110% Loading and 12% Unbalance on a Village Distribution Transformer
A 100 kVA, 11 kV/433 V ONAN distribution transformer (nameplate full-load current 139 A) supplies 180 domestic connections and 12 agricultural pump-sets through a 400 m LT line. During a June evening survey the lineman records: top-oil temperature 78 °C, ambient 38 °C, LT line currents L1 168 A, L2 172 A, L3 121 A, and a tail-end voltage of 372 V. The 60 kW rooftop solar plant on the panchayat building is, of course, not generating at that hour. Villagers report dim lamps and a burning smell near one LT pole joint. The sub-division can fund only one of three measures this quarter: fitting a forced-air fan bank on the transformer radiators, re-conductoring 200 m of the LT line with a larger conductor, or installing a second 63 kVA transformer with a split of the load. The junior engineer asks you which single measure to recommend, and what should be done before sunrise.
1. Compute the transformer loading and the phase-current unbalance. Does the 78 °C oil temperature prove the transformer is safe?
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Loading. Average line current = (168 + 172 + 121) / 3 = 153.7 A. Taking the LT voltage at the transformer as 415 V:
S = √3 × V × I = 1.732 × 415 × 153.7 ≈ 110.5 kVA, i.e. about 110% of the 100 kVA rating. The worst phase (172 A) is 124% of the 139 A nameplate current.
Unbalance. Deviation of the worst phase from the average = (172 − 153.7) / 153.7 ≈ 11.9%, above the usual 10% limit. At that level the negative-sequence component heats one winding and the rotor asymmetrically, so the unit cannot be treated as simply 'a bit overloaded'.
Oil temperature. 78 °C looks safe only against a top-oil limit of roughly 88 °C (38 °C ambient + 50 K rise). Top oil is not the hot-spot: at 110% load the winding hot-spot is far above 78 °C, and insulation ageing roughly doubles for every 6–8 K of extra hot-spot temperature. The oil reading therefore does not prove the transformer is safe - it only says the alarm has not been reached yet.
Also note the tail-end voltage: 372 V is about 7% below 400 V, outside the normal ±6% band.
2. Rank the three funded options and justify the one you would recommend with the money available.
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Recommend the second 63 kVA transformer with a split of the load.
- The problem is capacity plus unbalance, not primarily voltage. 100 + 63 kVA gives a real margin for a growing village, and separating domestic from pump-set feeders removes the evening unbalance at the same time.
- Fan bank (ONAN to ONAF): cheapest and can be fitted in a day; typically buys about 15–20% extra capacity. But it treats the symptom. The 12% unbalance, the 372 V tail end and the ageing of an already stressed unit all remain, and you still have a single transformer as the only supply.
- Re-conductoring 200 m of LT: improves voltage only. It does not reduce transformer loading, and by raising tail-end voltage it may actually increase connected load (people run more appliances).
If the budget cannot be released this quarter, fit the fans as a stop-gap and rebalance the single-phase connections immediately at no cost, while putting the 63 kVA unit into the next works estimate. What must not happen is another summer at 110% load with 12% unbalance and a hot joint on the line.
3. List the actions you would take tonight, in priority order, and identify the one safety item that cannot wait.
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- Rebalance first - it costs nothing. After isolating that section, move about 15–20 single-phase domestic connections from L2 (172 A) to L3 (121 A), targeting all three phases within 10% of the average.
- The burning smell cannot wait. A hot joint on an LT pole means a loose jumper, a worn connector or an undersized lug. Clamp the joint current, look for discolouration, and if it is hot, isolate that span and remake the joint with a proper connector at correct torque. This is the most likely source of a fire or a snapped conductor, and it is the one item that must be dealt with before the night is out.
- Reduce peak load: stagger the pump-set feeders by an hour through the local operator, and switch off non-essential street-lighting circuits on the loaded phase.
- Record and report: note currents, oil temperature, ambient and tail-end voltage with time stamps, and raise a written intimation so the loading is formally on record and augmentation is prioritised.
- Do not raise any relay setting, and do not open the transformer or top up oil - that is a separate authorised job.