What are the methods for monitoring transformer oil temperature? A detailed explanation of oil surface temperature and winding temperature
Date: May 28, 2026, 2:12:02 p.m.
- Oil surface temperature: This refers to the temperature of the insulating oil at the top of the transformer tank and serves as a direct indicator of the transformer’s overall thermal condition and the efficiency of its cooling system. It is measured using a PT100 platinum resistance thermometer immersed in the oil, providing intuitive readings and simple installation.
- Winding Temperature: Refers to the temperature at the hottest spot within the transformer windings. Oil-immersed transformers typically do not have sensors embedded directly in the windings; instead, the temperature is estimated indirectly using thermal simulation principles—a heating element simulates the temperature rise due to copper losses, and this value is combined with the oil surface temperature to estimate the winding hotspot temperature.
- Monitoring discrepancies: The oil surface temperature is the external indicator, while the winding temperature is the internal one. The oil surface temperature reflects the average thermal condition of the transformer oil, whereas the winding temperature reflects the actual temperature at the hottest internal point—and the rate of insulation aging is precisely determined by the temperature at this hottest point.
1. Oil surface temperature monitoring (BWY(Series)
The BWY series oil level temperature controllers are specifically designed to measure and control the temperature of the top layer of oil in oil-immersed transformers. The temperature sensor is immersed in the top layer of oil in the tank; as the internal temperature-sensing medium expands due to heat, it drives the pointer to indicate the temperature, while simultaneously actuating an electrical microswitch via a mechanical linkage. The instrument is equipped with 2 to 6 sets of adjustable electrical contacts, allowing separate settings for alarm temperature, trip temperature, andCooling FansStart/stop temperature.
The analog temperature scale is clear and intuitive, displaying the temperature without requiring a power source. The electrical contacts have sufficient capacity to drive standard alarms and contactors. The housing offers excellent protection and is suitable for outdoor substation environments. It is compatible with oil-immersed transformers of various specifications and models; simply select the appropriate model based on the oil temperature measurement range and the number of electrical contacts required.
2. Winding Temperature Monitoring (BWR(Series)
The BWR series winding thermometers utilize thermal simulation technology—the mainstream solution for measuring the temperature of oil-immersed transformer windings. The principle is as follows: a heating element is embedded within the temperature sensor; power is drawn from the transformer’s load current via a current transformer, and the heat generated by the heating element is proportional to the square of the load current (simulating copper losses). The temperature rise of the heated bulb, when combined with the oil surface temperature, results in a reading on the dial that represents the estimated temperature of the winding hotspot.
The ingenuity of this thermal simulation approach lies in the fact that it does not require any sensors to be embedded within the high-voltage windings. All temperature-sensing components are mounted on the exterior of the transformer tank, making installation and maintenance convenient and ensuring that the transformer’s insulation structure remains unaffected. Proven through decades of application, this thermal simulation technology is mature and reliable.
3. Comprehensive Analysis of Two Temperature Signals
| Temperature Performance | Possible causes | Recommendations |
|---|---|---|
| Oil temperature is normal, but the winding temperature is high | Excessive load factor leads to increased copper losses | Reduce the load or check the cooling system |
| Oil temperature is high, but the winding temperature is normal | Reduced cooling system efficiency or excessively high ambient temperature | Check the operating status of the radiator and fan |
| Both increased simultaneously | General overload or cooling system failure | Check the load and cooling system immediately |
4. Frequently Asked Questions FAQ
4.1 Q: Can a BWY and a BWR share the same transformer?
A: Yes, and it is the recommended approach. BWY monitors the oil temperature, while BWR monitors winding hotspot temperatures; together, they monitor the thermal condition of the transformer from different perspectives. For critical main transformers, it is recommended to install both series simultaneously.
4.2 Q: Are the winding temperatures in the thermal simulation accurate?
Answer: Thermal simulation offers good accuracy under steady-state loads and can meet the requirements of the vast majority of operation and maintenance scenarios. However, during transient processes involving significant load fluctuations, thermal simulation exhibits some thermal lag. For scenarios requiring extremely high accuracy, a direct fiber-optic temperature measurement solution may be considered.
4.3 Q: Where is the temperature sensor for the oil-surface thermostat located?
Answer: Install the sensor in the dedicated sensor housing on top of the transformer tank, with the sensor probe extending approximately 150–200 mm into the top layer of oil. Ensure that the sensor is fully submerged in the oil, and position it away from dead zones and areas of localized overheating.
4.4 Q: How do I determine the number of electrical contacts?
A: Determine based on control requirements: Select 2 sets of contacts if only an over-temperature alarm is needed; select 4 sets if an alarm and trip are required; select 6 sets if an alarm, trip, fan control, and communication are required. Clarify the design requirements for the control circuit before configuration to avoid an insufficient number of contacts.
4.5 Q: Does the thermostat require regular maintenance?
A: Yes. We recommend inspecting it once a year to ensure that the pointer reading matches the actual temperature, that the electrical contacts operate reliably at the set temperature, that the temperature-sensing capillary tube is free of damage or kinks, and that the dial glass is free of cracks and water ingress.
Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements.
Looking for a solution for transformer oil level or winding temperature controllers? Please contact Innotongda for product recommendations and technical documentation. Service Hotline: 13959168359 (also available on WeChat).
