What Is a Temperature Controller for Oil-Filled Transformers? A Detailed Explanation of How the BWY and BWR Series Work
Date: June 6, 2026, 2:12:02 p.m.
- The Importance of Oil-Filled Thermostats: In the operation of oil-immersed transformers, the temperature of the insulating oil and the windings directly affects the safety and service life of the equipment. Oil surface temperature and winding hotspot temperature are two distinct yet complementary monitoring parameters—the former measures the transformer’s external surface temperature, while the latter identifies the hottest internal points; both are essential.
- BWYSeries (Oil-Filled Thermostats): Designed specifically for measuring and controlling the oil temperature at the top of transformers, this device features a mechanical temperature-measuring structure consisting of a temperature sensor, capillary tube, and Bourdon tube. The analog temperature display is intuitive and reliable, requiring no external power source. It is equipped with adjustable electrical contacts to enable alarm, trip, and cooling control functions.
- BWRSeries (Winding Thermostats): This method employs thermal simulation principles—power is drawn from the transformer load via current transformers, and heating elements simulate the temperature rise caused by copper losses, which is then combined with the surface temperature to indirectly calculate the temperature of the winding hot spots. There is absolutely no need to embed sensors inside the high-voltage windings.
- The complementary relationship between the two series: For BWY transformers, oil surface temperature is monitored to assess the overall thermal condition and cooling efficiency of the transformer; for BWR transformers, winding hotspots are monitored to determine the temperature at the most critical locations of the insulation material. For critical transformers, it is recommended to install both systems.
1. Detailed Overview of the BWY Series Oil-Immersion Thermostats
The core components of the BWY series oil-level temperature controllers consist of a temperature-sensing bulb, a capillary tube, and a Bourdon tube. The temperature-sensing bulb is immersed in the top layer of oil in the transformer tank and is filled with a temperature-sensing medium. When the oil temperature changes, the pressure generated by the volume change of the medium is transmitted through the capillary tube to the Bourdon tube, driving the pointer to rotate and indicate the temperature. At the same time, a cam mechanism on the pointer shaft drives a microswitch, triggering an alarm or trip signal when the temperature reaches the set value.
The BWY series includes several models: the BWY-802A and BWY-803A are basic oil level temperature monitoring models, while the BWY-804 and BWY-806 offer additional adjustable contacts and functional options. The instruments are equipped with multiple sets of adjustable electrical contacts, allowing users to select models with 2, 4, or 6 sets of contacts based on control requirements. The analog temperature display is intuitive and clear, offering excellent protection and making it suitable for outdoor substation environments. These models are compatible with oil-immersed transformers of various specifications.
2. Detailed Overview of the BWR Series Winding Thermostats
The BWR series winding temperature controllers utilize thermal simulation technology—a mature, reliable, and mainstream solution in the field of oil-immersed transformer winding temperature measurement. The temperature sensor contains a heating element that draws power from the transformer’s load current via a current transformer. The heat generated is proportional to the square of the load current (simulating copper loss I²R). The resulting temperature rise is added to the oil surface temperature and displayed on the dial, representing the calculated winding hotspot temperature.
The primary advantage of this indirect measurement method is that it requires no sensors to be embedded within the high-voltage windings. All temperature-sensing components (temperature sensors, capillary tubes, and gauges) are installed on the exterior of the transformer tank, making installation and maintenance safe and convenient, and having no impact on the transformer’s insulation structure. The BWR series has been proven through decades of application and features mature technology.
3. Comparison of BWY and BWR
| comparison dimension | BWY Series (Oil-filled) | BWR Series (Windings) |
|---|---|---|
| Monitoring Objects | Oil temperature at the top of the transformer | Winding Hot Spot Temperature (Estimated from Thermal Simulation) |
| Temperature Measurement Method | The temperature sensor is immersed directly in the oil for measurement | Oil surface temperature + thermal simulation temperature rise of heating elements |
| Sensor Location | Inside the temperature sensor housing at the top of the fuel tank | Temperature sensor on top of the fuel tank + CT on the low-voltage bushing |
| Display mode | Analog temperature gauge | Analog temperature gauge |
| Electrical contacts | Adjustable from 2 to 6 groups | Multiple adjustable settings |
| responsiveness | Direct measurement, fast response | Thermal simulations involve thermal inertia, and transients exhibit hysteresis. |
4. Frequently Asked Questions FAQ
4.1 Q: Do the BWY and BWR have to be installed at the same time?
A: It is not mandatory, but it is recommended. For BWY transformers, monitoring the oil temperature provides an indication of the overall thermal condition and cooling efficiency; for BWR transformers, monitoring hot spots in the windings indicates the temperature at the most critical points for insulation. For main transformers rated at 110 kV and above, it is recommended to install both types simultaneously to protect the transformer from multiple perspectives.
4.2 Q: Are the winding temperatures in a BWR accurate?
Answer: The thermal simulation method 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 with severe load fluctuations, there is some thermal lag, causing the indicated values to lag behind actual changes in winding temperature. For special scenarios requiring extremely high accuracy, a direct fiber-optic temperature measurement solution may be considered.
4.3 Q: What happens if a capillary breaks?
A: A broken capillary tube can cause the internal temperature-sensing fluid to leak, rendering the thermostat completely inoperable. Capillary tube leaks usually cannot be repaired on-site and require replacement of the entire thermostat. Therefore, special care should be taken during installation to protect the capillary tube route—avoid sharp bends, avoid contact with sources of vibration, and avoid mechanical compression.
4.4 Q: Do BWY and BWR require periodic calibration?
A: Yes. We recommend calibrating the thermostat once a year during the annual maintenance. Compare the pointer reading with a standard temperature source to ensure the deviation is within the permissible range. Also, check whether the electrical contacts operate reliably at the set temperature.
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 temperature control solution for oil-immersed transformers? Contact Innotongda for BWY/BWR model recommendations and product documentation. Service Hotline: 13959168359 (also on WeChat).
