Transformer Temperature Rise Explained

Transformer Temperature Rise Explained

One of the most transformer critical issues I meet in my work is temperature rise.

We take temperature rise seriously. Once, we replaced a transformer because it was overheated.

In this article, I will answer the most common questions about transformers temperature rise. Let’s move forward.

Is it normal for a transformer to produce heat?

We know that transformer is a machine used to step up or step down the electrical voltages.

The process of stepping up/down voltage causes heat, the main source of the heat inside transformers is the core and windings because of the following:

  • Because, the process includes electromagnetic induction in order to transfer energy from one circuit to another, due to magnetic losses i.e. hysteresis and eddy current losses in the core the heat is generated.
  • Also, when electric current passes through a conductor (like winding in transformer) in normal operation it produces heat due to its resistance and electrical losses.
transformer temperature limits on nameplate
transformer temperature limits on nameplate

Depending upon the quality, the best quality transformer will have less temperature rise, still heats up, while low-quality transformer will heat up more.

Transformer temperature rise is the average temperature rise of the windings and transformer oil, in case of oil immersed transformer, above the allowable nameplate temperature.

Each transformer has a nameplate which has all necessary data about it. Of course transformer maximum temperature rise is one of these data on the nameplate.

Why does a transformer get hotter than usual?

Different losses that cause a transformer to get hot, in normal conditions, are losses due to resistance of the windings, Hysteresis, and eddy current losses as I mentioned above.

But, in some cases the transformer gets hotter than usual. There are other reasons due to which the transformer gets hotter. I will mention the most common reasons:

  1. When the load exceeds transformer rating “overloading the transformer”.
  2. Higher ambient temperature.
  3. High moisture in the transformer oil increases the temperature rise because, the transformer oil ability to dissipate heat decreases.
  4. Blocked ventilation openings of transformers, some time by birds nests.
  5. When connecting loads with poor Power Factor.
  6. Misaligned or broken cooling fan of the transformer.
  7. Any issues with the transformer cooling system i.e air fans or oil pumps.
  8. Any internal fault can increase the temperature of the transformer.

For instance, we in my work were making a regular inspection for one of power transformers, we noticed a temperature rise by the thermal camera.

We hired a third party company to make oil analysis, it found the reason of the transformer temperature rise was a higher moisture in the oil.

Can Reactive Power Produce Heat In Transformer?

Yes, reactive power can overheat the transformer. Too much value of the reactive power overheats the transformers, due to the increased current drawn by the reactive power component of the load.

Reactive power still a type of power, i.e the load with a high reactive power draws more current than another load with a lower reactive power. The higher the current the higher the temperature rise.

As you know, the higher the reactive power the lower the power factor.

Normally, if the apparent power exceeds the real power, the components overheat. This overheating leads to the shortening of the life of those components.

To cater this situation, we should keep the power factor equal to 1. In the real-life scenario, it is almost impossible to keep the power factor on a utility grid equal to 1.

So, the permitted range is the pf 0.8 to 1 leading, or 0.8 to 1 lagging. That keeps the temperature in a suitable range and does not let the transformer overheat.

I’ve written a detailed article about power factor correction, read it for more information.

How to check transformer oil and windings temperature?

thermal imaging for a transformer
Me, doing thermal imaging for a transformer

A good maintenance program should include regular check of the temperature rise of transformers.

In my work, I use thermal imaging to detect any abnormal temperature rise, this works very good for us.

As I mentioned above, we detected a faulty transformer with a higher temperature, and we replaced it.

Not all workplaces has thermal camera, In this case we use temperature indicators for oil and windings OTI and WTI, I will discuss this below in this article.

It is important to record temperature in transformer history datasheet to detect any change or abnormal rise.

If the loads and ambient temperature have no changes, then you should expect no changes in the transformer temperature. If it has, then it needs attention and special care.

Now let’s discuss OTI and WTI in details.

What is Transformer OTI and WTI?

OTI stands for Oil Temperature indicator, while WTI stands for Winding Temperature indicator.

These are a type of indicating devices used to indicate the real-time temperature of the transformer.

Oil temperature indicator (OTI) is used to measure the Top transformer oil temperature. It is a protective device, placed on the top of the transformer. OTI is used for controlling and protecting the transformer against temperature rise.

Some types of oil temperature indicators are used to just indicate the temperature of the oil, while other types are connected to the circuit breaker in the switchgear to isolate the transformer in case of any abnormal temperature rise.

Of course the indicator in the case has two contacts, one for alarm level and the other is to make a shutdown.

Winding Temperature indicator (WTI) directly measures temperature of transformer windings, which is always higher than the oil temperature because, the winding is the source of the temperature in the transformer as well as the core.

How OTI works?

While talking about the function of OTI, It consists of a sensor bulb, capacity tube, & a dial thermometer. The sensor bulb is placed near the hottest oil.

Similarly, the capacity tube is fitted with evaporation liquid.  When the temperature increases e or decreases, the vapor pressure varies.

The pressure is further transmitted to a tube inside the dial thermometer, which moves according to the changes in pressure, which is proportional to the temperature.

OTI consists of two switches. Depending upon the situation, the one switch act as an Alarm and the other is used for tripping.

How WTI works?

Just like OTI, a WTI also consist of a sensor bulb. The sensor bulb is placed on the top of the oil-filled pocket in the transformer.

Two capillary tubes connect the measuring instrument. The measuring system is filled with a liquid, which changes its volume by rising of temperature.

