evention 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
T
he “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 lo cost compare to other systems.
Fire - Anatomy & Classifiation !hat is Fire" !ire is an exothermic chemical chemical reaction in hich combustible combustible material combines combines ith oxygen in the atmosphere to give out heat and flame. "ut 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, li#uid or gaseous state. They are also identified as to or three$dimensional •
To$dimensional fires are those here fire and fuel are on a single plane or flat surface. %round fire or trench fire fall in this group of fires.
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Three $ dimensional fires are complicated fires of falling li#uid streams or fuel under pressure escaping from a container. !ire resulting from lea& of transformer oil falls un der this category.
!ire is not happen by their on self. 't re#uire some elements,!our factors/ (lements (lements must be re#uired in order before combustion occurs. They are, •
)eat$*ufficient to raise the temperature to ignition point
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+xygen at sufficient #uantity
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!uel
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hemical hain reaction of above three mixtures.
These !our elements may be represented in the form of a Tetrahedron. Tetrahedron. -ust as removal of one or more sides of a tetrahedron tetrahedron causes its brea&don, the removal removal of one or more factors causes the fire to cease. Stages of Fire
The Fire Triangle#Fire Tetrahedron (xcept explosion, most fires fires have humble starting and gro through through the folloing four stages stages of combustions •
$ni%ient stage t stage t this stage decomposition is occurring at the surface of the fuel due to the influence of some form of heat. 0roducts of combustion given off at this stage are invisible to the eye.
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Smouldering Stage t Stage t this *tage, up to 123 of the decomposing products released at the surface of the fuel are visible.
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Flaming Stage 4apour Stage 4apours s from the decomposing fuel have ignited and are at the stage here flames are self$propagating.
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'eat stage t stage t this stage the burning has progressed to the point here the fire is still small but generating sufficient heat to arm the air immediately around the fire sending arm products of combustion upards by convection. The above principle is used for the selection of fire detectors.
Classifiation of Fire as %er Bureau of $ndian Standards (B$S)
ombustible substance can be present present in three states $ solid, li#uid li#uid and gaseous state. The burning of most materials produces a flame, this occurs hen vapour or gas given off by the li#uid or solid materials is ignited. 5epending upon the nature and state of the burning b urning material, fires are generally classified into , ", or 5 type. Class - A Fire These are fires involving involving solid materials, normally normally of carbon compounds in hich combustion. combustion. %enerally occurs ith ith the formation formation of gloing embers. embers. These types of fires are most common.(xamples of lass $ fires are fires involving ood, rags, paper, plastic coal etc. Class - B Fire These fires involve flammable li#uids or li#uefiable li#uefiable solids. That is, fires occurring in vapour$air mixture mixture over the surface of flammable li#uids such as gasoline, oils, paints 6 thinners. These types of fires occur in storage tan&s and chemical storages. Class - C Fire These are fires involving involving li#uefiable gases li&e propane, butane, butane, 70%, )8, etc. Class - * Fire These are fires that occur in combustible metals such as magnesium, Titanium, Titanium, *odium etc. !or ontrolling such fires, special extinguishing agents/e#uipment and techni#ues are to be used. Eletrial Fires s per latest classification classification of "ureau of 'ndian *tandards, electrical fires do not constitute a 0articular class. The normal procedure is to cut off electric supply and extinguish the fire using (#uipment 6 medium appropriate for the burning material 9burning material may be 04 'nsulation around the conductors or the conductor material themselves such as aluminium and opper:. (xtinguishing agents hich are non$conductors 6 non$damaging to the e#uipment should be used. 'n case of doubt positive isolation of electric supply should be done.
