PT100 / T1-T2 Tesar — Transformer Overtemperature
At a glance
- Severity
- FAULT STOP
- Domain
- CABINA_ELETTRICA
- Reference standard
- IEC 60076-2, IEC 60076-7
- Product / context
- Tesar cast-resin transformer — PT100 probes in windings with temperature control unit (typ. Tecsystem NT935), alarm 140°C / trip 155°C class F
What it means
MV/LV transformer overtemperature detected by the thermal sensor (PT100 in windings or contact thermometer in oil). The transformer is disconnected by the thermal protection to prevent insulation degradation. For oil transformers the trip occurs at 105°C (top oil), for dry-type class F transformers at 155°C (winding). Exceeding the threshold indicates prolonged overload, insufficient ventilation, or cooling system failure.
Probable causes
- Prolonged overload above the transformer rated power
- Insufficient substation ventilation or blocked ventilation grilles
- Failed or inactive forced cooling fans
- High ambient temperature (sun-exposed substation without insulation)
- High harmonics causing additional winding losses
Reference values
| oil transformer alarm | 95°C (top oil) |
|---|---|
| oil transformer trip | 105°C (top oil) |
| dry transformer class F alarm | 140°C (winding) |
| dry transformer class F trip | 155°C (winding) |
| dry transformer class H trip | 180°C (winding) |
| hot spot gradient oil | 13°C above top oil |
| overload limit oil | 1.5x In for max 30 minutes (IEC 60076-7) |
Where to start
Work per CEI 11-27 (qualified person only). Before any intervention apply the 5 golden rules: disconnect, secure against reclosing, verify absence of voltage, earth and short-circuit, delimit the work area. Check the actual transformer temperature with an IR thermometer on the casing and compare with the value read from the PT100 sensor/panel thermometer. Are the values consistent?
Full guided diagnosis in the app
The diagnostic tree for this fault has 7 guided steps with measurements, checks and resolutions. You'll find it in ElectraIQ, one of the five IndexOS apps: free, works offline.
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More Tesar codes
- BUCHHOLZ — Buchholz Relay Trip (Gas Detection)