COOLING OF TRANSFORMERS

INTRODUCTION

The load that a transformer carries without heat damage can be increased by using

an adequate cooling system. This is due to the fact that a transformer's loading

capacity is partly decided by its ability to dissipate heat. If the winding hot spot

temperature reaches critical levels, the excess heat can cause the transformer to fail

prematurely by accelerating the aging process of the transformer's insulation. A

cooling system increases the load capacity of a transformer by improving its ability to

dissipate the heat generated by electric current. In other words, good cooling

systems allow a transformer to carry more of a load than it otherwise could without

reaching critical hot spot temperatures.

One of the more common types of transformer cooling equipment is auxiliary fans.

These can be used to keep the radiator tubes cool, thereby increasing the

transformer's ratings. Fans should not be used constantly, but rather only when

temperatures are such that extra cooling is needed. Automatic controls can be set up

so that fans are turned on when the transformer's oil or winding temperature grows

too high.

Maintenance of Cooling Systems

 Dry-Type Transformers:For dry-type transformers, the area in which the

transformer is to be installed should have proper ventilation. This ventilation

should be checked prior to installation to make sure it is adequate.

Additionally, the transformer's radiator vents should be kept clear of

obstructions that could impede heat dissipation.

  Forced Air: If the transformer's temperature is being kept at acceptable

levels by forced air from a fan, the fan's motors should be checked

periodically to make sure they are properly lubricated and operate well. The

thermostat that ensures the motors are activated within the preset

temperature ranges should be tested as well.

 

Water cooled systems: Systems that are cooled by water should be tested

periodically to make sure they operate properly and do not leak. Leaks can be

checked by raising the pressure within the cooling system, which can be done

in various ways. If the cooling coils can be removed from the transformer,

internal pressure can be applied by adding water. Otherwise, pressure checks

can also be made using air or coolant oil, if the coils need to be checked

within the transformer itself.

If the cooling coils are taken out of the transformer, the water cooling system

as a whole can be tested. Here, the coils are filled up with water until the

pressure reaches 80 to 100 psi, and left under that pressure for at least an

hour.  Any drop in pressure could be a sign of a leak. The other equipment

linked to a water-cooled system can be tested at the same time, such as the

alarm system, water pump and pressure gauges. Also, the water source

should be tested to make sure it has sufficient flow and pressure.

 

Liquid coolants: When oil coolants are prepared they are dehydrated, and

processed to be free of acids, alkalis, and sulfur. They should also have a low

viscosity if they are to circulate easily. If a transformer is cooled by oil, the

dielectric strength of the oil should always be tested before the transformer is

put into service.

Types of Cooling Systems

For oil immersed transformers, the options for cooling systems are as follows:

 

Oil Immersed Natural Cooled (ONAN): Here, both the core and the windings

are kept immersed in oil. The transformer is cooled by the natural circulation

of this oil. Additional cooling can be provided by radiators, which increase the

surface area over which a large transformer can dissipate heat.

 

Oil Immersed Air Blast (ONAF): In this case air is circulated and the

transformer cooled with the help of fans. Fans allow one to have a smaller

transformer for a given rating, since not as much surface area is needed for

heat dissipation. This in turn can cut costs.

 

Oil Immersed Water Cooled (ONWN): Here the transformer is cooled by an

internal coil through which water flows. This method is feasible so long as

there is a readily available source of a substantial amount of water, which is

not always the case. This kind of cooling has become less common in recent

years, abandoned in favor of Forced Oil Water Cooled (OFWF).

 

Forced Oil Air Blast Cooled (OFAF): In this case, cooling is accomplished in

two ways. Oil circulation is facilitated by a pump, and fans are added to the

radiators to provide blasts of air.

 

Forced Oil Natural Air Cooled (OFAN): For this type of cooling, a pump is

included within the oil circuit to aid in oil circulation.

 

Forced Oil Water Cooled (OFWF): Here, a pump within the oil circuit forces the

oil to circulate out through a separate heat exchanger in which water flows.

Types of Cooling Systems

Oil Immersed Natural Cooled

Oil Immersed Air Blast

Oil Immersed Water Cooled

Forced Oil Air Blast Cooled

Forced Oil Natural Air Cooled

Forced Oil Water Cooled

Forced Directed Oil and Forced Air Cooling

 

The most dependable type of cooling system for a transformer is the oil-immersed

naturally cooled (ONAN), which also produces the least noise. A forced-air cooled

transformer (OFAF) is more efficient, but it is also noisier and less reliable on account

of the possibility of fan malfunction.