While a current transformer connected load winding produces heat resistance. A sensor bulb notes down the temperature of the hottest oil. There are four different switches in WTI.

The function of these switches is to control the motor pump, fan, Alarm, and tripping.

Read also my other articles:

Can we keep transformer temperature within limits?

Power transformer

We can control the transformer temperature rise by cooling it and increasing its oil heat dissipation efficiency.

However, the procedure is different for Dry Type Transformers and oil immersed transformers. Temperature controlling methods of both types of transformers are discussed here briefly.

I have written a more detailed article about transformer cooling methods, you can check it here.

Dry type Transformer temperature rise control

Smaller size dry type transformers temperature can be controlled by natural air flow (AN).

AN dry type transformer cooling the natural air passes around across the transformer. And dissipate heat.

On the other hand, if the transformer rating is larger, then cooling fans are installed in order to force the air on the transformer for more enhanced cooling.

Dry type transformer cooling depends on air, even natural or forced.

I’ve written a detailed article about Transformer Cooling Methods 5 Answers For Beginners, read it now for more information.

Oil immersed transformers temperature rise control

The temperature controlling method is somehow different, we use both air and transformer oil to cool down the transformer.

Read my detailed article about ONAN cooling and other transformer cooling here


(Oil Natural, Air Natural method) is used to transfer generated heat from transformer core and windings to the oil then to the atmosphere. This method is based on the principle of convection.

Where, heated oil goes up, and the lower part is replaced by the cooled oil. While, the heat will be dispersed in the air by natural airflow.


Another method is Oil Natural Air Forced, In this method, the same technique is followed for oil, but a fan is installed, in order to force the flow of air.


Oil Forced Air Forced In this type of method oil, is circulated in the heat exchangers with help of a pump. Also, compressed air is passed through heat exchangers via fans.


Another method is the use of water instead of air and which is called Oil Forced Water Forced.

It is suitable for high rating transformers. In this method, water is passed through heat exchangers in order to control the transformer temperature.

While oil is forced to flow with the help of a pump.


ODAF vs OFAF Transformer Cooling
ODAF vs OFAF Transformer Cooling

ODAF, is an improved OFAF cooling method, by directing the oil to go through the windings then to the radiator.

This method is improved to get the highest cooling efficiency from oil flow inside the transformer and between its winding. .


What are temperature limits for transformers?

It’s worth mentioning that different types of transformers have different temperature ratings on full load capacity.

For liquid-filled transformers, common ratings are 55°C and 65°C. While dry transformer comes in the standard range of temperature rise of 80C, 115°C, and 150C. Please note that all these values are based on the ambient temperature of 40°C.

So, temperature rise of 55°C will mean that the transformer will operate at a winding temperature of 95°C at full load operations.

Although it totally depends upon the transformers rating and connected loads. According to international standards, the inner side of the oil immersed transformer temperature can reach up to a maximum of 65°C, If we add the ambient temperature of 40°C, the total value of inner side temperature can reach up to 105°C.

You should always check transformer data sheet for detailed information about its temperature rise limits.

What happens if a transformer gets too hot?

If a transformer overheats from its rated temperature, the following issues could increase:

While there is a chance of catching fire. Please note that the normal transformer is rated for 65°C inner sides and 22°C overall.

What is normal range of transformer oil temperature?

transformer oil temperature rise

As oil has excellent electrical insulating properties at high temperatures. That is the reason that most transformers are oil-filled. It also stops corona discharges and arcing effects.

Depending upon different working conditions the normal range of transformer oil temperature is set between 40°C to 60°C. The nameplate of the transformer usually has the exact oil temperature rise.

Temperature rise test of transformer

To check the temperature of transformer oil and windings we perform a test called the Temperature rise test of transformer.

In this test, we check whether the oil and winding are according to the rising limit of the transformer or not. The procedure of this type of test is given below.

  • Short circuited the LV winding of the transformer.
  • Place one thermometer in the transformer top cover.
  • Place two thermometers in the inlet and outlet of the cooler bank of the transformer.
  • Apply a specific amount of voltage to the HV windings, so that the power input is equal to no-load losses plus load losses corrected to a reference temperature of 75 Degree C.
  • Note down the total loss by three wattmeter’s method.
  • Take the readings of top oil temperature from the thermometer placed in the top cover each hour. Also, note down the hourly readings of the thermometers placed at the inlet and outlet of the transformer.
  • Now you can calculate the mean temperature of the oil from the noted values.
  • Until the top oil temperature reach an approximate steady-state figure keep making the temperature rise test for the top oil of the transformer . This value of top oil is determined as the final temperature rise of transformer insulating oil.

Now we will discuss how to check the temperature rise of transformer windings

Once the temperature rise test for the top oil of the transformer is completed. Reduce the current to its rated value for transformer and is maintained for one hour.

After an hour short circuit and supply connection to the HV side. While open the short circuit connection to the LV side.

Now measure the resistance of the windings. But keep in mind to maintain a gap of 3 to 4 minutes between the first measurement of resistance and the instant of switching off the transformer,

Measure the resistance at the 3 to 4 minutes’ time intervals over a period of 15 minutes.

Plot the Graph of hot resistance versus time. You can use the following formula to find the winding resistance (R2) at the instant of shut down can be extrapolated.


where R1 is the cold resistance of the winding at temperature t1.

I’ve written a detailed article about How temperature rise affect electrical equipment, Read it for more information.

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