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When designing a transformer substation, or even buying a transformer, you need to account for several factors. Some of the key factors you need to keep in mind include the transformer’s size, operating frequency, voltage, and ambient conditions.
One of the most important aspects of transformer design is ensuring that the substation does not overheat. If a transformer substation is not cooled properly, it can cause short-term losses and long-term maintenance issues.
From the type of cooling method you choose to the maintenance requirements of your transformer, there are a lot of factors to keep in mind. This article covers everything you need to know about transformer cooling methods.
Why transformer cooling?
Before we go into the cooling methods, it will be smart to know why you should be concerned about the cooling methods for the transformers. Here are some of the top reasons why transformers, whether step-down or step-up need functional cooling systems:
To enhance the efficiency of a transformer
Transformer efficiency can be referred to as the ratio of the input power to the output power. Transformer efficiency is defined as:
Efficiency = Output Power/Input Power = V*I/V*I = V^2/V^2
It is important to note that the efficiency of a transformer has two factors: “power factor” and “losses”. The power factor is a measure of how effectively a system converts alternating current to direct current. The losses are due to ohmic losses, iron loss, and eddy-current loss. The cooling system plays an important role in improving the overall efficiency of a transformer.
For long life expectancy
The core inside the transformers gets heated during its operation. This leads to some breakdowns in the core which can cause short-circuiting and failure in the transformer. It is therefore advisable that you have proper cooling systems installed inside your transformers to ensure their long life expectancy.
Proper cooling systems will ensure that there are no overheating issues and hence will contribute to the long life expectancy of the transformers.
For safe operations
The transformers are used in homes and offices for various applications and there are chances that people may come in contact with it while carrying out their daily activities. In order to ensure that no one gets injured or hurt, proper safety systems should be installed inside the transformer so as to ensure that no one comes in contact with any live wires or anything else which can lead to accidents.
To achieve high peak loads
The right cooling system can help a transformer achieve high peak loads during its operation. Peak load in this context refers to the maximum load that a transformer can carry at any given time. In order to achieve high peak loads, the transformers must be cooled properly to avoid overheating and failure.
Increase the power of the transformer
The transformers can be operated at an optimum level if they are properly cooled. This is because the cooling systems will help in the regulation of the temperature inside the transformer. The higher the temperature, the more power that can be produced by a transformer.
Now that you know the reasons, we can go ahead and analyze the different techniques used for cooling transformers.
Dry-type vs Oil-immersed transformers
When it comes to cooling methods, transformers are classified into two broad categories. These are dry type and oil-immersed transformers. What is the difference between these two?
Dry type refers to the transformers that are cooled by a forced draft or natural draft. They have a dry insulating system and cooling fans. These fans are used to remove the heat from inside the transformer.
Oil-immersed transformers, on the other hand, use oil for cooling. They have an oil-filled tank that is used for cooling. The oil inside this tank is circulated using a pump to remove the heat from inside the transformer.
Dry type transformers are more popular than their oil-immersed counterparts because they can be manufactured easily and at lower costs.
However, they also have some drawbacks like higher maintenance costs and lower efficiency in comparison to their oil-immersed cousins.
Under the dry type transformers, we have;
-Air Forced AF or Air Blast
Oil-immersed transformers are more expensive but they have higher efficiency and can operate at much higher temperatures without any problem at all.
Under the oil-immersed transformers, we have;
-Oil Natural Air Natural (ONAN) cooling
-Oil Natural Air Forced (ONAF) cooling
-Oil Forced Air Forced (OFAF) cooling
-Oil Forced Water Forced (OFWF) cooling
In order to be able to make an informed decision about which type of transformer you should buy for your home, we need to understand how each one of them works.
Air Natural (AN) Cooling
As we have mentioned, AN is a cooling method for the dry type transformer. This is a very simple method that uses only ambient air to cool the transformer. This type of cooling is not suitable for all types of environments, so it is advisable to check the ambient temperature before purchasing AN transformer.
How does AN work?
The transformer is housed in a metal case, which acts as a heat sink. The metal case is connected to the earth via a grounding rod. The case acts as a conductor of heat, which dissipates the heat generated by the transformer to the ground.
The ambient air that comes into contact with the metal case gets heated up and carries away some of the heat from the transformer. This way, both, the ambient air and the transformer get cooled down. If you are living in an area where there is not much wind or if you are living in an area where there is no wind at all, then this cooling method will fail for you since there will be no air movement to carry away excess heat from your transformer.
Air Forced (AF) Cooling
This is another method used for cooling dry type transformers and this method uses forced air circulation instead of natural air circulation to cool down your transformer. It is sometimes known as the Air Blast cooling method since streams of air are forced into the transformer.
