transformer oil
TRANSCRIPT
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