1、Designation: D3756 97 (Reapproved 2010)D3756 18Standard Test Method forEvaluation of Resistance to Electrical Breakdown by Treeingin Solid Dielectric Materials Using Diverging Fields1This standard is issued under the fixed designation D3756; the number immediately following the designation indicates
2、 the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONWhen failure occurs in solid organic dielectric
3、s that are subjected to very high, continuous, andnonuniform electrical gradients, it generally occurs by a mechanism called treeing. Materials ofdifferent molecular structures have different degrees of resistance to failure by treeing, and thisresistance can sometimes be increased by the addition o
4、f other materials in low concentration.2Trees that grow by a molecular degradation mechanism resulting from partial discharge (corona) arecalled electrical trees to distinguish them from water and electrochemical trees which are quitedifferent.This test method makes use of two opposing thin cylindri
5、cal electrodes, one sharpened to a point,the other with a hemispherical end. They are molded or inserted into blocks of the material to betested. Because of the shape of the electrodes this is often called a needle test. This test provides astatistical estimate of electrical treeing initiation and p
6、ropagation of solid dielectric materials in high,diverging electrical fields.1. Scope1.1 This test method covers the evaluation and comparison of the resistance of solid organic dielectric materials to the initiationor growth, or both, of tubular tree-like channels resulting from partial discharge (
7、corona) and molecular decomposition that occurin the region of very high, diverging electric fields.3,41.2 This test method is primarily for use at a power frequency of 50 or 60 Hz.1.3 The test may is able to be carried out at room temperature or temperatures above or below room temperature. Thetemp
8、erature shouldshall not exceed the softening or melting point of the sample material.1.4 This test method can be used for any solid material into which needles can be cast, molded, or inserted with heat aftermolding. The resistance to tree initiation is measured by the double-needle characteristic v
9、oltage, which is only applicable tonon-opaque materials so that tree can be observed optically. The resistance to tree initiation and growth is reported by thedouble-needle voltage life, which is applicable to both opaque and non-opaque materials.1.5 The values stated in SI units are to be regarded
10、as the standard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulato
11、ry limitations prior to use.1.7 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organizat
12、ion Technical Barriers to Trade (TBT) Committee.1 This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of SubcommitteeD09.12 on Electrical Tests.Current edition approved Oct. 1, 2010Nov. 1, 2018. Published
13、 October 2010November 2018. Originally approved in 1990. Last previous edition approved in 20042010as D3756 97 (2010).(2004). DOI: 10.1520/D3756-97R10.10.1520/D3756-18.2 Symposium on Engineering Dielectrics, ASTM STP 783, ASTM, 1982, and Symposium on Engineering Dielectrics, ASTM STP 926, ASTM, 1986
14、.3 W. D. Wilkens, Chapter 7, “Statistical Methods for the Evaluation of Electrical Insulating Systems,” Engineering Dielectrics, Vol IIB, Electrical Properties of SolidInsulating Materials, Measurement Techniques, R. Bartnikas, Editor, ASTM STP 926, ASTM, Philadelphia, 1987.4 R. M. Eichorn, Chapter
15、4, “Treeing in Solid Organic Dielectric Materials,” Engineering Dielectrics, Vol IIA, Electrical Properties of Solid Insulating Materials:Molecular Structure and Electrical Behavior, R. Bartnikas and R. M. Eichorn, Editors, ASTM STP 783, ASTM Philadelphia, 1983.This document is not an ASTM standard
16、and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases onl
17、y the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States12. Referenced Documents2.1 ASTM Standards:5D149 Test Method for Dielectric Breakdown V
18、oltage and Dielectric Strength of Solid Electrical Insulating Materials atCommercial Power FrequenciesD1711 Terminology Relating to Electrical InsulationD1928 Practice for Preparation of Compression-Molded Polyethylene Test Sheets and Test Specimens (Withdrawn 2001)6D2275 Test Method for Voltage End
19、urance of Solid Electrical Insulating Materials Subjected to Partial Discharges (Corona) onthe Surface2.2 Other Document:ANSI/IEEE 930-1987 IEEE Guide for the Statistical Analysis of Electrical Insulation Voltage Endurance Data73. Terminology3.1 Definitions:3.1.1 partial discharge, nrefer to D1711.3
