Transformer temperature rise is the amount by which the temperature of a transformer component exceeds a reference temperature, usually the ambient air or cooling-medium temperature. It is a difference, not a single reading. The same idea applies to top-oil temperature, winding temperature and hot-spot temperature, and each of these can have its own temperature rise.
Confusion arises because "temperature" and "temperature rise" are often used interchangeably. A winding that measures 80°C on a cold day and one that measures 80°C on a hot day have the same absolute temperature but different temperature rises. This article explains how the two concepts differ, why transformers heat up, how temperature rise is measured, tested and calculated, and where direct fiber optic winding measurement fits in.
Key Takeaways
- Temperature rise is not the same as absolute temperature. It is the difference between a component temperature and a reference temperature.
- Oil, winding and hot-spot locations have different thermal behavior, so they should not be treated as one value.
- Higher loading generally increases heat generation, and therefore winding and oil temperature rise.
- Cooling performance affects the final temperature rise reached at a given load.
- A temperature rise test and continuous temperature monitoring serve different purposes and do not replace each other.
- Fluorescence-based fiber optic probes measure actual temperature at selected winding locations, one point per probe.
What Is Transformer Temperature Rise?
In its simplest form, temperature rise is a component temperature minus a reference temperature:
Temperature rise = component temperature − reference temperature
The component might be the top oil, the winding, or a specific location inside the winding. The reference is typically ambient temperature or the temperature of the cooling medium, depending on the definition being used. Which reference applies depends on the transformer design, the cooling arrangement and the specification or standard in use.
Absolute temperature tells you how hot a location is. Temperature rise tells you how much of that heat comes from the transformer's own operation rather than from its surroundings. Both are useful, but they answer different questions. Absolute temperature is closely related to the thermal stress on insulation, while temperature rise helps compare thermal performance across different ambient conditions.
Different parts of a transformer can be described by their own temperature rise: top oil, winding, hot spot, and structural or tank regions. This article focuses on oil, winding and hot spot, because these are the values most often discussed in design, testing and monitoring.
Why Does Transformer Temperature Rise?
Transformers are efficient, but they are not lossless. The energy lost during operation appears as heat, and that heat has to be transferred to the surrounding oil, air or other cooling medium. Temperature rise is the result of heat generation balanced against heat removal.
Winding Losses
Load current flowing through the windings produces resistive losses. In general, higher load current means higher winding losses and more heat generated in the conductors. This is a main reason why winding temperature rises when loading increases.
The relationship is not instantaneous. Windings, oil and insulation have thermal mass, so temperatures respond to a change in load over time rather than immediately. Oil generally responds more slowly than the winding conductors.
Core and Stray Losses
The core produces losses whenever the transformer is energized, largely independent of load. Stray losses arise from leakage flux in structural parts such as tank walls, clamping structures and other metallic components, and they also contribute to local heating. Both add to the overall thermal picture, although the details depend on the design.
Cooling Conditions
Heat has to leave the transformer to keep temperatures stable. Depending on transformer design, this may involve natural oil circulation, forced oil flow, radiators, fans or pumps, and the surrounding air. If heat removal is less effective, the same losses will produce a higher temperature rise. This can happen because of higher ambient temperature, restricted airflow, or a change in the cooling arrangement.
For this reason, temperature rise is best understood as the outcome of load, losses and cooling together, not a fixed property of the transformer.
Ambient, Top-Oil, Winding and Hot-Spot Temperature
These temperatures are related, but they are not interchangeable. The table below summarizes how each one connects to temperature rise.
| Temperature | What It Represents | How It Relates to Temperature Rise | Why It Matters |
|---|---|---|---|
| Ambient temperature | Temperature of the air or surroundings | Commonly used as the reference from which temperature rise is counted | Sets the baseline that the transformer heats above |
| Top-oil temperature | Temperature of oil in the upper region of the tank | Top-oil temperature rise is the difference between top-oil and the reference temperature | Indicates the overall oil thermal state, but not the temperature at each winding location |
| Winding temperature | Temperature of the winding, often as an estimated or average value | Winding temperature rise is typically higher than oil temperature rise under load | Reflects heating of the conductors and adjacent insulation |
| Winding hot-spot temperature | Highest temperature within the winding | Hot-spot temperature can be expressed as an absolute value or as a rise above the reference | Represents the most thermally stressed location in the winding |
| Direct fiber optic winding temperature | Actual temperature at the probe sensing tip | Can be used to calculate rise at that location when a reference temperature is available | Gives measured data from a selected physical location |
Three distinctions are worth keeping in mind. Top-oil temperature is not the same as winding temperature, because the winding usually runs hotter than the oil around it. An average winding temperature is not the same as hot-spot temperature, because the average blends hotter and cooler regions. And a direct probe reading is the actual point temperature at the sensing tip location, which is not necessarily the hottest point in the winding.
