The oil temperature indicator (ANSI 26) measures the top oil of the tank and the winding indicator (ANSI 49) estimates the hot spot by adding a load-proportional gradient to it, using a thermal image fed by a CT. Their contacts start the fans, raise an alarm and trip; large transformers add fiber-optic sensors that measure the hot spot directly.
What they are and where they are
The life of paper insulation depends on the temperature of the winding hot spot. Because that point is inside the active part, the transformer carries instruments that watch it in different ways:
- Oil temperature indicator (OTI; ANSI 26). Its bulb sits in a thermowell in the cover, where the oil is hottest.
- Winding temperature indicator (WTI; ANSI 49). It uses another bulb in the oil and adds a gradient proportional to load.
- RTD sensors (for example, Pt100) to send the temperature to the relay, the controller or monitoring.
- Fiber-optic sensors inside the winding, in transformers where they are specified.
Dial indicators are mounted on the side of the tank or in the transformer control cabinet, at a comfortable reading height. IEC 60076-22-1 (clauses 7.8 and 7.9) sets the requirements and tests for mechanical liquid and winding temperature indicators, and IEC 60076-22-7 covers thermowells. For the hot-spot temperature calculation, see transformer hot-spot temperature.
How they work
Oil (26). A fluid-filled sensor, connected by a capillary to the instrument, converts temperature into a displacement that moves the pointer through a Bourdon tube or spring. It needs no power supply. The dial has several adjustable microswitches and often a maximum-reading pointer.
Winding (49) by thermal image. In most transformers the hot spot is not measured directly. The thermal image estimates it: a winding CT, normally one core of a bushing CT, feeds a heating resistor placed next to the sensor. That resistor raises the reading above the oil temperature by an amount that follows the load. In one commercial model, the thermal image transmitter receives a 2 A rating from the CT and the gradient is set with DIP microswitches. The setting is made to reproduce the winding-to-oil gradient obtained in the factory heat-run test.
The logic follows the IEC 60076-7 loading guide. In steady state, the hot spot is the sum of three terms: the ambient temperature, the top-oil rise, and the average winding-to-oil gradient multiplied by the hot-spot factor H. The standard treats that factor in its clause 8.1.4. With a load other than rated, both rises are corrected with exponents that depend on the cooling mode. For the thermal short-circuit withstand over 2 s, see thermal short-circuit withstand of 2 s.
Fiber optic. Probes, for example with a gallium arsenide crystal, are placed in the winding spacers where the thermal design predicts the hot spot, and an optical unit reads the temperature of each channel. IEC 60076-2 provides for direct hot-spot measurement during the heat-run test and devotes its informative annex E to installing these sensors in the windings. IEEE C57.169 gives the equivalent guide for determining the maximum winding temperature.
Electronics. A transformer relay or controller receives the top oil through an RTD and the current through a CT, and calculates the hot spot with the thermal model. With that it can run the cooling in stages.
Differences between distribution, power and extra-high-voltage
| Aspect | Distribution | HV power | EHV and autotransformer |
|---|---|---|---|
| Oil | Simple thermometer, with or without contacts | Indicator 26 with several stages | Same, plus RTD for monitoring |
| Winding | In dry types, sensors in the low-voltage windings with a temperature relay | Indicator 49 with thermal image | Thermal image for more than one winding, as specified |
| Direct measurement | No | Optional | Fiber optic when specified |
See the high-voltage power transformer and, for small units, the distribution transformer guide.
Typical data
The fan-start, alarm and trip thresholds are set by the transformer manufacturer according to its insulation class and cooling, and by the owner according to its loading policy; there are no universal values. What is checked in the field is that each contact operates at the set value and that the thermal image reproduces the gradient of the factory protocol.
Connection to protection and control
- Contacts of the 26 and the 49: fan stage 1, fan or pump stage 2, alarm and trip. The stages go to the cooling control; the trip goes to the 86 lockout relay.
- Analog outputs (Pt100 or 4–20 mA) to the relay, the controller or the monitoring system.
- Thermal image CT circuit: it is part of the transformer’s current wiring and must have short-circuiting terminals.
- Numerical relay: it may have its own thermal function; see thermal overload 49. In IEC 61850-7-4, STMP is temperature supervision and PTTR is thermal overload.
The set of the transformer’s own protections is explained in transformer mechanical protections.
Common faults and what to check first
- Winding and oil very different at no load: winding indicator out of calibration or with the wrong gradient setting; compare both with a reference.
- Winding equal to oil under load: CT circuit open, shorted or with the wrong ratio.
- Fans that start late: badly adjusted microswitch or low gradient.
- Lost or erratic reading: damaged capillary, bulb out of the thermowell or moisture in the dial.
- Temperature trip: before blaming the indicator, confirm the load, fans, pumps and radiator valves.
Tests and maintenance
- Comparison of the indicator with a reference, for example in a thermal bath, at several points.
- Test of each microswitch at its set value, through to the destination contactor or relay.
- Injection of the thermal image current with a secondary source and verification of the gradient and the time it takes to settle.
- Ratio and polarity of the CT that feeds the thermal image.
- Check of the fiber-optic channels and their readout unit where they exist.
Testing and configuration with Tiza Group
At Tiza Group we calibrate and test oil and winding indicators, the thermal image and their contacts on site, in Colombia or elsewhere on request, as part of power transformer testing.
References
- IEC 60076-2, Temperature rise for liquid-immersed transformers (direct hot-spot measurement and fiber-optic annex E).
- IEC 60076-7, Loading guide for mineral-oil-immersed power transformers.
- IEC 60076-22-1, Power transformer and reactor fittings – Protective devices (liquid and winding temperature indicators).
- IEC 60076-22-7, Accessories and fittings (thermowells).
- IEEE C57.91, Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators.
- IEEE C57.169, Guide for Determination of Maximum Winding Temperature Rise in Liquid-Immersed Transformers.
- IEEE C37.2, function numbers 26 and 49.
- IEC 61850-7-4, logical nodes STMP and PTTR.
Frequently asked questions
Why does the winding temperature read higher than the oil temperature?
Because the winding indicator adds to the oil temperature a gradient that grows with load current. At no load the two should read almost the same; if they differ a lot without load, check the winding indicator and its current circuit.
What happens if the CT circuit of the thermal image opens?
The winding indicator stops adding the gradient and reads only the oil temperature, so the fans start late and the alarm may not come. Also, a CT with an open secondary under load develops dangerous voltages: before working on the indicator, the CT is short-circuited at its terminals.
Does the relay's function 49 replace the winding temperature indicator?
Not entirely. The relay's 49 calculates the temperature from current alone and a model; the winding indicator starts from the real oil temperature. Many owners use both: the indicator for cooling and alarm, and the relay as backup and for recording.