The method of forced cooling has been used for many years now to increase the

loading capacities of transformers. A transformer's thermal performance can be

directly improved by the implementation of cooling systems. Consequently, it makes

sense to avoid excess heating and accelerated aging within a transformer by using

the appropriate cooling system.

Transformer oil

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Transformer oil or insulating oil is usually a highly-refined mineral oil that is stable

at high temperatures and has excellent electrical insulating properties. It is used in

oil-filled transformers, some types of high voltage capacitors, fluorescent lamp

ballasts, and some types of high voltage switches and circuit breakers. Its functions

are to insulate, suppress corona and arcing, and to serve as a coolant.

Explanation

The oil helps cool the transformer. Because it also provides part of the electrical

insulation between internal live parts, transformer oil must remain stable at high

temperatures for an extended period. To improve cooling of large power

transformers, the oil-filled tank may have external radiators through which the oil

circulates by natural convection. Very large or high-power transformers (with

capacities of thousands of KVA) may also have cooling fans, oil pumps, and even

oil-to-water heat exchangers.

Large, high voltage transformers undergo prolonged drying processes, using

electrical self-heating, the application of a vacuum, or both to ensure that the

transformer is completely free of water vapor before the cooling oil is introduced.

This helps prevent corona formation and subsequent electrical breakdown under

load.

Oil filled transformers with a conservator (an oil tank above the transformer) tend to

be equipped with Buchholz relays. These are safety devices that detect the build up

of gases (such as acetylene) inside the transformer (a side effect of corona or an

electric arc in the windings) and switch off the transformer. Transformers without

conservators are usually equipped with sudden pressure relays, which perform a

similar function as the Buchholz relay.

The flash point (min) and pour point (max) are 140 °C and −6 °C respectively. The

dielectric strength of new untreated oil is 12 MV/m (RMS) and after treatment it

should be >24 MV/m (RMS).

Oil transformer

Large transformers for indoor use must either be of the dry type, that is, containing

no liquid, or use a less-flammable liquid.

Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric

fluid since they are not flammable. They are toxic, and under incomplete combustion,

can form highly toxic products such as furan. Starting in the early 1970s, concerns

about the toxicity of PCBs have led to their being banned in many countries.

Today, non-toxic, stable silicon-based or fluorinated hydrocarbons are used, where

the added expense of a fire-resistant liquid offsets additional building cost for a

transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and

synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming

increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic

to aquatic life, readily biodegradable, and have a lower volatility and a higher flash

points than mineral oil.

Testing and oil quality

Transformer oils are subject to electrical and mechanical stresses while a

transformer is in operation. In addition there is contamination caused by chemical

interactions with windings and other solid insulation, catalyzed by high operating

temperature. As a result the original chemical properties of transformer oil changes

gradually, rendering it ineffective for its intended purpose after many years. Hence

this oil has to be periodically tested to ascertain its basic electrical properties, make

sure it is suitable for further use, and ascertain the need for maintenance activities

like filtration/regeneration. These tests can be divided into:

1. Dissolved gas analysis

2. Furan analysis

3. PCB analysis

4. General electrical & physical tests:

Color & Appearance

Breakdown Voltage

Water Content

Acidity (Neutralization Value)

Dielectric Dissipation Factor

Resistivity

Sediments & Sludge

Interfacial Tension

Flash Point

Pour Point

Density

Kinematic Viscosity

The details of conducting these tests are available in standards released by IEC,

ASTM, IS, BS, and testing can be done by any of the methods. The Furan and DGA

tests are specifically not for determining the quality of transformer oil, but for

determining any abnormalities in the internal windings of the transformer or the

paper insulation of the transformer, which cannot be otherwise detected without a

complete overhaul of the transformer. Suggested intervals for these test are:

General and physical tests - bi-yearly

Dissolved gas analysis - yearly

Furan testing - once every 2 years, subject to the transformer being in operation

for min 5 years.

