ASTM D117 guide for sampling, test method and specification for insulating oil
ASTM D117 standard guide for sampling, test methods and specifications for electrical insulating oils of petroleum origin
ELECTRICAL PROPERTIES
17. Dielectric Breakdown Voltage
17.1 Scope:
17.1.1 There are two standard test methods for determining the dielectric breakdown voltage of electrical insulating fluids at commercial power frequencies, D877 and D1816, and one standard test method for determining the dielectric breakdown voltage of insulating oils under impulse conditions, D3300.

17.1.2 Test Method D877 - Applicable to liquid petroleum oils, hydrocarbons, and askarels commonly used as insulating and cooling media in cables, transformers, oil circuit breakers, and similar apparatus. The suitability of Test Method D877 for testing liquids having viscosities exceeding 900 cSt (5000 SUS) at 40°C (104°F) has not been determined.

17.1.3 Test Method D1816 - Applicable to liquid petroleum oils commonly used as an insulating and cooling medium in cables, transformers, oil circuit breakers, and similar apparatus. The suitability of Test Method D1816 for testing oils having viscosities of more than 19 cSt (100 SUS) at 40°C (104°F) has not been determined.

17.1.4 Test Method D3300 - Applicable to any liquid commonly used as an insulating and cooling medium in high-voltage apparatus subjected to impulse conditions, such as transient voltage stresses arising from such causes as nearby lightning strikes and high-voltage switching operations.

17.2 Definition:
17.2.1 dielectric breakdown voltage - the potential difference at which electrical failure occurs in an electrical insulating material or insulation structure, under prescribed test conditions.

17.3 Summary of Test Methods:
17.3.1 Test Method D877 he insulating liquid is tested in a test cup between two 25.4-mm (1-in.) diameter disk electrodes spaced 2.54 mm (0.100 in.) apart. A 60-Hz voltage is applied between the electrodes and raised from zero at a uniform rate of 3 kV/s. The dielectric breakdown voltage is recorded, prior to the occurrence of disruptive discharge, when the voltage across the specimen has dropped to less than 100 V. In the referee procedure, one breakdown test is made on each of five fillings of the test cup, and the average and individual values of breakdown voltage are reported.

17.3.2 Test Method D1816 - The oil is tested in a test cell between spherically capped (VDE) electrodes spaced either 1 mm (0.040 in.) or 2 mm (0.080 in.) apart. The oil is stirred before and during application of voltage by means of a motor-driven stirrer. A 60-Hz voltage is applied between the electrodes and raised from zero at a uniform rate of 1/2 kV/s. The voltage at which the current produced by breakdown of the oil reaches the range of 2 to 20 mA, tripping a circuit breaker, is considered to be the dielectric breakdown voltage. In the procedure, five breakdown tests are made on one filling of the test cell. If the five breakdowns fall within the statistical requirements, the average value is reported. If not, five additional breakdowns are required with the average of the ten values reported.

17.3.3 Test Method D3300 - The electrode system consists of either: (1) two 12.7-mm (0.5-in.) diameter spheres spaced 3.8 mm (0.15 in.) apart or (2) a 12.7-mm (0.5-in.) diameter sphere and a steel phonograph needle of 0.06-mm radius of curvature of point, spaced 25.4 mm (1.0 in.) apart. The polarity of the needle with respect to the sphere can be either positive or negative. The electrodes are immersed in the oil in a test cell. An impulse wave of 1.2 by 50 µs wave shape (times to reach crest value and to decay to half of crest value, respectively) is applied at progressively higher voltages until breakdown occurs.

17.4 Significance and Use:
17.4.1 Power Frequencies (Test Methods D877 and D1816) - The dielectric breakdown voltage of an insulating liquid at commercial power frequencies is of importance as a measure of the liquid's ability to withstand electric stress. It is the voltage at which breakdown occurs between two electrodes under prescribed test conditions. It also serves to indicate the presence of contaminating agents, such as water, dirt, moist cellulosic fibers, or conducting particles in the liquid, one or more of which may be present when low dielectric breakdown values are found by test. However, a high dielectric breakdown voltage does not indicate the absence of all contaminants. See Appendix X1 of either test method for other influences that affect the dielectric breakdown voltage of a liquid.

17.4.1.1 The ability of a fluid to resist breakdown under the test conditions is an indication of the ability of the fluid to perform its insulating function in electrical apparatus. The average breakdown voltage is commonly used in specifications for the qualification and acceptance of insulating fluids. It is also used as a control test for the refining of new or reclaiming of used insulating fluids. Because of the complex interactions of the factors affecting dielectric breakdown voltage the values obtained cannot be used for design purposes.

17.4.1.2 The square-edged disk electrodes of Test Method D877 are relatively insensitive to dissolved water in concentrations below 60 % of the saturation level. This method is recommended for acceptance tests on unprocessed insulating liquids received from vendors in tank cars, tank trucks, and drums. It also may be used for the routine testing of liquids from selected power systems apparatus.

17.4.1.3 The more uniform electric field associated with VDE electrodes employed in Test Method D1816 is more sensitive to the deleterious effects of moisture in solution, especially when cellulosic fibers are present in the oil, than is the field in Test Method D877. Test Method D1816 can be used for processed or as received oils. Filtering and dehydrating the oil may increase Test Method D1816 dielectric breakdown voltages substantially.

17.4.2 Impulse Conditions (Test Method D3300):
17.4.2.1 This test method is most commonly performed using a negative polarity point opposing a grounded sphere (NPS). The NPS breakdown voltage of fresh unused oils measured in the highly divergent field in this configuration depends on oil composition; decreasing with increasing concentration of aromatic, particularly polyaromatic, hydrocarbon molecules.

