In the field of power systems, temperature gauges for transformer ratings are more than just a numerical value; they define the limits of the thermal performance of a transformer when it comes to safe and efficient operation. These ratings cover concepts such as power transformer temperature rise, and winding temperatures, as well as insulation material properties such as Class H insulation. A correct understanding of these parameters not only optimizes transformer performance but also extends its service life and prevents overheating failures.
This article will take an in-depth look at everything from transformer insulation temperature of transformer ratings to dry type transformer temperature rise monitoring, including how to use transformer thermometers and analyze time-temperature transition diagrams. Whether you are in maintenance, design, or operations, this article will provide you with practical knowledge and a forward-looking perspective.
Figure 1 transformer temperature rise
What are Transformer Temperature Rise?
Transformers generate heat during operation due to winding resistance and core losses. The transformer insulation temperature rating defines the maximum amount of heat a transformer can withstand to ensure structural and functional integrity.
Key indicators of Transformer Temperature Rise
Figure 1-1 transformer rated
Temperature Rise：
Temperature rise is the portion of the transformer’s internal temperature that is higher than the ambient temperature during operation. It reflects the thermal performance of the transformer and has a direct impact on insulation aging, load capacity and efficiency. Excessive temperature rise accelerates insulation ageing, shortens equipment life and increases energy loss. Controlling temperature rise improves safety and extends life while optimising transformer performance. Typical ratings are 30°C to 40°C.
Hot Spot Temperature
Hot spot temperature is the highest temperature point in the transformer winding, reflecting the heat load concentration when the transformer is running. It directly affects the rate of insulation aging and equipment life, and excessive hot spot temperature can lead to deterioration or even failure of insulation materials. Controlling the hot spot temperature can improve the safety and reliability of the transformer, and at the same time enhance the load adaptability and extend the service life of the equipment.
Winding Temperature
Winding temperature refers to the transformer temp rise winding produced by the passage of current during operation. It reflects the thermal performance of the transformer, and has a direct impact on the aging rate of insulation materials and equipment life. The high winding temperature will accelerate insulation loss and reduce the reliability and service life of the transformer. By monitoring and controlling the winding temperature, it can effectively optimize the operating condition of the transformer and ensure the safe and stable operation of the equipment.
Oil Temperature in Transformer
Transformer oil temperature is the transformer temp rise of the insulating oil when the transformer is running, which is used to reflect the working condition of the cooling system and the thermal performance of the equipment. High oil temperature will reduce the performance of the insulating oil, accelerate the winding insulation aging, and affect the thermal efficiency of the equipment. By monitoring and controlling the oil temperature, the normal operation of the transformer can be ensured and the service life of the equipment can be prolonged while preventing failures caused by overheating.
How to Check Temperature Ratings from the Nameplate
The temperature information on the nameplate, including kVA, ambient temperature, average winding temperature rise, and insulation rating, provides comprehensive thermal performance parameters. By interpreting the nameplate, the user can ensure that the transformer is operating within safe limits and avoiding overheating damage, whilst prolonging the service life of the equipment.
Figure 2-1 what is kva
Transformers are rated using power capacity (kVA or MVA), voltage, frequency, and insulation class to indicate their operational limits and performance.
Transformers are rated in kVA or MVA, indicating their power-handling capacity under specified conditions. The following are the main methods of viewing the temperature ratings:
Why transformer are rated in kva?
Figure 2-1 define kva
The nameplate usually indicates the rated capacity of the transformer in kVA and the corresponding temperature rise (e.g. 55°C or 65°C). How is transformer rated? The temperature rise is the range of temperature increase of the insulation of transformer windings at the rated load. For example, ‘1500 kVA, 65°C temperature rise’ means that the temperature of the windings of the transformer at rated load is 65°C above the ambient temperature. kVA ratings on nameplates are usually related to the following factors:
- Ambient temperature: the nameplate usually assumes an ambient temperature of 30°C (40°C maximum).
- Load: The load level determines the winding heat generation and affects the overall temperature rise.
