Comprehensive guide to hot spot temperature monitoring for dry-type transformers

• Core definitionsDry-type transformer hot spot temperature monitoring refers to the use of specific technical means, the dry-type transformer in the operation process due to the loss of the highest temperature point (i.e. “hot spot”) for real-time, accurate measurement and monitoring.
• Need for monitoring: The insulation aging rate of a transformer is directly determined by its hot spot temperature. Accurate temperature monitoring is a fundamental prerequisite for preventing premature insulation failure, evaluating equipment health, optimizing load capacity and ensuring safe operation.
• technical program: A variety of monitoring technologies exist, including traditional embedded Pt100/thermocouples, non-contact infrared thermography, and state-of-the-art fluorescent fiber optic thermometry capable of directly measuring internal hot spots.
• Key Technology Comparison:: Of all the technologies.Fluorescent fiber optic temperature measurementDue to its complete electromagnetic immunity and high voltage insulation, it is the only technology that can safely and accurately measure real hot spots directly inside the windings; andPt100On the other hand, it is widely used in the monitoring of secondary locations such as the outside of windings or cores due to its cost-effectiveness and maturity.
• system configuration: A complete monitoring system consists of front-end sensors, a data acquisition and demodulation host, and application layer interfaces for control and remote communication, which together form the core of intelligent thermal management of a transformer.

Table of Contents for this article

• 1. What is a dry-type transformer?
• 2. What is the normal temperature of a dry-type transformer?
• 3. Why monitor temperature?
• 4. What malfunctions can be caused by elevated temperatures?
• 5. What are the types of technology available for hot spot temperature monitoring?
• 6. Comparison table of major temperature measurement technologies
• 7. Components of a dry-type transformer temperature monitoring system
• 8. Frequently Asked Questions (FAQ)

1. What is a dry-type transformer?

A dry-type transformer is a type of power transformer in which the core and windings are not impregnated with any insulating liquid (e.g. transformer oil). Its insulation and cooling are accomplished by relying on air (natural or forced air cooling) and solid insulating materials (e.g. epoxy resin, insulating paper/film, etc.). Because they are oil-free, flame-retardant, fire- and explosion-proof, easy to maintain and environmentally friendly, dry-type transformers are widely used in indoor locations with high requirements for safety and environmental protection, such as high-rise buildings, commercial complexes, data centers, airports, subways, hospitals and power plants.

2. What is the normal temperature of a dry-type transformer?

The normal operating temperature of a dry-type transformer depends on the type of transformer used.Insulation heat resistance grade. The insulation class defines the maximum permissible temperature at which an insulating material can be subjected to for a long period of time without significant deterioration of its properties. Common insulation classes and their corresponding maximum allowable temperature rise and hot spot temperature limits are listed below:

• Class F insulation: This is the most common class. Its maximum permissible working temperature is155°C. At an ambient temperature of 40°C, the average temperature rise limit of its winding is 100K, and the permissible hot spot temperature rise is 115K, i.e., the hot spot temperature should not be more than155°CThe
• Class H insulation: Higher thermal class. The maximum permissible working temperature is180°C. At an ambient temperature of 40°C, the average temperature rise limit of its windings is 125K, and the permissible hot spot temperature rise is 140K, i.e. the hot spot temperature should not exceed180°CThe
• Class C insulation: Extremely high thermal class, maximum permissible operating temperature up to220°CThe

Typically, transformers are equipped with a temperature control system that protects the transformer by activating the cooling fan when the temperature reaches a preset value (e.g., 95°C), issuing an alarm at higher temperatures (e.g., 130°C), and issuing a trip command when the insulation limit (e.g., 150°C) is approached.

