Transformer Cooling

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8.1. Cooling methods

As an apparatus that is used for changing voltage in alternating current, it is a common phenomenon of transformers getting hot over a certain time period. This heating is further aided by the loss of copper and iron content from them. To ensure that temperature of windings are kept below a certain point, so that insulation does not deteriorate over a period, heat has to be dissipated from the body of the transformer.

Now here a comparison can be drawn regarding cooling of transformers with that of rotating machines. When machines are rotated, there is air flow created that is quite tumultuous in nature. It is this air flow that creates the required cooling effect and hence loss of heat acts as heat removal agent. In quite comparison to this, loss of heat from transformers is a difficult process, though this loss is quite low in terms of other apparatus.

Here is a detailed analysis of the ways in which transformers are cooled.

Types of transformers:

There are specifically 2 types of transformers that are available:

  • Oil immersed type
  • Dry type

Oil immersed transformers:

Given that most of the transformers are of this type, it can be stated that this is the most-used option and maintaining it is also comparatively easier. The oil that is present in these transformers, act as an insulating medium, and also acts, as one of the best conductors of heat than air.

There are certain specific categories associated with this:

  1. Oil immersed forced air-cool transformers:

In this case tank of the concerned transformer is immersed in oil while air is passed over outer surfaces of this tank. This external cooling process helps in the final cooling of these transformers.

  1. Oil immersed forced air cooled transformers:

This is another process that uses air for cooling but the process is quite different. Here, oil is pumped upward via winding process. This winding process is carried out by external radiators that have fans attached to them for cooling purpose. After oil reaches that standard level, heat is finally extracted from this process, and then this cooling process gets the required pushing.

A very important point to note in this regard is the costs that are associated with pumping of oil. It is interesting to note how this whole process results in reducing temperature levels of two ends of the enclosing tank.

  1. Oil immersed water-cooled transformers:

Below the topmost height of the tank there is a metallic coil that is immersed within oil. Now, a stream of water is pumped via this coil and heat is extracted from this over a certain time period. Naturally, this water obtained is heated and therefore it is cooled with help of a specific cooling tower or a spray pond according to its temperature demands.

  1. Oil immersed self-cooled transformers:

In this case, transformer is immersed within oil and it is from here that the process starts off. There is a certain amount of heat that is generated from oil transformers and passing of it happens via conduction. The oil that has heated up is removed from its place and is taken by cold oil.

Now from the bottom layers, natural oil that is heated transfers heat content to tank walls where it cones with contact with a comparatively ambient air. Finally this oil is cooled and it falls into lower part of the machine which further helps in dissipating this oil in various directions.

A point to note is that tank surface acts as dissipater and this energy is increased via: fins, radiator tanks, tubes, corrugators.

Dry type transformers:

This is generally the small type of transformers whose energy standards come down to 25kVA. Given that this is a process that is associated with smaller ones, there are specifically 2 facets to this.

  • Air blast format:

In this case a last of cooled air is used for lowering temperature rates of transformer. The windings and core that is present has openings, from where a blast of cool air is supplied to transformer for cooling over a period of time.

However, when this air supply is given it has to be well ventilated to ensure that there is no additional dirt material that is added on to it. Therefore, blast is produced by a fan that has specialized ducts added to it as per the cleaning system.

  • Air natural format:

In this mode, heat loss from transformers is carried by natural process. The surrounding air that is generated is specialized and used for cooling of the transformer. There is a specific metal sheet that is attached to it which is used for preventing any type of added mechanical injury.

Thus it can be said that this whole process is extremely systematic and controlled.

Thus, with help of the above-mentioned processes, it can be said that transformers will be cooled and you will have it for a longer period of time.

8.2. Transformer oil

Specifically used in functioning of transformers, transformer oil is a type of insulating oil that is known for its electrical insulation properties. Having the capacity to remain stable at high temperatures, this is used in multiple areas as capacitors having high voltage, lamp ballasts that are fluorescent, switches of high voltage, transformers that are filled with oil, and circuit breakers.

Being a mineral oil in nature that is obtained from refining of crude petroleum, it is specifically used for dealing in these areas:

  • It protects the paper from any type of moisture or dirt elements from external sources.
  • It helps in carrying that heat away which is generated via coils and core of the transformer.
  • It helps in providing an extra insulation cover.

Thus, while buying, it is important that you check out that these purposes are fulfilled completely.

Before buying you need to also find out if the transformer oil has certain specific properties.

  • The viscosity should be low as this will aid in heat transfer process
  • It should have a high dialectic strength and resistance to emulsion of any kind
  • Under normal operating conditions at a given time, it should be free from corrosive sulphur/sludge/alkali/inorganic acid
  • The fire point of this oil should be on the higher side

Once these factors match, you can get the perfect transformer oil. It is very important that a particular transformer makes use of this oil since; the insulating property of this ensures that the machine can be maintained for a longer time period under trying situations as well.

8.3. Conservator and breather

The oil in the transformer has to be protected under strict circumstances. With change in temperature, it is quite natural to have expansion and contraction of oil and to ease this process both these apparatus are required.

Here is a detailed understanding of these concepts.

What is a Conservator?

The primary function of a conservator is to determine expansion and contraction of oil without letting it to come in contact with external air conditions.

The reason why oil is not allowed to come in contact with air is: oil absorbs moisture from air that reduces its insulting properties. Another major issue is sludging of oil as well as acidic properties enhancing. It is primarily due to this reason that transformers need to have conservators that have an air-tight metal drum associated with it and tank on the overhead with a pipe attached to it.

In most of the cases, conservator tank is half-filled and the main tank that is there is completely filled with water during cooling period. In this way, air that is in contact with oil surface is reduced comparatively and on the whole the issues that arise with this contact with each other are negated to a great extent.

Thus, issues as formation of sludge or acidic formation dose not happen and on the contrary it is kept out of reach of main tank.

Understanding sludging process:

This is an issue that always starts off during air and oil contact. In general terms, this is slow formation of hydrocarbons from chemical processes as oxidation and heating. It is a continuous process, wherein a great amount of sludge is consistently produced and it further results in overheating of transformer.

This consistent overheating renders in transformers becoming useless over a certain time period. Hence, it is best that oil and air are kept in distance from each other for betterment of the conservator.

What is Breather?

When a temperature change occurs within the transformer, there is an issue regarding changing of oil. At times, oil content expands and at times it contracts as per the temperature changes. Associated with it is a displacement of air that is part of the whole process.

On cooling in this transformer, there is a reduction in oil content and in that space air is drawn in. This whole process is known as breathing process.

The breather is an apparatus that is used for allowing the air to come in and extract moisture content from it. There is an associated vessel that has silica gel crystal associated with cobalt crystal that helps in this whole process. This combination is the drying agent that helps in removing moisture content and ensuring that the machine is dry.


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