1、Designation: D3756 97 (Reapproved 2010)An American National StandardStandard 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
2、designation indicates 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 sol
3、id organic dielectrics 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 increa
4、sed by the addition of 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 o
5、pposing thin cylindrical 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 tre
6、eing initiation and propagation of solid dielectric materials in high,diverging electrical fields.1. Scope1.1 This test method covers the evaluation and comparisonof the resistance of solid organic dielectric materials to theinitiation or growth, or both, of tubular tree-like channelsresulting from
7、partial discharge (corona) and molecular decom-position that occur in the region of very high, diverging electricfields.3,41.2 This test method is primarily for use at a powerfrequency of 50 or 60 Hz.1.3 The test may be carried out at room temperature ortemperatures above or below room temperature.
8、The tempera-ture should not exceed the softening or melting point of thesample material.1.4 This test method can be used for any solid material intowhich needles can be cast, molded, or inserted with heat aftermolding. The resistance to tree initiation is measured by thedouble-needle characteristic
9、voltage, which is only applicableto non-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 opaqueand non-opaque materials.1.5 The values stated in SI units are to be regarded a
10、s thestandard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2
11、. Referenced Documents2.1 ASTM Standards:5D149 Test Method for Dielectric Breakdown Voltage andDielectric Strength of Solid Electrical Insulating Materialsat Commercial Power FrequenciesD1711 Terminology Relating to Electrical Insulation1This test method is under the jurisdiction of ASTM Committee D
12、09 onElectrical and Electronic Insulating Materials and is the direct responsibility ofSubcommittee D09.12 on Electrical Tests.Current edition approved Oct. 1, 2010. Published October 2010. Originallyapproved in 1990. Last previous edition approved in 2004 as D3756 97(2004).DOI: 10.1520/D3756-97R10.
13、2Symposium on Engineering Dielectrics, ASTM STP 783 , ASTM, 1982, andSymposium on Engineering Dielectrics, ASTM STP 926, ASTM, 1986.3W. D. Wilkens, Chapter 7, “Statistical Methods for the Evaluation of ElectricalInsulating Systems,” Engineering Dielectrics, Vol IIB, Electrical Properties of SolidIns
14、ulating Materials, Measurement Techniques, R. Bartnikas, Editor, ASTM STP926 , ASTM, Philadelphia, 1987.4R. M. Eichorn, Chapter 4, “Treeing in Solid Organic Dielectric Materials,”Engineering Dielectrics, Vol IIA, Electrical Properties of Solid Insulating Materi-als: Molecular Structure and Electrica
15、l Behavior, R. Bartnikas and R. M. Eichorn,Editors, ASTM STP 783, ASTM Philadelphia, 1983.5For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Sum
16、mary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.D1928 Practice for Preparation of Compression-MoldedPolyethylene Test Sheets and Test Specimens6D2275 Test Method for Voltage Endurance of Solid Elec-trical
17、 Insulating Materials Subjected to Partial Discharges(Corona) on the Surface2.2 Other Document:ANSI/IEEE 930-1987 IEEE Guide for the StatisticalAnalysis of Electrical Insulation Voltage Endurance Data73. Terminology3.1 Definitions:3.1.1 partial discharge, nrefer to D1711.3.2 Definition of Terms Spec
18、ific to This Standard:3.2.1 characteristic voltage or DNCV (double-needle char-acteristic voltage), nthat voltage which, when applied for 1h between the ends of two thin cylindrical electrodes (onesharpened to a point, the other with a hemispherical end) in agroup of replicate specimens, produces ob
19、servable dielectricdamage at the point of the sharp electrode in half of thespecimens.3.2.2 median voltage life (t50), nthe time, determinedfrom a Weibull plot, when 50 % failure occurs from a group of10 identical specimens subjected to the same voltage stress.4. Summary of Method4.1 In this test me
20、thod, specimens are prepared and needlesinserted to serve as electrodes. Voltage is applied to the needlesand continued for1hinthedouble-needle characteristicvoltage test or until electrical breakdown occurs in the double-needle voltage life test. Results are expressed as the voltage atwhich half of
21、 the specimens show dielectric damage in 1 h, orthe median time to failure of a group of specimens subjected toa given continuous voltage, at a selected or predeterminedtemperature.5. Significance and Use5.1 This is a laboratory test designed to simulate the effectsof (1) the presence of rough inter
22、faces between conductor orsemiconductive screen and primary insulation in an insulationsystem, (2) the presence of foreign particles (contaminants) inan insulation system, and (3) the presence of small voids orcavities within the insulation.5.2 This test method provides comparative data. The degreeo
23、f correlation with actual performance in service has not beenestablished.6. Apparatus6.1 Power SupplyA high voltage supply having a sinusoi-dal voltage output at a power frequency equipped with con-tinuous voltage control and an adjustable protective automaticcircuit-breaking device that operates at
24、 a controllable currentlevel. See Test Method D149.6.2 CurrentSensitive Individual Specimen DisconnectWhen ten specimens are tested to failure for the voltage lifetest, use a disconnect circuit for each.6.3 ElectrodesThe critical electrode is a round steel rod, 1mm in diameter, sharpened at one end
