ASTM International - ASTM D2275-14

Standard Test Method for Voltage Endurance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) on the Surface

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Organization:	ASTM International
Publication Date:	1 November 2014
Status:	inactive
Page Count:	9
ICS Code (Insulating materials in general):	29.035.01

significance And Use:

5.1 This test method is useful in research and quality control for evaluating insulating materials and systems since they provide for the measurement of the endurance used to compare different materials to the action of corona on the external surfaces. A poor result on this test does not indicate that the material is a poor selection for use at high voltage or at high voltage stress in the absence of surface corona; surface corona is not the same as corona that occurs in internal cavities. (See Test Methods D3382.)

5.2 This test method is also useful for comparison between materials of the same relative thickness. When agreed upon between the buyer and the seller, it is acceptable to express any differences in terms of relative time to failure or the magnitude of voltage stress (kV/mm or kV/in.) required to produce failure in a specified number of hours.

5.3 It is possible for this test method to also be used to examine the effects of different processing parameters on the same insulating material, such as residual strains produced by quenching, high levels of crystallinity or molding processes that control the concentration and sizes of gas-filled cavities.

5.4 The data are generated in the form of a set of values of lifetimes at a voltage. The dispersion of failure times is analyzed using one of the methods below:

5.4.1 Weibull Probability Plot.

5.4.2 Statistically (see IEEE 930-1987 for additional information), to yield an estimate of the central value of the distribution and its standard deviation.

5.4.3 Truncating a test at the time of the fifth failure of a set of nine and using that time as the measure of the central tendency. Two such techniques are described in 10.2.

5.5 This test method intensifies some of the more commonly met conditions of corona attack so that materials are able to be evaluated in a time that is relatively short compared to the life of the equipment. As with most accelerated life tests, caution is necessary in extrapolation from the indicated life to actual life under various operating conditions in the field.

5.6 The possible factors related to failures produced by corona are:

5.6.1 Corona eroding the insulation until the remaining insulation can no longer withstand the applied voltage.

5.6.2 Corona causing the insulation surface to become conducting due to carbonization, so that failure occurs quickly.

5.6.3 Forming of compounds such as oxalic acid crystals causing the surface conductance to vary with ambient humidity. It is possible conductance will be at a sufficient level to reduce the potential gradient at the electrode edge at moderate humidities, and thus cause either a reduction in the amount of corona, or its cessation, thus retarding failure.

5.6.4 Corona causing "treeing" within the insulation and consequently accelerating the time to failure.

5.6.5 Gases released within the insulation that change its physical dimensions.

5.6.6 Changes in the physical properties of an insulating material; embrittlement or cracking, for instance, causing the material to lose flexibility or crack, or both, and thus make it useless.

5.7 Tests are often made in open air, at 50 % relative humidity. In cases agreed upon between the buyer and the seller, additional information can be obtained for some materials with tests in circulating air at 20 % relative humidity or less (see Appendix X1).

5.7.1 If tests are made in an enclosure, the restriction in the flow of air can trap ozone and influence the results (see Appendix X2).

5.7.2 When tests are done outside the standard conditions, the report shall note the deviation and the alternative conditions.

5.8 The variability of the time to failure is a function of the consistency of the test parameters, such as voltage levels, which shall be monitored. The Weibull slope factor, β, is recommended as a measure of variability. β is the slope obtained when percent failure is plotted against failure time on Weibull probability paper. Such a plot is called a Weibull Probability Plot (see Fig. 1).

Note 1: Plotting percentage are 100 times the average of (n − ¹/₂ )/N and n/(N + 1). Artificial data were placed on a line (dashed) drawn to illustrate a Weibull line with a β of 4. A second line (not dashed) illustrates the distribution of failure times which are characteristic of materials with very flat volt-time curves, such as mica composites. This line has a β value of 0.7.

5.9 The shape of the Weibull Probability Plot can provide additional information. It is possible that a non-straight-line plot will indicate more than one mechanism of failure. For instance, a few unaccountably short time failures in the set indicating a small portion of defective specimens with a different failure mechanism from the rest of the lot.

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scope:

1.1 This test method determines the voltage endurance of solid electrical insulating materials for use at commercial power frequencies under the action of corona (see Note 1). This test method is more meaningful for rating materials with respect to their resistance to prolonged ac stress under corona conditions for comparative evaluation between materials.

Note 1: The term "corona" is used almost exclusively in this test method instead of "partial discharge," because it is a visible glow at the edge of the electrode interface that is the result of partial discharge. Corona, as defined in Terminology D1711, is "visible partial discharges in gases adjacent to a conductor."

1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see Section 7.