning record; turns ratio on every tap; tap position; silica gel breather condition; thermometer; and vector group/polarity confirmed before paralleling. Record everything in the test register.