ASTM (American Society for Testing and Materials) has developed the standards for oil testing. The following tests we recommended for a complete analysis of a transformer’s oil:
Dielectric breakdown (ASTM D-877 & D-1816)
The dielectric breakdown is an indication of the oil’s ability to withstand electrical stress. The most commonly performed test is ASTM D-877, and because of this, it is more readily used as a benchmark value when comparing different results. The oil sample is placed in a test cup and an AC voltage is impressed on it. The electrodes are two discs, exactly 1 in. in diameter and placed 0.10 in. apart.
The voltage is raised at a constant rate, until an arc jumps through the oil between the two electrodes. The voltage at which the arc occurs is considered the dielectric strength of the oil.
For systems over 230 kV, this test is performed using spherical electrodes spaced 0.04 or 0.08 in. apart (ASTM D-1816). Portable equipment is available for performing both levels of this test in the field.
Neutralization number (ASTM D-974)
Acids are formed as by-products of oxidation or sludging, and are usually present any time an oil is contaminated.
The concentration of acid in an oil can be determined by the amount of potassium hydroxide (KOH) needed to neutralize the acid in 1 g of oil. Although it is not a measure of the oil’s electrical strength, it is an excellent indicator of the pressure of contaminants. It is especially useful when its value is monitored over a number of sampling periods and trending data is developed.
Interfacial tension (ASTM D-971 & D-2285)
The interfacial tension of an oil is the force in dynes per centimeter required to rupture the oil film existing at an oil-water interface. when certain contaminants, such as soaps, paints, varnishes, and oxidation products are present in the oil, the film strength of the oil is weakened, thus requiring less force to rupture. For in-service oils, a decreasing value indicates the accumulation of contaminants, oxidation products, or both.
ASTM D-971 uses a platinum ring to physically break the interface and measure the force required. ASTM D-2285 measures the volume of a drop of water that can be supported by the oil without breaking the interface.
Power factor (ASTM D-924)
The power factor is an indication of the amount of energy that ls lost as heat to the oil. When pure oil acts as a dielectric, very little energy is lost to the capacitance charging. Contaminants will increase the energy absorbed by the oil and wasted as heat. The power factor ls a function of the phasor angle (the angular displacement) between an AC potential applied to the oil and the resulting current.
The test is performed by passing a current through a test cell of known gap, and “sing a calibrated capacitance or resistance bridge to separate and compare the reactive and resistance portions of the current passing through the oil.
Color (ASTM D-1500)
The color of a new oil is generally accepted as au Index of refmement. For in-service oils, a darkening of the oil (higher color number), observed over a number of test intervals, is an indication of contamination or deterioration.
The color of an oil is obtained by comparison to numbered standards. Although there are charts available, the most accurate way to determine the oil’s color is by the use of a color wheel and a comparator. An oil sample is placed in the comparator, and the color wheel is rotated until a match is obtained. This test is most effective when results are compiled over a series of test intervals, and trending data is developed.
Table 1 – Insulating fluids suggested test values
|Laboratory Test Values|
|ASTM Test||Oil||High Molecular
|30kV minimum||30kV minimum||30kV minimum||30kV minimum|
|Neutralization number D-974||0.04 MG-KOH/GM maximum||0.03 MG-KOH/GM maximum||0.01 MG-KOH/GM maximum||0.25 MG-KOH/GM maximum|
|Interfacial Tension D-971
|35 Dynes/cm minimim||33 Dynes/cm minimim||–||–|
|Color D-1500||1. Maximum||N/A Maximum||0.05 D-2129||–|
|Visual Condition D-1524||Clear bright pale straw||N/A||Crystal clear D-2129||Clear, Slightpink Iridscent|
|Power Factor D-924 & 25
|0.1% maximum||0.1% maximum||0.1% maximum||2% maximum|
|Water Content D-1533
15kV and below
|35 PPM*maximum||35 PPM*maximum||80 PPM*maximum||25 PPM*maximum|
|115kV – 230kV||20 PPM*maximum||–||–||–|
|Above 230kV||15 PPM*maximum||–||–||–|
* Or in accordance with manufacturer’s requirements. Some manufacturers recommend 15 PPM maximum for all transformers.
Moisture content (ASTM D-1533)
Moisture content is very important in determining the seniceability of an oil; the presence of moisture (as little as 25 parts per million) will usually result in a lower dielectric strength value. Water content is especially important in transformers with fluctuating loads. As the temperature increases and decreases with the changing load, the transformer’s oil can hold varying amounts of water in solution.
Large amounts of water can be held in solution at higher temperatures, and in this state (dissolved) the water has a dramatic effect on the oil’s performance.
Water contamination should be avoided.
(1) Water content is expressed in parts per million, and although water will settle to the bottom of the tank and be visible in the sample, the presence of free water is not an indication of high water content, and it is usually harmless in this state.
The dissolved water content is the dangerous factor; it is usually measured by physical or chemical means. A Karl Fischer titrating apparatus is one of the more common methods of measuring the dissolved water content.
(2) There are other tests available, such as Flashpoint, Viscosity, and Specific Gravity. They are of limited value for interpretation of the oil’s quality, but can be used for further investigation if unsatisfactory results are obtained for the tests listed above.
(3) Table 1 lists the acceptable values for the laboratory test results for various insulating fltids.