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 Oil

Effect 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 Tests

Four 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.
Dielectric Test
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.
Acidity Test
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

10 years
15 years
50 years
67 years

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 25^oC. 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.
IFT Test
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 expected.

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.
Circuit Breakers
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.