1

The Power of a Transformer Fire in the Context of Brazilian Substations

Duarte D., Universidade Federal de Pernambuco, Brazil Ancelmo Pires, T, Universidade Federal de Pernambuco, Brazil, Medina Pena, Companhia Hidroeletrica do São Francisco; and

Bastos, G, FURNAS.

SUMMARY

A fire in a transformer can damage life, property and the environment. There is also mental distress

and financial losses suffered by society in cases of a blackout. The summer of 1997 was known as the

summer of blackouts (verão dos apagões) by the people who were living in Rio de Janeiro due to

several fires in transformers.

Traditionally, expected damages can be estimated by studying loss histories, i.e. by compiling

previous fires´ amounts. However, loss expectancy (measured in dollars) will depend on the substation

layout and fire protection technology. In a substation in the North of Brazil a fire in one transformer

spread to other units due to the poor design of the firewall and the tanker under the transformer. In

another substation, the transformer was operating under normal conditions but for some reason the

spray system was activated by a false alarm. Since the sprinkler head was mounted facing the bushing

instead of the tanker, an external arc was created causing the rupture of the bushing and leading to a

fire. The objective of this paper is to estimate the power of a transformer fire on a target as well as on

the organization mission and objectives.

KEYWORDS: Pool Fire, Transformer, Brazilian Substations.

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INTRODUCTION

Brazil has an installed capacity of about 94,000MW greater than England and less than France. Most

of this energy is coming from hydroelectric power plant. They are responsible for around 81,000MW.

The Brazilian demand is approximately 60,000MW. Brazil has about 70,000Km of 230kV

transmission line. Our transformation capacity is about 171 GVA in the next eight years it will

increase 50%.

Electricity is taken for granted and it only when light go out and our day to day routine seen to move

like a slow motion picture that suddenly we become aware of our dependence on power plants,

transmission lines and substations, and so on. Over the last 20 years blackouts have been included in

the list of major disaster, such as storm, hurricane, earthquake and floods. Therefore consumers have

become more aware of an increasing fire and explosion involved oil filled transformers in generation,

transmission and distribution substations. A fire initiated within the transformer can completely

destroy a substation and burn down any associated plant or building located nearby. Many transformer

manufacturers try to minimize the size of the problem, but the outcome is that the fire spread and

envelops the transformer and the direct and indirect financial impact could be unacceptable for

society. Some blackouts resulted from fire and explosion in Brazilian substations are listed below:

1) In the summer of 1997 nine transformers exploded or caught fire in Rio de Janeiro.

2) In November 2003 a fire in a substation at Goiana city left thousands people without energy

during 1h and 30min.

3) On March 2004 a fire at Pirituba substation left the suburbs in northeastern of São Paulo

without energy during 1h 30min.

4) In January 2005 a circuit break exploded at a substation in northeast of Brazil left around six

thousands people in Paraiba and Rio Grande do Norte without energy during 39 minutes.

5) Frebuary 2005 a fire in one of the generator step up transformer at Tucuruí Power Plant spread

to an auxiliary transformer and a compact substation nearby. It left 600MW unavailable. The

replacement and repair costs only were estimated around 21million of dollars.

6) On May 2005 a cable fire shut down Paulo Afonso III power plant.

The losses incurred by a blackout are not merely financial. In fact the fire loss of a substation forms

only a minor part of the total overall cost in terms of energy availability, human loss and organization

reputation. The costs due to the lack of fire risk management in the electric sector are no longer

acceptable by the Brazilian society.

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POTENTIAL LOSS FROM TRANSFORMER FIRE

Estimating the loss arising from a fire is rarely easy and indeed could be to some extent not possible

due to the knowledge available and technology limitations. It may appear simple to estimate the

replacement costs of substation equipments or other structures nearby which has been damaged or

destroyed. This is frequently a comparatively minor part of the total loss incurred. If a fire has been

serious enough to cause a blackout the social cost will be a significant component of the overall cost.