Polychlorinated biphenyls (PCBs) were extensively used in indoor, fire-resistant,

liquid-filled transformers until they were banned in 1979 in the US. Since PCB and

transformer oil are miscible in all proportions, and since sometimes the same

equipment (drums, pupmps, hoses, and so on) was used for either type of liquid,

contamination of oil-filled transformers is possible. Under present regulations,

concentrations of PCBs exceeding 5 parts per million can cause an oil to be

classified as hazardous waste in California (California Code of Regulations, Title 22,

section 66261). Throughout the US, PCBs are regulated under the Toxic Substances

Control Act. As a consequence, field and laboratory testing for PCB contamination is

a common practice. Common brand names for PCB liquids include "Askarel",

"Inerteen", "Aroclor" and many others.

On-site testing

Some transformer oil tests can be carried out in the field, using portable test

apparatus. Other tests, such as dissolved gas, normally require a sample to be sent

to a laboratory. Electronic on-line dissolved gas detectors can be connected to

important or distressed transformers to continually monitor gas generation trends.

To determine the insulating property of the dielectric oil, an oil sample is taken from

the device under test, and its breakdown voltage is measured on-site according the

following test sequence:

In the vessel, two standard-compliant test electrodes with a typical clearance of

2.5 mm are surrounded by the insulating oil.

During the test, a test voltage is applied to the electrodes. The test voltage is

continuously increased up to the breakdown voltage with a constant slew rate of

e.g. 2 kV/s.

Breakdown occurs in an electric arc, leading to a collapse of the test voltage.

Immediately after ignition of the arc, the test voltage is switched off automatically.

Ultra fast switch off is crucial, as the energy that is brought into the oil and is

burning it during the breakdown, must be limited to keep the additional pollution

by carbonisation as low as possible.

The root mean square value of the test voltage is measured at the very instant of

the breakdown and is reported as the breakdown voltage.

After the test is completed, the insulating oil is stirred automatically and the test

sequence is performed repeatedly.

The resulting breakdown voltage is calculated as mean value of the individual

measurements.

DIELECTRIC FLUIDS FOR TRANSFORMER COOLING

This discussion is intended to provide the reader with some level of insight into

the appropriate selection and application of dielectric fluids used in transformer

cooling. We will attempt to provide both a historical perspective as well as a

discussion on the various types of fluids available today by most if not all

manufactures of liquid filled transformers.

Before we begin to compare the relative merits of different fluid types, it would

first seem appropriate to discuss the purpose of dielectric fluids in transformers

as a baseline for discussion. filled transformers, dielectric fluid is used to cool

the windings and provide optimal performance in the following manner. From

the bottom of the tank where the dielectric fluid is at it’s lowest or “bottom”

temperature, the fluid flows vertically up the winding ducts and is heated by the

windings. At the top of the tank, where the fluid is at its highest or “top oil

temperature”, it exits the main tank and enters a series of radiators or cooling

fins. It then flows downward through the radiators, where it is cooled, and

reenters the main tank at the bottom. In self cooled transformers this cycle is

governed naturally by convection. Natural convection can also be assisted by a

series of fans directing air against the radiators increasing the rate of heat

transfer and subsequent rate of cooling in the windings. In some large power

transformers it is also possible to have a level of forced oil circulation where a

pump assists in the circulation of the fluid. This generally provides a lower top

oil temperature and more uniform temperatures within the windings. From a

historical perspective there have been several fluid types offered by a variety of

different manufactures that have come and gone with the winds of time. Even

though discontinued, it is important to have a basic understanding of these

fluids and any special treatment that they may command should you encounter

them in the field. In 1978 General Electric began marketing a new transformer

design called “Vaportran®”. This transformer used R-113 as the dielectric

coolant and was very effective as a replacement for PCB units because of its

relatively small footprint, non-flammable nature, and excellent performance. R-

113 was a form of Freon that was in liquid state in the transformer tank,

evaporated and turned into a gaseous state as it entered the cooling radiators,

and then recondensed as it heated and reentered the tank. As most of you may

know, with global concerns about damage to the ozone layer, it didn’t take long

before government regulation set in again and the design was conscientiously

withdrawn from production. There are still respectable numbers of Vaportran®

units in service today, and it should be noted that there are more

environmentally friendly fluid substitutes available. 3M manufactures a fluid

called PF-5060 that is generally used as a replacement, but the cost may be a

bit burdensome. Mixtures of polychlorinated biphenyls (PCBs) were anufactured

commercially in the United States until 1977 and used as a transformer

dielectric fluid because of their non-flammable nature and chemical stability.