- Average Winding TemperatureRise (AWR): The average increase in winding temperature relative to the ambient temperature.
Hot spot temperature: maximum temperature of the hottest winding. The hottest transformers deliver high efficiency, advanced cooling, and cutting-edge design for demanding applications.
Transformer Environmental Temperature
Figure 2-2 environmental temperature
The ambient temperature is the most common parameter on the nameplate and is usually 30°C (standard value) or 40°C (limit value). Without load, the winding temperature is close to the ambient temperature. At full load operation, the ambient temperature is superimposed on the temperature rise and determines the final temperature of the winding.
Transformer Average Winding Temperature Rise (AWR)
Figure 2-3 transformer temperature rise rating
AWR=95°C−30°C=65°C The nameplate will indicate the average temperature rise of the windings, usually 55°C or 65°C.
This indicates the degree to which the temperature of the windings is above the ambient temperature at the rated kVA load.
Example:
A transformer with a nameplate marked ‘65°C Temperature Rise’ has a winding temperature of 95°C at an ambient temperature of 30°C.
Assumptions:
- Winding temperature (tested): 95°C
- Ambient temperature: 30°C
- AWR = 95°C-30°C = 65°
At this point, the transformer has an average winding temperature rise of 65°C, which is usually labeled as the temperature rise rating on the nameplate.
Hot Spot Temperature
Figure 2-4 hot spot temperature transformer
The hot spot temperature is the location of the highest temperature inside the winding, usually 10°C to 15°C above the average temperature rise. hot spot temperatures are not directly labelled on the nameplate, but can be calculated using the following formula:
Hot spot temperature = ambient temperature + average temperature rise of the windings + hot spot correction value
The hot spot temperature is a key indicator of transformer life. Elevated hot spots accelerate insulation ageing and shorten the service life of the equipment.
Insulation class
Figure 2-5 impact insulation class
The insulation class on the nameplate (e.g. class A, F or H) determines the maximum withstand temperature of the transformer windings. Example:
| Insulation Class | Temperature Limit (℃) | Commonly Used Insulation Materials |
| Transformer Class A rise | 105 | Cellulose materials treated by impregnation (such as insulation paper used in transformers, paperboard, etc.) |
| Class E rise | 120 | Polyester film, Polyester fiber paper, etc. |
| Class B rise | 130 | Mica, Fiberglass, etc. |
| Insulation Class F rise | 155 | Epoxy resin, Nomex paper, etc. |
| Class H rise | 180 and above | Silicone rubber, Ceramic fiber, etc. |
| Class C rise | 220 | Polyimide film, Mica products, and other high-temperature resistant materials |
A Class F insulation system can withstand a maximum operating temperature of 155°C, offering reliable thermal performance. Temperature classification defines the maximum operating temperature a transformer’s insulation system can withstand for reliable performance.
Typical materials used in a Class F insulation system include epoxy resins, mica, fiberglass, and polyester films, withstanding temperatures up to 155°C.
Typical materials used in a Class H insulation system include silicone resins, mica, fiberglass, and epoxy-based compounds, capable of withstanding temperatures up to 180°C. Transformer heating occurs due to electrical losses in windings and cores, requiring effective cooling systems to maintain performance and safety.
Dry-type transformers vs. oil-immersed transformers Transformer temperature ratings
| Comparison Dimension | Dry-Type Transformer | Oil-Immersed Transformer |
| Insulation Material Temperature Rise Limit | 155°C (Class F insulation) or 180°C (Class H insulation) | 95°C (Class A insulation) or 105°C (Class B insulation) |
| Winding Temperature Limit | 180°C (Class F) or 200°C (Class H) | 115°C (Class A) or 120°C (Class B) |
| Transformers Hotspot Temperature Rise Limit | ≤30°C | ≤10°C |
| Cooling Method | Natural air cooling (AN) or forced air cooling (AF) | Natural cooling (ONAN) or forced oil circulation cooling (OFAF) |
| Cooling Efficiency | Lower efficiency, requires better ventilation | Higher efficiency, utilizing oil circulation for heat transfer |
| Heat Resistance | Higher, suitable for high-temperature environments | Lower, performance may be limited in high-temperature conditions |
Three key insulating materials for transformers
In maximum transformers, the choice of insulating materials is critical, which directly affects the temperature rise insulation performance, insulation effect and operating life of the equipment. Common insulating materials include insulating oil, liquid insulating materials (e.g. natural ester) and solid insulating materials (e.g. electrical insulating paper). The following is a detailed description of these three materials and their role in transformers:
Transformer Insulating Oil
Figure 3-1 insulating oil
What is insulating oil?
