An alarm or trip for high oil temperature (26) or winding temperature (49) on a transformer is usually caused by load or cooling: overload, fans or pumps that do not start, dirty radiators or closed valves, or a hot ambient. The problem is also often in the indicator. If the temperature does not follow the load and the indicator is good, look for an internal fault with dissolved gas analysis.
Symptom
An alarm for high oil or winding temperature arrives, or the trip stage of the indicator operates. Sometimes it happens at peak load hours or on the hottest days; other times with no apparent change in operation. In substations with transformers in parallel, one may run hotter than its neighbor at the same load.
The transformer temperature indicators are two: the oil indicator (ANSI 26), which measures the top oil in the tank, and the winding indicator (ANSI 49), which estimates the hot spot with a thermal image fed by a CT. Both switch on the stages of the cooling system and give alarm and trip. The hot-spot temperature is what defines insulation aging, according to the loading guides IEC 60076-7 and IEEE C57.91.
Possible causes, from most to least likely
| Cause | Why it happens | What gives it away |
|---|---|---|
| Overload | Load above the rating or above the capacity of the cooling stage in service | Temperature that follows the load; oil and winding rise together |
| Poor cooling | Fans or pumps that do not start because of control, contactors or protections; dirty radiators or closed valves | Temperature higher than on comparable days at the same load |
| Hot ambient or poor ventilation | Closed enclosure, radiators near walls, direct sun | Temperature that follows the ambient and the time of day |
| Faulty indicator | Out-of-calibration instrument, dry thermowell, damaged sensor, wrongly connected thermal image | Difference with another indicator or with an independent measurement |
| Harmonics | Nonlinear loads that increase losses in windings and structural parts | Heating higher than expected for the fundamental current |
| Overexcitation | High voltage or an unsuitable tap that increases core losses | High temperature at low load and voltage above rated |
| Internal fault or low oil level | Internal connections, core with circulating currents, insufficient oil | Rising dissolved gases; level alarm |
What to check first
- Load and temperature on the same record. If the temperature follows the load, think overload or cooling; if not, the indicator or a fault.
- Cooling on site: every fan and pump running, direction of rotation, radiator valves open, radiators clean.
- Indicators: compare oil and winding with each other, with the other transformer if there is one in parallel, and with an independent measurement (thermography of the tank or a reference thermometer in the well).
- Voltage and tap changer position. Sustained voltage above rated increases core losses; see overexcitation protection 24.
- Oil level and, if the cause does not appear, a sample for gas analysis.
- Does the temperature follow the load?
- Yes: Does the load exceed the rating or the capacity of the cooling in service?
- Yes: Overload: apply the loading guide and verify the cooling stage
- No: Are all fans, pumps and radiators working?
- No: Poor cooling: control, motors, valves or dirty radiators
- Yes: Check harmonics, overexcitation and dissolved gases
- No: Does an independent measurement confirm the temperature?
- No: Faulty indicator: calibration, well, sensor or thermal image
- Yes: Ambient, ventilation or internal fault: check the enclosure and dissolved gases
- Yes: Does the load exceed the rating or the capacity of the cooling in service?
How it is confirmed
- Indicator test: calibration against a reference, verification of the thermal image with injected current, and of the contacts of each stage. See transformer control cabinet.
- Functional test of the cooling: start of each stage by temperature and in manual, current of each motor.
- Harmonic measurement on the secondary when there are significant nonlinear loads; IEEE C57.110 covers evaluating transformer capability with nonsinusoidal currents.
- Dissolved gas analysis: rising methane and ethylene confirm an internal thermal fault. See elevated dissolved gases in the transformer.
- Thermography of the tank and radiators: a cold radiator among hot ones indicates a closed or blocked valve.
How to prevent it from happening again
- Bring temperatures and load to the SCADA as values, not just alarms, and review their trend.
- Test the cooling at every maintenance, including the stages that rarely start.
- Clean the radiators and keep the airflow clear.
- Review the transformer capacity when the load grows or changes in nature (drives, rectifiers).
- Calibrate the indicators at the interval the manufacturer indicates.
