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| A typical power transformer with oil cooling tank. Sensenschmied, CC BY-SA 3.0, via Wikimedia Commons. |
You're
probably aware of the major disruption at Heathrow Airport this morning
because of a fire at a nearby electrical substation. According to the
BBC, 25,000 litres of cooling oil in a transformer at the substation
(20,000 according to The Guardian)
had caught fire. London Fire Brigade said this was due to transformer
failure igniting the oil. Transformers are ubiquitous components in
electrical and electronic appliances and gadgets, and larger types have
to be cooled. Just like the way a car engine has coolant flowing through
its core to carry away waste heat, transformers have a similar cooling
system. Water can't be used, so the cooling medium is usually a
specialised high-grade mineral oil.
Why do transformers get hot?
All
electrical conductors have electrical resistance which restricts
current flow. You've probably noticed the phenomenon where a power flex
in an electrical extension reel gets hot if the reel isn't fully
unrolled. Transformers have thousands of turns of wire wound onto
laminated iron cores and this wire gets hot (known as copper loss). The iron core of a transformer also gets hot because of magnetic fields inducing current flow in the core (known as eddy currents).
The small transformer in a power adaptor can usually air cool, however
room-sized transformers in substations because of their size have issues
with heat making its way to the surface and dissipating. Without being
cooled, heat can build up and melt insulation on the windings,
potentially causing the transformer to short out. Large transformers
therefore have a cooling system, and oil circulates through or around
the device, either by natural thermal convection or by being pumped.
This carries heat to the surface of the transformer casing where a heat
exchanger (just like the radiator in your car or home) dissipates it to
the ambient air. Sometime fans are used to increase airflow through the
cooling fins of the heat exchanger. The cooling system is usually
sealed, just like in a sealed, pressurised central heating system, so a
tank mounted on the transformer is often used to cater for expansion as
temperature changes. Sealing is necessary to prevent the oil absorbing
water, compromising its electrical insulating properties.
Counter-terror
officers are investigating the fire, so it's not known, or at least no
information has been released to the public yet about whether the fire
was due to a system fault or sabotage by terrorists.
The image above is of a typical transformer with conservator tank mounted on top.