63 kVA Village Transformer at Its Limit: Replace or Rebalance?
A 63 kVA, 11 kV/433 V distribution transformer on a village feeder in Assam supplies 96 single-phase domestic connections and one 5 HP irrigation pump. During a July evening peak the lineman records phase currents of 82 A (R), 76 A (Y) and 71 A (B) at the LT busbar, with a power factor of 0.86. The top-oil thermometer on the transformer reads 78 °C against an ambient of 34 °C. No single-phase load schedule exists; connections were added over five years by whichever phase was physically nearest. The pump runs only in the morning. Two new connections are pending sanction. The area supervisor wants to raise an indenting note for a 100 kVA replacement immediately and asks you, a vocational trainee attached to the sub-division, to justify the decision in writing before the note goes out.
1. Calculate the transformer's loading from the recorded data and decide whether they justify immediate replacement. Show your figures and state the margin that remains.
Read the worked answer
Full-load current: I = 63000 / (1.732 × 433) = 84 A.
Loading: average phase current = (82 + 76 + 71)/3 = 76.3 A, so S = 1.732 × 433 × 76.3 = 57.2 kVA, i.e. about 91% of rating - before any correction.
Unbalance: the worst phase (82 A) deviates 5.7 A from the average, about 7.5% current unbalance. Negative-sequence current heats the windings without producing useful output, so the transformer cannot be credited with its full 63 kVA. At this level of unbalance the usable capacity is roughly 0.9 × 63 ≈ 55–57 kVA. The unit is therefore effectively at or just past its derated limit during the evening peak, not comfortably inside it.
Thermal evidence: top-oil rise = 78 − 34 = 44 K, close to the design limit of about 45–50 K, so there is little margin left.
Judgement: the data show a genuine margin problem, but the cheapest large part of the fix is balancing, not a new transformer. Replacement is justified only if, after balancing, peaks still exceed ~90% of rating for a sustained period, or confirmed growth (pending connections plus pump load) pushes the derated capacity past the load.
2. State the measurements and records you would collect over the next two weeks and the decision rule you would apply to them.
Read the worked answer
Collect:
- A 15-minute-interval load logger on the LT busbar for at least 7 days - three phase currents, voltage, power factor - so the peak magnitude and its duration are known (a 20-minute peak and a 4-hour peak are different problems).
- A per-phase connected-load inventory built from the connection register, not from guesswork, including the pump's starting times.
- Top-oil and ambient temperature twice daily at peak, logged, not recalled.
- Voltage at the tail end of each phase (a low tail-end voltage confirms a loading/unbalance problem rather than meter error).
- The two pending applications' connected load.
Decision rule: after redistribution, if the highest phase current stays below 75% of FLC (63 A), unbalance is under 5% and oil rise stays under 40 K, defer replacement and review after one more summer. If the highest phase current still exceeds 90% of FLC (76 A) for more than about two hours a day, or unbalance stays above 5% after balancing, raise the indenting note - then the replacement is defensible on recorded data rather than on one evening's reading.
3. Replacement is deferred. Describe the sequence you would follow to redistribute the single-phase connections, including the safety steps and the measurement that closes the job.
Read the worked answer
Plan: prepare a reconnection schedule in advance so no consumer is off longer than necessary; work in the low-load window (early morning, pump idle).
Safety sequence:
- Inform consumers and obtain shutdown permission from the sub-division.
- Open and lock/tag the 11 kV drop-out or isolator; confirm the correct feeder.
- Test dead on both LT and HT side with a voltage detector of the correct range.
- Apply earths on the HT side and on the LT busbar before touching conductors.
- Work with insulated tools, rubber mat and gloves; one hand where possible; no live LT work.
Technical sequence: 6. Identify each consumer's phase (clamp meter at the service with load on, before shutdown, or continuity after isolation) and tabulate connected load per phase. 7. Transfer connections so that each phase carries connected load within about ±5% of the others; retorque lugs, replace damaged lugs. 8. Insulation-resistance test the LT side, remove earths in reverse order, close the isolator, energize.
Closing measurement: re-measure the three phase currents and voltages at the same point at the next evening peak and compare with the baseline; if unbalance is still above 5%, the problem is inside the transformer or its joints, not in the consumer distribution.