Transformers generally contain the
largest quantity of combustible substance located in a substation.
Therefore, special attention should be given to their location, relay
protection, and fire protection.
Fires in oil-insulated transformers
result principally from the breakdown of insulation. This may be
caused by overloads, switching or lightning surges, gradual
insulation deterioration, low oil level, moisture or acid in the oil,
or failure of an insulating bushing.
Arcing that follows an electrical
breakdown can burn through the tank or vaporize the oil, thereby
creating pressure sufficient to force off the cover or rupture the
tank. A considerable amount of burning oil may be expelled over a
large area, and an intense fire may follow.
Fixed water extinguishing systems
The most common type of extinguishing
agent employed to protect transformers is water. A water system is a
special fixed pipe system connected to a reliable source of water and
equipped with discharge nozzles to provide a specific water discharge
pattern and distribution over the transformer.
The piping system is connected to the
water supply through an automatically actuated valve that initiates
flow of water to the nozzles. The automatic valve is actuated by
heat-sensing devices located around the perimeter of the transformer.
The system should be designed so that
the water discharge will not cause a flashover. This is recommended
to avoid spurious tripping caused by false operation of the water
extinguishing system.
Consideration should be given to the
automatic tripping of the transformer and the deenergization of its
pumps and fans upon the activation of the water system. Sometimes,
because of low ambient temperatures or high wind, spot-type
heat-sensing devices located around the perimeter of outdoor
transformers can operate too slowly to activate the
fire-extinguishing systems.
This delay could allow the transformer
sufficient time to fail catastrophically and possibly render the
fire-extinguishing system inoperative. For general design and
guidance, see ANSI/NFPA 15-1990.
Water pressure
Substations often must be located in
areas where the local community’s water mains may not be of
sufficient size to supply the fixed water extinguishing system or
where water mains may not exist. At sites such as these, alternate
methods may have to be utilized.
A series system of nitrogen bottles or
an engine-driven pump can be utilized to supply the pressure to drive
the water from a holding tank. The holding tank may be filled from
low-pressure mains, a private well, or a tank truck.
Another method is to use a pressurized
tank designed to meet NFPA standards. For general guidance, see
ANSI/NFPA 20-1993 [B14], ANSI/NFPA 22-1993 [B15], and ANSI/NFPA
24-1992 [B16].
Water systems freeze protection
In areas that are subject to freezing
temperatures, consideration should be given to the installation of
freeze protection (heat tracing) or to providing a “dry system.”
Water capacity
The water system should be capable of
supplying 0.25 gal/min/ft2 (0.17 L/s/m2) of transformer area,
including the tank and radiators, for a minimum of 30 min for each
transformer protected.
Cable buses
The water spray system should also be
installed to protect any enclosed transformer cable bus systems that
could be exposed to a transformer fire.
Foam systems
Occasionally, foam systems are
installed to extinguish transformer fires. These systems are
activated similarly to fixed water systems.
The advantage of this system is that it
forms a blanket of foam over the engulfed area and provides a
smothering effect, thus reducing the spread of fire. The disadvantage
of a foam system is that all foams are electrically conductive and
could cause flashover problems. For special design features, see
ANSI/NFPA 16-1995 [B12].
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