Why is transformer insulation system rating important?

Just like any other electrical device, transformers generate waste heat as a byproduct of their operation. The heat generated in the process typically increases the internal temperature of the transformer, which occurs when the average temperature of the windings is higher than the ambient (surrounding) temperature during normal full load. This often creates a huge challenge for design engineers who are trying to improve the reliability and performance of their products in the field. The situation is even worse if these products are deployed in extreme environments.

Since transformers with lower temperature rise ensure more efficient performance and vice versa, manufacturers aim to design transformers with lower temperature rise, and this is where thermal insulation systems come into play.

The insulation system is the maximum internal temperature that the transformer can withstand. The expected life of a transformer operating within the temperature range of its insulation system is usually 20-25 years, and higher temperatures are usually the main reason for reducing transformer life. In addition, insulation system failure will trigger the maximum transformer failure, so basically, any behavior that interferes with the internal insulation characteristics of the transformer will have an adverse effect on the transformer life.

At the maximum temperature rating, the insulation system rating is in degrees Celsius and can be defined as the maximum allowable operating temperature for the normal expected life of the transformer. These values ​​are based on a maximum ambient temperature of 40°C. The insulation rating is determined by the sum of the temperature rise, ambient temperature and hot spot margin (see figure below). For example, this means that in a 40°C ambient, when fully loaded, a transformer with a temperature rise of 80°C will operate at an average winding temperature of 120°C.

Graph showing maximum temperature limits in relation to insulation ratings

These maximum temperature limits are set by NEMA standards and exceeding these limits will shorten the life expectancy of the transformer.

Previously, insulation classes were indicated using a letter system. This has been changed to actual temperature ratings (Class A = 05°C Class, Class B = 150°C Class, Class F = 180°C Class, Class H = 220°C Class).

In addition to life expectancy, the insulation system rating also helps determine the overload capacity of the transformer. Lower temperature rise generally translates into higher overload capacity. Most dry-type transformers use the same insulation on their windings, which is typically rated for 220°C regardless of the design temperature rise. This is why an 80°C rise unit provides more room for occasional overloads than a 150°C rise unit without damaging the insulation or affecting the life of the transformer.

Additionally, when evaluating transformers and making purchasing decisions, insulation system rating can be an important factor considering the growing demand for energy efficiency in these devices.

The level of insulation that should be used in a particular transformer is certainly a key design consideration and should be considered along with other aspects such as voltage regulation, material cost and availability for optimal results.