3. Why monitor temperature?

Temperature, especiallyhot spot temperature, is the single most critical and direct parameter affecting the life and reliability of dry-type transformers. The necessity of monitoring temperature is reflected in the following aspects:

1. Determinants of insulation life: The aging of electrical insulation materials is an irreversible chemical process, the rate of which is closely related to temperature. According toMontsinger's LawFor Class A insulation, every 8-10°C rise in temperature doubles the rate of insulation ageing, i.e. halves the life expectancy. Accurate monitoring and control of hot spot temperatures is essential to ensure that the transformer reaches its design life.
2. Safe and secure operations are guaranteed: Continuous over-temperature operation will degrade the mechanical and electrical strength of the insulating material, which may ultimately lead to turn-to-turn, layer-to-layer, or phase-to-phase insulation breakdowns, triggering short-circuit failures and even fires.
3. Optimization basis for load capacity: The rated capacity of a transformer is defined at a specific temperature rise limit. With accurate, real-time hotspot temperature monitoring, O&M personnel can understand the thermal margin of the transformer at the current load, so that short-term overloads can be safely implemented to address temporary power spikes and improve asset utilization.
4. Fault diagnosis and condition assessment: Abnormal temperature rise patterns are important clues for diagnosing internal defects. For example, a three-phase winding temperature imbalance may indicate a fault such as a three-phase load imbalance or the presence of a turn-to-turn short in one phase. Trend analysis of long-term temperature data is the basis for assessing transformer health and performing predictive maintenance.

4. What malfunctions can be caused by elevated temperatures?

Persistent or sharp temperature increases, if not intervened in time, may directly or indirectly lead to the following serious malfunctions:

• Accelerated aging and carbonization of insulation: Solid insulating materials (e.g., Nomex paper, epoxy resins) become brittle, shrink, and carbonize at excessive temperatures, losing their original insulating strength and mechanical support.
• Winding turn-to-turn short circuit:: Insulation deterioration may first occur in the weak insulation layer between the turns of the conductor. Once this layer of insulation fails, a turn-to-turn short-circuit is formed, which generates a huge short-circuit current and leads to rapid burnout of the windings.
• Damage to the mechanical structure:: Conductors and cast bodies of epoxy resin cast windings have different coefficients of thermal expansion. Repeated over-temperature cycling can lead to significant mechanical stresses between the two, which may cause cracking of the casting body and create conditions for moisture ingress and insulation breakdown.
• Connection point overheating and burnout:: Poor contact between the winding leads and terminals and other connection parts will generate additional resistance heat under high currents, resulting in localized hot spots, which can seriously burn the connections and cause open-circuit failures.

5. What are the types of technology available for hot spot temperature monitoring?

For hotspot monitoring of dry-type transformers, a variety of technical solutions exist with different principles, applicability and accuracy.

5.1 Embedded contact temperature measurement

Pt100 Platinum Resistance Thermometer

Working Principle: Based on the physical property that the resistance value of a platinum wire changes precisely, steadily and linearly with temperature. The temperature is converted by measuring its resistance value.
vantage: Pt100Mature technology, good linearity, high accuracy and relatively economical cost. It is very suitable for monitoring the core, the outer surface of the winding, clamps and other parts without high potential, and is the most commonly used type of sensor for standard temperature controllers for dry-type transformers.

Thermocouple

Working Principle: Based on the Seebeck effect, a loop made of two different conductor materials connected together produces a weak voltage that is related to the temperature difference when the temperatures of the two connection points are different.
vantage: Wide temperature measurement range and fast response speed. However, in transformer applications, because its output is a weak mV voltage signal, very susceptible to electromagnetic interference, and the need for cold-end compensation, so the application is not as extensive as Pt100.

Fluoroptic Temperature Sensing

Working Principle: A pulse of light is transmitted through an optical fiber to a fluorescent material at the end of the probe, and the temperature is demodulated by measuring the fluorescence decay time. The decay time is a function of temperature and is completely independent of light intensity and electromagnetic field.
vantage: Fluorescent Fiber Optic SensorIt is the only technology that can safely and accurately measure real hot spots directly inside the winding. Its probes and fiber optic cables are composed entirely of dielectric material with aPerfect Electromagnetic Immunityrespond in singingHigh-voltage insulationThis is its fundamental advantage over all other technologies. This is its fundamental advantage over all other technologies.

