Transformer Breather: Everything You Need to Know

Oil immersed transformers have a simple system called transformer breather!

This system lets the transformer oil expansion process to happen during load peaks. Let’s discuss transformer breather in this article.

What is transformer breathing?

Transformer Breathing is the process happens due to oil expansion and contraction by the effect of temperature rise.

Current passing through transformer windings generates heat, the main job of transformer oil is to transfer this heat outside the transformer.

The oil temperature rises with the increase of the winding temperature, and makes the oil to expand.

Transformers breath in and out during oil expansion and volume reduction due to temperature rise and drop.

This transformer breathing process, pushes the air out and draw it in, this is called transformer breathing and out.

Transformer Breathing in and out process:

  • The temperature of the oil varies due to loads on the transformer secondary side. This temperature variation makes oil to expand, gets out to the conservator tank and pushes the dry air out of the tank through the transformer breather. This process is called transformer breathing out.
  • When the oil gets cooled down, its volume decreases and gets back to its normal level. The air from outside the transformer is drawn in to the transformer tank, this process is called breathing in.

What is Transformer Breather?

Transformer breather is a simple system to allow the expansion of the transformer oil. This system contains silica gel breather, to prevent moisture from entering into the transformer tank.

Silica gel is used in transformer breather because, it has the ability of moisture absorbing and holding water vapor.

This prevents water vapor and moisture from entering into transformer oil during the breathing in and out process.

A small amount of transformer oil is used in a small cab in the breather. Let’s find out why?

Why is Transformer Oil Used in Breather?

transformer breather oil cup
transformer breather oil cup

A simple, but effective protection method, for both transformer oil and silica gel, is using oil cap, or transformer breather oil cup, at the bottom of breather and serves two purposes.

  1. Firstly, it absorbs the dust particles that are present in the air during breathing in process.
  2. Secondly, it acts as barrier between silica gel and air when there is no breathing in or out.

Do All Transformers Have Breathers?

Not all transformers require breather and conservator tanks. 

Breathers are required for transformers in which the breathing in and out can cause the oil to come out of the transformer.

Dry type transformers has no oil and then no breather is required. Also hermetically sealed transformers are without breather. These type of transformers are completely sealed and has no connection to outside atmosphere.

For heat dissipation, Corrugated fins are used instead of breathers. Corrugated fins have enough flexibility to allow space for oil expansion

Similarly, Current Transformers (CT) and Potential Transformers (PT) which are dry type transformers, there is no need of breathers as temperature inside the transformers remains within the limit, and require no oil.

This leads us to another type of transformer oil protection system called hermetically sealed transformers. Let’s know more about it!

Why Hermetically Sealed Transformers require no breather?

Hermetically sealed transformers have no contact to the outside environment. These transformers are designed without any conservator tank, breather, or silica gel.

Hermetically sealed transformers require no breather because, its sealing is done in such perfection that, the oil expansion is allowed within the transformer body.

The absence of the transformer breather makes the  use of these transformers very common in environments where rain or snow percentage is quite high, also environments with high level of gases in air, and placing a transformer with a breather outside could cause destructive for transformer’s life.

Is Silica Gel Effective in transformer breather?

transformer breather oil cap and silica gel
transformer breather oil cap and silica gel

Silica gel breather is a way to allow the oil expansion and in the same time to prevent moisture from entering into the transformer.

Breather and Silica Gel protect oil from moisture during transformer breathing.

Silica gel plays the main role to protect transformer oil from moisture. It has a great ability to absorb moisture from air and passing on the dry air only into the transformer tank. So, it is effective and does its role perfectly.

Silica gel in transformer breather changes its color if it adsorbs moisture and water, so we are indicated to replace it.

Silica gel colors

White silica gel: is a non-indicating silica gel, which means no color changing will happen if it adsorbs water.

Blue silica gel: This type of silica gel has cobalt chloride which changes its color to to be pink after adsorbing water.

When it becomes pink this means it reached its adsorption maximized  capacity.

Orange silica gel: This is an other color indicating type which suitable to be used in silica gel breather of transformers.

Why to prevent moisture from entering into transformer?

If transformer breather has no silica gel to absorb moisture from the air which comes from outside atmosphere, then transformer oil may contain water particles, which decreases its insulation.

Passing air through the silica gel ensure that only dry air to enter the transformer. and protects transformer oil. The oil insulation degree decreases to a bad situation if it contains water, that why we use silica gel.

I have written a detailed article about transformer moisture causes and solutions. You can find it here.

When Should I Replace transformer Silica Gel?

damaged silica gel by bad oil
damaged silica gel by bad oil

It is recommended to change the silica gel at 1/3 stage of saturation because, waiting for complete saturation could lead to some moisture transferring inside the transformer.

Silica Gel keeps on absorbing moisture from the air flowing inside the conservator until it reaches a saturation point.

The saturation point comes at different times for different transformers, but the principle of saturation is the same.

Moisture starts from the bottom of the breather and keeps going upward, moisturizing all the silica present in the container.

For Orange silica gel, saturation level changes the color to transparent, whereas in purple silica, the saturation color would be pink.

If the silica gel absorbed by oil, specially after a fault like the one in the photo, you should replace it. In this case silica gel becomes bad.

The photo is of one 500KVA transformer in my workplace, it had a short circuit and the oil came out of the transformer through the silica gel breather. The silica gel became bad.

Can I reuse absorbed silica gel?

General advice is to at least maintain 2-4 inches of silica buffer dry. However, replacing silica is not necessary always, because the alternative option of drying out the moisture through specific level of heat is more cost efficient.

Drying silica gel is usually done under a temperature of 60°C for about 6 hours.

In case of absorbing oil, silica gel can’t be reused again, and should be replaced.

How to Replace power Transformer Silica Gel?

The conventionally used process for replacing silica gel from power transformers involves following steps:

  • First thing is to remove the conservator from the transformer and take out all the absorbent silica Gel present inside it.
  • Second step involves cleaning the glass cover to remove any dirt attached.
  • In the next step, new silica gel is filled into the conservator. At this stage, it is preferred to use silica gel without any color impurities, for monitoring the performance of absorber.
  • Finally, transformer oil used for absorbing impurities is poured into the seal cup attached down to the conservator. The level of oil is carefully maintained to the safe limit.

Can We Change Silica Gel of Energized Transformer?

I recommend never to change silica gel while the transformer is energized, specially for power transformers, for safety purposes.

However, in some cases experienced electricians can do this job while the transformer is in service. 

The main point to take into consideration is the safe distance between the breather and the power lines of the transformer. Besides, the expertise of the person is a vital key too.

Also, I never recommend doing this job during windy days and high moisture hours of the day. In general, its better to do the job indoors i.e inside a workshop if possible.