Temperature monitoring of dry-type and oil-immersed transformers

I. Temperature monitoring of dry-type transformers

1. Core monitoring sites

• Winding Temperature: The windings of a dry-type transformer are the main source of heat (heat generated by copper or aluminum losses), and the insulation of the windings is directly affected by the temperature, so it is the core of the monitoring.
• Core temperature: The core heats up due to hysteresis and eddy current losses, and the temperature is usually slightly lower than that of the windings, but it may overheat in abnormal situations (e.g., when the core is grounded at multiple points) and requires auxiliary monitoring.
• environmental temperature: Dry-type transformers rely on air heat dissipation, the ambient temperature directly affects the efficiency of heat dissipation (e.g., a high temperature environment will lead to a rise in winding temperature), which needs to be used as a reference benchmark.

2. Monitoring methodology

• Direct measurement (recommended)::
  When the windings are manufactured, thePlatinum resistance sensors (Pt100) Pre-embedded in the winding inside (usually three-phase winding hot spot location), direct collection of the real temperature of the winding. pt100 high accuracy (error ± 0.1 ℃), strong stability, is the first choice for temperature monitoring of dry-type transformers.
• Indirect monitoring::
  For dry-type transformers without pre-embedded sensors, theinfrared thermometerNon-contact measurement of the winding surface temperature (note that the surface temperature is lower than the internal hot spot temperature, with an error of about 5-10°C), or estimation by means of "ambient temperature + calculated temperature rise of the winding current" (lower accuracy, only for temporary monitoring).

3. Temperature limits (core safety indicators)

Note: "Temperature Rise Limit" refers to the difference between the winding temperature and the ambient temperature (ambient temperature is based on 40°C), exceeding the limit will accelerate the aging of the insulation.

4. Monitoring systems and protection functions

• The temperature controller receives a Pt100 signal, displays the winding temperature in real time, and has built-in protection logic:
  • Over-temperature alarm: When the temperature reaches the "alarm threshold" (e.g., Level F is set to 130°C), an audible and visual alarm is triggered, prompting O&M personnel to check;
  • Over-temperature trip: When the temperature reaches the "trip threshold" (e.g., level F is set to 150℃), a trip signal will be output to cut off the power supply of the transformer to avoid insulation burning.
• In the case of forced air-cooled dry-type transformers, the temperature controller will also automatically start and stop according to the temperature.Cooling Fans(e.g., starts when the temperature exceeds 100°C and stops when the temperature falls below 80°C) to enhance heat dissipation.

5. Monitoring maintenance elements

• Periodic calibration of the Pt100 sensor (1 time per year) ensures measurement accuracy;
• Check that the contact between the sensor and the winding is tight (looseness will result in a low measurement value);
• Clean winding surfaces (dust buildup can impede heat dissipation and lead to falsely high temperatures);
• Verify the alarm and trip functions of the temperature controller (tested by analog signals).

II. Temperature monitoring of oil-immersed transformers

1. Core monitoring sites

• Top oil temperature: The insulating oil is heated upward convection, the top oil temperature is the highest temperature in the oil, directly reflecting the transformer overall heat dissipation status, is the most critical monitoring point.
• Winding hot spot temperature: The maximum temperature inside the windings (5-15°C higher than the top oil temperature, depending on the cooling method) is a direct influence on the aging of the insulation (the hot spot temperature limit for oil-immersed transformer insulation paper is 105°C).
• bottom oil temperature: To assist in determining whether the oil circulation is normal (normally the temperature difference between the top layer and the bottom layer is about 10-20°C, too small a temperature difference may be a circulation fault).

2. Monitoring methodology

• Oil Temperature Measurement::
  Installation at the top of the tank (10-20cm below the top oil level)temperature sensor, common types include:
  • Expansion thermometer (mercury/alcohol, direct reading, for in situ monitoring);
  • Platinum resistance (Pt100) or thermocouple (for remote transmission to a monitoring system, accuracy ±0.5°C).
• Winding hot spot temperature calculation::
  When direct measurement is not possible, the temperature is calculated indirectly by "top oil temperature + additional temperature rise of winding current":
  Winding hot spot temperature = top oil temperature + (hot spot temperature rise at rated current) x (actual current / rated current)²
  (Note: Hot spot temperature rise at rated current is typically 10-15°C, as supplied by the manufacturer).

3. Temperature limits (core safety indicators)

• Top oil temperature: Not more than 85℃ during normal operation (the hot spot temperature of the winding is about 95-100℃ at this time), and not more than 95℃ for a short period of time (the hot spot temperature is ≤105℃);
• Winding hot spot temperature: Maximum permissible 105°C (above this temperature, the aging of the insulation paper is dramatically accelerated and the life span is drastically shortened);
• environmental temperature: 40°C as a reference (above 40°C, reduce the load to limit the oil temperature).

4. Monitoring systems and protection functions

• Oil temperature controller: Monitor top oil temperature and set up two levels of protection:
  • Alarm: Alarm when the oil temperature reaches 80℃ (suggesting that the heat dissipation may be insufficient);
  • Tripping: Tripping when oil temperature reaches 90℃ (avoid hot spot temperature exceeding 105℃).
• Winding Temperature Analog Controller: Capture the winding current through current transformer, calculate the winding hot spot temperature by combining with the top oil temperature, and provide the alarm and trip function similar to the oil temperature controller (closer to the real state of the winding).
• Forced oil circulation transformers also require monitoring of the operational status of the oil pump and radiator fan (failure can result in a sudden rise in oil temperature).

5. Monitoring maintenance elements

• Periodically check the oil temperature sensor for oil leaks (oil immersed sensors may leak due to seal failure);
• Calibrate the temperature measuring device (once a year) to ensure that the top oil temperature is measured with an error of ≤2℃;
• Check the oil level (a low oil level reduces the heat dissipation area, resulting in higher oil temperatures) and oil quality (oil deterioration reduces heat dissipation efficiency);
• Verify the linkage function of the cooling system (oil pump, fan) (e.g. automatic start-up when oil temperature reaches 60°C).

Third, the core difference between dry-type and oil-immersed transformer temperature monitoring

summarize