5.2 Non-contact temperature measurement

Infrared Thermography

Working Principle:: Any object with a temperature above absolute zero radiates infrared radiation. An infrared camera calculates the temperature distribution of an object by detecting the intensity of infrared radiation on its surface.
vantage: It is capable of providing a two-dimensional temperature cloud over the entire surface of the transformer, allowing for the visualization of abnormal hot spots in areas such as external connection points and winding surfaces. However, itUnable to measureHot spots inside the winding encapsulated by solid epoxy resin.

Wireless Passive Temperature Sensing

Working Principle: Based on surface acoustic wave (SAW) or RFID technology. The sensors do not require batteries, they receive electromagnetic energy emitted by an external reader and feed back a signal containing temperature information.
vantage: No wiring is required. However, it is still an electronic device, and its stability under strong electromagnetic fields and the limitation of its mounting position limit its application in hot spot monitoring inside the winding.

6. Comparison table of major temperature measurement technologies

7. Components of a dry-type transformer temperature monitoring system

A complete dry-type transformer temperature monitoring system, especially one based on advanced fluorescent fiber optic technology, typically consists of three components:

1. Sensing layer: temperature sensor
   This is the front end of data acquisition. For full-scale monitoring, a mixed multi-point arrangement is usually performed: pre-built in hot spots inside the three-phase windingsFluorescent Fiber Optic Probes; mounted on the outer surface of the core, windings, etc.Pt100 SensorThe
2. Acquisition and Processing Layer: Intelligent Thermostat/Data Acquisition Mainframe
   This is the core brain of the system. It is responsible for connecting all sensors, demodulating (fiber optic) or acquiring (Pt100) the signals, and displaying the temperatures of all measurement points in real time through the LCD screen. What's more, it has built-in control logic such as:
   • Fan control: Automatically starts and stops the cooling fan based on a set temperature point (usually based on the highest phase's winding hot spot temperature).
   • Alarms and Protection: Provide multiple programmable relay dry contacts for over-temperature alarm and over-temperature trip signal output.
3. Communications and application layer: remote interface
   In order to realize intelligent operation and maintenance, hosts usually have standard communication interfaces:
   • Analog output (4-20mA): Transmit critical temperature points as standard analog signals to a PLC or DCS system.
   • Digital communication (RS485/Modbus): Transmit the temperature and equipment status of all measurement points to the backstage monitoring system by digital signals to realize remote centralized monitoring and data analysis.

8. Frequently Asked Questions (FAQ)

1. What is a dry-type transformer “hot spot”? Why is it so important?

The hot spot is the point in the transformer winding that has the highest temperature in operation. Due to uneven cooling and leakage field distribution, it is usually located in the upper part of the winding near the outlet end. The rate of aging of the insulation is determined by the temperature at this point, so accurate monitoring of the hot spot is key to ensuring the life and safety of the transformer.

2. How is the fluorescent fiber optic probe installed inside the winding?

This has to be done during the manufacturing process of the transformer. The fiber optic probe is designed to be mechanically strong enough to be wound and secured directly between specific turns or layers of the winding during the winding process, and then epoxy cast or VPI impregnated along with the winding to eventually become part of the winding structure.

3. Can't Pt100 be used to measure winding temperature?

Pt100 is a metallic conductor and cannot come into direct contact with windings carrying high voltages. It can be pre-buried on the outside of the winding (e.g. in the interlayer insulation or on the winding surface), but this is not a real hot spot and the measured value will be 10-20°C or even more below the real hot spot temperature, with the risk of insufficient safety margins.

