Transformer Overload Capacity & Tap Setting — Traction Distribution Notes

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Traction transformers are deliberately designed to handle short bursts of overload beyond their rated capacity — real, standardised figures govern exactly how much and for how long, since exceeding these limits risks permanent insulation damage.

Overload Rating — Real Figures

Overload rating: 50% overload for 15 minutes, OR 100% overload for 5 minutes — after the transformer has attained steady temperature during continuous operation at full load.
Maximum permissible temperature rise over a 45°C ambient: Winding 50°C (measured by resistance method), Oil 40°C (by thermometer), Current-carrying parts 35°C (by thermometer).
The hot-spot temperature after either 50% overload for 15 min OR 100% overload for 5 min must NOT exceed 100°C for a 45°C ambient temperature.
Interval between two successive overloads (after continuous working at maximum 45°C ambient) is 3 HOURS — applying identically to both the 50%/15min and 100%/5min overload types.

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Exam Focus — Three Different Temperature Rise Limits

A genuinely easy set of numbers to mix up: winding rise limit (50°C) is HIGHER than oil rise limit (40°C), which is in turn higher than current-carrying-parts rise limit (35°C) — a descending order (winding > oil > current-carrying parts) worth memorizing as a sequence rather than three isolated facts.

Tap Setting — Real Procedure

Traction transformers usually have OFF-LOAD tap changers (operated locally or by remote control), with taps ranging from +10% to (-)15% in steps of 5%. To decide the correct tap setting, a recording voltmeter is connected at the traction substation to the secondary side of a Potential Transformer, tracking the voltage variation pattern over 24 hours, across at least 3 TYPICAL days.

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Exam Focus — The Two Real Voltage Targets

Based on the recorded readings, tap position is fixed so the DAILY OHE voltage PEAK at the traction substation lies JUST BELOW 27.5kV but never touches it — ensuring OHE voltage stays well above the minimum even at the FARTHEST point on the system when heavily loaded. Separately, once-a-year 24-hour voltage records on BOTH SIDES of every neutral section must confirm voltage never falls below 19kV at any time — two genuinely distinct thresholds (27.5kV peak-avoidance at the substation vs. 19kV minimum-floor at neutral sections) that are easy to conflate.

Since any change in grid inter-connections affects traction substation voltage, SSE(PSI) must keep in touch with Supply Authorities regarding system changes — arranging a fresh 24-hour voltage-reading cycle and re-adjusting the tap setting if needed whenever such a change occurs.