What happens when a transformer fire occurs in a substation

Transformer fire in a substation

If a fire occurs as a result of a failure in a transformer, then the transformer is nearly always a total write off. However, the total cost of a transformer fire is typically in the order of 2-3 times the cost of the replacement transformer and can in unfavourable events be many times the cost of the transformer, even without including the cost of loss of supply for customer.

The strategy therefore is:

1. Minimize the risk of the transformer fire occurring.
2. Protect the potential fire victims, humans and the remainder of the substation installation from also being fire damaged.
3. Maintain supply during the fire, or if not possible then restore the supply as early as possible after the fire.
4. Avoid pollution and contamination of the environment.

Combustion products and their effect on life and safety

The product of combustion can be divided into four categories:

1. Fire gases
2. Smoke
3. Heat from fire and
4. Loss of oxygen

In addition to the product from combustion, there is also the risk of pollution from oil spill and contamination by products use in transformer fire fighting such as foam and possibly contaminated water! Each of these can have damaging effect on humans, other equipment and the environment.

Each item will be discussed briefly below:

1. Fire Gases

Transformer oil and cellulose insulation burn to mostly carbon dioxide, or carbon monoxide if air supply is restricted. Other more toxic or corrosive gases can be released from burning of cable insulation.

2. Smoke

Smoke consist of very fine solid particles and condensed water vapour. In many cases smoke reaches untenable levels before the temperature does. This is especially so when fire occurs indoors or in confined areas.

Smoke particles can cause damage to the respiratory system and it may impair vision if lodges in eyes and thus impair the ability to escape the transformer fire.

3. Heat from fire

Heat from flame can cause dehydration and exhaustion and if intense and conducted into the lungs, cause serious decline in blood pressure and failure of blood circulation. Burns can be caused from contact with flames, heated objects or from radiation.

4. Loss of Oxygen (fire consumes oxygen)

The oxygen level in normal air is 21% and if it drops below 15% then muscular skills diminish, at a further reduction to 14 – 10%, fatigue sets in and judgement becomes impaired. If oxygen reduced to the range from 10 to 6%, complete collapse and unconscious occurs, but revival may still be effected if fresh air or oxygen becomes available.

Classification of fires and extinguishing agents

Classification of fires as determined by National Fire Protection Association [NFPA]:

• Class A – Fires in ordinary combustible materials (glowing after burning). The extinguishing agent is water.
• Class B – Fires in flammable liquids. The extinguishing agent is fine spray of water (water mist of fogging). The blanketing or smothering effect keeps the oxygen away from the fuel.
• Class C – Fires in electrical equipment. The extinguishing agent must be non-conducting (powder, carbon dioxide, vaporizing liquid (foams or water sprays at safe distance).

Fire resistance classification

The resistance of a substation structure and its construction material including buildings is normally indicated by a combination of code letter and number. Significant variations exist between countries in the test methods used and the classification code applied. But it is common to use a combination of letters and numbers as is done using the REI classification.

This classification method assign a fire rating grading period in minutes based on three distinct criteria using the letters as follows:

• R – Structural adequacy – The ability to maintain stability and load bearing capacity.
• E – Integrity–The ability to resist flame and passage of hot gases.
• I – Insulation – The ability to maintain a temperature on the non exposed surface below the specified limit.

However to compare different classification it is necessary to understand both the test method and the classification coding applicable for the specific classification.

Extinguishment of fires (Fire triangle)

The fire triangle provides a very good graphic presentation of how a fire can be extinguished.

Remove the heat

The fire can be extinguished if the heat is removed and the fuel is cooled below its fire point temperature. Water can be very efficient as a cooling medium to extinguish external fires and to protect adjacent asset from being heated to their flash point.

Also for the same reason water alone is not efficient in extinguishing oil pool fires. Whereas water with foam can be very efficient for this purpose as it excluded oxygen from the oil surface.

Oxygen displacement or dilution

Removal of oxygen can be a very effective method of extinguishing fires where this method is possible. Only a slight decrease of the oxygen concentration in air decreases the fire intensity and below 16 % oxygen in the air there is no risk for a fire.

Many alternative gases have been used successfully to displace or dilute oxygen and thus extinguish the transformer fire.

Gases commonly used for this purpose include carbon dioxide, halon and nitrogen. (halon is now disappearing from use, as it is considered a non-environmentally friendly gas).

The disadvantage for all of these gases has been that human beings could be suffocated, if the gas is injected before all humans have been evacuated. CO₂ is heavier than air and is often used in buildings and other areas where the gas can be contained and the displaced air can raise above the fire.

Some manufactures of transformer fire extinguishing systems, have used nitrogen for injection into the base of oil filled transformers to extinguishing a fire burning from the oil surface. In this application nitrogen will stir and cool the oil in the transformer tank and displace the air above the oil and suppress the fire.

Foam and high pressure water fogging can also be used to displace oxygen.

Foam can be very effective for use on oil pool fires, but is less effective on oil fires where oil is spilling over a vertical surface and it is often difficult to get foam into a fire burning inside a transformer tank. Water deluge and high pressure water fog or water mist have the benefit of oxygen dilution as well as providing cooling.

Removal of fuel

Removal of fuel can be effective, but is often not possible. Some strategies for fuel removal exist for transformer oil, as it is possible to equip transformer tank with oil dump valves which can be opened by remote control. Dumped and/or spilled oil can be directed into oil/water separation tanks or into gravel or crushed rock beds or other safe holding areas.

Transformer as a fire victim

The transformer can become a victim to a fire started elsewhere! As large power transformers contain large quantities of mineral oil, the potential for the transformer to add considerable quantities of high energy fuel to an existing fire exist and strategies to minimize the risk of releasing and igniting this fuel source should be considered as part of the substation design process.

The most effective protection is probably fire barriers and water spray for cooling and keeping the oil within the transformer tank, away from oxygen and below the flashpoint temperature.

Reference // Guide for Transformer Fire Safety Practices by Working Group A2.33 (Arne PETERSEN – Convenor Rudy BLANC, Kjell CARRANDER, Dayse DUARTE, Yoshihito EBISAWA, Elisa J. FIGUEROA, Marc FOATA, Makoto KADOWAKI, Takayuki KOBAYASHI, Terence LEE, Russell MARTIN, Sidwell MTETWA, Hiroshi MURAKAMI, Uwe RIMMELE and Yukiyasu SHIRASAKA