On the other hand, ANEEL (i.e the regulatory agency created by the government for the electric

sector) has emphasised that the electric organizations (including power plants, transmission

and distribution systems) should be punished with high fines whenever they cannot provide

electric energy to their consumers. ANEEL also will punished them if a transmission line, a

transformer or any other equipment is not available. These fines, if applied, could

compromise the organisation’s economic health and reputation.

The exploitation of people, fixed assets (such as equipment, property, etc.), information/knowledge

will yield cash flow. In addition business will usually also draw benefit from organization’s reputation

and goodwill and physical environment. A transformer fire will have an impact on most of these

assets.

A transformer fire or explosion could cause extent damage either to the substation fixed assets or other

plants nearby as a result of the thermal and blast waves impacts. In addition, transformer could be

installed underground, such as at Itaipu power plant. Another important undesirable effect that

should be considered which is associated with fire, explosion and fragment projections is the

domino effect. It is possible under certain conditions that the fire or explosion result from a

transformer is expanded to other neighbouring equipments of the substation, creating a chain

major accident with extended consequences. Attention should be given not only to health

effects, but also to the resistance of other equipment and structure to certain radiation level.

A step up transformer fire could put out of action for an extended period of time a natural gas

thermoelectric power plant, for example, and then the reduction of production could result in a

significant reduction in the cash flow to the plant owners. This situation may be exacerbated by having

to acquire alternative supplies to meet contractual commitments or by having to pay penalties as

impose by ANEEL due to the unavailability of the transformer and gas turbine.

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A poor mechanic project of the transformer could cause oil leakage, as result there will be the

contamination of the oil. The oil degradation could lead to a transformer fire. In addition, a

failure in the maintenance of the oil filling transformer could result in a pool fire, spray fire or

Boiling Liquid Expanding Vapour-BLEVE. The principal environment impact probably could

be from the discharge of oil on the soil, which could affect the phreatic surface. The berm

under the transformer should collect the oil in the case of a major leak in order to prevent soil

contamination.

What are the costs of a bad reputation? While a power plant will suffer from having a

transformer fire, other companies could be adversely affected by the impaired public

perception of the electric sector as a whole. The reputation of the power plants and

transmission and distribution sectors will be generally set by its poorer performers.

Although establishing the actual cost of a transformer fire can be difficult there is no doubt

that some large bill has to be paid. The organization will usually look first to his insurer for

recompense, and claims settlements may well cover a large part of the losses. It is, however,

important that the nature of insurance is understood. It will not cover the losses from a bad

reputation or the penalties from business interruption loss.

TRANSFORMER FAILURES versus GLOBALIZED ECONOMY

According to Bastos et al in the last 10 years the failure rate of the new transformers acquired

by FURNAS increased either during acceptance tests or operation, as shown in Table 1.

TABLE 1. Failure rate on FURNAS new transformers. Source: Bastos et al, 2006.

Transformer Voltage Number of Unit Acquired Number of Failure

Acceptance Tests Operation

800kV 11 5 8

500kV 33 17 8

>345kV 26 5 2

An investigation carried out by FURNAS showed that the main cause of failures of their new

transformers during operation has origin on the active part (include core, winding, etc.) and

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bushing. Active part failures related repair cost and time are normally much high compared to

failure from intrinsic protection devices, tap changes and bushing.

All failures are unwanted and cause of concern to utilities organizations as well as transformer

manufacturers. Repair cost and time depend on the affected component. The repair costs of a

component related to the transformer price according to equation 1, are listed below.

Active Part............................. 60-70%

Winding.............. 35-40%

Core.................... 25-30%

Tap Changer.......................... 10-15%

Bushing.................................. 4-6%

Self Protection Devices......... 1-2%

( ) ( ) βα −= kVAinPOWERNOMINALkVinVOLTAGEkrTransformeprice Equation 1

∴ k is the constant for each component, α is the voltage correction factor and δ is the

power correction factor.

