20th September 2020

Transformer Cooling Technique

Introduction

The losses in the transformer are mainly resistive losses which get converted in heat and increase the temperature of the transformer. So the transformer cooling needs to be proper or it affects the performance of transformer and sometimes it may damage the windings or core of the transformer.

The transformers are cooled by the process of radiation, convection, and conduction. The transformers can be cooled by ambient air or by immersing them in oil.

For better clarification, let us take an example of a transformer with of 100KVA.
The efficiency of transformer assume: 98.6%
The transformer is working on 0.9 pf

So the losses are = ((100*0.9/98.6)-(100*0.9))KW
= 1.28 KW

A 100KVA transformer has a loss of 1.28KW so this heat has to be removed from the transformers for the proper functioning of the transformer. The transformers are cooled by air directly or by oil which is further cooled using various methods.

Types of Transformer Cooling

  1. Air Cooled
    1. Air Natural (AN)
    2. Air Forced (AF)
  2. Oil Cooled
    1. Oil Natural (ON)
    2. Oil Natural Air Natural (ONAN)
    3. Oil Natural Air Forced (ONAF)
    4. Oil Forced Air Forced (OFAF)
    5. Oil Natural Water Forced (ONWF)
    6. Oil Forced Water Forced (OFWF)

1. Air cooled

Air cooled transformers are known as the dry type transformer. The size of the dry-type transformer is small compared to oil filled transformer. These types of transformers are used where the indoor placement of transformer is Required. There are two types of Air Cooling method.

1.1 Air Natural

In this type of cooling the transformer is kept in ambient, the ambient air is used as the cooling medium. This type of cooling method is used in small transformers, where the heat generated in the transformer can be completely transferred to the surroundings through the outer surface of the core and windings.

r Natural cooling of transformer
Air Cooled Transformer Used for small Watt Ratings.

1.2 Air forced

In this type of cooling method, the forced air is used as a heat removal medium from the transformers. In this method, external fans are used. This method is used for the dry type of transformer where natural ventilation of air is not proper.

2.Oil Cooled

In this type of cooling method, the core and windings are immersed in the oil. The heat generated in the windings and core get transformed to the oil, which further gets transferred to the surroundings from the oil. The various method is used for cooling the oil. In Oil type transformer the oil works as the primary* coolant, and air or water works as the secondary* coolant.

Primary coolant: – Primary coolant is defined as the medium which directly takes heat from the surface which should be protected.

Secondary Coolant: – Secondary coolant is defined as the medium which cools the primary coolant and never comes in contact with the equipment which should be cooled.

2.1 Oil Natural

In this method, the core and windings are emerged in the oil. The oil takes the heat from the windings and core and gets cooled by transferring the heat to the container body. In this type no, extra radiator or parts are not required to cool the oil. The cooling of takes place .

The example of this type of transformer is the small pole mounted transformer.

Oil Natural cooling of transformer
Pole Mounted Oil Natural Cooled transformer for Distribution purpose

2.2 Oil Natural Air Natural

In this method, the core and winding are emerged in the oil container. The oil removes heat from the windings and core. In this method, the heat exchange between oil and air happen at the radiators.

The hot oil in the tank rises flows to the radiator from top and In the radiator, the hot oil passes through the radiator fins and gets cool down and flows back to the transformer tank from the bottom of the radiator.

In the radiator, the oil flows in the radiator fins, which have a large surface area, for the exchange of heat from the oil to ambient air. This cooling method can be used to 20-30MVA transformer.

Oil Natural Air Natural cooling of transformer
Pole Mounted ONAN Cooled transformer for Distribution purpose

2.3 Oil Natural Air Forced

This method is similar to ONAN but in this method, heavy fans are used to blast the air into the radiator. This type of cooling is used for large KVA transformer. In this type of cooling the transformer required some auxiliary power for cooling. The Fan can be switched off if the transformer is running on low utilization factor.

Note:-  According to IEC the transformer shall be utilized to 60% of its maximum rating when the fans are not working.

2.4 Oil Forced Air Forced.

In this method the oil from the tanks is pumped to the radiators using the pumps and the air is blasted in the radiator using the fan. This type of cooling method is used for large transformers. In this type of transformers, the oil circulation pump shall be in continuous operation when the transformer is connected to the power supply.

2.5 Oil Natural Water Forced

In this type of cooling the oil is cooled using the water. The exchange of heat between the oil and water happens in the plate heat exchanger. In the plate exchanger, the oil is flown in the small fins which have high surface areas and the water comes in the contact with the fins/ Plates and gets heated. This type of cooling method is used where space is the main constraint, this method reduces the size of the equipment and the cooling gears.

The main disadvantage of this method is that water vapor can be formed inside the fins because the pressure of water is higher than the pressure of oil inside the fins.

This Cooling method is used for the Transformer used for Induction Furnace. These transformers have a huge heat dissipation, so to take that heat ONWF mehod is used.

2.6 Oil Forced Water Forced

In this type of cooling the oil from the tank is pumped to the plate heat exchanger. In the heat exchanger, the heat exchange takes place between water and oil. The advantage of this method is that it has high pressure inside the tubes compared to water so there is less chance of forming of water vapors.

The transformer with this type of cooling is used where space is the main constraint. This types of transformers are used in the ships. 

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