But how does the air forced cooling method work?
The air that is forced into the transformer passes through louvers that are located on the bottom of the transformer. The air will be forced to flow inside the transformer by using a fan.
The air from the fan will then flow through ducts and ledges in the case and will be forced to flow through the louvers in order to exit the secondary side of your transformer. This cooling method is not very effective in areas where there is no or little wind.
In order for this method to work, there has to be a sufficient volume of air flowing through your case with enough pressure and temperature changes as well as enough volume of air flowing through your case with enough pressure and temperature changes as well as enough volume of air flowing through your case with enough pressure and temperature changes.
Oil Natural Air Natural
This is the most basic cooling method used on the oil immersed transformers. It is also called the oil natural air natural cooling method. The oil and air mixture is circulated through the primary coil of the transformer.
The oil that is circulating through the primary coil of the transformer will be forced to flow through a tube that has been drilled into the transformer’s secondary coil. This allows for a high-temperature differential between both coils in order for air to be drawn into the transformer’s secondary coil.
The heat that is generated by the core is directly absorbed by the circulating oil, which will further transfer to the radiator. The heated oil gets lighter and will get to the top of the tank where it will cool down. On the other hand, dense oil will get down to absorb the heat and the cycle will continue.
Oil Natural Air Forced (ONAF) Cooling
As the name suggests, this type of cooling utilizes oil and as well as forced air. The heat generated by the cores and other power components of a transformer is transferred to the wall by the circulating oil.
On the other hand, a stream of forced air is also generated and circulated in the transformer by fans, fins, and other components.
A combination of oil and blasted air facilitates a faster cooling process of the transformer. This transformer cooling method is also more efficient than the natural air in combination with the oil.
ONAF cooling method is ideal for the large transformers, those that have the capacity of generating up to 60MVA.
Oil Forced Air Forced cooling
In this cooling method, all the cooling components are forced; air and oil. The oil is circulated in the transformer and is forced through pipes, fins, and other components. A stream of air is also generated by fans and fins to circulate in the transformer.
A heat exchanger facilitates the flow of air in the transformer. The operation of this exchanger is aided by a fan. There is no doubt that OFAF is one of the most efficient cooling methods for power transformers. This explains why transformers that have been fitted with this method are quite expensive.
Oil Forced Water Forced Cooling
This cooling method features the use of both water and oil. Heated oil flows from the top of the transformer down to the heat exchanger. While at the exchanger, high-pressure water is used to separate heat from the oil. This action means that the oil leaves the exchanger with less heat and will flow back to the transformer to perform its cooling task.
This is the most advanced cooling method and is used on powerful transformers rated above 100KVA.
How to choose the right cooling methods for a transformer
We have discussed different transformer cooling methods. Each method has its pros and cons. Which one should I use for a transformer?
Various factors dictate the choice of a transformer cooling method. They include;
-The power of the transformer: To keep the transformer cool, you will need to choose a cooling method that is appropriate for the power rating of the transformer. The higher the power rating, the more efficient and costly the cooling method should be.
-The location of the transformer: A transformer that is located in a hot place needs an efficient cooling method. A cooler place would necessitate a different type of cooling method.
-The type of insulation: The insulation type used on transformers affects how they are cooled. If you have a transformer with multiple layers of insulation, then it will take longer for heat to permeate through them, thus resulting in poor efficiency.
-The cost and size of the transformer: Higher cost means better quality and more efficient cooling methods are used for it. But if you have a small-sized transformer, then you may not need to spend much to cool it adequately as lesser quality materials will be used as compared to bigger transformers.
-Application: The type of application required by the transformer would influence the selection of the cooling method. For example, if the application requires a transformer to produce high voltage, then an air cooling method should be used. If you are in need of a transformer that has a high power rating and is located in an area that is hot, then a water-cooling method should be used for it.
Is the transformer cooling system controllable?
Yes, they are. They can be controlled either manually or automatically.
-Manual control: This kind of control is done manually by the user and it is done by controlling the cooling fan, which blows air through ductwork attached to the transformer.
-Automatic control: This type of control has a thermostat that regulates the temperature at which the transformer operates and this thermostat is typically set at a particular temperature. When the temperature of the transformer drops below this setting, it will automatically start cooling it down again by using an air conditioning system.
We have given you adequate information about transformer cooling systems. Next time you want to buy a transformer, find out the cooling technique that it uses.
A reliable sourcing agent such as ICRFQ should confirm the cooling method used in case you want to import transformers from China.
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