17.4.2.2 This test method may be used to evaluate the continuity of composition of an oil from shipment to shipment. The NPS impulse breakdown voltage of an oil can also be substantially lowered by contact with materials of construction, by service aging, and by other impurities. Test results lower than those expected for a given fresh oil may also indicate use or contamination of that oil.

17.4.2.3 Although polarity of the voltage wave has little or no effect on the breakdown strength of an oil in uniform fields, polarity does have a marked effect on the breakdown voltage of an oil in nonuniform electric fields.

18. Dissipation Factor and Relative Permittivity (Dielectric Constant)
18.1 Scope:
18.1.1 This test method covers new electrical insulating liquids as well as liquids in service or subsequent to service in cables, transformers, oil circuit breakers, and other electrical apparatus.

18.1.2 This test method provides a procedure for making referee and routine tests at a commercial frequency of approximately 60 Hz.

18.2 Summary of Test Method:
18.2.1 The loss characteristic is commonly measured in terms of dissipation factor (tangent of the loss angle) or of power factor (sine of the loss angle). For values up to 0.05, dissipation factor and power factor values are equal to each other within about one part in one thousand and the two terms may be considered interchangeable.

18.2.2 Test Method D924 - The oil test specimens are tested in a three-terminal or guarded electrode test cell maintained at the desired test temperature. Using a bridge circuit, measure the loss characteristics and capacitance following the instructions appropriate to the bridge being used. For routine tests, a two-electrode cell may be used.

18.3 Significance and Use:
18.3.1 Dissipation Factor (or Power Factor) - This property is a measure of the dielectric losses in an oil, and hence, of the amount of energy dissipated as heat. A low value of dissipation factor (or power factor) indicates low dielectric losses and a low level of soluble polar ionic or colloidal contaminants. This characteristic may be useful as a means of quality control and as an indication of oil changes in service resulting from contamination and oil deterioration.

18.3.2 Relative Permittivity (Dielectric Constant) - Insulating liquids are used in general either to insulate components of an electrical network from each other and from ground, alone or in combination with solid insulating materials, or to function as the dielectric of a capacitor. For the first use, a low value of relative permittivity is often desirable in order to have the capacitance be as small as possible, consistent with acceptable chemical and heat transfer properties. However, an intermediate value of relative permittivity may sometimes be advantageous in achieving a better voltage distribution between the liquid and solid insulating materials with which the liquid may be in series. When used as the dielectric in a capacitor, it is desirable to have a higher value of relative permittivity so the physical size of the capacitor may be as small as possible.

19. Gas Evolution
19.1 Scope - Test Method D 2300 describes a procedure to measure the rate at which gas is evolved or absorbed by insulating oils when subjected to electrical stress of sufficient intensity to cause ionization. The oil test specimen is initially saturated with a selected gas (usually hydrogen) at atmospheric pressure.

19.2 Summary of Test Method:
19.2.1 Test Method D2300 - After being saturated with a gas (usually hydrogen) the oil is subjected to a radial electrical stress at a controlled temperature. The gas space above the oil is ionized due to the electrical stresses; and therefore, the oil surface at the oil-gas interface is subjected to ion bombardment. The evolution or absorption of gas is measured with a gas burette and reported in µL/min.

19.3 Significance and Use - This test method indicates whether insulating oils are gas absorbing or gas evolving under the test conditions. Numerical results obtained in different laboratories may differ significantly in magnitude, and the results of this test method should be considered as qualitative in nature.

19.3.1 For certain applications when insulating oil is stressed at high voltage gradients, it is desirable to be able to determine the rate of gas evolution or gas absorption under specified test conditions. At the present time, correlation of such test results with equipment performance is limited.

20. Resistivity
20.1 Scope:
20.1.1 This test method covers the determination of specific resistance (resistivity) applied to new electrical insulating liquids, as well as to liquids in service, or subsequent to service, in cables, transformers, circuit breakers, and other electrical apparatus.

20.1.2 This test method covers a procedure for making referee and routine tests with dc potential.

20.2 Definition:
20.2.1 specific resistance (resistivity) - of a liquid, the ratio of the dc potential gradient in volts per centimetre paralleling the current flow within the test specimen, to the current density in amperes per square centimetre at a given instant of time and under prescribed conditions. This is numerically equal to the resistance between opposite faces of a centimetre cube of a liquid. It is measured in ohm centimetres.

20.3 Summary of Test Method:
20.3.1 Test Method D1169 - The oil test specimen is tested in three-terminal, or guarded-electrode test cell maintained at the desired test temperature. A dc voltage is applied of such magnitude that the electric stress in the liquid is between 200 and 1200 V/mm (5 to 30 V/mL). The current flowing between the high-voltage and guarded measuring electrode is measured at the end of 1 min of electrification and the resistivity calculated using specified equations appropriate to the method of measurement used. A two-electrode cell may be used for routine tests.

20.4 Significance and Use - The resistivity of a liquid is a measure of its electrical insulating properties under conditions comparable to those of the test. High resistivity reflects low content of free ions and ion-forming particles and normally indicates a low concentration of conductive contaminants.

21. Stability Under Electric Discharge
21.1 Scope - Test Method D6180 measures the relative stability of new, used, or reclaimed insulating oils of petroleum origin in the presence of a controlled electric discharge by monitoring the pressure increase in the evacuated discharge chamber.

21.2 Summary of Test Method - A test specimen is introduced into a discharge cell and degassed under vacuum at room temperature. An ac potential of 10 KV is applied for 300 min. The gradual rise of pressure inside the discharge cell is measured as a function of time. The dissipation factor of the oil at 100°C is determined before and after the stability test using Test Method D924.

21.3 Significance and Use - The changes observed in the generation of gases as noted by pressure change and the composition modification as reflected in dissipation factor increases may provide a relative assessment of the stability of the oil for high voltage application.