$m%ortant +ro%erties of Flamma,le aterials Flash +oint !lash 0oint of a li#uid is the minimum temperature at hich sufficient sufficient vapour is given off to form an ignitable mixture ith the air near the surface of the li#uid. "y 'gnitable mixture is meant a mixture that is ithin the flammable range and that is capable of propagation of flame aay from the ignition source. lthough vapours are are evolved at temperatures temperatures belo the flash flash point, the concentration concentration is not sufficient for ignition to occur. Flash %oint of transformer oil is ./0OC Fire +oint
The loest temperature temperature of a li#uid at hich vapours evolved evolved is hot enough to support continuous ombustion is its fire point. This temperature is usually a fe degrees above the f lash point. Auto $gnition Tem%erature uto$ignition uto$igni tion temperature is the minimum temperature re#uired re#uired to initiate self$sustained combustion in a substance ithout any apparent source of ignition 9spar& or flame:. The substance may be solid,li#uid or gaseous. Thus auto ignition is the ignition of a combustible material ithout initiation by any external agency li&e a spar& or flame $ hen the material has been raised to the auto ignition temperature. Auto ignition tem%erature of is transformer oil 1/2OC 3a%our +ressure ;hen a petroleum product is transferred into a container, itit 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 7i#uid and e#uilibrium is ultimately reached ith a certain amount of gas evenly distributed throughout. Fire Extinguishing ethods !or the purpose of extinguishing extinguishing fires, e have to basically limit limit or eliminate one or more more elements of the fire triangle/ tetrahedron namely fuel, source of ignition, oxygen 9air: and chain reaction for stopping the combustion process. ;hatever may be the e#uipment or the extinguishing media used for fire fighting, they follo the folloing four basic mechanisms for fire e xtinction. These are the commonly adopted methods of extinguishing fires •
*tarvation $ (limination of fuel.
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*mothering $ 7imitation of +xygen.
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ooling $
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'nhibition $ "rea&ing the chain reactions.
Starvation *tarvation can be be brought about in = ays •
"y 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, ta&ing material out of the are$house etc.
Smothering 'f the oxygen or air source to the burning material material can be minimi>ed or limited, limited, the combustion ill ill tend to retard. This method of extinguishment is accomplished by covering a burning surface ith a et blan&et, *and, 50, !oam, etc.
Cooling 'f the heat generated during combustion combustion can be dissipated at a faster rate than than generation by some means, the combustion cannot sustain. "y proper cooling, the heat of combustion is removed at a !aster rate thus reducing the temperature of the burning mass, continuously. 'n due course of time,the heat lost ill be more than the production and the fire ill die don. pplication pplication of ater 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 ell as the boiling point. !or these reasons, ater is a good cooling medium as its specific and latent heats are higher than those of other common extinguishing media. Chemial Chain $nhi,ition ombustion consists of rapid rapid chain reactions involving hydrogen hydrogen atoms and other active substances.
Reasons for Fire in Oil Filled Transformers Fire due to internal auses Those faults that arise ithin in the transformer are called internal faults. faults. These faults are very severe and there is alays ris& of fire. •
7ine to ground fault or line to line fault on )4 and 74 external terminals.
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7ine to ground or line to line fault on )4 and 74 indings.
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*hort circuit beteen turns of )4 and 74 indings.
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5ue to brea& don of insulation of lamination bolts etc, a poor electrical connection of conductors hich causes arcing under oil.
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oolant failure, hich ill cause a ris& of temperature rise even belo full load operation.
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"ad load sharing beteen transformers, in parallel, hich can cause overheating due to circulating current.
Fire auses due to external auses !aults hich occur in other part of the system outside the transformer, are called external faults. •
"ushing failure.
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+7T fire.
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*pread of fire from surroundings.
ll the above ill cause internal brea& don of insulating material material in the transformer and the resulting arc energy causes rapid rise of oil temperature. 5ue to rising of oil temperature the hot oil having exceeded the flash point temperature coming contact ith oxygen from the external atmosphere, causes fire in the oil filled transformer transformer.. *amages aused due to the fire in Oil filled Transformers •
(xplosion of tan&.
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0ersonnel injury.
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Neighboring e#uipment?s such as T?s, 7ightning arrestors, poer cables, marshaling box etc, catching fire.
5ue to fire in the oil temperature temperature rises rapidly and it ill ill exceed flash point temperature. temperature. )ence pressure in the transformer tan& rises rapidly 0<4 acts or even in some cases tan& rupture.