Transformer Temperature Rise vs Hot-Spot Temperature
Temperature rise describes how far a temperature sits above a reference. Hot-spot temperature describes the temperature at a specific local position in the winding, typically the location where the temperature is highest. They are different concepts, though they often appear in the same discussion.
A hot-spot temperature can be expressed as an absolute temperature, and it can also be analyzed as a rise above a reference. When someone refers to "temperature rise," they may mean an average winding rise, an oil rise or a hot-spot rise, so it is worth clarifying which one is meant. The origin of localized heating in windings is covered in our article on transformer hot spots, so this article treats them only in relation to temperature rise.
How Is Transformer Temperature Rise Measured?
Temperature rise is not measured by a single instrument. It is determined by combining a reference temperature with one or more component temperatures, each of which can be measured or estimated in a different way. For a broader look at monitoring approaches over the life of a transformer, see our page on transformer temperature monitoring.
Ambient / Reference Temperature Measurement
Because temperature rise is a difference, the reference temperature has to be known. Ambient temperature is commonly measured with a sensor placed outside the transformer, in a location representative of the cooling air. The choice of reference and its measurement location affect the result, so they should be defined consistently, particularly when comparing results over time or between units.
Top-Oil Temperature Measurement
Top-oil temperature is measured in the upper region of the tank, where heated oil collects. It is a widely available measurement and gives a useful indication of the overall thermal state of the oil. Subtracting the reference temperature gives the top-oil temperature rise. It does not, however, describe the temperature of individual winding locations.
Winding Temperature Estimation
Winding temperature is commonly obtained by calculation. A winding temperature indicator (WTI) or a thermal model typically combines measured top-oil temperature with a signal proportional to load current, then applies model parameters to estimate the winding temperature. WTI and thermal models provide calculated or simulated winding temperature based on measured inputs and model assumptions. They are well established and useful for monitoring and protection. For a closer look at these methods, see how transformer winding temperature is measured.
Direct Winding Temperature Measurement
Direct measurement uses a probe whose sensing tip is installed at a selected winding location. The measured value is the actual temperature at that physical point, not a calculated estimate. With a fluorescence-based fiber optic probe, the optical fiber carries the signal out of the transformer to an instrument that converts it to a temperature reading. Combined with a reference temperature, the result can be expressed as a temperature rise at that location. This is covered in more detail in a later section.
What Is a Transformer Temperature Rise Test?
A transformer temperature rise test, sometimes called a heat run, is a controlled test used to observe how a transformer heats up under defined loading and test conditions. It is generally performed on a transformer before it is put into service, as part of design verification or factory testing.
The purpose is to observe the thermal behavior of the oil and windings, evaluate how they respond to the applied test conditions, and compare the measured results with design values or specified requirements. The test is a snapshot under controlled conditions. It shows how the transformer performs in that setup, not how it will be loaded and cooled over years of service.
The details of a temperature rise test, including its test conditions, duration, and how results are evaluated, are defined by the applicable standard and project specification. Those documents should be consulted directly rather than relying on general descriptions.
Temperature Rise Test vs Continuous Temperature Monitoring
A temperature rise test and continuous monitoring both involve temperature, but they answer different questions.
| Aspect | Temperature Rise Test | Online Temperature Monitoring |
|---|---|---|
| Purpose | Verify thermal performance against design or specified requirements | Observe temperature behavior during actual operation |
| When performed | Typically at the factory or during defined testing | Continuously or periodically throughout service life |
| Operating conditions | Controlled and defined test conditions | Real load, cooling and ambient conditions |
| Data duration | Limited to the test period | Long-term trends over days, seasons and years |
| Typical temperature information | Oil and winding temperature behavior under test conditions | Temperature trends under varying load and cooling |
| Use in service | Reference for design verification | Operational awareness, trend comparison and thermal model validation |
A factory or controlled test is not the same as long-term operation under real conditions. Online monitoring can show how temperatures change with load and cooling in the field, but it does not replace a factory temperature rise test, and the test does not replace monitoring.
How Is Transformer Temperature Rise Calculated?
At the basic level, the calculation is simple subtraction:
Temperature rise = component temperature − reference temperature
As a purely mathematical example, suppose the ambient temperature is 25°C and a measured component temperature is 75°C. The temperature rise is 75 − 25 = 50°C. This number is only an illustration of the arithmetic. It is not an allowable temperature rise, a typical value or a transformer rating.
Calculating winding or hot-spot temperature from operating data is more involved. It can depend on load current, oil temperature, thermal time constants, the temperature difference between winding and oil, and cooling conditions. These calculations normally rely on a thermal model, a loading guide such as IEC 60076-7 or IEEE C57.91, and transformer design data. The parameters are specific to the design and should come from the manufacturer or the applicable guide rather than from general assumptions.
What Does Transformer Temperature Rise Rating Mean?