PCBs were widely used for about fifty years and produced under a variety of

trade names, the most common of which were Askarel® and Pyranol®.

Although chemically stable, PCBs would only slowly biodegrade. That is that

they tended to persist in nature as opposed to decomposing into basic

elements. There were numerous health studies conducted that documented

their potential effects on both humans and wildlife. As a result of increasing

public concern Congress reacted and passed the Toxic Substances Control

Act. This act singled out PCBs for regulation and directed the U.S.

Environmental Protection Agency to implement controls. These regulations

were published in the Federal Register in 1979. During much of the 80’s and

90’s a great deal of time, money, and effort was expended in complying with

federal mandates. The great majority of transformers containing PCBs were

either retro-filled with more acceptable fluids, or disposed of under

federaluidelines. It should be noted that it is still common to discover PCB filled

or PCB contaminated transformers in limited service todayrly 1980’s

Westinghouse began marketing and promoting a new fluid called “Wecosol®”.

Wecosol® was the Westinghouse trade name for tetrachloroethylene, also

called perchloroethylene, (PCE). This type of fluid was widely used in dry

cleaning processes. The major advantages of this fluid as a transformer

dielectric coolant were its nonflammability and low cost. The scientific data on

tetrachloroethylene with regards to both health and environmental issues was

far to similar to those that led to the regulatory

Now that we have done a postmortem on some of the industries more notable

stories, it would seem appropriate to take a look at some of the industries

current dielectric fluid offerings.

Today there are four generally accepted fluid types offered in the market,

Mineral Oil, Silicone, Beta fluid®, and Envirotemp®. While each has good

properties as a dielectric fluid, there are attributes unique to each that may

make one a better choice over the others depending on the users needs.

Mineral Oil has been used as a dielectric fluid in generations of transformers.

There is a longstanding, proven, track record of good performance and low

costs. Mineral Oil is generally considered as a top choice in outdoor

installations where its low first cost is of prime concern, and its flammable

nature is understood and accepted. Mineral Oil is considered to be a

“Flammable” fluid by Factory Mutual, and as such has certain restrictions

imposed on its use and containment that will be discussed later in this

document.

Silicone was for several decades the fluid of choice when a Factory Mutual

approved “Less-flammable” dielectric fluid was desired. It has a relatively high

fire point and is generally considered to self extinguish when the source of a fire

is removed. However, Silicone does contain Methylpolysiloxanes which can

generate Formaldehyde at around 300 degrees Fahrenheit. Formaldehyde can

be a skin and respiratory sensitizer, eye and throat irritant, and is believed to be

a potential cancer hazard. Silicone has been used for many years in both

. outdoor applications and indoor areas. When used indoors, it has been my

experience that transformers are generally in vaulted, contained areas. Silicone

is not miscible with conventional mineral oils and should not be mixed with other

fluids.

Beta fluid® meets NEC and Factory Mutual requirements for a “Less-

Flammable” dielectric fluid. It is a blend of petroleum oils and is 100%

hydrocarbon. Beta fluid® is fully miscible with conventional mineral oil and may

be used to retrofill or top off these units. Beta fluid® has high dielectric strength,

stability, and is non-toxic. While it does meet NEC requirements for a “Less-

Flammable fluid”, its fire point is significantly lower than either Silicone or

FR3™.

Cooper Power Systems offers a dielectric fluid called Envirotemp® or FR3™

which is available in transformers produced by most manufactures today. The

product is a soy-based, fire-resistant fluid that meets NEC requirements for a

“Less–flammable” fluid, and is listed by Factory Mutual and UL as such.