Insulating oil is the most traditional insulating and cooling medium in transformers. It is usually made of mineral oil and has excellent electrical insulation and thermal conductivity properties.
How does insulating oil affect a transformer?
- Transformerinsulating oil provides efficient cooling capacity to reduce the temperature rise of the windings and core.
- Transformerinsulating oil In the event of localised overheating, the insulating oil effectively disperses the heat and prevents hot spots from becoming too hot.
- Transformerinsulating oil is used as a liquid insulating medium in combination with paper insulation to form a solid insulation system.
Why is insulating oil important?
Insulating oil is critical to the stable operation of oil-immersed transformers. High quality mineral oils are not only less costly but also have a rich application history and extensive performance data to support them.
Commonly used materials:
- Mineral oil: the most widely used insulating oil, offering good insulation and thermal conductivity at a lower cost.
- Synthetic ester oils: Synthetic esters offer higher oxidative stability than mineral oils but at a higher cost.
- Silicone oils: suitable for high temperature environments and have excellent thermal stability, but are expensive.
Characteristics and applications: Mineral oils are the most common insulating oils used in conventional oil-immersed transformers. Synthetic ester and silicone oils, on the other hand, are mostly used for special applications, such as high temperature or environmentally demanding scenarios.
Liquid Insulation Material
Figure 3-2 liquid insulation
What are liquid insulating materials?
Liquid insulating materials include new environmentally friendly insulating media such as natural esters (e.g. FR3®) and synthetic ester oils. They are often an alternative to oil-immersed transformers, especially in environmentally demanding scenarios.
How do liquid insulating materials affect transformers?
- Thermal performance enhancement: natural esters have a higher flash point and are able to maintain stability at high temperatures.
- Environmental protection enhancement: natural esters are biodegradable, reducing environmental pollution.The hgtransformers temperature are designed to operate efficiently within specified temperature limits, ensuring reliability and thermal stability. .
- Insulation paper protection: natural esters can absorb moisture, thus improving the life of solid insulation.
Why is liquid insulation important?
Liquid insulation combines high performance with environmental friendliness, making it an ideal alternative to traditional mineral oils. The use of natural esters is growing rapidly in new energy projects or in ecologically sensitive areas.
Solid Insulation Material
Figure 3-3 insulation paper
What is solid insulation?
Solid insulating materials consist mainly of electrical insulation paper such as traditional kraft paper or modified aramid paper (Nomex®). They are used to wrap windings or as a support material to provide efficient insulation.
How do solid insulating materials affect transformers?
- Excellent insulating properties: electrical insulating paper combines with oil or liquid to form a composite insulating system.
- Good thermal stability: high-temperature insulating papers can withstand higher hot spot temperatures, e.g. aramid paper can withstand 220°C.
- Strong mechanical properties: insulating paper can maintain good strength and stability under high temperature and high pressure.
Commonly used material for solid insulation:
- Kraft Paper(Kraft Paper): traditional insulating paper made from wood pulp for standard oil-immersed transformers.
- Aramid Paper (Nomex®): a high-performance insulating paper, resistant to high temperatures and ageing, widely used in dry-type transformers.
- Epoxy resin: used for casting and moulding of dry-type transformers, with high mechanical strength and moisture resistance.
- Ceramic Insulation: Provides excellent heat and arc resistance in ultra-high voltage transformers.
