It isn’t unusual to see electrical power transformers catching fire and violently exploding during rainy seasons. In India, we do see a number of such cases from June to Sept/October.
What might be the physics behind such failures of power transformer that threaten life and assets?
Any short-circuit of an electric transformer would produce an electric arc that has very high heat energy. This high energy initiates a phenomenon called “oil cracking.”
What happens when the transformer oil cracks? It produces primarily acetylene and hydrogen which are 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 vapour phase or as mist having explosive properties. The high energy of the short circuit easily sets fire to the combustible gases, which then rapidly expands and explodes producing rapidly oscillating pressure waves. The pressure of such waves is directly proportional to square of acceleration. So, it is the acceleration component that explodes the transformer causing it to spill its oil to spread fire all over thus damaging property with possible loss of human lives.
But what can produce this short circuit? Well, an unwitting hawk or a crow can certainly cause it. But that alone is not the only reason. 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, may not be noticed by the engineering crew if they are not very aware of this phenomenon. 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 with consequent fire, explosion 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 may be 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 existing protective devices and monitor the general health of a transformer, especially for moisture ingress.