The Net Internal Product (PIB) is proportional to the electric demand. Before the 90 decade

only the Government invested in the electrical market. In addition, the government for quite

some time did not invested on power plant and distribution utilities, despite the electricity

consumption have been increasing over the years. As a result, aging transformers are often

overloaded. As it (the government) could not face the challenge of investing in the sector

alone, it decided to deregulate the Brazilian Market. As a result, new private utilities were

created as well as organization de capital aberto enter in the Brazilian Market.

The National Regulatory Agency of the Brazilian Sector System-ANEEL has been pushing

the new owners to optimize the time of the project and construction phases of the new utility

facilities. On the other hand, a globalize economy creates pressure on the manufactures to

reduce their costs. This could be achieved by reducing manufacturer time through either a

better tools or manufacturer processes.

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Operational condition has been become more severe in the last years. The transformers have

to operate in conditions more appropriated to very fast transient or resonant voltage surges.

These transient conditions could be the result of maneuvers in the power system. At the same

time the manufacturers tools have been improving, therefore there has been the tendency to

reduce the safety factor of the transformer intrinsic protections, active parts, tap change or

bushing to face the globalize market. In other words, the manufacturers have been reducing

the transformer safety margins as their intention is attend only the minimum requirements of

the valid standards. On the other hand, the acceptance tests (type and routine tests) do not

reflect the operational condition in the field.

The evolution of the transformer projects shown in Tables 2 and 3 shown a tendency of the

manufacturers to proposed a more compact projects probability due to an increase

competitiveness in a globalize market, which have been leading them to reduce their profit

margins.

TABLE 2. Core type transformer project evolution: Voltage per spiral.

Year Volts/Spiral

1915 2-4 volts/SP

1932 8-10 volts/SP

1975 19-20 volts/SP

1981 200 volts/SP

TABLE 3. Project evolution of the transformer oil volume.

Year Litter/kVA

1915 7,6

1930 3,8

1945 1,9

1960 1,3

1975 0,6

1977 0,5

1979 0,4

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TRANSFORMER FIRE SCENARIOS

There is an increasing risk associated with the expansion of the power system, mainly due to

transformers overload as they become older or the new ones have their safety margin reduced.

Transformer fires are associated to internal faults, dielectric degradation, either bushing or tap

change failures. According to D J Allan the incident of fires in utilities has generally

increased from one fire every five years to more than one fire a year over the last 10 years. If

the transformer are installed underground substations or in caverns the combined risk of a fire

and an explosion can lead to major losses.

Some bushing failure causes are represented in detail on Figure 1. If a bushing failure oil is

expelled. If there is a rupture at the oil end of the bushing housing, oil from the transformer

will feed a fire and the fire will be extend to the main tanker. In a similar situation a load tap

changer fitted in a separate compartment may explode and catch fire. Under these

circumstances a fire will spread through the pressboard barrier. Figure 2 shows how a bushing

failure could result in a transformer fire. When a transformer tanker rupture and oil is expelled

it does not necessarily lead to a fire. It depends on how quickly the transformer protections

system will operate. On the other hand, if no fire occurs a major ecological impact could

happen due to soil contamination.

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FIGURE 1. Probable bushing failures causes.

