What are the temperature monitoring methods and sensors for dry-type transformers?

The safe operation and service life of dry-type transformers are directly related to the health of their insulation materials, and temperature is the most critical factor affecting the aging rate of insulation materials. Therefore, accurate and reliable temperature monitoring is the core of the dry-type transformer protection system. Unlike oil-immersed transformers, which rely on transformer oil for insulation and cooling, dry-type transformers rely mainly on air (natural convection or forced air cooling) and solid insulating materials (e.g., epoxy resins, insulating paper, etc.), which makes the monitoring of internal hot spots a more stringent requirement.

Below is a detailed description of all the main ways in which dry-type transformer temperatures can be monitored, from the core to the auxiliary, and from the inside to the outside:

Mode 1: Embedded winding temperature direct measurement

This is the core and primary method of monitoring. Sensors are pre-embedded directly into the windings or close to the surface of the windings during the manufacturing process of the transformer to most directly reflect the actual temperature of the windings, which are the hottest component.

1. Platinum resistance temperature sensors (Pt100/Pt1000)

This is the most common, standard and widely used method of temperature measurement in dry-type transformers.

• Deployment Location: During the manufacture of the transformer, platinum resistance temperature sensing elements (usually three, corresponding to the three-phase windings A, B and C) are pre-embedded in the upper part of the three-phase low-voltage winding near the outlet end. This area is usually the hot spot where the winding dissipates the least heat and has the highest temperature. The sensors are firmly fixed in the gap between the winding coils and are epoxy cast or VPI vacuum pressure dipped together with the windings, becoming a permanent part of the transformer body.

• Working Principle: Utilizes the physical property that the resistance value of a pure platinum wire changes precisely and linearly with temperature. pt100 refers to a resistance value of 100.00 ohms at 0 °C. The temperature controller applies a very small, constant measuring current to the platinum resistance sensor via a special shielded cable and then precisely measures the voltage across the sensor. According to Ohm's law (resistance = voltage/current), the current resistance value of the sensor can be calculated. Since there is an internationally recognized standard correspondence between the resistance value of platinum resistors and temperature (ITS-90 temperature scale), the controller can look up the table to accurately convert the resistance value to the current temperature value.

• System Components::

  • temperature sensing element: Pt100 platinum resistors pre-embedded in the winding.

  • connecting lead: Specialized wires leading from the sensor, high-temperature resistant and shielded to prevent electromagnetic interference.

  • Temperature Controller: The core device that receives and processes sensor signals.

• Functions and Features::

  • highly accurate: Accurate measurement with good linearity.

  • good stability: Long-term operational performance is stable with low drift.

  • standardization: Pt100 is an international standard with good interchangeability.

  • limitationsThe accuracy of the readings is highly dependent on the precise alignment of the actual hotspot at the pre-embedded location during manufacture.

2. Fiber optic temperature sensors

This is a more advanced but also more costly way, mainly used for high voltage level, large capacity or special requirements of dry-type transformers.

• Deployment Location: The small size of the fiber optic sensor allows for more flexible placement inside the transformer. They can be attached directly to the surface of the high-voltage winding or even buried inside the high-voltage winding, which is not possible with traditional metal sensors (e.g. Pt100) due to insulation problems.

• Working Principle: Fiber optic temperature measurement technology has a variety of implementation principles, commonly used in transformer applications:

  • Fluorescent fiber optic temperature measurement: A small piece of special rare earth fluorescent material is coated on the end of the optical fiber. The temperature measurement host emits a specific wavelength of excitation light through the fiber, and the fluorescent material absorbs the energy and emits fluorescence. When the excitation light stops, the decay time (lifetime) of the fluorescence has a precise correspondence with the temperature. The host calculates the temperature by measuring this decay time.

• System Components::

  • Fiber Optic Sensor Probes: Optical fibers with temperature-sensitive materials or gratings.

  • Fiber optic demodulator (temperature measurement mainframe): Responsible for transmitting and receiving light signals, and performing calculations and displays.

  • optical connection: Used to connect the probe to the main unit.

• Functions and Features::

  • intrinsic safety: Complete electrical insulator, immune to any electromagnetic interference (EMI/RFI) and safe for direct contact with high-voltage components.

  • Measurement accuracy: High precision and fast response time.

  • Multi-point measurementFiber optic temperature measurement technology can realize the simultaneous monitoring of different hot spot temperatures of the transformer, which can capture the hot spot distribution more comprehensively.

