High Oil Temperature on a 250 kVA Transformer During a Village Fair
An 11/0.433 kV, 250 kVA ONAN distribution transformer supplies an agricultural feeder. The nameplate gives a full-load current of 333 A at 433 V. At 14:30, the lineman records secondary line currents of 342 A, 336 A and 329 A, a secondary line voltage of 398 V, an oil top temperature of 89 °C, and an ambient temperature of 44 °C. The tap changer is set at -5%. The transformer is mounted on a plinth with free air on three sides, but a temporary tarpaulin screen erected for a village fair is 1.2 m from the radiator bank. Consumers at the far end of the 500 m feeder complain of low voltage and pump motors tripping. The transformer had been filled to the correct oil level after a recent filter change, and the silica gel breather is blue. The lineman has an infrared thermometer and a clamp meter.
1. Calculate the approximate transformer loading and decide whether the oil temperature is explained by overload alone.
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Average secondary current = (342 + 336 + 329) / 3 = 335.7 A. Apparent power ≈ √3 × 398 V × 335.7 A ≈ 231 kVA. That is about 92% of 250 kVA, so the transformer is not grossly overloaded by current alone. But the ONAN rating assumes 40 °C ambient; at 44 °C the permissible load is lower, perhaps around 230 kVA or less depending on the manufacturer's derating. The oil top temperature of 89 °C is above the usual 85 °C alarm and leaves little margin to a 95 °C trip. The tarpaulin 1.2 m from the radiator bank blocks natural convection, so the oil cannot reject heat. Therefore the temperature is explained by a combination of near-rated load, high ambient and restricted cooling, not by current overload alone. The pump motors tripping at the far end also point to voltage drop, not only transformer loading.
2. Evaluate the -5% tap setting. Should the lineman move the tap to raise secondary voltage? Explain the effect on current and temperature.
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The -5% tap is already the boost direction: it reduces primary turns, so for a given primary voltage the secondary voltage is higher than at 0%. Moving to 0% or +5% would lower the secondary voltage further. The real question is where the 398 V is lost. Measure the primary voltage at the transformer and the secondary voltage at the transformer terminals, then at the far end. If the transformer terminal voltage is about 398 V and the far end is much lower, the 500 m feeder drop is the problem; changing the tap will not fix it. If the primary is low, take up the matter with the substation. Do not move an off-circuit tap with the transformer energized. Also remember that raising the secondary voltage increases current for resistive load and can make the thermal problem worse. Correct cooling and load first; adjust the tap only after a voltage study.
3. List the immediate and permanent actions, including safety.
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Immediate: remove or relocate the tarpaulin to restore free air around the radiators; switch off non-essential pumps in a planned sequence to reduce load; check oil level, leaks, gasket condition and breather; record oil and ambient temperature every 15 minutes. If oil exceeds the manufacturer's alarm or winding temperature indicates trip, de-energize and inform the control room. Do not hose the transformer or open it live. Permanent: reduce the feeder loading by adding a second transformer or by reconductoring the 500 m feeder; stagger pump loads; keep the plinth clear; check the tap after voltage measurements; investigate motor tripping. Also thermal-scan the bushings and joints. Restoration should follow the utility's shutdown and permit procedure, with the transformer isolated, tested and earths removed in the correct order.