20、.2 Definitions of Terms Specific to This Standard:3.2.1 characteristic voltage or DNCV (double-needle characteristic voltage), nthat voltage which, when applied for 1 h 1 hbetween the ends of two thin cylindrical electrodes (one sharpened to a point, the other with a hemispherical end) in a group of
21、replicate specimens, produces observable dielectric damage at the point of the sharp electrode in half of the specimens.3.2.2 median voltage life (t50), nthe time, determined from a Weibull plot, when 50 % failure occurs from a group of 10tenidentical specimens subjected to the same voltage stress.4
22、. Summary of Method4.1 In this test method, specimens are prepared and needles inserted to serve as electrodes. Voltage is applied to the needles andcontinued for 1 h in the double-needle characteristic voltage test or until electrical breakdown occurs in the double-needle voltagelife test. Results
23、are expressed as the voltage at which half of the specimens show dielectric damage in 1 h, or the median timeto failure of a group of specimens subjected to a given continuous voltage, at a selected or predetermined temperature.5. Significance and Use5.1 This is a laboratory test designed to simulat
24、e the effects of (1) the presence of rough interfaces between conductor orsemiconductive screen and primary insulation in an insulation system, (2) the presence of foreign particles (contaminants) in aninsulation system, and (3) the presence of small voids or cavities within the insulation.5.2 This
25、test method provides comparative data. The degree of correlation with actual performance in service has not beenestablished.6. Apparatus6.1 Power SupplyA high voltage supply having a sinusoidal voltage output at a power frequency equipped with continuousvoltage control and an adjustable protective a
26、utomatic circuit-breaking device that operates at a controllable current level. See TestMethod D149.6.2 CurrentSensitive Individual Specimen DisconnectWhen ten specimens are tested to failure for the voltage life test, usea disconnect circuit for each.6.3 ElectrodesThe critical electrode is a round
27、steel rod, 1 mm 1 mm in diameter, sharpened at one end to a controlled radiusof 3 6 1 m 1 m and an included angle of 30 6 1 degrees. 1.6.4 Needle-Grinding Lathe, for preparation of the sharpened electrodes to a controlled-point sharpness and included angle. Atypical lathe and grinder combination are
28、 shown in Fig. 1.6.5 Specimen Molding Chase, for single-step preparation of compression molded specimens containing needles.Atypical chaseis shown in Fig. 2.6.6 Needle-Insertion JigA jig is required for slow, controlled insertion of electrodes. Fig. 3 shows a specimen insertion jig.6.7 Test ChamberA
29、ny temperature-controlled test chamber or enclosure, which can hold at least ten specimens and maintainuniform temperature, is appropriate for this test.5 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM St
30、andardsvolume information, refer to the standards Document Summary page on the ASTM website.6 The last approved version of this historical standard is referenced on www.astm.org.7 Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.D3756 1827. S
31、ampling7.1 Sample so that the specimens tested will represent the entire lot.8. Test Specimens8.1 Test specimens are approximately 25-mm2 blocks, 6 mm thick, containing two needle-like electrodes as shown in Fig. 4.Prepare by compression molding, extrusion, or cutting from finished pieces. The tips
32、of the electrodes are separated by 12.0 6 0.5mm for characteristic voltage determination and 6.5 6 0.5 mm for voltage life determination. The number of test specimens is atleast 24 for characteristic voltage determination (i.e. (that is, six specimens for each of at least four different testing volt
33、ages) and10ten for voltage life determination.8.2 Preparation of Thermoplastic and Crosslinked SpecimensCompression-mold plaques, 6-mm thick, of the sample material,in a steam or electrically heated hydraulic press equipped for cold-water cooling of the platens. Use a positive pressure mold, whichma
34、y be of either the picture-frame type or the milled-cavity type. Use parting sheets of cellophane, polyester film, or aluminumfoil between the mold surfaces and the resin. The choice of parting sheet depends somewhat on the molding temperature, althoughaluminum sheets, washed with alcohol and thorou
35、ghly dried, are preferred. The size of the mold is not critical, 200 200 mm isrecommended. For peroxide-crosslinkable materials, the typical compression-molding conditions shouldmust follow the materialmanufacturers recommendation of temperature, time and pressure. The Remove the by-products of pero
