When selecting a transformer for industrial or commercial applications, it's essential to consider not only its rated capacity but also its ability to handle overload conditions. One of the key aspects that influence the performance and longevity of a transformer is its permissible overload capacity—how much extra load it can safely handle beyond its rated power. In this article, we'll discuss the overload capability of dry-type transformers, why it's important, and how it impacts your transformer’s efficiency and lifespan.

Understanding Overload Capacity in Dry-Type Transformers

Overload capacity refers to a transformer's ability to handle short-term spikes in electrical load without causing damage to its internal components, such as the windings or insulation. Dry-type transformers are designed with certain tolerances for overload, which allows them to accommodate temporary increases in demand, especially in environments where power fluctuations are common.

The permissible overload percentage for dry-type transformers is generally specified by the manufacturer and depends on several factors, including the transformer’s thermal class, insulation system, and ambient conditions. Overload capability ensures that a transformer can continue to operate safely even when the load exceeds its rated capacity for brief periods.

The rated overloading of a transformer depends on the transformer's previous load, the corresponding windings or oil temperature at the beginning of the overloading.
Examples of the permissible duration and the respective levels of the acceptable over loadings are shown below for dry type transformers according to IEC 60076-12 and for transformer with thermal class 155°C (F).
For example, if the transformer is loaded with 50% of its rated power continuously, then the transformer can be overloaded to 150% or to 120%, only the time will be the difference. 

Typical Overload Percent for Dry-Type Transformers

For dry-type transformers, the overload capacity is typically in the range of 125% to 150% of the rated capacity. This means that if your transformer is rated for 100 kVA, it can generally handle a load of up to 125 kVA to 150 kVA for a limited duration (usually between 30 minutes to 1 hour). However, it’s important to remember that these are general guidelines, and the exact overload percentage may vary depending on the transformer’s design and intended use.

Here’s a breakdown of the typical overload limits for dry-type transformers:

• 125% Overload: This is the most common overload limit for dry-type transformers with Class 155 (F) thermal insulation. Under this scenario, the transformer can handle a load up to 125% of its rated capacity for a short period (usually 30 minutes).
• 150% Overload: Transformers with Class 180 (H) insulation, which are designed to handle higher temperatures, may permit a higher overload, often up to 150% for a short duration (typically 30 to 60 minutes).

These overload capacities are intended to provide flexibility and prevent unexpected failures during periods of high demand. However, sustained overloads beyond these limits can result in overheating, premature aging of insulation, and ultimately failure of the transformer.

Factors Influencing Overload Capability

While the overload percentages mentioned above are common, there are several factors that influence a transformer’s ability to safely handle overloads. These include:

1. Thermal Class and Insulation Type

The thermal class of a transformer refers to the temperature range that the insulation can handle without degrading. Common thermal classes for dry-type transformers include Class 155 (F) and Class 180 (H). Higher thermal classes allow the transformer to tolerate higher temperatures, which in turn supports greater overload capacity.

2. Ambient Temperature

The temperature of the environment in which the transformer operates plays a significant role in its overload capability. Higher ambient temperatures reduce the available overload capacity since the transformer is already operating closer to its thermal limits. Dry-type transformers are typically rated for a standard ambient temperature of 40°C (104°F), but adjustments need to be made for hotter or colder environments.

3. Cooling Mechanism

The cooling method employed in the transformer also affects its ability to manage overloads. Natural air cooling (AN) is the most common, but transformers can also be fitted with forced air cooling (AF) or liquid cooling systems, which help maintain lower operating temperatures and improve the overload capacity.

4. Load Duration

The duration of the overload is also critical. Most dry-type transformers are designed to handle short bursts of increased load, but sustained overloads can cause insulation damage and lead to premature transformer failure. The overload duration is often specified in the transformer’s datasheet, and exceeding this duration can result in overheating.

Consequences of Overloading Beyond the Permissible Limits

While transformers are designed to handle brief overloads, sustained overloading or exceeding the permissible overload capacity can lead to several problems:

1. Insulation Breakdown

Overloading generates excessive heat within the transformer, which can cause the insulation to degrade faster than normal. This reduces the transformer's lifespan and increases the likelihood of a short circuit or failure.

2. Overheating of Windings

Excessive load can overheat the windings, leading to deformation and damage. Once windings become damaged, the transformer’s efficiency declines, and it may fail prematurely.

3. Reduced Efficiency

When a transformer operates above its rated capacity, it can become less efficient. This results in higher energy losses, which can impact the overall cost of operation for a business or facility.

4. Potential for System Failure

Overloaded transformers can strain the electrical system, potentially leading to failures in connected equipment or electrical faults that can disrupt operations or cause costly downtime.

How to Ensure Safe Overloading Practices

To ensure your dry-type transformer operates within safe overload limits, follow these best practices:

• Monitor Load: Keep track of the load on the transformer to avoid prolonged periods of overloading.
• Use Proper Cooling: Ensure that the transformer has adequate cooling (whether air or forced cooling) to maintain safe operating temperatures.
• Check Operating Conditions: Regularly assess the transformer’s environment, including ambient temperature and ventilation, to ensure it’s within recommended limits.
• Perform Regular Maintenance: Regular inspection and maintenance of the transformer can help detect issues like overheating or insulation degradation before they lead to failure.

Conclusion

The permissible overload capacity for dry-type transformers is a key factor in ensuring reliable performance during periods of high electrical demand. With typical overload limits ranging from 125% to 150%, dry-type transformers are built to handle temporary load increases, but they are not designed for sustained overloading. To maximize the lifespan and efficiency of your transformer, always adhere to the manufacturer's recommended overload limits, monitor environmental conditions, and perform regular maintenance.

At Varelen, we understand the importance of transformer performance in demanding applications. Our range of high-performance dry-type transformers is designed to offer reliable, efficient, and safe operation, even under fluctuating load conditions. If you have any questions about overload capabilities or need help selecting the right transformer for your needs, feel free to reach out to our team of experts.