What is a dry-type transformer thermostat?

I. Principle of operation

II. Main functions

1. Temperature monitoring and display
   • Real-time display of the temperature of each phase of the transformer winding, part of the thermostat can also display the core temperature or ambient temperature.
   • Support multi-channel temperature acquisition (e.g. three-phase windings are monitored separately), and have temperature patrol function.
2. temperature control
   • Fan start/stop control: When the winding temperature reaches the set value (e.g. 100℃), the cooling fan is automatically started; when the temperature is lower than the set value (e.g. 80℃), the fan is automatically stopped to reduce energy consumption and noise.
   • Over Temperature Alarm: When the temperature exceeds the alarm threshold (e.g., 130°C), the thermostat emits an audible and visual alarm signal to remind O&M personnel to check the equipment.
   • Over-temperature trippingIf the temperature continues to rise to the danger threshold (e.g. 150°C), the thermostat outputs a trip signal, which cuts off the power supply to the transformer and prevents serious damage to the equipment.
3. Fault diagnosis and protection
   • With sensor failure detection function (such as open circuit, short circuit alarm), to avoid misoperation due to abnormal sensors.
   • Part of the thermostat supports historical temperature data recording and querying, which is convenient for analyzing the operation status of the transformer and the trend of temperature change.
4. Communications and Remote Monitoring
   • Supporting RS485, Modbus and other communication protocols, it can be networked with substation automation system or monitoring center to realize remote temperature monitoring and control.
   • Some high-end thermostats are equipped with Ethernet interface or wireless communication function, which supports remote viewing of data by cell phone APP or web.

III. Common types and structures

1. typology
   • Pointer thermostat: Temperature display by means of a mechanical pointer, simple function, suitable for small transformers.
   • Digital Thermostat: The LCD or LED screen is used to display the temperature, which is feature-rich and highly accurate, and is the mainstream type nowadays.
   • Intelligent Thermostat: Integrated microprocessor, supports multiple communication protocols and extended functions (e.g. fault recording, data analysis), suitable for large or important transformers.
2. Structure Composition
   • hosts: Contains a display, operating buttons, control circuits and communication interfaces.
   • temperature sensor: Usually a platinum resistor (PT100), mounted in a critical position in the transformer winding or core.
   • output module: Includes relay contacts (for controlling fans, alarms and trips) and communication modules.

IV. Installation and application scenarios

• mounting position: The main body of the thermostat is usually mounted on the control panel of the transformer enclosure, and the sensor is connected to the winding or core by means of leads.
• application scenario: Widely used in dry-type transformers (e.g. distribution transformers, rectifier transformers, traction transformers, etc.), especially for temperature-sensitive applications (e.g. high-rise buildings, hospitals, data centers, subways, etc.).

V. Selection and maintenance points

1. Selection Points
   • Select the appropriate thermostat accuracy (e.g. ±1°C or ±2°C) and function configuration according to the transformer capacity, voltage class and operating environment.
   • Verify that the thermostat's input signal type (e.g., PT100) and output interface (number of relay contacts, communication protocol) meet system requirements.
   • Consider the level of protection (e.g. IP54) to suit different installation environments (e.g. waterproof for outdoor or wet locations).
2. Maintenance Points
   • Periodically check that the thermostat display is normal and that the sensor leads are not loose or deteriorated.
   • Clean the thermostat housing and display to avoid dust buildup that could affect heat dissipation and display.
   • Functional tests (e.g., simulated over-temperature alarms, trips) are performed once a year to ensure reliable control logic.
   • When the transformer is overhauled or the windings are replaced, the mounting position and accuracy of the temperature sensors need to be recalibrated.

VI. Difference with oil-immersed transformer thermostat

• Differences in cooling methodsDry-type transformers rely on air cooling (natural or forced air cooling), and the thermostat needs to directly monitor the winding temperature; oil-immersed transformers dissipate heat through the oil cycle, and the thermostat usually monitors the oil surface temperature.
• Different protection requirementsDry-type transformer thermostats are more concerned with dustproofing and heat dissipation, while oil-immersed thermostats need to consider waterproofing and oil leakage prevention.

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