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Transformers are essential to safely provide power to businesses, infrastructure, and neighborhoods. A transformer is designed to reduce the voltage before it enters the structure or panel, because power lines transmit energy at a very high voltage. The possibility of electrical fires and short circuits are diminished since transformers are made to offer voltage stability and overload tolerance. 

Unfortunately, transformers can catch fire—and even explode—after damage or deterioration. While it might be surprising to learn that transformers can catch fire, it’s important to ask and understand what is a transformer fire and how does it happen? We’ll also look at the best solution for transformer fire protection: installing a fire suppression system.

What Causes a Transformer to Catch Fire?

Transformers can catch fire as a result of electrical and mechanical failures. Internal damage to the transformer's components is what leads to mechanical failure of the transformer. Extreme weather, seismic activity, and physical handling of the equipment during shipping and installation can all result in mechanical damage. Transformers can also fail due to electrical disturbances, including animal intrusion and insulation deterioration. The most frequent cause of failure is a lightning strike, which can damage cables and/or equipment and result in an excessive amount of energy flowing into the transformer, leading to fire or explosion.

Although the likelihood of a transformer failing varies greatly between utilities and transformer types, the average annual rate of major failures is normally between 0.9% and 1.0%. In practice, this means that throughout a 40-year service life, 2.4% to 4% of all transformers should be expected to cause a fire.

According to the article Power Transformer Fire and Explosion: Causes and Control, transformer fires causes include:

  • Thermal failure caused by:
    • Overloading of transformer
    • Poor contacts at bolted connections with cables or draw rod of bushings
    • Inadequate oil flow inside the transformer
  • Voltage surges with:
    • High rate of rise of current
    • Short circuits in the secondary
    • Rapidly fluctuating load currents
  • Insulation failure between winding of coils due to:
    • Mechanical damage during manufacture
    • Infiltration of water
    • Long-term overload and over voltages
    • Mechanical movements of turns
    • Obstruction in oil flow
  • Winding-tank insulator failure due to:
    • Water contents in oil
    • Sludge in oil
    • Insulation between tank and winding
  • Partial discharges may result in heating in the range of 100 degrees to 700 degrees due to:
    • Incomplete oil impregnation leaving cavities in which discharges occur
    • High humidity in paper
    • Arcing between bad connections
    • Shielding rings
    • Adjacent disks or conductors of windings
    • Broken brazing
  • Poor mechanical contacts at bushings leading to:
    • Sparks
    • Loose springs in the tap changing contacts
    • Pitted contacts of tap changer
    • Fire in vapor accumulating above the oil
    • Bad maintenance practice and condition monitoring.
  • Eddy current heating in magnetic core leads to heating with temperatures rising above 700 degrees due to:
    • Malfunction of the magnetic circuit
    • Large negative or zero sequence currents
    • Circulating currents in the tank and core
  • Explosion of the Diverter Switch connected to on Load Tap Changer due to accumulation of combustible gasses.

When safety procedures are not followed, system failure, an emergency shutdown, and injury could occur, endangering your employees and business operations. Maintaining a safe environment for all components of the transformers will benefit from understanding the design and from correct installation.

What Happens When a Transformer Is on Fire?

Unfortunately, a transformer failure might result in a severe fire and, in turn, a violent explosion. The overheating of the cooling fluids like mineral oil in transformers poses a serious concern. Mineral oil is not only a coolant in transformers, but it is also used as insulation and to stop electrical arcing. 

According to an article in Scientific Reports, under typical weather circumstances, transformer fires and explosions are unlikely, but they might happen if there is a: 

  • Design issue
  • Hardware problem 
  • System overload

Transformers that sustain sudden damage may also cause electrical overload—excessive voltage and/or currents—which can result in enough heat and sparks to burn the mineral oil. It’s important to note that mineral oil has a fire point and flash point of ~330°F and ~295 °F, respectively. As a result of the gasses produced by boiling mineral oil, there is a significant buildup of pressure inside the sealed transformer, which causes the oil tank to rupture and release a lot of energy and strong thermal radiation that scatters several things over the surrounding area like: 

  • Flaming oil
  • Gaseous byproducts of decomposition
  • Solid insulation
  • Molten conductor material 

How to Fight a Transformer Fire

Transformer failures may be quite dangerous, but with the proper safety measures in place, fires can be managed to lessen damage to nearby components, save plant downtime, and increase worker safety. There is a great risk if a fire breaks out, since mineral oil fires can spread to surrounding machinery and buildings. To better minimize the harm and possible spreading of a fire, it is suggested by NFPA 850 to install physical separation and fire walls. But fire prevention extends beyond physical structures, extinguishing the fire should be the top priority.

So how do you extinguish a transformer fire? Although a burning transformer will probably be destroyed relatively quickly, the impact of the fire on nearby machinery and buildings can be reduced by using a fire suppression system. A sprinkler system for transformers is not only crucial to putting out a fire, but also to meet transformer fire protection standards like the NFPA 15. As another standard, the NFPA 851, section 5-7, states: “Oil-filled main, station service, and startup transformers should be protected with automatic water spray or foam-water spray systems.” It’s also important to install a containment pit and drainage system to effectively collect any spilled transformer oil or discharge from a fixed water spray system.

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