4. How many temperature measurement points are normally required for a transformer?

This depends on the capacity and importance of the transformer. A typical configuration is: 1 fiber optic temperature measurement point in the hot spot area of each of the three phases (A, B, C) of the LV winding, and 1 fiber optic temperature measurement point in the hot spot area of each of the three phases of the HV winding, for a total of 6 fiber optic points. In addition, 1-2 Pt100 points are arranged on the iron core, and 1 Pt100 is used to monitor the ambient temperature, constituting a comprehensive monitoring program.

5. Based on which temperature should the cooling fan be controlled?

The most scientific and efficient control is based on the true hot spot temperature** of the highest of the three-phase windings**. This ensures that the fan is only activated when absolutely necessary, enabling precise thermal management and energy savings.

6. Are thermal imaging cameras still useful for routine inspections?

Very useful. Although it can not measure the internal hot spot, but it is the most effective tool to quickly inspect the winding surface, the core, especially the terminals and other external connection points for abnormal heat, is a powerful complement to fiber optic temperature measurement.

7. Does the fluorescent fiber optic temperature measurement system require periodic calibration?

Not required. The technology is based on stable physical principles. The temperature characteristics of the fluorescent material are precisely calibrated at the factory and solidified in the host computer, providing excellent long-term stability and eliminating the need for recalibration throughout the life of the transformer.

8. How can temperature data help me with predictive maintenance?

By monitoring data on hot spot temperature and load current over time, you can model the health of your transformer. If you find that the hot spot temperature rise is higher than before under the same load and ambient temperature, this may indicate early defects such as clogged cooling ducts, fan failures, or internal turn-to-turn short circuits, thus guiding you to targeted maintenance.

9. What is “dynamic capacity building”? How does temperature monitoring support it?

Dynamic capacity increase refers to safely allowing a transformer to operate above its nameplate rated capacity for a short period of time based on real-time hot spot temperatures and cooling status. Only through direct, real-time hotspot monitoring, such as fluorescent fiber optics, can we accurately understand the current thermal margin of the transformer, thus providing a reliable data basis for decision-making on dynamic capacity increase.

10. What are the most important considerations when selecting a temperature monitoring system?

Most important are **applicability** and **reliability**. For hot spots inside the winding, fluorescent fiber is the only safe and reliable choice. For other locations, Pt100 is the proven and economical choice. The system chosen must have industrial grade reliability and be able to operate stably for long periods of time in substations with strong electromagnetic interference and wide temperature differences.

Why choose Inotera's dry-type transformer temperature monitoring solution?

INNOTD (Fuzhou) Sales Limited (INNOTD) We are the leading supplier of harsh environment temperature monitoring solutions in China, and we have deep technical accumulation and rich application experience in the field of dry-type transformer hot spot monitoring.

• Highly reliable industrial grade products: OurFluorescent Fiber Optic Temperature Measurement SystemDesigned specifically for transformer applications, from the probe's voltage rating and mechanical strength to the mainframe's anti-electromagnetic interference capability, all have been subjected to the most stringent testing and validation to ensure stability and reliability throughout the entire life cycle.
• Accurate temperature measurement performance: We use high-quality sensing materials and advanced demodulation algorithms to ensure superior measurement accuracy and long-term stability, providing you with data you can trust.
• Complete system integration program: Our temperature measurement hosts offer a wide range of interfaces and standard communication protocols, and can be used with ourTransformer Intelligent ThermostatSeamless integration provides you with a one-stop solution from data collection to intelligent control.
• Professional application support: Our technical team has a deep understanding of the manufacturing process and operating characteristics of dry-type transformers, and is able to provide transformer factories and end-users with professional technical support for the entire process, from the optimization of the sensor layout, installation guidance to the commissioning of the system.

By choosing Inotera, you are choosing an accurate, reliable and intelligent temperature safety guard for your critical dry-type transformer assets.

The content of this article is only a general technical science and does not represent the performance and specifications of any specific product of our company. For detailed product information, solutions and quotations, please be sure to contact us for...].

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