AGING

BUSHING FALIRE

DESIGN &

MANUFACTURE

RESIDUAL HUMIDITY

IMPREGNATION INADEQUATE

OIL CONTAMINATIO

ELECTRIC FIELD IS BAD

DIELETRIC STRESS

SHORT CIRCUIT BETWEEN LAYERS

COOLING SYSTEM

DEFICIENCIES

PAPER IS NOT WELL FIXED

INADEQUATE DRYING

CONDUCTIVE STRIPS

LOCATION IS INAPROPRIATE

CONTACTS OVERHEATING

CONTAMINATION BY HUMIDITY AND

OXIGEN

GASKET DEGRADATION

HOT SPOTS

DIRECT INCIDENCY OF

INFRARED RAYS

CORROSSION ASSEMBLY OPERATION MANTAINEC

E PORCELAIN

FISSURES

SCREW MOMENTUM WAS INAPPROPRIETE

ELECTRIC DISCHARGE ON THE PORCELAIN

OIL STAGNATION

ELETRIC DISCHARGE

OPERATION ON

HORIZONTAL POSITION

MECHANICAL FAILURES

STORAGE ON THE

HORIZONTAL POSITIONl

DIELECTRIC DEGRADATION

DIELECTRIC DEFORMATIO

PARTICLES DECATATIO

N

INSULATING PAPER OUTSIDE THE OIL

POLLUTION

OIL BUSHINH LEAKAGE TO

TRANSFORMER

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EXPLOSION OF THE BUSHING

BUSHING FIRE

THE CORRENT REQUIRED

BY TRANSFORMER IS ABOVE

IT UPPER LIMIT

THE CORRENT REQUIRED

BY TRANSFORMER IS BELOW

IT UPPER LIMIT

TANKER DO NOT WITHSTAND

THE PRESSURE

THE TANKER HOLE

IS NEAR THE BUSHING

IGNITION NO IGNITION

NO TRANSFORMER FIRE TRANSFORMER FIRE

THE TANKER HOLE IS

FAR FROM THE BUSHING

TANKER WITHSTAND

THE PRESSURE

NO BUSHING FIRE

NO EXPLOSION OF THE BUSHING

BUSHING FAILURE0

5

7

10

12

1413

11

9

8

6

43

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FIGURE 2. A fire scenario due to a bushing failure.

In Figure 2, the scenario represented by the path 0-1-3-5-7-9-11-13 shows that a transformer fire could

be the result of the follow event sequence: after the bushing failure, it explode and catch fire, the

current required by the transformer is above it upper limit, the main tanker do not withstand the

internal pressure, the hole in the tanker is located near the bushing and a ignition source is present.

The scenario represented by the path 0-2 shows that a transformer fire will not take place if the

bushing does not explode after it fails.

The scenario represented by the path 0-1-4 shows that even after the bushing failure and it explosion,

the transformer fire will not happen if the bushing explosion does not initiate a fire.

The scenario represented by the path 0-1-3-6 shows that as a result of a bushing failure it could

explode follow by a fire, but if the current required by the transformer is below it upper limit the

transformer fire will not take place.

The scenario represented by the path 0-1-3-5-8 shows that as a result of the bushing failure it explode

follow by a fire in addition the current required by the transformer is above it upper limit, but as the

tank withstand the internal pressure the transformer fire will not happen.

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The scenario represented by the path 0-1-3-5-7-10 indicates that a transformer fire could be avoided if

the tanker hole is located far from the bushing, even if the bushing fails, explodes and catches fire as

well as if either the current required by the transformer is above it upper limit or tank fails.

The scenario represented by the path 0-1-3-5-7-9-12 indicates that the transformer fire will not happen

if there are no ignition sources present, even if the bushing fails, explodes and catches fire, the current

required by the transformer is above it upper limit, as well as the tanker hole is near the bushing.

Figure 3 shows a power transformer involved in a pool fire. Even if there is not any nearby

ignition source, the loss of the dielectric fluid can cause an internal fault to develop in the

transformer and cause arcing at the bushing.

FIGURE 3. A transformer involved in a pool fire.

Another fire scenario for liquid-filled transformers could start with the degradation of the

insulating materials. The degradation velocity of the products based cellulose depend on the

temperature, moisture contents, the amount of oxygen and acids in the oil. Heat and moisture

are the main enemies of the solid paper insulation, and the oxidation process act as an

accelerator. When the degradation of the cellulose chains occurs products such as carbon

monoxide and carbon dioxide are found dissolved in the oil.

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The deterioration of the insulating oil is normally associated with oxidation. Due to the

presence of oxygen and water, insulating oil oxidizes even under ideal conditions. The

insulating properties of the oil are also affected by contaminants from other solid materials

inside the transformer. The first stage of the oxidation is the attack of oxigen on the

hydrocarbon molecule to form peroxides which dissociate to form free radicals which can act

as initiators for chain reactions involving free radicals and hydrocarbons. The propagation

reactions are caplable of repetition over a number of cycles for each hydrocarbon free radical

supplied by the initiation reaction. The final products of the oxidation process are acid

material, which can affect the characteristic of the insulating fluid as well as damage the

electrical components of the transformer.