36、xide decomposition shouldbe removed before testing by use of a vacuum oven at elevated temperatures (80C for 7seven days for XLPE using dicumylperoxide).FIG. 1 Machine Setup for Needle SharpeningFIG. 2 Chase for Preparation of Specimens Containing ElectrodesD3756 1838.2.1 After molding, cut the plaq
37、ue into 25-mm square blocks with square and smooth edges. Store the squares under standardlaboratory conditions, 23C and 50 % relative humidity, and protect them from dirt and atmospheric contamination until used.8.3 Insertion of NeedlesInsert the needle electrodes into the specimen blocks slowly an
38、d carefully to avoid orientation strains,formation of cavities, and damage to the sharp points. Use a jig, such as the one shown in Fig. 3, to ensure that every needle willbe inserted under identical conditions.8.3.1 Make the insertions as follows: Place twelve specimen blocks in the slots provided
39、for them and lightly clamp into place.Inspect the twelve sharpened needles, after cleaning with methyl ethyl ketone; then carefully place them into the needle slots onone side so their tips just touch the specimens. Use the individual adjusting screws for positioning the needles. Slip shims into the
40、needle slots above the needles, and use a cover plate to close the top of the needle slots. Secure this cover with small C-clampsat each end. The purpose of the shims and cover is to prevent the needle from cocking, and to force it to enter straight into thespecimen. Place twelve electrodes with hem
41、ispheric ends in the slots on the opposite sides of the specimens in the same manner.8.3.2 When the specimens and needles are mounted and the needles are adjusted into proper position, place the whole jig intoa circulating air oven at 105C, for low-density polyethylene or crosslinked materials, for
42、a 1-h preheating period. For otherpartially crystalline polymeric materials use a temperature approximately 10C below the thermodynamic first-order transitionpoint. After preheating, simultaneously advance the needles 1.30 mm by making one full turn of the large lead screw. Repeat at5-min intervals.
43、 Make sufficient turns to accomplish the insertion. Five turns are normally required for the characteristic voltagetest and seven for the voltage life test. An electrode gap of 12.0 6 0.5 mm is commonly used for characteristic voltagedetermination and 6.5 6 0.5 mm is preferred for voltage life tests
44、. Use constant gaps for comparison of materials by this test.8.3.3 When the insertions are complete, leave the jigs untouched for at least 30 min in the oven for stress relief or annealing.Remove the jigs from the oven and cool naturally to room temperature. Examine each specimen with a microscope,
45、if possible,to ensure that the needle point was not damaged during insertion.8.4 Preparation of Specimens with Molded-In ElectrodesCondition as specified in Section 9.8.4.1 The molding is 150 25 6 mm thick and contains six pointed and six hemispherical electrodes. Cut six squarespecimens from this b
46、lock. See Fig. 2.8.4.2 A compression mold that can be used for preparation of the six specimen plaques consists of a chase, in two layers,grooved to clamp the electrodes during molding. It is surrounded laterally by a steel backup plate to prevent distortion of the chase.Slits cut in the backup plat
47、e accommodate the ends of the needles. The top and bottom are polished stainless steel plates.8.4.3 To mount the needles, remove the top clamp plates A and B. Lay the hemispherical electrodes carefully in place in thegrooves so the ends project approximately 11 mm into the hollow section. Replace pl
48、ateAand tighten the screws. Follow the sameprocedure for the sharp electrodes under plate B, except do not tighten the screws. Position the sharp electrodes under a microscopewith a built-in optical scale. Carefully manipulate each sharp electrode so that the specified spacing between the opposingel
49、ectrodes is obtained. Then tighten the screws in plate B. Check to see that the spacing has been maintained. Care must beexercised throughout in order not to bend the needle points.8.4.4 Place one of the 200 250-mm polished plates on the lower platen of the molding press. Cover this with a sheet ofpolyester film or aluminum foil, at least 0.1-mm thick, to prevent sticking of the resin in the polished plate. Lay the chase centrallyon this polyester sheet or aluminum foil. Evenly distribute approximately 30 g of the molding material (for example, polyeth
