Transformer Fire Protection: Firewalls and Oil Pits

Short answer

An oil-filled transformer fire can destroy neighboring equipment and the control building. Protection combines separation or firewalls, containment of spilled oil with crushed stone, and active systems such as water spray or nitrogen injection, all coordinated with the transformer protections. IEC 61936-1, NFPA 850 and IEEE 979 give the reference criteria.

What it is and where it sits

The transformer fire protection system brings together the passive and active measures that keep the fire of an oil-filled transformer from spreading. An internal arc decomposes the oil, raises the tank pressure and can rupture the tank; spilled oil burns and can reach neighboring transformers, the control building or the buildings of a plant. The system sits in the switchyard, around each power transformer, and its control is in the control building or in a dedicated panel. Together with lightning shielding, it forms the physical protection of the site.

The parts and how each one works

Separation or firewalls. Distance is the first barrier. When it does not fit, a fire-resistant wall between transformers, or between a transformer and a building, does the same job. IEC 61936-1 gives as an example walls rated EI 60, with a height up to the top of the conservator tank and a length equal to that of the oil pit.
Oil containment. A pit or sump under the transformer collects the oil. With crushed stone, the flame goes out when the oil sits a few centimeters below the surface of the stone. If several transformers share a collecting tank, the design must prevent the fire of one from passing to the other. IEEE 980 is the guide for containment and control of oil spills in substations.
Water spray (deluge). Nozzles around the transformer, fed by a deluge valve that opens on the detection signal. The water cools the tank and the neighboring surfaces.
Nitrogen injection. It drains part of the oil to lower the pressure and remove fuel. It then injects nitrogen from the bottom of the tank to agitate the oil and bring its temperature below the flash point. The nitrogen that remains on top isolates the oil from oxygen. Another technology acts on pressure: it depressurizes the tank within milliseconds when it detects the pressure wave of an arc and then injects nitrogen.
Detection: heat detectors around the transformer and, as confirmation signals, the transformer’s own protections.
Control panel of the system, with its logic, its alarms and its supply from the AC station service panel and the DC system.

Distances according to IEC 61936-1 and NFPA 850

IEC 61936-1 requires that, in outdoor installations, a transformer with more than 1,000 liters of liquid does not create a fire hazard for other transformers or objects. It gives a table of guide distance values, which grows with the volume of liquid and is greater toward the surfaces of combustible buildings. These distances can be reduced if there is automatic extinguishing; if they cannot be met, firewalls are required. The layout of the equipment in the switchyard, with its distances, is covered in Physical layout of a substation: switchyard and clearances.

NFPA 850, the recommended practice for fire protection for electric generating plants and high voltage direct current converter stations, has an equivalent table for outdoor mineral-oil transformers. That table matches IEEE 979, the guide for substation fire protection, in the essentials. For transformers of less than 500 gallons (about 1,900 liters), NFPA 850 recommends at least 5 ft (1.5 m) to the exposed structure. Both documents allow the tables to be replaced by a site-specific risk analysis.

How it changes by voltage level

Like the other systems on the site, it grows with the voltage level; see station service by voltage level.

Medium voltage and industry: oil-filled distribution transformers with their containment, or dry-type transformers inside buildings. Less-flammable liquid (ester) transformers allow shorter distances according to the standards and the insurers.
High voltage: firewalls between neighboring transformers, oil pits with stone and, depending on the value of the equipment and its location, water spray.
Extra-high voltage: autotransformers and reactors with large oil volumes, where walls, pits with a collecting tank and active systems are combined, often nitrogen injection or deluge.
Solar plants: substation transformers with the same criteria, and the transformers of the power stations with their containment.

Design criteria

Volume and type of liquid of the transformer.
Available distances to other transformers and buildings, and the fire-resistance rating of the walls.
Volume of the oil pit, including rainwater and the water from the extinguishing system.
Available water and its reserve, in deluge systems.
Activation logic, so that it neither operates falsely nor fails to operate.
Requirements of the insurer and of the local authority.

Connection with protection, control and alarms

The nitrogen injection system must not operate with the transformer energized: its logic receives the position of the transformer breakers and the signals of the mechanical protections 63 and 26 (Buchholz, sudden pressure, temperature) in addition to the fire detectors. Electrical protections, such as the 87T differential, trip the breakers through the 86 lockout relay, which prevents reclosing onto the faulted transformer until it is reset. SCADA receives the system operation, its faults, the pressure of the nitrogen cylinder or of the water network, and loss of supply.

Common faults and what to check first

Oil pit stone clogged with soil and vegetation: it no longer extinguishes the flame.
Oil pit drain blocked or separator valve closed.
Deluge valve not tested or nozzles obstructed.
Nitrogen cylinder at low pressure.
Logic without the de-energized transformer condition or with detectors without redundancy.
False trip: check which signals coincided and whether a detector or a contact was left in a fault state.

Testing and maintenance

At commissioning: functional tests of the logic with each combination of signals, without discharging the agent. Verification of the wiring with the transformer protections and the 86. Tests of the deluge valve and of the flow rate according to the manufacturer. Test of the signals to SCADA. In maintenance: cleaning of the stone, inspection of pits and drains, cylinder pressure, detector testing and periodic drills of the logic.

We support you with your system

At Tiza Group we test the logic of fire protection systems together with the transformer protections in medium, high and extra-high voltage substations, solar plants and industry. Field testing is done in Colombia, or elsewhere on request. Learn about our commissioning or write to us.

Physical layout of a substation: switchyard and clearances

References

IEC 61936-1: power installations exceeding 1 kV AC.
NFPA 850: recommended practice for fire protection for electric generating plants and high voltage direct current converter stations.
IEEE 979: guide for substation fire protection.
IEEE 980: guide for containment and control of oil spills in substations.
IEEE C37.91: guide for protecting power transformers.

Frequently asked questions

At what oil volume does IEC 61936-1 require separation between outdoor transformers?

IEC 61936-1 requires that a transformer with more than 1,000 liters of liquid does not create a fire hazard for other transformers or objects, and gives guide distance values that grow with the volume. If they cannot be met, firewalls or automatic extinguishing systems are used.

What is the crushed stone in a transformer oil pit for?

So that spilled oil drains down between the stones and stops burning once it is below the surface. The tests cited in NFPA 850 and IEEE 979 show that the flame goes out when the oil sits a few centimeters below the top of the stone, so the stone has to be kept clean.

When does a nitrogen injection system operate?

Its logic requires the transformer to be de-energized and a combination of signals, such as pressure, gas or temperature, to avoid false operation. It first drains part of the oil and then injects nitrogen from the bottom to agitate the oil and cool it below its flash point. Each manufacturer defines its own logic.