Fires are damaging for two principal reasons. First they generates products that are harmful to

people, impeding escape and causing injuries and fatalities. Second, the heat transfer to items

of the substation or supporting structures cause collapse and escalation of the fire (i.e. domino

effect). Figure 4 shows the results of a transformer fire which render a substation unavailable

leaves about one million people without energy, because the structure that support the high

voltage bar collapse.

FIGURE 4. Collapses of the high voltage bar due to a transformer fire.

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Another fire scenario could involves a leaky indoor transformer casing. A poor mechanic

project of the transformer tanker or it degradation probably due to corrosion could cause oil

leakage that eventually produce a growing pool of transformer fluid on the floor. If the liquid

vapor reaches an external ignition source, peharps a pilot light on a gas-fired appliance in an

adjacent area, a pool fire will result.

After having identified and understood some possible transformer fire scenarios the next step

will be answer the question: What will be the extent of a transformer pool fire on it

surroundings? In the next section the physical phenomena involved will be modeled.

TRANSFORMER POOL FIRE

Liquid fuel may burn in an open storage container or on the ground in the form of a spill.

Figure 3 shows a transformer involved in a pool fire. When spilled the flammable liquid may

form a pool of any shape and thickness and may be controlled by the confinement of the area

geometry such as a curbing or dick. Once ignited a pool fire spread rapidly over the surface of

the liquid spill area. Pool fire in a transformer may result from either rupture at the oil end of

the bushing housing or due to the tank rupture when oil is expelled, Figure 2 shows the

dynamic feature of a transformer pool fire. For a given amount of fuel spill with a large

surface area burn with a high heat release rate-HRR for a short duration. And spill with a

smaller surface area burn with a lower heat release rate for a long duration.

The thermal radiation from a pool fire depend on a number of parameters such as composition

of the hydrocarbon, size and shape of the fire, duration of the fire, its proximity of the target

(i.e. the object at risk) and the thermal characteristics of the object exposed to the fire. The

objective of this paper is to identify the best and simple techniques which could be used in a

day to day routine of an electrical engineer to estimate the heat release rate from a transformer

fire.

The heat transfer from the fire to the liquid pool can be represents by equation 1.

radiativeconvectionconductive qqqq ++= ........................... Equation 1

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A pool fire with a diameter greater than about 1 meter the radiative term in equation 1

dominates the heat flux to the pool, because the flame becomes a large, optically thick,

radiating blackbody. This is the region of interesting since diameter of bushing base of a

power transformer is assumed to be greater than 1 meter.

The general equation for a pool fire heat release rates with unlimited air access is given by

equation 2. Where Q is the chemical heat release rate (kW), "m is the mass burning rate per

unit surface area (g/m2s), cH∆ is the net heat of combustion (kJ/g), chemx is the combustion

efficiency and D is the pool diameter (m).

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.... 2" DxHmQ chemc π∆= ........................... Equation 2

An equation for the mass burning rate of burning liquid surface under windless conditions has

been given by Burgess and Zabetakis (1962 BM RI 6099) as follows:

( )Dkemm ." 1 −∞ −= ........................... Equation 3

Where "∞m is the asymptotic burning rate for large pools and k is an effective absorption

coefficient including the mean beam length correction factor. The mass burning rate -"m and

the asymptotic burning rate - "∞m could be expressed in m/s. In other worlds, the mass

burning rate in g/m2s is determine by multiplying the burning rate (m/s) by the liquid fuel

density. According to Babrauskas the burning rate can either decrease or increase in a tank or

dike with a large freeboard or lip height, with decreased burning rates being more common at

a freeboard height greater than about 20% of the tank. For a transformer oil

)/(39 2" smgm −=∞ , )(7,0 1−= mk , )/(4,46 gkJH c =∆ and 84,0=chemx .