Extrat From $ndian Eletriity Rules .450 (Seond Amendment 6222) Setion 7R 0/(6) (e) (%age 7o-/.) ;here a substation or a sitching sitching station ith apparatus having more more than 8222 liters of oil is installed, installed, hether indoor or outdoor, the folloing measures should be ta&en namely •
0rovisions shall be made for suitable oil soa& pit and here use of more than @222 liters of oil in any one oil tan&, receptacle or chamber is involved, provision shall be made for the draining aay or removal of any oil hich may lea& or escape from the tan&s receptacle or chambers containing the same special precautions shall be ta&en to prevent the spread of any fire resulting from the ignition of the oil from any cause and ade#uate provision shall be made for extinguishing any fire hich may occur.
Setion !R 0/(6) (f) (%age 7o-/6) •
;ithout prejudice to the above measures, ade#uate fire protection arrangements shall be provides for #uenching the fire in the apparatus.
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The transformer shall be protected by an automotive high velocity ater spray system or by arbonAdioxide 9+8: or "! 9"romo chloro A difluro methane: or "TM 9"romotri fluro methane: fixed installation system or nitrogen injection and drain method.”
7itrogen $n8etion & *rain ethod 9eneral desri%tion
't onsists of fire extinguishing cubicle near the transformer, transformer, control box in control room, fire detector on transformer tan& top cover, specially designed non$return valve in conservator pipe beteen buchholt> relay and conservator tan&, and signal box placed on transformer transformer.. !ire extinguishing cubicle is connected to transformer tan& by pipes for oil drain and nitrogen injection. able connections are provided from signal box to ontrol box and from ontrol box to !ire ubicle. Nitrogen injection fire prevention prevention and extinguishing system designed for oil filled transformer, transformer, shall prevent, tan& explosion and the fire during internal faults resulting in an arc here tan& explosion ill normally ta&e fe seconds after arc gene ration and also extinguish the external oil fires on transformer top cover due to tan& explosion or external failures li&e bushing fires, due to +7T fires fires and fire from surrounding e#uipment?s. The system shall or& on the principle of “5<'N N5 N5 *T'<” and on activation, shall drain a predetermine #uantity of oil from the tan& top through the outlet valve, to reduce the tan& pressure and inject nitrogen gas at high pressure from the loer side of the tan& through inlet valves to create stirring action and reduce the temperature of top oil surface, belo flash point to extinguish the fire. onservator tan& oil shall be isolated during tan& explosion and oil fire, fire, to prevent aggravation of oil fire. fire. Transformer Trans former isolation shall be an essential precondition for activating the system. The system shall be designed to operate operate manually, in case of failure of poer source. source. System E:ui%ment;s •
Fire extinguishing u,ile ;ith u,ile ;ith base frame and containing oil drain assembly, nitrogen cylinder, electromechanical electromechanical control unit for oil drain and nitrogen release, pressure monitoring sitch for bac&up protection for release of nitrogen, detectors necessary for monitoring system, flanges ith gate / butterfly valves on top panel for connecting pipe connections from transformer, panel lighting etc.
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!ire extinction period
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+n commencement of N8 injection B C Maximum =2 seconds
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+n system activation up to post cooling B Maximum = minute.
Control ,ox !or ,ox !or monitoring system operation, automatic control and remote operation, ith alarms, indication light sitches, push buttons, audio signals, line fault detection suitable for tripping and signaling on station battery 5 supply.
+re-stressed non-return valve (+7R3) ;or&ing mechanically on transformer oil flo rate, ith proximity sitch for remote alarm indication and ith visual position indicator. 0N<4 setting setting for operation B Minimum D2 liters liters per minute !ire detectors The The re#uired number of fire detectors detectors rated for 1E1+ for heat sensing fitted fitted on the top of the transformers. )eat sensing temperature B 1E1+ )eat sensing area B F22 mm radius !or terminating cable connections from 0N<4 and fire detectors. Signal ,ox ,ox !or Connetion *etails *chematic arrangement of Nitrogen Nitrogen injection fire prevention 6 extinguishing system for oil filled filled poer transformers. •
!ire extinguishing cubical 9!(: to b e placed on plinth at about minimum G mtrs aay from the transformer it consists of
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Nitrogen cylinder ith regulator and falling pressure electrical contact manometer.
