Understanding Dry Type Transformer Temperature Rise

The dry-type transformer temperature rise—C-rise —is one of its most critical specifications. It indicates the allowable range before overheating occurs and shows how efficient the specific unit is. As you will learn below, buyers selecting these transformer types must understand this rating and its implications on the device’s lifespan. Let’s begin with its definition.

What is Transformer Temperature Rise?

A transformer temperature rise is the change it experiences above maximum ambient temperature when it operates at full or nameplate load. You can find this specification indicated in the product’s datasheet.

When under load, the windings and core generate heat due to various losses. The heat must not exceed the allowable limit. If it does, the insulation may break down, leading to faults.

Therefore, users must fully understand the rated rise of their transformers and how it affects the device’s lifespan. This rating is also different across transformer types. This insight will solely focus on the dry transformers.

Dry Type Transformer Temperature Rise

The rated dry-type transformer temperature rise is the expected increase during operation. The rise depends on the effectiveness of the windings’ insulating material. Consequently, the insulation is grouped into various classes and named using the letters of the alphabet.

Each insulation type or class represents a rated range within which heat can increase safely. Three of the most used classes are E, F, and H. Let’s see what each means regarding temperature limits.

• Class E Insulation – less than 75k at 120 degrees Celsius
  
• Class F Insulation – less than 100k at 155 degrees Celsius
  
• Class H Insulation – less than 125k at 180 degrees Celsius
  

Other classes include A, B, and C. However, these are not common for air-cooled transformer windings. In the next section, we will see how to use the C-rating to determine allowable heat limits.

Dry Type Transformer Temperature Rise Ratings

These devices use natural or forced air to cool, which makes them different from the liquid-cooled or oil types. They are typically designed in three main ratings depending on the insulation class: 80°C, 115°C, and 120°C.

The above ratings are specified using a 40°C maximum ambient temperature. When calculating the allowable limit for winding temperature, add the specific C-rise rating to this temperature. Here is what that means:

• A dry or air-cooled unit rated 80°C when under full load operates at 120°C (40+80).
  
• The 115°C type, on the other hand, operates at 155°C
  
• At a 150°C rating, the average temperature is 190°C
  

The insulation used in most dry transformers has a 220°C rating, which means the windings still have a measurable margin or allowance before the heat can reach unsafe levels. As you can see, a lower C-rise provides a broader margin.

How Does Temperature Rise Rating Affect Dry Transformers?

A rise above the dry transformer’s ambient limit profoundly impacts its performance and lifespan. As such, users must understand what a high or low rating means for their device—and the electrical system in general.

Efficiency Level

The efficiency level depends on the C-rise rating. A higher value means it generates too much heat. It also means a smaller allowance for overloading. Based on this fact, a lower rise value indicates a more efficient unit and vice versa.

Overload Capacity

At lower ratings, it has a higher overload capacity. Let’s take an 80°C rated type as an example. Let’s also assume 220°C insulation. At this rating, the unit has a 70°C capacity before it can reach its maximum temperature. On the other hand, a 115°C unit would only have a 40°C reserve capacity.

Application Environment

Because units with lower C-ratings generate less heat, they have higher safety capabilities. This property makes them usable in confined spaces with limited ventilation, such as electrical rooms, underground vaults, and other indoor spaces. Higher-rated types are mainly suited for outdoor uses.

Estimated Lifespan

Heat is a significant cause of damage and accelerated aging. It usually results from excessive heat damaging the insulation and causing electrical faults such as short circuits. If the windings run at low temperatures, the insulation and other components last long, usually estimated at 20 years or longer.

Dry Type Transformer Overheating

Overheating means the temperature range exceeds what the device can handle or tolerate. This problem can have many causes besides causing aging. The causes are divided into internal and external, as explained below.

Internal Causes

• Core and winding losses such as hysteresis and Eddy current losses
  
• Insulation damage from aging or other causes
  
• High winding resistance caused by the type of material and size
  
• Bad tap changer contacts due to dirt, weak spring, etc.
  

External Causes

• Cooling system failure, including inoperable fans
  
• Inadequate ventilation in the installation room
  

Dry Type Transformer Temperature Monitoring

Excessive heat levels break down the insulation, especially if experienced over extended periods. Users must use monitoring technologies to sense, communicate, or regulate to prevent that. The monitoring systems can be of different types. They include:

• Mechanical dials or gauges
  
• Electronic temperature monitoring devices and controllers
  

These systems use heat sensors to work, with some offering more versatility than others. For example, an electronic controller has sensing probes to detect excessive heat or hot spots.

These controllers can set alarms or start cooling fans. The controller disconnects the supply if the level exceeds limits to avoid accidents.

In addition to using monitoring tools, users must conduct regular physical examinations and other predictive maintenance practices, including temperature rise tests, to ensure the unit’s overall health.

Conclusion

The dry-type transformer temperature rise rating is a crucial parameter that buyers must consider when sizing the device. Depending on the system loads, the rating may be low or high. Lower ratings mean a more significant margin for increase, allowing occasional overloads. In addition to this rating, proper monitoring is essential to extend the transformer’s lifespan.