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When a transformer burn out, It makes a great miss in the location, the team work has to work hard to return the power back to. But why does a transformer burn? How to protect it? And what happen when a transformer burn? Let’s get started.

There are different factors that can cause a transformer to burn. Here we will discuss some common reasons responsible for transformer burning. But, What is meant by transformer burning? It means that the transformer actually burns out and can catch fire too, when its temperature rises to extreme level with out no cooling.

1. Table of Contents

   Oil Leakage

As we discussed the importance of oil in the transformer in our previous article, Similarly, oil leak is one of the main reasons that can cause transformer to burn. Oil main functions are insulation and cooling.

When the oil level falls from the standard level, the winding gets hot, and as a result the transformer temperature increases and causes burning. If the oil overheats it can also cause an explosion in the transformer.

I have written an article about transformer oil, you can read it here.

2. Natural Aging

Like all other working equipment, aging is one of the failure factors. However, with a good maintenance program and well trained workers the age of the equipment can be extended. While the same equipment with no or poor maintenance makes its lifetime shorter.

Transformers are not an exception, Natural aging is another reason of transformer burning. Transformer consist of Silicon coated paint having a small value of insulation resistance. Due to natural aging, the insulation resistance cannot withstand for a long time, and eddy current losses occur which causes damages to the transformer.

Also, with the factor of age, side by side to the environmental effects, the oil tank of the transformer is getting damaged due to corrosion. By this way the oil may leak and the transformer temperature rises and then, the transformer burn out.

3. Multiple point Core grounding

Multiple points core grounding also causes a transformer to burn.

4. Overloading is another reason that can cause a transformer to burn. 
   

5. Partial discharge (PD):

PD activity, is an electric spark that happens inside the transformer in small air bubbles due to the high value of the electric field. This PD causes the temperature of the transformer to rise up to a limit of burning out. Special high technology devices are used to monitor and detect partial discharge activity in the transformer.

Lightening, Flashover voltages cause an explosion and can cause transformer burning.

Similarly, In the case of a current Transformer if secondary voltage produces a large amount of voltage causing a transformer to burn.

How to Protect Transformer From Burn out?

A good and regular maintenance program, side by side to the main protection installation from day one of transformer working are the key of protecting the transformer, Protection devices like Circuit breaker, fuses and advanced protection relays are the combination of protection system. These protection devices detect any fault before it become a serious problem and lead to burning the transformer.

We had a transformer of 2.5 MVA in the work which completely burnt out when the protection devices failed to discover and isolate the fault. The main reason was an external short circuit. The large current damaged the winding.

The following precautions need to be taken in order to protect transformer from burnout.

• Regular check the transformer cooling system to make effective cooling. Check fans and oil pumps. Also check the radiator for any air blockage and make sure it is always clean.
• Properly check the oil level in the transformer tank. For this purpose, monthly inspection needs to be carried out. This is why its important to have a preventive maintenance schedule.
• Repair leaks, if any exist. Methods of leakage repair is i details in my other article here.
• Use painting and other weather-resistive material to protect the transformer tank from aging and corrosion.
• Install watchdog breakers to protect against power failure and overcurrent faults.
• Make sure that protection devices and fuses are designed properly to protect this transformer.
• Installing lightning arrestors can also protect transformers from burning and explosions. As it protects the transformer from lightning.
• Avoiding insulation resistance, internal arcing, and reducing flashover current also help to protect transformers.
• Using partial discharge devices to monitor the PD activity and detect it early before turning into an explosion.

What happens when a transformer burns out

If the protection devices failed to protect the transformer, then the oil will catch fire. This leads to an other common question that I found when I was searching this topic:

Is transformer oil flammable?

The direct answer is, Transformer oil is used as a spark extinguisher that can happen inside it, But, also it can catch fire because the oil vapor is flammable.

Let me describe it more. Transformer oil has strong insulating properties, it has high thermal conductivity, chemical stability, and dielectric strength. It consists of different types of hydrocarbon compounds such as paraffin, Iso paraffine or Naphthalene, etc. So, these are flammable materials and have a strong attraction toward the fire.

Oil Flash Point

The oil flash point is the minimum temperature at which the oil evaporates and produces ignitable vapor near its surface. This vapor can easily catch fire. Transformer oil flash point should be not less than 145°C

Signals That A Transformer Is About To Burn Out

A good maintenance team are always sensitive to any change or small faults, this is because small faults can be signals of large ones. 

Clear examples of theses faults that you should take seriously are listed below:

• Transformer temperature rise is another fault that can cause burning. many reasons can lead to temperature rise, I recommend reading my other article here.
• Spark noise inside the transformer is a serious fault, it can happen due to internal lose connection. Once, we found this fault in a new transformer, we heard an internal spark, When we opened the transformer for inspection, we discovered lose connection of terminals.
• Partial discharge activity, this one starts small and then becomes a serious problem.
• Oil leak. An oil immersed transformer without oil is an accident that is ready to happen. I have written a detailed article about oil leak reasons and repairs, you can find it here.
• Fuse blowing.

Although fuse blowing is not always a strong sign of expected burning of the transformer, but in case of repeated blowing with unknown reasons can be a serious internal fault in the winding. This internal fault can be sparking that heats the winding and the oil. Below are some causes that can make fuse to blow.

What Causes a Fuse to Blow in a Transformer?

• Short circuit. Internal or external
• Ground fault.
• The overloaded circuit on the secondary side
• Fault on the primary side.
• An arc faults.
• The wrong type or sizing of fuse or incorrect installation.