A transformer temperature rise rating refers to the temperature rise a transformer is designed for, as specified for its insulation system and cooling arrangement. It is tied to the design requirements, the insulation system, the cooling method and the applicable specifications. Different designs and standards use different specified limits or reference conditions, so no single value applies to all transformers.
To find the rating for a particular unit, check the transformer nameplate, the manufacturer's documentation, the project specification and the applicable standard. A value quoted for one transformer type should not be applied to another without confirming that the same reference conditions and requirements apply.
Direct Fiber Optic Measurement of Winding Temperature
INNO's technology is fluorescence-based point fiber optic temperature sensing. A probe is installed with its sensing tip at a selected winding location. An optical fiber carries the signal out of the transformer, and a compatible instrument reads the temperature.
A fiber optic winding temperature sensor offers several characteristics that suit winding measurement:
- Direct point temperature measurement at the sensing tip location
- Electrical isolation of the optical sensing path
- High resistance to electromagnetic interference
- Multi-point measurement using multiple probes
Because the probe reports actual temperature at a chosen location, it can be used to check thermal model output, compare temperatures under changing load, or calculate temperature rise at that point when a reference temperature is recorded. Where the measurement objective is the hot-spot region, direct hot-spot monitoring can measure temperature at winding locations that are expected to be thermally critical.
When several locations need long-term recording, multiple probes can be connected to an oil-immersed transformer temperature monitoring system, which records the channels together and follows their trends over time.
Point Measurement Has to Be Planned
One probe measures one selected physical location. If several locations need direct measurement, several probes are needed. A probe does not monitor the entire winding, and the fiber between the sensing tip and the instrument does not measure temperature along its length. Several point probes together do not form a distributed sensing system, and the system does not automatically find an unknown hot spot.
Probe placement therefore has to be decided in advance, based on:
- Transformer design
- Thermal analysis
- Factory test information
- Known or expected thermally critical locations
- The measurement objective
For this reason, direct winding probes are normally planned during design and manufacturing. Access to the interior of an existing transformer is usually limited, so retrofitting internal probes cannot be assumed.
How Temperature Rise Relates to Transformer Ageing
Higher operating temperature, and especially sustained temperature at a local hot spot, increases the thermal stress on winding insulation. Temperature rise is one way to describe the thermal conditions the transformer operates under, since it shows how much the transformer heats itself above its surroundings. Interpreting it in terms of insulation life requires more than temperature alone. Moisture, oil condition, loading history and other factors also play a role. Our article on transformer ageing covers that relationship in detail.
When Should Transformer Temperature Rise Be Reviewed?
Temperature rise is worth reviewing at specific points in a transformer's life. No single observation in the list below indicates a fault on its own. Each is a reason to look more closely at the data.
- Higher loading than usual: sustained or repeated heavy loading changes heat generation and temperature.
- Cooling changes: modifications, maintenance or changes in cooling operation can shift the temperature reached at a given load.
- Higher temperature at similar load: if temperatures trend upward under comparable load and ambient conditions, the cause is worth investigating.
- Differences between phases or windings: unequal temperatures across similar windings may point to differences in loading, cooling or measurement.
- Factory temperature rise testing: to verify thermal performance against design and specification.
- Commissioning: to establish baseline temperature behavior in service.
- Thermal model validation: to compare calculated winding temperatures with measured values.
- Long-term monitoring: to track seasonal and load-related trends over time.
Which Temperature Measurement Tells You What?
Different measurements answer different questions. The table is not a ranking, and no single row is the best in every situation.
| Measurement | Useful For | Direct or Indirect | Main Limitation |
|---|---|---|---|
| Ambient temperature | Providing the reference for temperature rise | Direct measurement of surroundings | Depends on sensor location and how representative it is |
| Top-oil temperature | Overall oil thermal state and oil temperature rise | Direct for oil, indirect for winding | Does not represent individual winding locations |
| WTI | Estimated winding temperature for operation and protection | Indirect | Depends on measured inputs and model assumptions |
| Thermal model | Calculating winding and hot-spot temperature from load and oil data | Indirect | Depends on design data and parameters |
| Direct winding fiber optic probe | Actual temperature at one selected winding location | Direct | Measures one point only, and placement must be planned |
| Multiple winding probes | Actual temperatures at several selected locations | Direct | Covers only the locations where probes are installed |
Conclusion
Transformer temperature rise describes how much a component heats above its reference temperature. It results from load, losses and cooling acting together, and it appears differently in the oil, in the winding and at a hot spot. Top-oil temperature, average winding temperature and hot-spot temperature are related but distinct values, and each can be discussed in absolute terms or as a rise above the reference.
Direct fiber optic probes can provide actual temperature values at selected winding locations, which is useful for comparing measured data with calculated values and following trends under real operating conditions. Interpreting any temperature rise still depends on transformer design, cooling arrangement, loading condition and the applicable specifications.
If a transformer project requires direct winding temperature or hot-spot measurement, INNO can help review probe locations, fiber routing and monitoring channel requirements.