Dry-type transformer temperature monitoring sensors

Date: August 7, 2025 08:48:29

Dry-type transformers are widely used in power distribution systems, industrial plants and other scenarios because of their oil-free cooling, compact structure, easy maintenance and other features. However, its windings, core and other components in operation will be due to loss of heat, if the temperature is too high will accelerate the insulation aging (or even breakdown), and in serious cases, cause a fire.Temperature Monitoring SensorsAs the core component of dry-type transformer condition monitoring, the ability to capture temperature changes in real time and provide early warning is the key to ensuring the safe operation of the equipment.

I. The need for temperature monitoring of dry-type transformers

Insulation materials for dry-type transformers (e.g. epoxy resin, Nomex paper, etc.) have defined temperature tolerance limits (e.g. maximum permissible temperature 155°C for class F insulation, 180°C for class H). If the temperature exceeds the limit value:

 

  • Insulation material mechanical strength, electrical strength significantly reduced, life expectancy is dramatically shortened (for every 10 ℃ increase in temperature, the life expectancy may be halved);
  • Winding resistance increases and losses rise further, creating a vicious cycle of "overheating - increased losses - more overheating";
  • Extreme cases may cause accidents such as short-circuiting of the winding and fire.

 

Therefore, it is necessary to monitor the temperature of key parts in real time through sensors to provide data support for heat dissipation control (e.g., starting the fan) and failure warning.

Second, common temperature monitoring sensor types and characteristics

Dry-type transformer temperature monitoring sensors need to be adapted to theirHigh electromagnetic interference, dusty, compact spaceThe common types of operating environments are listed below:

1. Platinum resistance sensors (Pt100/Pt1000)

principle: Based on the characteristics of platinum metal resistance as a function of temperature (100Ω/Pt100 or 1000Ω/Pt1000 at 0°C, with resistance linearly increasing at higher temperatures), the temperature is converted by measuring the resistance value.
specificities::

  • High precision (error usually ≤±0.1℃~±0.5℃), good linearity, suitable for high-precision monitoring;
  • Faster response time (milliseconds), high stability and long life (up to 10+ years);
  • Requires direct connection to the measurement circuit (wired transmission) and is susceptible to electromagnetic interference (shielding required).
    Applicable Scenarios: Direct contact temperature measurement of critical parts of dry-type transformer windings and cores is the most widely used type.

2. Thermocouple sensors

principleThe temperature difference between two different metals (e.g. NiCr-NiSi type K, Fe-Cu type J) in a closed loop generates a thermoelectric potential (Seebeck effect), which is converted to temperature by the potential value.
specificities::

 

  • Wide temperature measurement range (-200℃~1300℃), can adapt to the high temperature scenario of dry-type transformer short-time overload;
  • Simple structure, low cost, vibration and shock resistant;
  • Lower accuracy (error ±1℃~±3℃), requires cold-end compensation (ambient temperature affects measurement), and linearity is worse than that of platinum resistors.
    Applicable Scenarios: Temperature monitoring for non-high precision needs such as iron cores, housings, etc., or as a backup sensor for platinum resistors.

3. Infrared sensors (non-contact)

principle: Converts surface temperature by detecting the energy of infrared radiation emitted from the surface of an object (following Planck's law).
specificities::

 

  • Non-contact measurement without direct connection to the device and without compromising insulation;
  • Flexible installation allows monitoring of winding surfaces, housings, and other inconveniently accessible areas;
  • It is not possible to measure the internal temperature because it is greatly affected by the environment (e.g., dust and water vapor absorb the infrared radiation, and the error may reach ±2℃~±5℃).
    Applicable Scenarios: Supplementary to contact sensors for monitoring the overall surface temperature of the transformer or in areas where it is not convenient to install contact sensors.

4. Fiber optic sensors

principle: Using optical fibers to transmit optical signals, the temperature signal is converted into a change in light wavelength through the refractive index of the fiber optic material (e.g., fluorescent fibers) that changes with temperature, and then the temperature is restored by the demodulation equipment.
specificities::

 

  • Extremely strong anti-electromagnetic interference ability (fiber optic is not conductive), suitable for the high-voltage side of dry-type transformers, strong electromagnetic environment;
  • Corrosion-resistant, high-pressure resistant (can be embedded inside the winding), high measurement accuracy (error ≤ ± 0.5 ℃);
  • Higher cost, easy to break optical fiber (need protective measures), complex demodulation equipment.
    Applicable Scenarios: High-voltage dry-type transformers, applications sensitive to electromagnetic interference, or where high-precision monitoring needs to be embedded inside the winding.

Third, the sensor installation position

Uneven temperature distribution in dry-type transformers.Windings (especially hot spots), cores, environmentIt is a core monitoring point and the installation location needs to be selected in conjunction with the monitoring objectives:

 

monitoring point Purpose of monitoring Recommended Sensor Types Installation
Winding hotspot Reflects the most dangerous overheating location (highest temperature) Pt100 (pre-embedded), fiber optic Manufactured to be embedded inside the winding (without damaging the insulation)
Winding surface Aids in determining overall heating trends Pt100 (adhesive), infrared Adhesion to the winding surface (insulating glue is required for fixing)
iron core Monitoring of overheating caused by eddy current losses in the iron core Thermocouple, Pt100 Fixed to core clamps or surfaces
environmental temperature Aids in determining thermal conditions (e.g., high room temperature) Pt100, thermocouple Installed in the air environment 1~2m around the transformer.

IV. Key factors for selection

Selection of the sensor needs to be combined with the model of dry-type transformer, operating environment, accuracy requirements comprehensive judgment:

 

  1. Temperature range: Need to cover the normal operating temperature of the equipment (30℃~120℃) and short-term overload temperature (e.g. above 150℃);
  2. accurateWinding hot spot monitoring needs to be within ±0.5°C (e.g. Pt100, fiber optic), environmental monitoring can be relaxed to ±2°C;
  3. anti-interference capabilityFiber optic sensors are preferred for high pressure and strong electromagnetic environments, while Pt100 (shielded) is available for low pressure;
  4. Installation CompatibilityEmbedded windings should be selected from miniaturized, well-insulated sensors (e.g., Pt100 with a diameter of ≤3 mm) to avoid damaging the insulation;
  5. stability: Requires resistance to vibration, dust, humidity (e.g., IP65 rating) and a life expectancy of ≥ 5 years.

V. Temperature monitoring system components

Sensors are front-end "sensing" components that need to work with the back-end system to realize the complete monitoring function:

 

  • Data Acquisition Module: Converts sensor signals (resistance, potential, light signals) into digital signals;
  • Display and Alarm Units: Real-time display of the temperature value, and warning through sound and light, SMS etc. when the limit is exceeded;
  • control module: Linkage of cooling devices (e.g. fans) to automatically start cooling when the temperature is too high.

summarize

The core role of dry-type transformer temperature monitoring sensors is "early detection, early warning", different types of sensors have their own advantages and disadvantages:Pt100Best value for money and suitable for most scenarios;Fiber Optic SensorsIt is the first choice for strong electromagnetic environments;infrared sensorSuitable for supplementary monitoring. In practice, the combination of "contact + non-contact" is often used, taking into account the accuracy and safety, and providing a reliable guarantee for the stable operation of dry-type transformers.