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MAINTENANCE OF LIQUID INSULATION
Using Dielectric, Acid No., IFT, & Power Factor
The following article is an excerpt from the United States
Department of the Interior Bureau of Reclamation's Maintenance of Liquid
Insulation: Mineral Oils and Askerals, Facilities Inustructions, Standards,
& Techniques, Volume 3-4, 1992.
Insulating liquids used in transformers, circuit breakers, and
high-voltage bushings require proper testing and maintenance to keep them
in good condition. Liquid insulation that is not in good condition may
cause damage to equipment and become a hazard to personnel.
Deterioration of Insulating OilEffect of Oxygen on Oil
Moisture contamination is one of the most common causes of
deterioration in the insulating quality of oil. This contamination can be
eliminated by purification. A less rapid but more serious characteristic
deterioration, the formation of acids and sludge, is caused by
oxidation. Thus, the exclusion of oxygen is of prime importance.
In free-breathing transformers, the oxygen supply is virtually unlimited
and oxidative deterioration is consequently faster than in sealed
transformers. Atmospheric oxygen is not the only source of oxygen
available for the oxidation of insulating oils; water also serves as a
source of oxygen and, therefore, leaky gaskets constitute a very real
hazard due to both oxidation and moisture contamination. The rate of
oxidation also depends on the temperate of the oil; the higher the
temperature, the faster the oxidative breakdown. This fact points to the
importance of avoiding overloading of transformers, especially in the
summertime. Oxidation results in (a) the formation of acids in the
insulating oil, and (b) the formation of sludge.
Moisture in Oil
Water can be present in oil (a) in a dissolved form, (b) as tiny droplets mixed with the oil (emulsion), or (c) in a free state at the bottom of the container holding the oil. Demulsification
occurs when the tiny droplets unite to form larger drops which sink to the
bottom and form a pool of free water. Emulsified water or water in the
free state may be readily removed by filtering or centrifugal treatment;
the filtration process can partially remove dissolved water if the filter
papers are thoroughly dried before filtration and are replaced frequently.
Effect of temperature on moisture
The amount of moisture
which can be dissolved in oil increases rapidly as the oil temperature
increases. Therefore, an insulating oil purified at too high a temperature
may lose a large percentage of its dielectric strength on cooling because
the dissolved moisture is then changed to an emulsion.
Oil Deterioration in Transformers
In transformers, sludge
sticks to the surfaces through which heat should be dissipated; the sludge
forms a blanket barrier to the flow of heat from the oil to the coolant and
from the core and coils to the cool oil. If allowed to continue long
enough, the sludge may even block off the flow of oil through the cooling
ducts. As a result, the transformer insulation gets too hot and is
damaged, particularly between turns of the windings. Deterioration of the turns insulation may eventually lead to short circuits between turns and the breakdown of the transformer. When oxidation progresses to the point where sludge is being precipitated, the first step should be to remove the sludge from the transformer by a high-pressure stream of oil and to either replace the sludged oil or treat it with activated clay to remove the acid and sludge precursors. Complete treatment of the oil is normally less costly than replacing it with new oil.
Types of Oil TestsFour basic tests on insulating oil, when considered collectively, give a reasonably accurate diagnosis with respect to the serviceability of an insulating oil. The tests are (a) dielectric, (b) acidity, (c) power factor, and (d) IFT (interfacial tension). Other tests such as water content and oxidation inhibitor content may be required due to the operating environment and the equipment age.
The dielectric test measures the voltage at which the oil beaks down. The breakdown voltage is indicative of the amount of contaminant (usually moisture) in the oil, and the voltage should be checked frequently. The generally accepted minimum dielectric strength is 30 kV for transformers with a high-voltage rating 287.5 kV and above and 25 kV for transformers with a high-voltage rating below 287.5 kV. New oil should test at least 35 kV by ASTM methods of testing. Oil is not necessarily in good condition even when the dielectric strength is adequate because this tells nothing about the presence of acids and sludge.
New transformer oils contain practically no acids if properly refined. The acidity test measures the content of acids formed oxidation. The oxidation products polymerize to form sludge which then precipitates out. Acids react with metals on the surfaces inside the tank and form metallic soaps, another form of sludge. Sludging has been found to begin when the acid number reaches or exceeds 0.4, and 0.4 is considered to be the normal service limit. New oil has as acid number of less than 0.05. the acid number (formerly referred to as neutralization number) equals the milligrams of KOH (potassium hydroxide) required to neutralize the acid contained in 1 gram of oil. It is questionable whether an oil that is deteriorated to the point where its acid number exceeds 0.6 can be put back into lasting good condition by a single renovation. It is almost certain that two or more renovations, spaced 6 months to 1 year apart, would be necessary. It is recommended that an upper limit of 0.2 be used to determine when oil should be renovated, as a single renovation would most probably restore such an oil to very good condition. Oil showing an acid number of 0.15 or larger can be expected to show accelerated acid formation. Tests have been conducted which indicate the acidity is proportional to the amount of oxygen absorbed by the oil. It is estimated that 0.0015 m3/L (0.2 ft3/gal) of oxygen absorbed in oil will cause an acidity of about 0.4 mg of KOH, which is the approximate acidity number at which sludging is assumed to start. On the basis of this, equal loading test cycles and other assumptions, it has been estimated that different types of transformers would take the following periods of time before sludge would appear.:
|Transformers with free air access|
Transformers with conservators
Transformers bolted tight
Transformers with nitrogen over oil
While the above periods may not correspond to actual field examples due to different load conditions than those assumed, they are illustrative of the relative periods of serviceability for the different types of transformers.
