What is Transformer Breathing?

What is Transformer Breathing?

Understanding the Natural Breathing Process in Oil-Filled Transformers

In power distribution systems, oil-filled transformers are common because they provide excellent insulation and cooling. These transformers exhibit a unique phenomenon known as “breathing”, which is essential for their safe and reliable operation.

Transformer breathing refers to the process where a transformer inhales and exhales air due to changes in oil volume caused by temperature fluctuations. It’s a natural response to thermal expansion and contraction of the insulating oil inside the transformer.

When a transformer operates under electrical load, the internal temperature rises. This causes the insulating oil to expand, increasing its volume.

As the oil expands, it pushes out the air present inside the conservator tank — a cylindrical oil reservoir mounted above the main tank. This is known as “breathing out.”

Later, when the transformer load reduces or it cools down during lower ambient temperatures, the oil contracts, reducing its volume.

This contraction creates a vacuum, drawing outside air into the conservator tank to compensate for the reduced oil volume. This is referred to as “breathing in.”

However, the incoming air may carry moisture, which can degrade the quality of the transformer oil and harm the insulation system.

To prevent this, transformers are equipped with a breather unit. This is a small chamber filled with silica gel, a desiccant material.

Silica gel absorbs moisture from the air entering the transformer. This ensures that only dry, clean air enters, preserving the oil’s insulating properties.

A small oil cup is also placed at the base of the breather. The air passes through this oil layer, which acts as an additional filter before reaching the silica gel.

As silica gel absorbs moisture, it changes color — usually from blue to pink — indicating saturation. Once saturated, it must be replaced or reactivated.

This breathing mechanism is critical because moisture contamination can reduce the oil's dielectric strength, leading to insulation failure or internal arcing.

Proper transformer breathing ensures that internal pressure remains balanced and that the insulation system stays dry and intact.

Some small or sealed transformers don’t have conservator tanks. Instead, they use nitrogen gas to balance pressure changes without air exchange.

In conclusion, transformer breathing is a simple yet vital self-regulating process that safeguards the transformer’s longevity and performance by managing pressure and moisture control effectively.