Mode 2: Non-contact external surface temperature measurement

This approach does not go inside the transformer, but rather aids monitoring and inspection by detecting its external surface temperature.

3. Infrared thermography detection

This is a very effective tool for inspection and diagnostics, but not as a primary means of real-time protection.

• Deployment Location: The operator holds an infrared camera, or installs a fixed infrared camera in the transformer room, and scans the surface of the windings, the core, the terminals, the bushings, etc. of the transformer.

• Working Principle: Any object with a temperature above absolute zero radiates infrared energy outward. The higher the temperature of the object, the more infrared energy it radiates. The thermal imaging camera receives the infrared radiation from the surface of the object by means of its internal infrared detectors (focal plane array) and converts it into an electrical signal. After processing, the system generates a "heat map" in pseudo-color, with different colors representing different temperatures on the image, allowing the human eye to visualize the temperature distribution on the surface of the object.

• System Components: Handheld or fixed thermal imaging cameras.

• Functions and Features::

  • non-contact: There is no need for a power outage, and the test can be performed while the transformer is running, which is very safe.

  • comprehensive and intuitiveThis provides an image of a "temperature surface" rather than a "temperature point", which makes it possible to quickly detect localized overheating defects, and is particularly effective for checking for loose terminals and poor contact.

  • Surface temperature measurement: It measures only the surface temperature of the transformer windings or core and does not reflect the maximum temperature of the internal windings, and its reading will be much lower than the actual hot spot temperature inside the windings.

  • complementary diagnosis: It is mainly used for regular inspections and preventive maintenance to identify potential problems, rather than for real-time control and protection.

Approach III: Other critical components and ambient temperature monitoring

In addition to monitoring the core winding temperature, it is also important to monitor the core and ambient temperatures.

4. Core temperature monitoring

• Deployment Location: A temperature sensor (either Pt100 or thermocouple) is usually mounted on the yoke (upper yoke) or clamp of the core.

• Working Principle: The same principle as the winding temperature measurement, used to measure the temperature of the iron core.

• Functions and Features: Abnormally high core temperatures may indicate a fault such as multiple points of grounding in the core, excessive eddy currents due to damage to the insulation between silicon steel wafers, and so on. Monitoring the core temperature can provide an alarm and diagnostic basis for these specific faults.

5. Ambient temperature monitoring

• Deployment Location: In the room or cabinet where the transformer is located, select a location for the temperature sensor that is representative of the surrounding cooling air temperature and is not affected by direct heat radiation from the transformer.

• Working Principle: Measures the temperature of the cooling medium (air).

• Functions and Features: The ambient temperature is the benchmark for calculating the temperature rise of the transformer. Excessive ambient temperatures can significantly reduce the transformer's ability to dissipate heat, thus limiting its load capacity. Monitoring the ambient temperature can be used to control the ventilation system in the transformer room or to provide an early warning if the ambient temperature is too high.

Summarization and system integration: temperature control systems

All of the above sensors are just "eyes", they see the data ultimately need to be brought together in a "brain" for processing, this "brain" is the Dry-type transformer temperature controllerThe

A completeDry-type transformer thermostat, usually in a combination of the above, and through a smart thermostat that fulfills the following core functions:

1. Three-phase temperature patrol and display: Automatically displays the real-time temperature of A, B and C phase windings in turn, and can manually switch to view the highest phase temperature.

2. Automatic fan controlWhen the temperature of any phase of the winding reaches the preset "fan start" value, the controller will automatically close the relay contacts to start the fan.Cooling FansForced air cooling; when the temperature drops to the "fan stop" value, the fan is automatically turned off to save energy and reduce noise.

3. Over Temperature AlarmWhen the temperature of any one phase of the winding reaches the preset "alarm" value, the controller will issue an audible and visual alarm signal to remind the operator to pay attention.

4. Over-temperature trippingWhen the temperature of any phase of the winding reaches the preset "trip" value (which is the last line of defense to protect the insulation), the controller will output a set of passive trip contact signals to the transformer's high-voltage side of the switch, so that it will trip, cut off the power supply of the transformer, to achieve the final protection.

5. Sensor Failure Detection: The controller can automatically detect if there is a broken or short circuit fault in the sensor and issue a fault alarm.

6. Data Remote TransmissionModern temperature controllers usually have communication interfaces such as RS485 and support standard protocols such as Modbus, which can remotely transmit all temperature data and equipment status to the back-end monitoring system (SCADA).