Transformer Protective Devices — Traction Distribution Notes

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Protective devices ensure safe transformer operation under both normal and extended-feed conditions. Alarm/trip circuit operations are tele-signalled and indicated at the RCC — TPC must advise SSE in every such case, so the sub-station can be inspected, the cause investigated, corrective action taken, and a detailed report submitted to Sr.DEE/DEE(TrD).

Temperature Alarm & Trip

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Exam Focus — The Real 150% Threshold

Instantaneous overloads of load up to 150% of full load are handled by OVERLOAD RELAYS. Sustained overload BELOW 150% is instead cleared by THERMAL PROTECTION — a genuinely important distinction between two different protective mechanisms, split at exactly the 150% threshold, not a vague 'high load = overload relay' rule.

A recording ammeter connected to get a 24-hour current-loading chart gives valuable clues about corrective action needed — the shape of the load curve itself is diagnostic.
If alarm/trip circuits operate frequently during peak periods, first attempt spacing out trains more uniformly with the Operating Department to reduce peak load.
In suburban sections where peak load genuinely can't be reduced, the second standby transformer may be pressed into service for the peak duration — this parallel operation can ALSO reduce total losses (an economy side-benefit) and increases OHE voltage, since combined transformer impedance is halved when the transformers are identical.
If one sub-station is persistently overloaded while an adjacent one is appreciably underloaded, shifting the Neutral Section between them may be considered as a structural fix.

Differential Protection — Real Mal-Operation Causes

The differential relay isn't purely a fault-only device — it can genuinely mal-operate for non-fault reasons too, and knowing why is exactly what lets a supervisor tell a real internal fault apart from a false trip.

3 Real Causes of Differential Relay Operation

A genuine internal fault in the transformer — the actual fault condition the relay is designed to catch.
Current inrush from core magnetisation at switching-on time — a normal transient, not a real fault.
Spill current from imperfect balance between EHV-side and 25kV-side current transformer secondaries — a mal-operation cause traced to defective harmonic restraint filters or wrong CT ratios, both of which should be eliminated as the actual fix.

Buchholz Relay

The Buchholz relay assembly detects gas evolution from internal faults. Right after first commissioning, the upper (alarm) assembly of the relay may sometimes trip — this doesn't automatically mean a real fault. Analysing the composition of the collected gas reveals the actual nature of the issue: if it's MERE AIR BUBBLES, the transformer is sound (this commonly happens on new/recently-filled transformers as trapped air works its way out).

It's always wise policy, when the Buchholz relay trips, to get the dielectric strength of the oil tested, measure insulation resistance, and carry out a ratio test — a real, prudent 3-check response rather than assuming the worst or dismissing it outright.