Characteristics and applications: Kraft paper is the most economical choice and is mostly used in conventional transformers. Whereas aramid paper and epoxy resins are suitable for high temperature or dry transformer scenarios, ceramic insulations are more commonly found in speciality transformers.
Why are solid insulation materials important?
Solid insulation is the core component of the internal insulation of a transformer, especially when subjected to high voltages and complex loads. The use of high quality insulation paper extends the life of the transformer and enhances the reliability of the equipment.
What is the effect of transformer temperature ratings on transformers?
Figure 4-1 transformers overload
Effect of temperature rise ratings on transformer overload transformers capacity
- Core point: the lower the temperature rise rating, the greater the transformer overload capacity.
- Examples: a dry-type transformer with 220°C insulation class has no excess transformer overload capacity with a 150°C temperature rise design; if designed for an 80°C temperature rise, the hot spot temperature transformer can be reduced to 150°C, providing 47 percent continuous overload capacity; a 115°C temperature rise design can provide 19 percent continuous overload capacity. If the transformer is cooled by forced air (e.g. a fan), the overload capacity can be increased by an additional 33 per cent, but only for short periods of use, not continuous operation.
- In summary: Low temperature rise designs increase overload capacity by reducing hot spot temperatures and providing more thermal headroom for the windings.
Temperature Rise Ratings on Transformer Life
- Lower hot spot temperatures slow down the rate of thermal ageing of the winding insulation.
- Lower hot spot temperatures mean less thermal stress on the insulation, so the rate of aging slows down significantly.
- The life of the insulation is extended and the overall life of the transformer is increased. All values of a transformer are proportional to its power rating. The trafo room is designed to house transformers, ensuring safe operation and efficient cooling.
- Reducing the temperature rise not only extends the life of the insulation material but also improves the long-term reliability of the transformer.
Effect of temperature rise rating rise on transformer efficiency
Core point: Low temperature rise design improves the load efficiency of the transformer.
Efficiency Boost Principle:
- The optimum operating efficiency point for a transformerrating is usually around 50% of the load capacity.
- With a low temperature rise design (e.g. 80°C or 115°C), the transformer performs closer to this efficiency sweet spot under heavy load.
- The additional load capacity maximises operating efficiency.
Long-term benefits: Despite the higher initial cost of a low temperature rise design, the long-term benefits are significant through reduced energy consumption and extended equipment life.
In summary: By reducing temperature rise, transformers can be operated in a more efficient manner, especially in application scenarios where loads are highly variable.
Conclude
Keeping track of safe transformer temperature ratings is key to ensuring the safety and efficiency of power systems. The industry is innovating through the use of advanced tools and materials such as transformer thermometers and Class H insulation.
FQA
When to use type from class transformer?
Temperature standards for transformers ensure safe operation by defining maximum allowable limits for winding, core, and insulation temperatures.Transformers are rated according to their power capacity, voltage, frequency, and cooling class method.
How transformers are rated?
Transformer ratings based on their power capacity (measured in kVA or MVA), which indicates the maximum load they can handle without exceeding thermal limits. The rating also considers:
- Primary and Secondary Voltage: Specifies the input and output voltage levels.
- Frequency: Indicates the operating frequency (typically 50 Hz or 60 Hz).
- Cooling Method: Defines how the transformer dissipates heat (e.g., AN, AF, ONAN, OFAF).
- Insulation Class: Determines the maximum temperature the insulation system can handle.
- Phase Type: Specifies whether the transformer is single-phase or three-phase.
These ratings are crucial for selecting a transformer suitable for specific applications while ensuring safe and efficient operation.
What is the hot spot temperature transformer?
The hotspot transformers is the highest temperature point within the winding, critical for assessing insulation lifespan and performance.
No, a time-temperature transformation (TTT) diagram, also known as a time-temperature transformation diagram for steel, illustrates the relationship between time, temperature, and phase transformation in steel during heat treatment. time temperature transformation ttt diagram