Power Factor Test
The power factor of an insulating oil equals the cosine of the phase
angle between an AC voltage applied to an oil and the resulting current.
Power factor indicates the dielectric loss of an oil and, thus, its
dielectric heating. The power factor test is widely used as an acceptance
and preventive maintenance test for insulating oil. Oil power factor
testing in the field is usually done with the Doble type MH or M2H test set
in conjunction with power factor tests of the oil-filled equipment.
The power factor of new oil should not exceed 0.05 percent at
25oC. A high power factor in used oil indicates
deterioration, contamination, or both with moisture, carbon, varnish,
Glyptal, sodium soaps, or deterioration products. Used oil with a power
factor in excess of 0.5 percent should be further analyzed in a laboratory
to determine the cause of the high power factor. Oil with a power factor
in excess of 2.0 percent may be an operational hazard. It should be
investigated and either reconditioned or replaced.
It should be recognized that the acidity test alone
determines conditions under which sludge may form but does not necessarily
indicated that actual sludging conditions exit. The IFT (interfacial
tension) test is employed as an indication of the sludging characteristics
of power transformer insulating oil. It is a test of IFT of water against
oil, which is different from surface tension in that the surface of the
water is in contact with oil instead of air. The attraction between the
water molecules at the interface is influenced by the presence of polar
molecules in the oil in such a way that the presence of more polar
compounds causes lower IFT. The test measures the concentration of polar
molecules in suspension and in solution in the oil and thus gives an
accurate measurement of dissolved sludge precursors in the oil long
before any sludge is precipitated. It has been established that an IFT of
less the 0.015 N/m (15 dyne/cm) almost invariably shows sludging. An IFT
of 0.015 to 0.022 N/m (15 to 22 dyne/cm) shows an uncertain condition, and
an IFT value of more than 0.022 N/m (22 dyne/cm) is generally indicative
of no sludging. Transformer oils whose IFT is in the range of 0.015 to
0.022 N/m (15 to 22 dyne/cm) should be scheduled for reclaiming, regardless
of acidity values.
Periodic Testing Program
From the aspects of safety, continuity of service, and of efficient,
low-cost maintenance, it is desirable to monitor the condition of the
insulating oil by testing and to take remedial measures before the oil
reaches a point of deterioration beyond which failure of equipment can be
The condition of the oil and the load conditions should
determine whether an annual, biannual, or more frequent schedule should be
followed. Normally, acidity, IFT, power factor, and dielectric tests
should be done on oil in major electrical equipment at least once a year.
Permanent records should be kept of all test results. Whenever the test
results show that accelerated deterioration is occurring, more frequent oil
tests should be made to forestall trouble.
Idle, Oil-filled equipment
Idle, oil-filled equipment may also accumulate moisture and should be
tested at least once a year. The cooling coils of water-cooled transformers
sometimes develop leaks, and water may enter the oil so fast that even
weekly dielectric tests would not catch the trouble. A rise in the
transformer oil level is the best indicator for this condition. Most
water-cooled transformers are equipped with cooling tubes of double-wall
construction. The intent of this construction is to bleed off any leaking
water or oil into the space between the two walls of the tube to avoid
contamination. Usually, the oil pressure, and a double-wall leak causes
oil to be lost into the cooling system discharge. The loss would be
indicated by low oil level in the transformer. Hence, change of oil level
is an important indication and should be checked frequently. Distribution
transformers need not be tested as frequently as once a year, unless they
are serving critically important loads such as the main power station
auxiliary motors and lights.
The presence of carbon in circuit breaker and step-voltage regulator oil
introduces a hazard due to the tendency of the carbon to lower the
dielectric strength of the oil and also to form deposits on insulating
surfaces, reducing the insulation resistance. Carbonized oil is more
vulnerable to moisture than clean oil. The quantity of carbon is
proportional to the number and severity of the arcs interrupted. Samples
of oil for dielectric test should be obtained from oil circuit breakers
after a heavy fault or series of faults and from both types of equipment
(circuit breakers and step-voltage regulator) at least twice a year. If an
oil sample is black from suspended carbon, the oil should be filtered or
centrifuged even though the dielectric test is good.