The Physics of Failure of Transformer

Last time I promised to answer Mr. Chattopadhayay's queries on the blog "Funny and Freak failure mode of a transformer".

First of all I would like to start the explanation with my most favorite style -- i.e. by understanding the Physics of failure of a transformer and then explain the physics of how the explosion would occur and what its consequences might be. I would then try to highlight other possible reasons of such explosions taking place and conclude by listing out the possible measures that can be taken by us to prevent such life threatening explosions.

Any short circuit of such a transformer would produce an electric arc that contains very high heat energy. This high energy is then capable of initiating a phenomenon called 'oil cracking' (this principle I believe, is also used favorably in petrochemical industries). What happens when the transformer oil cracks? It produces primarily acetylene and hydrogen as dangerous combustible gases. However, with high di-electric strength of the oil along with the inherent good design of a transformer it is difficult, if not impossible, to produce the cracking phenomenon. This is because all high voltage power transformers are to be tested for short circuit after these are manufactured. This is done as per IEE norms of short circuiting. The transformer is short circuited and kept that way for at least 2 sec, which is sufficient for the cracking to take place, since such cracking would take around 0.1 sec time to happen. Unfortunately very few transformer companies have such testing facilities and very few users specify such a test during procurement stage of a power transformer.

But coming back to the point, these combustible gases can now exist in at least two ways -- either as oil in vapor phase or as mist having explosive properties. The high energy of the short circuit then easily sets off the fire to the combustible gases, which then rapidly expands and explodes producing rapidly oscillating pressure waves. And as we know, the pressure of such waves is directly proportional to square of acceleration. So, it is the acceleration component that produces the high force that explodes the transformer spilling the oil and spreading fire all over thus damaging property and possibly human lives.

But what can produce this short circuit? Well the unwitting hawk could have certainly caused it. But there appears to be another common reason. Such short circuiting might also happen if the bus bar support barrier catches moisture from the atmosphere when we open up the transformer for repair and maintenance (especially in places having high relative humidity). This, I presume would generally go unnoticed by the maintenance crew if they are not very enlightened about the physics of failure. The moisture then compromises with the insulating property of the barrier and is subjected to electrical tracking that leads to a definite possibility of short circuit and consequent fire and general all round failure. In that case the cover of the link box explodes first. The failure analyst can look for such important clues at site to establish the cause of failure.

But what can we do to prevent such explosions and loss?

There are a number of possibilities some of which are as follows:

a) Specify short circuiting test during procurement stage (the best option)

b) Install Transformer protector. This is a passive mechanical system that is activated by the level of transformer tank internal pressure reached during a short circuit. At least two transformers can be simultaneously protected by one device.

c) Install protective system based on the release of superheated water at 180 degrees C.

d) Install protective systems based on the release of powder to suppress the fire and subsequent explosion.

e) Install protective systems based on water and salts.

f) Regularly monitor the condition of the existing protective devices and the general health of the transformer.

It might be of some interest to see the photograph of the burning transformer and the unfortunate dead bird, which I have pasted on this blog.

Hope this would be of help to Project Managers, Plant Heads, Safety Managers, Maintenance Managers, Electrical Engineers and Reliability Specialists to prevent such unfortunate events in our plants.

Signing off for now,

Dibyendu