• Electrical India
  • Mar 5, 2015

Nitrogen Injection Fire Prevention and Extinguishing System for Oil Filled Transformers

Different types of Transformers are used in power system. Commonly used power transformer is oil filled power transformer. In the oil filled power transformer oil act both as insulating and cooling media. During an internal fault resulting in an arc will cause fire. The fire may spread resulting from ignition of oil, causing explosion of tank, rupture of tank, core and winding and also burning the neighbouring equipment’s.

- Kiran Kumar R B


 The “Nitrogen injection and drain method” is one of the best fire prevention and extinguishing system for oil filled transformers for indoor/outdoor use. This system is fully automatic and unattended, Maintenance free and low cost compare to other systems.

Fire - Anatomy & Classification

What is Fire?

  Fire is an exothermic chemical reaction in which combustible material combines with oxygen in the atmosphere to give out heat and flame. But once fire has begun, the heat produced may often be sufficient to support or even accelerate the process. The materials involved in the combustion process may be in solid, liquid or gaseous state. They are also identified as two or three-dimensional

  • Two-dimensional fires are those where fire and fuel are on a single plane or flat surface. Ground fire or trench fire fall in this group of fires.
  • Three - dimensional fires are complicated fires of falling liquid streams or fuel under pressure escaping from a container. Fire resulting from leak of transformer oil falls under this category.

  Fire is not happen by their own self. It require some elements,Four factors/ Elements must be required in order before combustion occurs. They are,

  • Heat-Sufficient to raise the temperature to ignition point
  • Oxygen at sufficient quantity
  • Fuel
  • Chemical Chain reaction of above three mixtures.

  These Four elements may be represented in the form of a Tetrahedron.

  Just as removal of one or more sides of a tetrahedron causes its breakdown, the removal of one or more factors causes the fire to cease.

Stages of Fire

The Fire Triangle/Fire Tetrahedron

  Except explosion, most fires have humble starting and grow through the following four stages of combustions:

  • Incipient stage: At this stage decomposition is occurring at the surface of the fuel due to the influence of some form of heat. Products of combustion given off at this stage are invisible to the eye.
  • Smouldering Stage: At this Stage, up to 10% of the decomposing products released at the surface of the fuel are visible.
  • Flaming Stage: Vapours from the decomposing fuel have ignited and are at the stage where flames are self-propagating.
  • Heat stage: At this stage the burning has progressed to the point where the fire is still small but generating sufficient heat to warm the air immediately around the fire sending warm products of combustion upwards by convection. The above principle is used for the selection of fire detectors.

Classification of Fire as per Bureau of Indian Standards (BIS)

  Combustible substance can be present in three states - solid, liquid and gaseous state. The burning of most materials produces a flame, this occurs when vapour or gas given off by the liquid or solid materials is ignited. Depending upon the nature and state of the burning material, fires are generally classified into A, B, C or D type.

Class - A: Fire

  These are fires involving solid materials, normally of carbon compounds in which combustion.

  Generally occurs with the formation of glowing embers.

  These types of fires are most common.Examples of Class A - fires are fires involving wood, rags, paper, plastic coal etc.

Class - B: Fire

  These fires involve flammable liquids or liquefiable solids. That is, fires occurring in vapour-air mixture over the surface of flammable liquids such as gasoline, oils, paints & thinners. These types of fires occur in storage tanks and chemical storages.

Class - C: Fire

  These are fires involving liquefiable gases like propane, butane, LPG, H2, etc.

Class - D: Fire

  These are fires that occur in combustible metals such as magnesium, Titanium, Sodium etc. For Controlling such fires, special extinguishing agents/equipment and techniques are to be used.

Electrical Fires

  As per latest classification of Bureau of Indian Standards, electrical fires do not constitute a Particular class. The normal procedure is to cut off electric supply and extinguish the fire using Equipment & medium appropriate for the burning material (burning material may be PVC Insulation around the conductors or the conductor material themselves such as aluminium and Copper). Extinguishing agents which are non-conductors & non-damaging to the equipment should be used. In case of doubt positive isolation of electric supply should be done.

