NTT performs tests according to standards approved by the American Society for Testing and Materials (ASTM). These tests are designed to evaluate the ability of the oil to perform its job. Tests include moisture in oil, interfacial tension, acid number, color number, visual, dielectric strength, viscosity, specific gravity, power factor at 25oC & 100oC, oxidation inhibitor, refractive index, pour point, and flash point.
Water in Insulating Fluids
ASTM D 1533B
The presence of water can adversely affect the dielectric strengths of an insulating fluid. Water content is reported in parts per million.
Acid Number
ASTM D 974
Oxidation of insulating fluids and/or additives in the fluid results in the production of acidic compounds. The periodic measurement of acidity provides a means of monitoring the progress of oxidation. The build-up of acidic compounds precedes the formation of sludge in the transformer which is the end-product of oxidation.
Interfacial Tension
ASTM D 971
Determining the presence of polar contaminants in insulating oil is accomplished by measuring the tension of oil against water. The interfacial tension of an oil is sensitive to the presence of the products of oxidation of the oil and can be used, together with acidity measurements, as an indicator to monitor sludge development. Foreign substances such as dissolved varnishes andother organic coating materials can also affect IFT. The presence of polar contaminants generally lowers the interfacial tension value.
Visual Examination and Color
ASTM D 1524
Monitoring the color and visual appearance of an insulating oil provides a rapid assessment of oilquality. Insulating oils tend to darken due to oxidation and/or presence of contamination. Materials suspended in the oil or sediment are assessments of oil quality and/or samplingtechniques.
Dielectric Breakdown Voltage
ASTM D 877
ASTM D 1816
The dielectric breakdown voltage is the voltage at which an insulating fluid begins to conduct. This voltage denoted the electrical stress that an insulating fluid can withstand without failure.
Specific Gravity
ASTM D 1298
The specific gravity of an oil is the ratio of the weights of equal volumes of oil and water at the same temperature. Specific gravity is applicable in determining suitability for use in specificsituations. In cold climates, specific gravity can be used to determine whether ice resulting from freezing water in the oil will float on the oil.
Kinematic Viscosity
ASTM D 445
ASTM D 2161
Viscosity is the measured resistance of the insulating fluid to flow under specific conditions. Viscosity influences heat transfer characteristics of the insulating fluid.
Power Factor
ASTM D 924
Power factor is a measure of the dielectric losses in an insulating fluid due to heat dissipation when the fluid is placed in an electric AC field. Power factor is usually performed in conjunction with other oil quality tests to determine the state of the insulation fluid.
Oxidation Inhibitor
This is the measurement of the amount of inhibitor remaining in the fluid after oxidation has reduced its concentration. Inhibitors can be added to oil in prescribed amounts to increase the service life of the oil.
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