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The Temperature Rise of Dry-type and Oil-immersed Transformers As Well As Their Temperature Rise Range
There are numerous factors that affect the service life of dry-type transformers and oil-immersed transformers Among them, temperature is the most crucial factor that leads to insulation aging and subsequently affects the lifespan of the transformers. Due to the uneven heat distribution within the transformers, there are significant temperature differences in various parts. Therefore, the allowable temperature rise for each part under the rated load is specified to ensure safe operation.
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I. Basic Principles and Calculations of Temperature Rise
Insulation class and maximum allowable temperature
The general oil-immersed transformers adopt class A insulation, and their maximum allowable temperature is 105℃.
- Calculation of allowable temperature rise (taking the coil as an example)
Allowable temperature rise = Maximum allowable temperature - Ambient temperature (with 40℃ as the reference point)
That is: 105℃ - 40℃ = 65℃ (Allowable temperature rise of the coil)
- The relationship between oil temperature and winding temperature
The temperature of transformer oil is usually about 10℃ lower than that of the windings. Therefore:
- The allowable temperature rise of the oil is: 65℃ - 10℃ = 55℃
- To prevent oil from aging, the temperature rise of the upper oil surface must not exceed 45℃
The actual operating temperature range
If the upper oil temperature is between 80℃ and 90℃, then the winding temperature will be approximately 95℃ to 105℃. Staying in this range for a long time will accelerate insulation aging.
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II. Hazards of High-Temperature Operation for Dry-Type and Oil-Immersed Transformers
Insulation material deterioration
Long-term exposure to high temperatures will cause the insulation paperboard to become brittle and prone to cracking, losing its insulation properties and increasing the risk of breakdown.
- Insulation oil aging
Severe aging of the winding insulation will accelerate the deterioration of the insulation oil, further shortening the overall service life of the transformer.
- Conclusion
It is advisable to avoid operating the transformer at high temperatures, especially for prolonged periods of high temperature operation.
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III. Classification of Transformer Insulation Grades
The insulation class indicates the heat resistance performance of the insulating material, and different classes correspond to different maximum allowable temperatures.
Oil-immersed transformers (with 7 grades)
A, E, B, F, H, N, C
- Dry-type transformers (3 different grades)
B, F, H
Take Class A as an example. Its meaning is: The sum of the transformer's operating temperature and the ambient temperature should not exceed 105℃; for other classes, the principle is the same.
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IV. Temperature Rise Limit Diagram (Explanation)
- The temperature rise limit for oil-immersed transformers is shown in the diagram.
| Transformer Insulation Grades | A | E | B | F | H | N | C |
| Maximum allowable temperature(℃ ) | 105 | 120 | 130 | 155 | 180 | 200 | 220 |
| Winding temperature rise limit(K) | 60 | 75 | 80 | 100 | 155 | / | 150 |
- The temperature rise limit for dry-type transformers is shown in the diagram.
| Transformer Insulation Grades | B | F | H |
| Maximum allowable temperature(℃ ) | 130 | 155 | 180 |
| Winding temperature rise limit(K) | 80 | 100 | 125 |
Note: The magnitude of the temperature rise limit directly reflects the heat resistance performance of the insulating material, and is an important basis for selection and operation monitoring.
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V. Core Security Principles
As long as the temperature rise does not exceed the permitted limit, regardless of how the ambient temperature changes, the transformer can operate safely within the specified service life.
