A transformer heats up because of its losses: core losses, which depend on voltage, and winding losses, which grow with the square of the current and with harmonics. It overheats when those losses rise (overload, harmonics, high voltage, unbalance, an incipient internal fault) or when the heat cannot get out (dirty radiators, fans that do not start, a poorly ventilated room, low oil level). Heat accelerates the aging of the paper insulation, which is what sets the life of the unit.
Where the heat comes from
A transformer converts almost all the energy it receives, but not all of it. What it loses becomes heat in two places:
- The core. No-load losses (hysteresis and eddy currents in the steel laminations) depend on voltage and frequency, not on load. They are present 24 hours a day, even if the plant is shut down.
- The windings and metal parts. Load losses grow with the square of the current: at twice the current, four times the heat. Harmonics add eddy-current losses in the conductors and in the tank, and these grow with frequency.
That heat has to reach the surroundings. In an oil-filled transformer it travels by oil convection to the radiators (ONAN) and, at higher load, by fans (ONAF) or pumps. In a dry-type transformer it is carried away by the air that flows through the coils. We explain the types in transformer cooling system.
The point that matters is not the tank or the top oil, but the hot spot of the winding: the hottest place in the insulating paper. Most plant transformers do not measure that value directly; it is estimated with the thermal image of the winding temperature indicator, which combines oil temperature with current. See transformer temperature indicators. More on the hot-spot temperature calculation in transformer hot-spot temperature.
Why it overheats
| Cause | What happens | How it shows up |
|---|---|---|
| Load above rated | Winding losses grow with the square of the current | Temperature that follows the load curve; alarm at peak hours |
| Harmonics | Extra losses in windings, joints and tank even though the RMS current is not high | Heat that does not match the measured load; drives, UPS units or rectifiers on the bus |
| High voltage | More core losses and more magnetizing current | High temperature even at light load; wrong tap-changer position |
| Phase unbalance | One phase carries more than the others | Current difference between phases; neutral current |
| Poor cooling | The heat cannot get out | Dirty or painted radiators, fans that do not start, a room without ventilation, blocked grilles |
| Low oil level | Part of the windings or radiators loses circulation | Low-level indicator, oil stains |
| Incipient internal fault | A loose connection, a turn-to-turn short circuit or a circulating current in the core produces localized heat | Dissolved gases in the oil, a hot spot in thermography, a temperature rise with no change in load |
| Loose external connections | Heat at bushings and terminals, not in the transformer | Thermography of bushings and terminals |
The most underestimated cause in industry is harmonics. A transformer that feeds many variable frequency drives can overheat at a load that looks comfortable in amperes. For that case there are the K-factor transformer and derating according to IEEE C57.110; the source is covered in harmonics from variable frequency drives.
What a transformer running hot puts at risk
Paper-and-oil insulation ages faster the hotter it runs. The IEC 60076-7 loading guide takes a hot-spot temperature of 98 °C as the reference for non-thermally-upgraded paper, and assumes the aging rate doubles for every 6 K above that value. IEEE C57.91 uses 110 °C as the reference for the thermally upgraded paper of 65 °C-rise transformers. Aged paper loses mechanical strength: the transformer can keep working for years, but the next external short circuit or the next overvoltage finds it weaker.
Heat also has immediate effects:
- With a very high hot spot, gas bubbles can form in the oil and lower its dielectric strength.
- Moisture migrates from the paper into the oil and back again as it cools, which speeds up deterioration.
- In a dry-type transformer, the resin or varnish degrades and cracks appear.
- The transformer’s temperature protection (device 26 for oil, 49 for winding) first alarms and then trips; a temperature trip in the middle of production is the most expensive way to find out.
The transformer’s own protections are explained in Buchholz 63, pressure and temperature 26/49.
How it is diagnosed
Diagnosis combines three sources: load, temperature and internal condition.
- Does the temperature rise and fall with the load?
- Yes: Does the measured load exceed the rating?
- Yes: Real overload; review capacity and redistribute loads
- No: Are there drives, UPS units or rectifiers on the bus?
- Yes: Measure harmonics and assess the additional losses
- No: Check cooling, fans and room ventilation
- No: Does the gas analysis show thermal-fault gases?
- Yes: Incipient internal fault; electrical tests and an intervention plan
- No: Check service voltage, tap position and indicators
- Yes: Does the measured load exceed the rating?
- Load and power quality measurement. A power quality analyzer on the secondary, during at least one week of normal operation, shows current per phase, unbalance, voltage and harmonic content. It is the way to tell a real overload from a harmonic load.
- Thermography. It shows radiators that do not circulate, hot terminals and abnormal spots on the tank. It is done with the transformer loaded. See the guide on thermography in substations and switchboards.
- Oil analysis. Dissolved gases reveal whether there is a thermal or electrical fault inside the tank; moisture and dielectric strength show the condition of the insulation. See dielectric oil analysis.
- Offline electrical tests, when the oil or the thermography points to an internal problem: winding resistance (loose connections, tap-changer contacts), turns ratio (shorted turns), excitation current and insulation power factor.
- Checking the indicators. An uncalibrated thermometer or a badly set thermal image can show a problem that does not exist, or hide one that does.
What to do next
It depends on the cause. An overload can be solved by redistributing loads, compensating reactive power to reduce current, adding forced cooling if the design allows it, or replacing the transformer; see overloaded transformer. Harmonics are handled with filters or with a suitable transformer. Cooling is recovered by cleaning and by maintaining fans and controls. An internal fault requires tests and a repair decision that should not be postponed. What is not advisable is raising the alarm setting so that it stops sounding: the alarm is the only warning before damage. On what the rated power of a transformer means: transformer rated power.
How we help
Our team diagnoses the transformers in your plant: load and harmonic measurement, thermography, oil analysis and on-site electrical tests, in Colombia or elsewhere on request, with a report that says what is heating the unit and what to do. See our power transformer testing and our testing and commissioning service. If the problem turns out to be harmonics or capacity, we run the study remotely, worldwide, from your measurements. Contact us with the transformer nameplate and the temperature readings.
Related guides
- Why a transformer burns out
- Transformer temperature indicators
- Transformer cooling system
- Dielectric oil analysis
- Transformer Hot-Spot Temperature Calculation
- Transformer Rated Power: What It Means (IEC 60076-1)
References
- IEC 60076-7, Power transformers – Part 7: Loading guide for mineral-oil-immersed power transformers.
- IEEE C57.91, Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators.
- IEEE C57.110, Recommended Practice for Establishing Liquid-Immersed and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents.
- IEEE C57.104 and IEC 60599, interpretation of dissolved gases in oil.
Frequently asked questions
Is it normal for a transformer to feel hot to the touch?
A transformer in service always runs above ambient temperature, and the tank of an oil-filled unit can be too hot to touch without that being a fault. What points to a problem is an oil or winding temperature above the usual value for the same load and the same ambient, or a spot much hotter than the rest in a thermographic survey.
Why does a transformer overheat if it is not overloaded?
Because harmonics from variable frequency drives and electronic power supplies increase losses in the windings and in the metal structure even when the RMS current does not look high, and because a voltage above rated raises the core losses. It also overheats if the cooling does not work or if there is an incipient internal fault, such as a loose connection or a turn-to-turn short circuit.
How much life does running hot take from a transformer?
According to the IEC 60076-7 loading guide, for non-thermally-upgraded paper the aging rate doubles for every 6 K that the hot-spot temperature exceeds 98 °C. A few hot hours do no harm, but months of it consume life cumulatively and irreversibly.