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+il drain pipe ith mechanical #uic& drain valve.
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(lectromechanical control e#uipment?s for oil drain and pre$determined regulated nitrogen release.
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0ressure monitoring sitch for bac&up protection for nitrogen release.
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ontrol box ith activating, monitoring devices and lin e fault indicators, 9to be placed in control room:.
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0re$*tressed Non$return 4alve 90N<4: 90N<4: to be fitted in the conservator pipelineH beteen conservator and buchholt> relay operating mechanically on transformer oil flo rate ith electrical signal for monitoring.
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!ire detectors fixed on transformer tan& top cover for sensing fire.
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*ignal box fixed on transformer side valve for terminating cable connections from fire detector and 0N<4 0N<4..
+ther connection re#uirements re#uirements for system system installation installation •
+il drain and nitrogen openings ith gate valves on transformer tan&, flanges ith dummy piece in conservator pipe and fire detectors brac&ets on transformer top cover.
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*pare potential free contacts for system activating signals that is differential relay, buchholt> relay, and pressure relief valve for transformer isolation.
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0ipe connections beteen transformer to fire extinguishing cubicle 6 fire extinguishing cubical to oil pit.
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abling on transformer top cover all fire detectors to be connected in parallel and inter cabling beteen signal box to control box and control box to fire extinguishing cubicle.
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0linth for fire extinguishing cubicle, oil pit ith capacity as 123 of total oil #uantity of transformer.
O%eration 5uring fire due to internal internal or external faults in the transformer transformer the fire detector senses senses and gives activating signals to the system, Transformer is isolated from the supply and a predetermined #uantity of oil drain commences, *imultaneously Nitrogen injected under pressure at a p redetermined flo rate creates stirring action 6 temperature of top oil surface is brought belo ignition point and fire extinguishers ithin ithin =2 seconds. Nitrogen gas occupies space created by oil drain and act as an insulating layer beteen tan& oil and fire on top cover. 0re$stressed non$returned valve bloc&s oil passage. lso isolates conservator tan& oil and prevents escalation of fire.
System ativating signals For +revention 5ifferential relay trip I buchholt> or pressure relief valve trip I circuit brea&er trip 9Transformer 9Transf ormer isolation:. For Extintion !ire Extintion !ire detector operation I buchholt> or pressure relief valve trip I circuit brea&er trip 9Transformer 9Transf ormer isolation:. aintenane
0ractically Nitrogen Nitrogen 'njection and 5rain method of !ire 0rotection and (xtinguisher system is maintenance free. 't is recommended for monthly monthly visual inspection for about 1G$82 minutes minutes and annual operational test schedule.
Advantage of 7itrogen $n8etion & *rain ethod of Fire +rotetion •
7o investment cost as compared to other conventional system.
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4ery lo post fire and no secondary damages.
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Minimum maintenance and running cost.
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No climatic effects.
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*uitable for indoor / outdoor installation.
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Minimum space re#uirement.
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Multi signals for activation, eliminates false alarms.
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llos system testing on operational transformer not possible possible ith conventional fire system.
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No moisture absorbing in inside the transformer due to presence of nitrogen.
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%reat saving in cost, due to absence of moisture.
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!ully automatic, unattended and a fool proof system.
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't can be operated manually / automatically, local / remote control.
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No ater reservoir or major civil or& re#uired.
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0revents transformer explosion ensuring s ystem remains functioning.
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0revention of unplanned outages.
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onsiderable savings.
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'mproves overall poer system s ystem reliability.
Conlusion
The JNitrogen injection 6 5rain methodJ to protect the 0oer Transformer Transformer from catching catching fire is a best method techni#ue suited for 'ndoor/+utdoor application because of its lo investments, lo maintenance 6 ith practically no running cost beside assuring K!ool proof operation? 6 is being satisfactorily adopted by many state electricity "oards.J!ire is good servant but bad masterJ safe ay is best ay.