“Because Envirotemp® FR3™ fluid is derived from 100% edible seed oils and

uses food grade additives, its environmental and health profile is unmatched by

other dielectric coolants. Its biodegradation rate and completeness meets the

U.S. Environmental Protection Agency (EPA) criteria for “Ultimate

Biodegradability” classification.” Cooper also claims “Envirotemp® FR3™ fluid

extends insulation life by a factor of as much as 5-8 times because it has the

unique ability to draw out retained moisture and absorb water driven off by

aging paper. It also helps prevent paper molecules from severing when

exposed to heat. These properties can result in an increase of overloadability

and/or longer transformer insulation life, resulting in both lower life cycle costs

and delayed asset replacement.” (www.cooperpower.com/FR3/) FR3™ is fully

miscible with conventional mineral oil or R-Temp®, and may be used to retrofill

or top off units filled with these fluid types. It appears the only negative that can

be attributed to this fluid is the fact that it has a relatively high first cost relative

to Mineral Oil and could easily add 15-30% to the transformer first cost.

The following chart is intended to outline some of the key thermal properties of

the various fluids discussed. It should be noted there is a large amount of

additional data that can be viewed and compared. You should

request a Material Data Sheet for full descriptive information on each of the

fluids presented.

Key Thermal properties

Mineral Oil Beta Fluid® Silicone Envirotemp®

Fire Point 165 Deg C 308 Deg C 371 Deg C 360 Deg C

Flash Point 145 Deg C 285 Deg C 268 Deg C 330 Deg C

When it comes to selecting a dielectric fluid that best meets the needs of a

particular installation and customer, there are several factors that need to be

considered. Not the lease of these would be first cost. The following table is

intended to provide the reader with an approximation of the relative first cost of

transformers filled with each of the four dielectric fluids previously discussed.

Please note that these are only approximations and the relative costs can vary

depending on the volume of liquid contained in the transformer.

Fluid Type Relative First Cost

Mineral Oil 1.00

Beta Fluid® 1.20

Silicone 1.30

Envirotemp®, FR3™ 1.30

In determining first cost there is more to consider than just the initial equipment

cost. There are installation requirements specific to different fluid types that can

add a significant burden to project costs. It is generally considered that Factory

Mutual is the ruling authority when it comes to standards and requirements for

the installation of any liquid filled transformer. Factory Mutual has published

data sheets that define separation distances

between transformers and buildings, fire barrier requirements, and liquid

containment systems specific to the various fluid types and ratings. These

requirements are very specific and should be consulted along with local building

codes when determining the requirements specific to any installation.

The following tables published by Factory Mutual are used in determining separation

distances between transformers, buildings, and other equipment.

As outlined by Factory mutual, containment systems are required when:

1) “A release of Mineral Oil would expose buildings.”

2) “More than 500 gallons of Mineral Oil could be released.”

3) “More than 1320 gallons of FM approved less flammable fluid could be

released.”

CONCLUSION

4) “More than 2640 gallons of biodegradable FM approved less flammable fluid

could be released. The fluid must be certified as a biodegradable fluid by the

environmental protection agency. A release of this fluid must not expose

navigable waterways. The transformer must be properly labeled. You should

refer to Factory Mutual and local codes for complete definitions and

requirements for compliance.

While the determination as to which fluid constitutes an individual users “fluid of

choice” can vary greatly, it would seem clear that the industry is migrating in the

direction of environmental awareness. Perhaps, unlike many of mans other

decisions, we are not determined to repeat the mistakes of the past.

I hope this article provides you with a better understanding of the history,

current offerings, and practices concerning dielectric fluids. It is ultimately the

users decision based on design, cost, location, and potential environmental

impact that should define the fluid type to be used.

Reference Material

www.cooperpower.com/FR3/

www.dsifluids.com/Beta%20Fluid%20Page.htm

“Envirotemp® FR3™ Fluid”, Cooper Power Systems, Bulletin B900-00092

“Three-Phase Padmounted Transformers”, GE publication #JVB-005

“Material Safety Data Sheet”, GE Silicones

“Factory Mutual Global Property Loss Prevention Data Sheets”, Factory Mutual

Insurance Company, 2005.

“Cost Comparison - Cooling Options”. GE Prolec

“Beta Fluid – Fire Resistant Insulating Oil”, DSI Ventures, Inc.

“Material Safety Data Sheet – Beta Fluid”, DSI Ventures, Inc., David Sundin, Ph. D.,

Effective Date 12/01/05

“SPQR – Westinghouse “Wescosol” Transformer”, 11/15/1982, GE publication

#GIZ-1768A

“Meeting Federal PCB Regulations for the Food and Feed Industry”, by Edward

W. Feuerstein and William K. Mallon, 8/84, GE Publication #GER-3381A