Thomas has developed a correlation for the visible length of flame take into account the wind

velocity, which is expressed in equation 4. Where H is the pool flame height (m), aρ ambient

air density (kg/m3), g gravity acceleration (m/s2) and *u nondimentional wind velocity given

by equation 5. wu is the wind velocity (m/s).

14

21,0*

67,0"

..

55 −

= u

gD

m

D

H

aρ........................... Equation 4

3/1"

*

..

=

v

w

Dmg

uu

ρ........................... Equation 5

Thomas gave the following correlation for flame tilt based on data from two dimensional

wood cribs.

49,0

"..7,0cos

−

=Θ

a

w

Dmg

u

ρ

........................... Equation 6

Wind can significantly increase the effective pool diameter and corresponding mass burning

rates. The wind tends to both tilt and increase the flame diameter in the downwind direction.

Mudan and Croce suggest the following correlation to estimate the increase in flame diameter.

But, equation 7 should be used with caution at very large wind velocities or small pool

diameter due to a possible flame blow off.

48.0069.02

25.1

⋅⋅=

a

vww

Dg

u

D

D

ρρ

........................... Equation 7

Where wD is the effective flame diameter in the presence of wind, wu is the wind velocity

(m/s) measured at an elevation of 10m,vρ and aρ are the densities of vapour and air,

respectively.

The thermal radiation flux from a pool fire can be estimate by a point source model or a solid

flame model. The point source model removes most of the geometrical parameters from the

calculation. It assumes that all of the radiative energy from the fire is emitted at a single point

rather than distributed over an idealized shape meant to represent the fire. It requires an

estimate of the total heat release rate-HRR of the fire, and the fraction of that energy that is

15

emitted as thermal radiation. If atmospheric absorption effect are negligible, and the target

distance is large compared to the flame height the radiant point source approximation

provides an attractive simplification to obtain the incident heat flux on the target, equation 8.

2"

..4 x

Qqr π

= ........................... Equation 8

In a solid flame radiation model the thermal radiation flux, "rq ,from a fire to a nearby object

is given by equation 9. Where F is a geometric view factor that intercepted by the receiving

object, i.e. target. ζ is the atmospheric transmissivity to thermal radiation, it is a function of

humidity and the distance between the radiation source and receiver. fε is the effective

emissivity of the flame, expressed by ( )Df e ⋅−−= κε 1 where κ and D are the attenuation

coefficient and pool diameter respectively. fE is the total emissive power of the flame at the

flame surface.

ffr EFq ⋅⋅⋅= εζ" ........................... Equation 9

For pool fires greater than a few meter in diameter the effective emissivity- fε is

approximately equal to one. If the atmospheric transmissivity-ζ was taken as one equation 9

become fr EFq ⋅=" , which is the equation proposed by STD 979-20NN from the IEEE.

The thermal impacts caused as a consequence of a transformer pool fire with about 40,000

litter of mineral oil were estimated using the equations mentioned in the previous paragraphs

(i.e. equation 1 to 8). The radiative energy in target from the pool fire of various pool

diameters is presented in Table 4. It was assume that the point source model will provide

reasonable results for target double the flame height (i.e. 2H meter) away from the fire. A

pool fire of 5 meter in diameter is superimposed over a power plant layout, Figure 5.

The high radiate energy from a transformer pool fire increase the temperature of the structures

nearby, therefore its strength and stiffness could be reduced. This effect may lead to

unacceptable deformations or structure failure, such as take out of service a substation

because either the high voltage bar, Figure 4, or the rely house, Figure 6, suffer irreversible

16

thermal damages. At about 70oC damage to electronic components are irreversible. What

could happen if a transformer fire causes irreversible damage to the electronics equipment

placed inside the rely house (Figure 6), either due to the transformer’s close proximity to the

house, or difficulties in controlling and extinguishing it before the heat transfer from flame

takes the substation out of the network? In this case, about 2 million people would be without

electricity for quite some time. Imagine hospitals without electricity, a big city without traffic

lights, and so on.

