;)
The types of insulating materials used in the construction of transformer windings are what determine the maximum working temperature of the equipment.
The greater suitability and temperature resistance of the insulating materials used will determine the classification of their thermal class, establishing the maximum temperature of the hottest point of the windings and the temperature elevation of the same with respect to the ambient temperature.
The IRAM 2276 / IEC 60076-1 standard establishes the following classification in this regard:
| Insulation System Temperature | Average heating limit of windings with the assigned current |
| °C | K |
| 105 (A) | 60 |
| 120 (E) | 75 |
| 130 (B) | 80 |
| 155 (F) | 100 |
| 180 (H) | 125 |
| 200 | 135 |
| 220 | 150 |
This table sets out the maximum heating limits in transformer windings under normal operating conditions. In this way, the temperature limit values in windings are indicated in the IRAM 2276 / IEC 60076-11 standards. On the other hand, its classification and thermal evaluation are treated in the IEC 60085 standard.
The maximum temperature that occurs in any part of the winding insulation system is what is called the hottest point temperature, according to the IEC 60076-12 standard, which establishes the aging of the insulating materials.
The temperature at that point must not exceed the value set out in the first column of the preceding table (“Insulation System Temperature”).
The second column of the table (“Average heating limit”) reflects the temperature elevation of the windings, in relation to the temperature of the cooling or ambient air of the equipment. The unit of measurement is °C, but by convention it is identified with K, to avoid confusion with the insulation temperature.
It is also established that the core of the transformer, the metal parts and accessory elements must not reach a temperature that could cause damage to the equipment.
The temperature class of the insulation determines the constructive form of the equipment and must be considered when designing it.
The higher the class, the transformer will support higher working temperatures, making it possible to use higher current densities in the windings and making them more compact.
In situations where there are space restrictions at the installation site, it is advisable to use equipment with higher thermal classes, given their smaller size.
There are also situations where, given the type of service to which the equipment will be subjected, the user decides to use a thermal class higher than that necessary as a power reserve, in the face of higher specific demands, taking into account the increase in short-circuit losses of the machine.
Statistically, the most widely used thermal class in the region for this type of transformer is F (155°C with an over-elevation of 100 K) and, secondly, the H class (180°C with an over-elevation of 125 K).
| Insulation Class | Room Temperature | Hot Spot | Overlift | Total Temperature |
| F | 40°C | 15°C | 100°C | 155°C |
| H | 40°C | 15°C | 125°C | 180°C |
There are particular cases where the user finds a transformer of a certain class useful, but which has a lower temperature elevation than that established for it. For example, a transformer with class H of 180°C, but which has an elevation of 100 K corresponding to a class F equipment.
This condition implies that the equipment is built with materials in accordance with the insulation level of class H, but that at nominal power it will not exceed 100 K of elevation. This, added to the maximum allowed ambient temperature of 40°C and the 15°C that the standard establishes as the difference to the hottest spot (Hot Spot), results in 155°C of total temperature (25°C below the maximum of class H).
This declassification of the equipment allows it to work well below the insulation limit, allowing for less insulation degradation and extending the life of the equipment beyond what is determined by the standard.
Additionally, declassification would be useful when the transformer is subjected to ambient operating temperatures higher than those allowed by the standard, or in situations where the load has a high or unknown harmonic content at the time of installation of the equipment. The additional heating generated by these situations in the transformer windings could be easily supported by that available reserve.
Given the integration of all the calculation, design, manufacturing, testing and certification processes that our company has, we can offer any solution according to the needs of the client, satisfying the most demanding technical specifications, providing tailor-made solutions and collaborating to optimize the economic equation of the project.
Optimize Performance with Dry Isolation Transformers
The choice of insulating materials and the right thermal class, such as the popular F (155°C) and H (180°C) classes, are crucial to ensure the durability and efficiency of transformers, especially in environments with high temperatures or space constraints. Dry-insulated transformers, with their compact design and high thermal resistance, are ideal for minimizing insulation degradation and supporting harmonic loads, extending the life of the equipment. For customized solutions that comply with regulations such as IRAM 2276 and IEC 60076, discover Miron’s dry isolation transformers and quote the best option for your projects with us at Miron.
