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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