Design “Away” Fires In Substations And Transformers
By : Eugene Pretorius. N.Dip. (Fire Tech)
Many fires can be prevented and or reduced to insignificant losses by introducing minuscule changes
at the incipient design stages of new substations and the placement of new transformers at sites. We have been involved in various projects and I have drafted a standard, which ensures limited fixed fire installations need to be installed to “make safe” these electrical installations. This document is limited to the event of fire and its potential impact on facilities. Recommendations are predominately confined to the design of the building.
Fire hazards in substations occur mainly because the insulating liquids in the circuit breakers / switchgear are combustible. Faults could ignite these liquids, resulting in equipment destruction and possibly the destruction of adjacent units or buildings. Statistically, the likelihood of such an event is remote and damage is normally confined to the equipment specifically involved.
Most substation design promote fire spread. The nature and extent of the inter-leading floors and tunnels makes the impact of fire appreciable, and the typical lack of fire fighting measures increases the potential for total involvement once a fire develops.
Considerations for substations design
Substations should preferably be constructed on ground floor level.
At least two entrances on opposite walls must be installed on each substation.
Multi-level substations should be avoided as far as practicable.
If, however this is unavoidable the cable trenches (vertical and horizontal) should have fire stopping between floors.
Ventilation and other services penetration openings further complicate the design of any system and these will still allow the fire to spread into other areas. Therefore, minimal openings should be allowed, and services should be contained within one area, easily accessible for the purpose of extinguishing a fire.
Cables should be routed overhead, rather than in cable trenches otherwise provision should ensure adequate access to a trench from at least two sides. Also, the use of open grills (expanded metal grids) rather than solid steel plates for the covering of the trenches, is advised. This will ensure firefighting operations can be performed from overhead rather than from below, thereby reducing personal exposure.
Routing of main cables vital to production should avoid areas having a high fire risk or locations that
expose them to mechanical damage. Where this cannot be achieved mechanical protection in the form of barriers and protection giving a minimum 1-hour resistance must be provided e.g. intumescent coatings.
Dry switchgear should be used, rather than oil filled.
HT, LT, PLC’S, UPS’S and transformers should be separated by firewalls. These walls should extend to
the underside of the roof and sealed accordingly (whether it is concrete or corrugated sheeting used
for the roof). A blast wall, to prevent explosion damage, should separate the HT and LT switchgear.
All openings in the cable trench intersections should be closed with fire retardant walls / seals. This
is to ensure that fire will not spread through from one section to the next.
Non-combustible ceilings should be used, and the ceiling voids should not be common.
During risk evaluations, it is important to determine whether the equipment is critical to the company, either in financial or logistical terms.
Replacements are not always readily available and extended business interruption can be caused by delays in supply of replacements. Also, fire or explosions could involve critical areas of plant.
Methods of achieving risk improvement involves the combination of two basic protection principals, namely active and passive. Transformers require total containment (passive) and in most cases protection by an automatic water deluge or a gaseous flooding system (active).
Passive protection involves the separation or enclosure of risks and even, the provision of early warning devices. Where this is not possible subdivision by using firewalls or other approved separation methods are recommended. The following considerations are relevant:
Liquid seal traps should be installed in the oil drainage system, to prevent burning oil from travelling along any drainage system.
Automatic fire detection
Equipment should be installed which will automatically detect; flame, smoke, or other conditions likely to produce fire, and cause the automatic actuation of an alarm and or fire suppression system.
Openings in fire walls or floors through which pipes, cables, conduit, or any other services pass, should be fire-stopped. For this purpose, fire doors, dampers, or penetration seal material having a similar fire rating to that of the wall or floor, in which they are to be installed (usually two hours).
Non-combustible liquid-tight retaining walls should surround the containment area, which will prevent burning oil from endangering adjacent equipment and buildings. An oil pool fire burning in this containment would be difficult to extinguish, without hand-held hose lines. If the bund were filled with small stones, however, only small amounts of flammable vapour will pass through the voids between the stones, making the fire more controllable. The capacities of bunds should be 110% of the contents of the protected transformer: Determination of this capacity includes the volume occupied by the stone. It should be constructed of non-combustible liquid-tight material, extending to a minimum of 1 metre beyond any transformer protection in plan view, and at least 150mm above the base of the containment. Should the location and criticality of the transformer or lack of manpower dictate the use of high velocity water spray systems, stones inside the bund are unnecessary.
If this method of protection is utilised, a siphon should be installed to drain the bund's contents to a
safe, remote holding area.
Any leaks from oil-filled transformers will run through the cable openings, directly into the cable trenches and underneath switchgear. It is recommended that the transformer bund levels are well below the cable penetration openings and that any leak from a transformer will be adequately contained in the bund without it running into the trench.
A separating wall should be constructed between transformers to prevent radiant heat transfer. Firewalls should extend to at least 500 mm above the highest projection of the transformer and one metre beyond any projection when viewed in plan. Brick, concrete or steel and other fire-resistant materials are suitable materials for the firewall's construction. Firewalls should be designed and constructed to have a fire resistance rating of at least two hours.
Roof structures should not extend over transformer bays, especially if they are constructed of wood and corrugated sheeting. The outside walls where transformers are located should extend 0.5M past the ends of the roof structure.
Facilities are to be located externally in open compounds. These may be located adjacent to other buildings provided the exposed structures are of non-combustible construction, giving two hours fire resistance.
Openings, windows, doors or roof over-hang of substations above the compound or within three meters either side should not be accepted. Should adjacent structures not meet these requirements the minimum separation distance of six meters should be maintained.
With indoor transformers, stone-filled bunds may not be necessary, those with a siphon and drains are recommended to reduce fire intensity. Circumstances may dictate the total enclosure of critical transformers. The fire resistance of the compartment should be at least two hours and all openings in enclosing walls should be fire-stopped.
If the criteria for passive protection is not instituted on the transformers, they should be protected with an automatic “high velocity” water spray systems designed in accordance with the standards laid down in NFPA 15.
Active protection of transformers consists of installation of water spray systems to protect oil-filled equipment and designed to discharge water at the equipment and auxiliary equipment, which may be exposed to a fire. Total flood gaseous systems can be used for indoor locations.
To achieve effective means of active protection the following points should be noted:
Water spray installations should comply with NFPA Code 15.
Local containment areas should be provided for each unit of oil-filled equipment with sufficient drainage to prevent any overflow of oil and the applied water.
Containment areas surrounding apparatus protected by water spray systems should not be filled with stone ballast. A siphon utilised to drain the bunds contents in an emergency is more satisfactory and reduces the risk of overflowing bunds.
Action plan implementations
After succeeding in effectively fire separating substation sections, the next logical step would be to detect the fire. The time between the earliest fire condition and the transition into the flaming and heat-stages is crucial. Therefore, it is necessary to ensure that an early warning detection system be installed. The system should be capable of detecting a fire condition, sending a signal to a fire alarm control panel and activating relay contacts to ensure shut-off of systems, to reduce further electrical overloads.
Additionally, this alarm should be connected to a plant’s Scada/PLC system and main control centre, to alert personnel to the emergency, which has evolved. Also, all ventilation/air-conditioning systems should be inter-inked to automatically shut off, if a fire is detected.
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