Important Properties of Flammable Materials

Flash Point

  Flash Point of a liquid is the minimum temperature at which sufficient vapour is given off to form an ignitable mixture with the air near the surface of the liquid. By Ignitable mixture is meant a mixture that is within the flammable range and that is capable of propagation of flame away from the ignition source. Although vapours are evolved at temperatures below the flash point, the concentration is not sufficient for ignition to occur.

Flash point of transformer oil is 146OC Fire Point

  The lowest temperature of a liquid at which vapours evolved is hot enough to support continuous Combustion is its fire point. This temperature is usually a few degrees above the flash point.

Auto Ignition Temperature

  Auto-ignition temperature is the minimum temperature required to initiate self-sustained combustion in a substance without any apparent source of ignition (spark or flame). The substance may be solid,liquid or gaseous. Thus auto ignition is the ignition of a combustible material without initiation by any external agency like a spark or flame - when the material has been raised to the auto ignition temperature.

Auto ignition temperature of is transformer oil 340OC

Vapour Pressure

  When a petroleum product is transferred into a container, it commences to vaporise, that is, it liberates gas into the space above it. There is also a tendency for this gas to re-dissolve in the Liquid and equilibrium is ultimately reached with a certain amount of gas evenly distributed throughout.

Fire Extinguishing Methods

  For the purpose of extinguishing fires, we have to basically limit or eliminate one or more elements of the fire triangle/ tetrahedron namely fuel, source of ignition, oxygen (air) and chain reaction for stopping the combustion process. Whatever may be the equipment or the extinguishing media used for fire fighting, they follow the following four basic mechanisms for fire extinction. These are the commonly adopted methods of extinguishing fires:

  • Starvation - Elimination of fuel.
  • Smothering - Limitation of Oxygen.
  • Cooling - Removal of temperature (Ignition Source).
  • Inhibition - Breaking the chain reactions.

Starvation

  Starvation can be brought about in 3 ways:

  • By removing combustible material from the vicinity of fire such as transfer of fuel from burning transformers, isolating or closing off valve on oil or gas line leading to fire, taking material out of the ware-house etc.

Smothering

  If the oxygen or air source to the burning material can be minimized or limited, the combustion will tend to retard. This method of extinguishment is accomplished by covering a burning surface with a wet blanket, Sand, DCP, Foam, etc.

Cooling

  If the heat generated during combustion can be dissipated at a faster rate than generation by some means, the combustion cannot sustain. By proper cooling, the heat of combustion is removed at a Faster rate thus reducing the temperature of the burning mass, continuously. In due course of time,the heat lost will be more than the production and the fire will die down. Application of water jet or spray for this cooling purpose is based on this method and principle. The efficiency of an extinguishing agent as a cooling medium depends upon specific and latent heats, as well as the boiling point. For these reasons, water is a good cooling medium as its specific and latent heats are higher than those of other common extinguishing media.

Chemical Chain Inhibition

  Combustion consists of rapid chain reactions involving hydrogen atoms and other active substances.

Reasons for Fire in Oil Filled Transformers

Fire due to internal causes

  Those faults that arise within in the transformer are called internal faults. These faults are very severe and there is always risk
of fire.

  • Line to ground fault or line to line fault on HV and LV external terminals.
  • Line to ground or line to line fault on HV and LV windings.
  • Short circuit between turns of HV and LV windings.
  • Due to break down of insulation of lamination bolts etc, a poor electrical connection of conductors which causes arcing under oil.
  • Coolant failure, which will cause a risk of temperature rise even below full load operation.
  • Bad load sharing between transformers, in parallel, which can cause overheating due to circulating current.

Fire causes due to external causes:

  Faults which occur in other part of the system outside the transformer, are called external faults.

  • Bushing failure.
  • OLTC fire.
  • Spread of fire from surroundings.

  All the above will cause internal break down of insulating material in the transformer and the resulting arc energy causes rapid rise of oil temperature. Due to rising of oil temperature the hot oil having exceeded the flash point temperature coming contact with oxygen from the external atmosphere, causes fire in the oil filled transformer.