In a relay house some of the protection, bypass and control circuits which control the

transformers and other substation equipments are placed. In the substation shown in Figure 6

there are two relay houses. Their structure is masonry with glass windows. The structural

thermal performance simulation of one of the relay houses during a transformer pool fire was

carried out by use of the finite elements method. The distance between the transformer and

the relay house is 3.50 meters. Ninety minutes after the fire was initiated, the wall temperature

distribution showed some wall temperatures to be above 300oC. Such high temperatures can

cause wall failure. Despite this, the structural integrity of the walls, pillars and beams occurs

only 117 minute after the fire is started. On the other hand, 45 minutes after the fire is

initiated, the glass windows will break. Therefore there will be a rapid growth of the gas

temperature inside the relay house. The heat impact will cause irreversible damage to the

electronic panels. The gas temperature distribution analysis inside the house showed that

temperatures higher than 70oC could be reached in approximately 30 minutes.

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TABLE 4. Radiate energy (kW/m²) in a target from a transformer pool fire.

Pool Diameter and Radiate Energy-kW/m2 Distance to the Target

m 1 m 2 m 3 m 4 m 5 m

1 -- -- -- -- --

2 3,6 -- -- -- --

3 1,6 -- -- -- --

4 0,9 5,4 -- -- --

5 0,6 3,4 -- -- --

6 0,4 2,4 6,3 -- --

7 0,3 1,7 4,6 -- --

8 0,2 1,7 3,5 6,7 --

9 0,2 1,1 2,8 5,3 --

10 0,1 0,9 2,3 4,3 7,0

11 0,1 0,7 1,9 3,5 5,7

12 0,1 0,6 1,6 3,0 4,8

13 0,1 0,5 1,3 2,5 4,0

14 0,1 0,4 1,1 2,2 3,5

15 0,1 0,4 1,0 1,9 3,0

18

Legend

Zone Energy Target Distance

> 5 kW/m² 11 metros

> 2kW/m² 18 metros

FIGURE 5. A pool fire transformer superimposed on step by transformer bay.

19

FIGURE 6. Detail of the layout of the substation.

A number of workers have correlated threshold of pain and blistering due to a thermal impact,

Table 5 shows some limits to pain and injury given in the literature. The energy liberate from

the transformer fire illustrated in Figure 5 is on the threshold of pain.

TABLE 5. Threshold to pain.

Thermal Radiation Intensity

1,5 kW/m² Threshold of pain

2,5 kW/m² Level at which pain is felt after 1minute

1 kW/m² Level just tolerable to a clothed man

8 kW/m² Level which causes death within minutes

4,7 kW/m² Threshold of pain. Average time to

experience pain 14.5 seconds

When the load bearing capacity becomes equal to the applied load there will be the structure

failure. During the present study was carried out two structural analysis of an unprotected

steel member (i.e. tension and compression member) as presented below. The analysis was

based on the EUROCODE 3 part 1-2. It suggests a simplified method to estimate the

temperature and the load bearing capacity. In other words, the temperature is assumed

20

uniform over the cross section, and it does not take into account the effects of the restrictions

of the real structures.

• Case 1- Tension member:

o Unprotected steel section – MR250 steel;

o ½ I cross section 152x18.6;

o Length of member: 4,5 m

o Tension effort in member: 224 kN;

• Case 2- Compression member:

o Unprotected steel section – MR250 steel;

o 2 C cross section 305x30.8;

o Length of member: 4,0 m

o Tension effort in member: 200 kN;

Possible transformer pool fire radiate energies as a function of the time to the structure

member fails and its temperature for both case 1 and 2 are relate in Table 6.The Figures 7, 8

and 9 show the temperature versus time and the load bearing capacity versus time graphs to

19kW/m2, 20kW/m2 and 7kW/m2, respectively.

TABLE 6. Time to fail of the structure member of cases 1 and 2.