What not to do
- Do not raise the alarm or trip settings to make the problem go away without a loading study. The thermal protection 49 protects the life of the insulation.
- Do not leave the fans in permanent manual without fixing the cause in the control.
- Do not ignore a difference between twin transformers at the same load: one of them has a problem.
- Do not disconnect the temperature trip to keep the load.
Which temperature matters and what the loading guides say
Transformer life is consumed by the winding hot spot, not by the oil temperature. IEC 60076-7 expresses insulation aging as a relative rate that depends on the hot-spot temperature and doubles with increases of a few degrees; the parameters differ depending on whether the paper is normal or thermally upgraded. As a result, a transformer that runs a few hours hotter than planned consumes life nonlinearly, and that consumption is not recovered.
The same guide sets limits for loading. For the hot spot of the windings and of the metallic parts in contact with cellulosic insulation, normal cyclic loading is limited to 120 °C. Above 140 °C gas bubbles can form and reduce the dielectric strength, and emergency loadings must be evaluated with that risk in mind. The top-oil and emergency values vary with the edition and with the transformer class; take them from the edition in force and from the nameplate data.
Cooling classes and why they matter
The nameplate of a transformer states its cooling method with a four-letter code in IEC 60076-2 and a two- or three-letter code in IEEE nomenclature:
| IEC | IEEE | Meaning |
|---|---|---|
| ONAN | OA | Natural oil and natural air circulation |
| ONAF | FA | Natural oil, forced air with fans |
| OFAF | FOA | Forced oil with pumps, forced air |
A unit with several stages has a rated power for each one. If the fan control fails, the transformer does not shut down: it is left with the capacity of the previous stage, and that capacity is lower than what the operator believes is available. It is one of the most frequent causes of “high temperature with no apparent overload”.
How the winding indicator is set
The winding indicator does not measure the hot-spot temperature: it calculates it. It adds to the top-oil temperature a gradient obtained from the load current, which is fed from a winding CT through a heater resistor in the thermal image. The setting of that gradient is taken from the manufacturer’s heat-run test report, which is specific to each transformer. An indicator replaced with a generic setting, or one from another unit, reads reasonable values at low load and wrong values at high load, which is exactly when they matter. That is why the indicator test checks the thermal image with injected current as well as the oil sensor.
Illustrative example
Two twin transformers in parallel at the same load: one reads ten degrees more than the other. The load is shared equally, so the difference does not come from overload. On inspection, a group of radiators on the hot transformer has had its valves closed since the last maintenance, and thermography shows cold radiators among hot ones. With the flow restored, the temperatures equalize. The comparison with the twin is what revealed it.
When to call us
If your transformer runs hot and it is not clear whether the cause is the load, the cooling, the indicator or an internal fault, we perform the power transformer tests: indicators and thermal image, cooling, gases and electrical tests, in Colombia or elsewhere on request. If the capacity or the harmonic load needs review, we evaluate it with a power quality study. Contact us.
Related guides
- Transformer temperature indicators
- Transformer cooling system
- Thermal overload protection 49
- Transformer mechanical protections 63 and 26
- Elevated dissolved gases in the transformer
- Transformer control cabinet
- Transformer maintenance
References
- IEC 60076-2, Power transformers – Part 2: Temperature rise for liquid-immersed transformers.
- IEC 60076-7, 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.
Frequently asked questions
Why does the winding indicator read higher than the oil indicator?
Because the winding indicator estimates the hottest spot by adding to the oil temperature a gradient proportional to the load, using a thermal image fed by a CT. That difference is normal. What is abnormal is when it grows without a load increase or does not change with load.
How much can a transformer be overloaded?
It depends on its design, the ambient temperature, the load cycle and the loss of life that is accepted. The loading guides IEC 60076-7 and IEEE C57.91 relate load, hot-spot temperature and insulation aging, and the manufacturer sets the limits of each unit. There is no universal percentage.
What happens if the transformer runs hot for a long time?
The insulating paper ages faster the higher the hot-spot temperature, and that aging cannot be recovered. At high temperatures the oil also degrades and gas can form. The transformer's life is shortened even if no immediate fault occurs.