Damages caused due to the fire in Oil filled Transformers

  • Explosion of tank.
  • Rupture of tank, Winding and core.
  • Personnel injury.
  • Neighboring equipment’s such as CT’s, Lightning arrestors, power cables, marshaling box etc, catching fire.

  Due to fire in the oil temperature rises rapidly and it will exceed flash point temperature. Hence pressure in the transformer tank rises rapidly PRV acts or even in some cases tank rupture.

Extract From Indian Electricity Rules 1956 (Second Amendment 2000)

Section NR 64(2) (e) (page No-41)

  Where a substation or a switching station with apparatus having more than 2000 liters of oil is installed, whether indoor or outdoor, the following measures should be taken namely

  • Provisions shall be made for suitable oil soak pit and where use of more than 9000 liters of oil in any one oil tank, receptacle or chamber is involved, provision shall be made for the draining away or removal of any oil which may leak or escape from the tanks receptacle or chambers containing the same special precautions shall be taken to prevent the spread of any fire resulting from the ignition of the oil from any cause and adequate provision shall be made for extinguishing any fire which may occur. 

Section WR 64(2) (f) (page No-42)

  • Without prejudice to the above measures, adequate fire protection arrangements shall be provides for quenching the fire in the apparatus. 
  • The transformer shall be protected by an automotive high velocity water spray system or by Carbon–dioxide (CO2) or BCF (Bromo chloro – difluro methane) or BTM (Bromotri fluro methane) fixed installation system or nitrogen injection and drain method.”

Nitrogen Injection & Drain Method

General description

  It Consists of fire extinguishing cubicle near the transformer, control box in control room, fire detector on transformer tank top cover, specially designed non-return valve in conservator pipe between buchholtz relay and conservator tank, and signal box placed on transformer. Fire extinguishing cubicle is connected to transformer tank by pipes for oil drain and nitrogen injection. Cable connections are provided from signal box to Control box and from Control box to Fire Cubicle.

  Nitrogen injection fire prevention and extinguishing system designed for oil filled transformer, shall prevent, tank explosion and the fire during internal faults resulting in an arc where tank explosion will normally take few seconds after arc generation and also extinguish the external oil fires on transformer top cover due to tank explosion or external failures like bushing fires, due to OLTC fires and fire from surrounding equipment’s.

  The system shall work on the principle of “DRAIN AND STIR” and on activation, shall drain a predetermine quantity of oil from the tank top through the outlet valve, to reduce the tank pressure and inject nitrogen gas at high pressure from the lower side of the tank through inlet valves to create stirring action and reduce the temperature of top oil surface, below flash point to extinguish the fire.

  Conservator tank oil shall be isolated during tank explosion and oil fire, to prevent aggravation of oil fire.

  Transformer isolation shall be an essential precondition for activating the system.

  The system shall be designed to operate manually, in case of failure of power source.

System Equipment’s

  • Fire extinguishing cubicle: With base frame and containing oil drain assembly, nitrogen cylinder, electromechanical control unit for oil drain and nitrogen release, pressure monitoring switch for backup protection for release of nitrogen, detectors necessary for monitoring system, flanges with gate / butterfly valves on top panel for connecting pipe connections from transformer, panel lighting etc. 
  • Fire extinction period
  • On commencement of N2 injection " → Maximum 30 seconds 
  • On system activation up to post cooling " Maximum 3 minute.

Control box: For monitoring system operation, automatic control and remote operation, with alarms, indication light switches, push buttons, audio signals, line fault detection suitable for tripping and signaling on station battery DC supply.

Pre-stressed non-return valve (PNRV): Working mechanically on transformer oil flow rate, with proximity switch for remote alarm indication and with visual position indicator.

  PNRV setting for operation " Minimum 60 liters per minute

  Fire detectors: The required number of fire detectors rated for 141OC for heat sensing fitted on the top of the transformers.

  Heat sensing temperature " 141OC
  Heat sensing area " 800 mm radius
Signal box: For terminating cable connections from PNRV and fire detectors.

