In the present electrical energy dependent living, electrical transformer is a major component of the electrical distribution infrastructure and plays a great role in making the electrical energy available to every user with its desired characteristics of voltage and current. Electrical transformer is also one of the costliest equipment of the electrical energy supply chain infrastructure. There are different types of power or current distribution transformers i.e., step up & step down for single phase-two phase-three phase applications with various versions like air, oil, paper etc. depending on what makes the medium of the transformer for cooling. These transformers are coming with different capacities and costs. In general these cost from few thousand to hundred of lakhs of rupees depending on their capacity and quality.
Transformer on fire
The transformers are used to transfer current from higher voltage distribution lines to lower voltage that can be used by buildings and equipment. With the growing load towards electricity and compromising with quality between the suppliers and buyers, recently these pillars of electrical distribution system are frequently heard catching fires not only at the stations, substations but also inline system. Electrical transformers transfer energy between circuits, switching energy from one voltage to another. But when flooded with too much electricity, the sudden surge can cause a
transformer explosion. Transformers catching fires not only deprive the users of their life line (electricity) for a quite long time but also creating pollution, fire hazards and a great loss to exchequer. 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. In order to control or minimize such incidents it is interesting to understand what might be the science 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”. These can explode when their insulating materials begin to fail. It depends on the type of oil. Modern silicone based oil have a quite high flash point (in the range of 3000C) so is not easy at all to ignite them: even at high ambient temperature a spark does not have enough energy to ignite it. Transformer oil being a mineral oil, it is quite flammable, and its flash point can be as low as 1400C. Every precaution is necessary to keep it away from direct flame as also extreme heat. Older transformers can also explode when their insulating materials begin to fail. This happens as cellulose and oil absorb water over time and degrade cellulose's ability to insulate, triggering an explosion. Older transformers which have already met or are nearing the end of their operational lives should be replaced with new ones.
How transformer can catch fire?
Electrical transformers transfer energy between circuits, switching energy from one voltage to another. But when flooded with too much electricity, the sudden surge can cause a transformer explosion. During a transformer short-circuit, the electrical arc vaporizes oil and creates a dynamic pressure peak which travels at the speed of 1,200 meters per second and this phenomenon occurs within a few milliseconds. Because of reflections in the tank the pressure peak will generate pressure waves and the integration of all of the wave’s pressure peaks creates static pressure. Then, the pressure becomes equal throughout the entire transformer tank within 50 to 100 milliseconds after the electrical arc, and causes the transformer tank to rupture. A chamber full of several gallons of mineral oil keeps the circuits cool, but given too much electricity, the circuits fry and melt, failing in a shower of sparks and setting the mineral oil a flame. Mineral oil, in turn, combusts explosively and rockets transformer scything into the air. All it takes is a trigger, a corroded or faulty wire, and the circuits surge will get ahead of the breaker.
The transformer oil on its cracking process produces primarily acetylene and hydrogen which are dangerous combustible gases. However, with high dielectric 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. Unfortunately, very few transformer companies have such testing facilities and very few users specify such a test during procurement stage of a power transformer. 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. Short circuiting might 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). 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.
Lack of standards and regulations
Transformers can only withstand a small overpressure and are not designed as pressure vessels according to ASME Codes and Controls. Consequently, transformers have proven to be very dangerous because transformer standards describe electrical requirements but do not cover mechanical design.
- Pressure relief valve inadequacy: Pressure Relief Valves are suitable for slow pressure rise whereas pressure gradients developed during low impedance faults are extremely fast. Transformers that have exploded were usually equipped with Pressure Relief Valves.
- Buchholz and rapid pressure relay inefficiency: transformer electrical protections are not designed to react to sharp pressure gradients. During the transformer protection tests, the Buchholz always failed to detect any gas and oil movement or pressure variation.
- Electrical breaker opening time: the best breaker technology trips in 50 milliseconds, far too late to prevent the explosion because most of the gases are generated within milliseconds after short-circuit. Consequently, the tank pressure keeps increasing even after the breaker has opened.
Precautions to be taken
There may be a number of possibilities some of which are as follows:
- Specify short circuiting test during procurement stage (the best option)
- 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.
- Install protective system based on the release of superheated water at 1800C.
- Install protective systems based on the release of powder to suppress the fire and subsequent explosion.
- Install protective systems based on water and salts.
- Regularly monitor the condition of existing protective devices and monitor the general health of a transformer, especially for moisture ingress.