Oil Vs. Winding Temperature Indicators For Transformers

What is the difference between oil and winding temperature monitoring?
A transformer oil temperature indicator measures the temperature associated with the transformer oil, while a winding temperature indicator is designed to represent the winding hot-spot condition through an additional temperature-rise principle. They answer related but different questions:

Oil temperature: How hot is the transformer oil at the measurement point?
Winding temperature: What is the estimated or measured thermal condition of the transformer winding?
Using both measurements can give operators a more complete view of transformer thermal performance. The correct instrument depends on the transformer design, thermal model, current-transformer arrangement, alarm philosophy, and remote monitoring requirements.

 

How does a transformer oil temperature indicator work?
Oil temperature indicator designed for transformers using a composite temperature-rise principle. Depending on the selected configuration, it can provide a DC 4–20 mA current signal or a Pt100 platinum resistance signal corresponding to the temperature value.

The listed functions include local temperature indication, control-switch operation, and remote data transmission through a circuit module. The product information gives the following reference values for the described configurations:

Measuring range: 0–150°C, customizable
Accuracy class: listed as 1.5 for the technical parameters
Switch operation error: ±2°C
Switch differential: 6 ± 2°C
Protection grade: IP55
Ambient temperature: –40°C to +55°C
Relative humidity: up to 95% under the stated condition
AC/DC 100–260 V power supply options and model configurations with multiple temperature-control switches. These values should be verified against the exact model, because the model code determines the scale, switch quantity, signal output, and environmental version.

 

How does a transformer winding temperature indicator work?
A winding temperature indicator is intended to represent winding temperature by combining a measured oil temperature with an additional temperature-rise signal. The reviewed BWR-04 series product is described as a transformer winding temperature thermostat based on an additional temperature-rise principle.

Depending on the selected version, the available remote outputs may include:

Pt100
4–20 mA
0–5 V
RS485
100–800 Ω
The product built-in mechatronic structure that removes the need for a traditional external converter in the specified arrangement. It can be connected to a current transformer through CT calculation when the project design calls for that interface.

Reference operating and technical parameters include:

Measuring range: 0–150°C, customizable
Accuracy class: listed as 1.5 in the technical parameters
Switch operation error: ±2°C
Switch differential: 6 ± 2°C
Protection grade: IP55
Ambient temperature: –40°C to +55°C
Relative humidity: up to 95% under the stated condition
The CT ratio, heating-current calculation, switch settings, output type, and transformer thermal design must be confirmed before a winding temperature indicator is selected.

 

Oil temperature indicator vs. winding temperature indicator

Comparison point	Oil temperature indicator	Winding temperature indicator
Main purpose	Monitor transformer oil temperature	Represent winding thermal condition
Temperature principle	Composite temperature-rise principle in the described series	Additional temperature-rise principle in the described series
Remote outputs	4–20 mA or Pt100, depending on configuration	Pt100, 4–20 mA, 0–5 V, RS485, or 100–800 Ω, depending on configuration
Common range in reviewed data	0–150°C, customizable	0–150°C, customizable
Control function	Temperature alarms or control switches	Temperature alarms or control switches
Critical selection input	Oil measurement point and control requirements	CT data, thermal calculation, winding arrangement, and control requirements

 

Which indicator should a transformer use?
The choice should follow the control objective. An oil temperature indicator may be appropriate when the project needs direct oil-temperature display, oil-temperature alarms, and a remote oil-temperature signal. A winding temperature indicator may be required when the protection and cooling strategy is based on winding thermal condition and current loading.

For large or heavily loaded transformers, engineering teams may use both. The oil temperature value helps evaluate the cooling medium, while the winding temperature value helps evaluate the thermal stress associated with load current.

The selection process should review:
Transformer rating, cooling designation, and temperature-rise data.
Required alarm and trip temperatures.
Current-transformer ratio and available CT signal.
Required output: Pt100, 4–20 mA, 0–5 V, RS485, or resistance output.
Number of temperature-control switches.
Power supply and contact capacity.
Ambient temperature, humidity, and enclosure protection.
Local scale, dial size, mounting, and wiring space.
Integration with fans, pumps, PLC, SCADA, or substation automation.

How do temperature indicators control transformer cooling?
Temperature indicators can provide the signal used to stage fans, start oil pumps, issue alarms, or initiate a protective response, depending on the approved control logic. For example, a transformer oil air cooler may use oil or winding temperature signals to control fan operation. A transformer oil water cooler may use temperature, flow, and pressure signals together with the transformer control system.

The temperature instrument should not be specified separately from the cooler. Confirm the following interfaces:
Sensor output and scaling
Alarm and trip setpoints
Fan or pump start thresholds
Number of stages
Fail-safe behavior when the sensor or signal is lost
Local/remote selector logic
SCADA data mapping
Test and calibration procedure

What do the model codes usually tell you?
A model code may identify the design series, dial or scale arrangement, control-switch quantity, signal output, and environmental version. For example, the reviewed product information describes configurations in which a model suffix identifies a linear scale, four temperature-control switches, a Pt100 output, or use in a specified tropical environment.

Because model-code conventions are product-specific, buyers should request a model-code explanation and a complete datasheet instead of decoding a suffix by assumption. The quotation should clearly state the selected outputs, switches, power supply, protection grade, and temperature range.

 

FAQ
Is a winding temperature indicator the same as a temperature sensor installed in the winding?
Not necessarily. The reviewed winding temperature indicator uses an additional temperature-rise principle and may use CT-based calculation. The actual arrangement depends on the transformer design and instrument configuration.

What is the advantage of a 4–20 mA output?
A 4–20 mA output provides an analog signal that can be connected to a compatible remote monitoring or control system. The receiving system must be configured for the correct range and signal type.

When is Pt100 preferred?
Pt100 can be selected when the monitoring system is designed to receive a platinum resistance signal. Cable length, wiring method, input type, and calibration requirements should be checked.

Can RS485 be used for remote monitoring?
Yes, when the selected indicator provides RS485 and the monitoring system supports the required communication arrangement. Confirm the protocol, address, wiring, and data map.

Are the 0–150°C and IP55 ratings universal?
No. They are reference values listed for the reviewed configurations. The exact model and approved datasheet determine the final rating.

Can the indicator operate the transformer fan automatically?
It may provide the control signal, but automatic fan logic depends on the transformer cooling design, setpoints, contact arrangement, control panel, and approved electrical scheme.

How can VRCOOLER support selection?
VRCOOLER can review the transformer cooling system together with the required temperature indicator, including oil or winding signal type, fan or pump control, remote transmission, and replacement installation details.

Oil and winding temperature indicators are complementary instruments. Oil temperature monitoring focuses on the thermal condition of the insulating oil, while winding temperature monitoring addresses the winding-related thermal condition calculated or represented by the selected instrument. The right choice depends on transformer data, CT inputs, signal interfaces, control stages, and the cooling system.

For project-level design, consult the applicable transformer requirements and the relevant edition of IEC 60076-1. Confirm every rating and output against the final selected model before purchase.