Connection Details

  Schematic arrangement of Nitrogen injection fire prevention & extinguishing system for oil filled power transformers.

  • Fire extinguishing cubical (FEC) to be placed on plinth at about minimum 5 mtrs away from the transformer it consists of 
  • Nitrogen cylinder with regulator and falling pressure electrical contact manometer. 
  • Oil drain pipe with mechanical quick drain valve. 
  • Electromechanical control equipment’s for oil drain and pre-determined regulated nitrogen release. 
  • Pressure monitoring switch for backup protection for nitrogen release. 
  • Control box with activating, monitoring devices and line fault indicators, (to be placed in control room). 
  • Pre-Stressed Non-return Valve (PNRV) to be fitted in the conservator pipeline; between conservator and buchholtz relay operating mechanically on transformer oil flow rate with electrical signal for monitoring. 
  • Fire detectors fixed on transformer tank top cover for sensing fire. 
  • Signal box fixed on transformer side valve for terminating cable connections from fire detector and PNRV.

  Other connection requirements for system installation

  • Oil drain and nitrogen openings with gate valves on transformer tank, flanges with dummy piece in conservator pipe and fire detectors brackets on transformer top cover. 
  • Spare potential free contacts for system activating signals that is differential relay, buchholtz relay, and pressure relief valve for transformer isolation. 
  • Pipe connections between transformer to fire extinguishing cubicle & fire extinguishing cubical to oil pit. 
  • Cabling on transformer top cover all fire detectors to be connected in parallel and inter cabling between signal box to control box and control box to fire extinguishing cubicle. 
  • Plinth for fire extinguishing cubicle, oil pit with capacity as 10% of total oil quantity of transformer.

Operation

  During fire due to internal or external faults in the transformer the fire detector senses and gives activating signals to the system, Transformer is isolated from the supply and a predetermined quantity of oil drain commences, Simultaneously Nitrogen injected under pressure at a predetermined flow rate creates stirring action & temperature of top oil surface is brought below ignition point and fire extinguishers within 30 seconds. Nitrogen gas occupies space created by oil drain and act as an insulating layer between tank oil and fire on top cover. Pre-stressed non-returned valve blocks oil passage. Also isolates conservator tank oil and prevents escalation of fire.

System activating signals

  For Prevention: Differential relay trip + buchholtz or pressure relief valve trip + circuit breaker trip (Transformer isolation).
  For Extinction: Fire detector operation + buchholtz or pressure relief valve trip + circuit breaker trip (Transformer isolation).

Maintenance

  Practically Nitrogen Injection and Drain method of Fire Protection and Extinguisher system is maintenance free.

  It is recommended for monthly visual inspection for about 15-20 minutes and annual operational test schedule.

Advantage of Nitrogen Injection & Drain Method of Fire Protection

  • Low investment cost as compared to other conventional system.
  • Very low post fire and no secondary damages. 
  • Minimum maintenance and running cost. 
  • No climatic effects. 
  • Suitable for indoor / outdoor installation.
  • Minimum space requirement.
  • Multi signals for activation, eliminates false alarms.
  • Allows system testing on operational transformer not possible with conventional fire system. 
  • No moisture absorbing in inside the transformer due to presence of nitrogen. 
  • Great saving in cost, due to absence of moisture. 
  • Fully automatic, unattended and a fool proof system. 
  • It can be operated manually / automatically, local / remote control. 
  • No water reservoir or major civil work required.
  • Prevents transformer explosion ensuring system remains functioning.
  • Prevention of unplanned outages.
  • Considerable savings.
  • Improves overall power system reliability.

Conclusion

  The 'Nitrogen injection & Drain method' to protect the Power Transformer from catching fire is a best method technique suited for Indoor/Outdoor application because of its low investments, low maintenance & with practically no running cost beside assuring ‘Fool proof operation’ & is being satisfactorily adopted by many state electricity Boards.'Fire is good servant but bad master' safe way is best way.


Author is from Mysore region Electricity Supply Company.