Transformer Insulating Oil Test Methods (ASTM)

Insulating fluid is the lifeblood of a liquid-filled apparatus, and regular sampling and laboratory analysis can give great insight into the condition of the equipment that regular electrical tests may not detect. Test methods developed by ASTM support the quality, distribution, and delivery of insulating liquids.

Each apparatus, voltage class, fluid type and service age will require its own unique set of tests and parameters based on these standards.

Recommended: Insulating Liquids: Basic Properties, Types, and Applications Explained

Color D1500

Units: 0-5

Used to observe darkening of the oil by comparing it to previous samples of oil from the same transformer. Oil color is determined by means of transmitted light and given a numerical value (0-5) based on comparison with a series of color standards. Noticeable darkening oil indicates either contamination or that arcing is taking place.

Dielectric Breakdown Voltage D877

Units: kV

Measures the voltage at which the oil fails electrically, which is indicative of the amount of contaminant (usually moisture) in the oil. The dielectric breakdown voltage is measured by applying a voltage between two electrodes under the oil. New oil should have a minimum dielectric strength of 35 kV by ASTM methods of testing.

Dielectric Breakdown Voltage D1816

Units: kV

Measures dielectric breakdown voltage uses VDE electrodes. This method is run at one of two gap setting: 1 mm (0.04 in) or 2 mm (0.08 in). Stirring the liquid is optional and this is achieved with a magnetic stirring bar. Because of a greater sensitivity over the D877 method, the rate of voltage rise for this test is lower.

Dissolved gas analysis (DGA) D3612

Units: ppm

Identifies various gas ppm levels that are present in transformer insulating oil. Different gasses will dissolve in the oil that indicate various types of thermal and electrical stress occurring within the transformer.

An oil sample tube and syringe is used to draw, retain and transport the oil sample in the same condition as it is inside a transformer with all fault gases dissolved in it. An internal fault in a transformer can be discovered earliest using this test method.

Dissolved metals D7151

Units: µm

Identifies any dissolved metals that could originate from overheating or arcing and a portion of the particulate metals that may originate from mechanical wear. Measured by inductively coupled plasma atmonic emission spectrometry (ICP-AES) and expressed in micrometers.

Flash point, fire point D92

Units: °C

Indicates the volatility of insulating oil by measuring the minimum temperature at which the heated oil gives of sufficient vapor to form a flammable mixture with air.

Furanic compounds D5837

Units: ppb

Determines the presence of degradation in cellulosic materials such as paper, pressboard, and cotton, which generate furanic compounds in the insulating oil. Measurements are made using high-performance liquid chromatography (HPLC).

Interfacial tension D971

Units: mN/m

Measures the presence of soluble contaminants and oxidation products in transformer oil. Expressed in mN/m, it is a test of interfacial tension 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.

A decreasing value indicates an increase in contaminants and/or oxidation products within the oil.

Kinematic Viscosity D445

Units: mm2/s

Measurement is based on tracking the flow time of transparent or opaque liquids through a calibrated glass capillary. To calculate the kinematic viscosity, the flow time is multiplied by the capillary constant, which is obtained by the measurement of certified viscosity reference material.

Temperature is one of the most important factors influencing viscosity measurement.

Moisture (Water) Content D1533

Units: ppm

Measured in parts per million (ppm) using the weight of moisture divided by the weight of oil. Moisture content in oil lowers the insulating system dielectric strength and allows flashover that can damage a transformer.

For mineral oil, a generally accepted maximum moisture content is 35 ppm. This test does not measure the water content in the transformer paper insulation.

Neutralization (Acid) number D974

Units: mg KOH/g

New transformer oils contain practically no acids. The acidity test measures the content of acids formed by oxidation and contaminates.

Measurements are made by the amount of potassium hydroxide (KOH in mg) required to neutralize the acid in one gram of oil. Typically, results of 0.10 mg KOH/gram of oil or less are considered good.

Higher values are indicative of a problem.

Oxidation inhibitor content D2668

Units: %

Measures the amount of 2,6-ditertiary-butyl paracresol and 2,6-ditertiary-butyl phenol that has been added to new insulating oil as protection against oxidation. In a used oil it measures the amount remaining after oxidation has reduced its concentration.

Polychlorinated biphenyls (PCB) content D4059

Units: ppm

Detects the concentration level of polychlorinated biphenyls in transformer oil by gas chromatography. Measured in ppm, it also applies to the determination of PCB present in mixtures known as askarels.

Pour point D97

Units: °C

Indicates the lowest temperature at which the insulating oil will flow. This test is particularly useful in cold climates to ensure that the oil will circulate and serve its purpose as an insulating and cooling medium.

Power Factor D924

Units: %

Indicates the dielectric losses of the oil, or energy that is dissipated as heat. Useful for measuring changes within the insulating oil resulting from contamination or deterioration.

The power factor of insulating oil equals the cosine of the phase angle between an ac voltage applied and the resulting current. For mineral oil, the power factor of new oil should not exceed 0.05 percent at 25 degrees C.

Relative density (specific gravity) D1298

Units: numerical

Determines the density, relative density (specific gravity), or API gravity of transformer oil by use of hydrometer at a reference temperature. A high specific gravity indicates the oil’s ability to suspend water. In extremely cold climates, specific gravity can be used to determine whether ice will float on the oil.

Resistivity D1169

Units: ohms

Measures the electrical insulating properties of transformer oil under conditions comparable to those of the test. A low resistivity reflects a high content of free ions and ion-forming particles in the insulating oil, and normally indicates a high concentration of conductive contaminates.

Visual examination D1524

Units: Bright, dark, clear of particles etc.

Oil is visually examined by passing a beam of light through it to determine transparency and identify foreign matters. Contamination of the oil is exhibited by poor transparency, cloudiness, or the observation of foreign particles.