Case 1: Tension Member

Section Factor: 271 m-1

Case 2: Compression Member

Section Factor:175 m-1 Energy

(kW/m²) Temperature

(°C)

Failure Time

(min)

Temperature

(°C)

Failure Time

(min)

20 10 11

18 12 13

16 16 15

14 NF 18

12 NF 23

10 NF 35

08

507

NF

415

NF

21

FIGURE 7. Case 1: Temperature versus time and load bearing capacity versus time graphs to

an exposed energy of 19kW/m².

FIGURE 8. Case 2: Temperature versus time and load bearing capacity versus time graphs to

an exposed energy of 20kW/m².

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FIGURE 9. Case 2: Temperature versus time and load bearing capacity versus time graphs to

an exposed energy of 7kW/m².

The structure failure depends on fire severity, steel area exposed to the flames, the applied

load and support conditions. The tension member failures (case 1) are associated with

energies higher than 16kW/m2. The compression member failure is probable with energies

around 10kW/m2. For the fire simulates, Figure 5, either tension or compression members

failures are not foreseen, but scenarios are subject to uncertainty.

The basic of fire hazard assessment is a set of scenarios. In this paper only the scenario of

pool fire transformer was analyzed. Both completeness and realism of these scenarios are

related to the release itself and to the escalation both are subject to uncertainty, Figure 10.

There is a further uncertainty concerning to the geometry of the release. The transformer oil

will not necessarily come out as a pool, but may issue as a spray fire and may impinge on

other equipment.

An autotransformer of 150MVA protected by a spray system caught fire as a result of a bushing

failure, Figure 10. The spray system did not operate when the fire began. This delay cause the fire

spread, due to the oil leakage through the base of the bushing. NFPA 15 describes water spray design

for transformer. Even if the suppression systems are available there are many uncertainties for their

success or failure in controlling the fire. Questions such as, Can water discharge from the spray

system? Can water terminate the fire? Water will be discharged from the spray system if all water-

supply valves are open when the sensor fuses will enough water reaches the spray head? On the other

hand, the violent nature of a transformer fire could render automatic spray system useless. Although

23

this may have occurred in may cases the automatic spray system did survive the explosion and were

credited with controlling fire, limiting damage and minimizing system (i.e. a substation or a power

plant) downtime. As the system in which the transformer is insert has a dynamic behavior, the fire

scenarios are subject to many uncertainties.

Figure 10. Escalation of a transformer fire.

FINAL CONSIDERATIONS

Power plants and substations have been around for quite some time, so ample engineering

experience exists and the public is familiar with their equipment and structures (i.e.

transformer, circuit break, transmission lines, etc). They also have a substantial economic

incentive to prevent accidents. In spite of mature technology, good management, and

incentives to keep the plant or substation from blowing up, uncontrollable fire rages within

them on occasion, killing operators and causing substantial losses.

Fire in substations range from those which have a relatively minor impact, in which there is

little or no interruption of the operation to the interconnect network to major catastrophe: the

blackout in Southeast of Brazil in 1995 being synonymous. While the engineers who design

the substation have the knowledge and understanding to recognise the fire hazard throughout

the system interactions and take measures, which will reduce the risk of a fire occurring, it is

the substation operators who are responsible for its safe operation on a day-to-day basis. They

Time t 2 t 2

24

must be aware, not only of the inherent hazard of the process of which they are in charge, but

also of what can go wrong and, perhaps more importantly, how it can go wrong.

An insurance company, a fire officer or an industrial company organization all have different

ways of dealing with the fire risk. It involves understand what is at risk, have a sense of the

fire severity and how to dealing with it. In other words, we should answer questions such as:

What is the risk? How serious is it? And what are the alternatives for dealing with it? Some

view of fire risk management in Brazil focus only on a decision of what type of insurance to

purchase. Some organizations ignore the risk in the hope that misfortune will not happen to

them. Based on the assumption that if a transformer fire occurs decisions of what to do will be

handled at the time. An engineering of a suitable protection measures for a suitable protection

for a transformer fire should involved three steps: scenario identification, consequence

analysis and protection evaluation. Some considerations about transformer fire scenarios and

its consequences were discussed in this paper.

25

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