25、to a controlled radius of3 6 1 m and an included angle of 30 6 1 degrees.6.4 Needle-Grinding Lathe, for preparation of the sharpenedelectrodes to a controlled-point sharpness and included angle.A typical lathe and grinder combination are shown in Fig. 1.6.5 Specimen Molding Chase, for single-step pr
26、eparation ofcompression molded specimens containing needles. A typicalchase is shown in Fig. 2.6.6 Needle-Insertion JigA jig is required for slow, con-trolled insertion of electrodes. Fig. 3 shows a specimeninsertion jig.6.7 Test ChamberAny temperature-controlled test cham-ber or enclosure, which ca
27、n hold at least ten specimens andmaintain uniform temperature, is appropriate for this test.7. Sampling7.1 Sample so that the specimens tested will represent theentire lot.8. Test Specimens8.1 Test specimens are approximately 25-mm2blocks, 6 mmthick, containing two needle-like electrodes as shown in
28、 Fig. 4.Prepare by compression molding, extrusion, or cutting fromfinished pieces. The tips of the electrodes are separated by 12.06 0.5 mm for characteristic voltage determination and 6.5 60.5 mm for voltage life determination. The number of testspecimens is at least 24 for characteristic voltage d
29、etermination(i.e. six specimens for each of at least four different testingvoltages) and 10 for voltage life determination.8.2 Preparation of Thermoplastic and CrosslinkedSpecimensCompression-mold plaques, 6-mm thick, of thesample material, in a steam or electrically heated hydraulicpress equipped f
30、or cold-water cooling of the platens. Use apositive pressure mold, which may be of either the picture-frame type or the milled-cavity type. Use parting sheets of6Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.7Available from American National Standards
31、Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036. FIG. 1 Machine Setup for Needle SharpeningD3756 97 (2010)2cellophane, polyester film, or aluminum foil between the moldsurfaces and the resin. The choice of parting sheet dependssomewhat on the molding temperature, although aluminumshee
32、ts, washed with alcohol and thoroughly dried, are pre-ferred. The size of the mold is not critical, 200 3 200 mm isrecommended. For peroxide-crosslinkable materials, the typi-cal compression-molding conditions should follow the materialmanufacturers recommendation of temperature, time and pres-sure.
33、 The by-products of peroxide decomposition should beremoved before testing by use of a vacuum oven at elevatedtemperatures (80C for 7 days for XLPE using dicumylperoxide).8.2.1 After molding, cut the plaque into 25-mm squareblocks with square and smooth edges. Store the squares understandard laborat
34、ory conditions, 23C and 50 % relative humid-ity, and protect them from dirt and atmospheric contaminationuntil used.8.3 Insertion of NeedlesInsert the needle electrodes intothe specimen blocks slowly and carefully to avoid orientationstrains, formation of cavities, and damage to the sharp points.Use
35、 a jig, such as the one shown in Fig. 3, to ensure that everyneedle will be inserted under identical conditions.8.3.1 Make the insertions as follows: Place twelve specimenblocks in the slots provided for them and lightly clamp intoplace. Inspect the twelve sharpened needles, after cleaning withmethy
36、l ethyl ketone; then carefully place them into the needleslots on one side so their tips just touch the specimens. Use theindividual adjusting screws for positioning the needles. Slipshims into the needle slots above the needles, and use a coverplate to close the top of the needle slots. Secure this
37、 cover withsmall C-clamps at each end. The purpose of the shims andcover is to prevent the needle from cocking, and to force it toenter straight into the specimen. Place twelve electrodes withhemispheric ends in the slots on the opposite sides of thespecimens in the same manner.8.3.2 When the specim
38、ens and needles are mounted and theneedles are adjusted into proper position, place the whole jigFIG. 2 Chase for Preparation of Specimens ContainingElectrodesFIG. 3 Needle Insertion JigFIG. 4 Finished SpecimenD3756 97 (2010)3into a circulating air oven at 105C, for low-density polyeth-ylene or cros
39、slinked materials, for a 1-h preheating period. Forother partially crystalline polymeric materials use a tempera-ture approximately 10C below the thermodynamic first-ordertransition point. After preheating, simultaneously advance theneedles 1.30 mm by making one full turn of the large leadscrew. Rep
40、eat at 5-min intervals. Make sufficient turns toaccomplish the insertion. Five turns are normally required forthe characteristic voltage test and seven for the voltage life test.An electrode gap of 12.0 6 0.5 mm is commonly used forcharacteristic voltage determination and 6.5 6 0.5 mm ispreferred fo
41、r voltage life tests. Use constant gaps for compari-son of materials by this test.8.3.3 When the insertions are complete, leave the jigsuntouched for at least 30 min in the oven for stress relief orannealing. Remove the jigs from the oven and cool naturally toroom temperature. Examine each specimen
42、with a microscope,if possible, to ensure that the needle point was not damagedduring insertion.8.4 Preparation of Specimens with Molded-In ElectrodesCondition as specified in Section 9.8.4.1 The molding is 150 3 25 3 6 mm thick and containssix pointed and six hemispherical electrodes. Cut six square
43、specimens from this block. See Fig. 2.8.4.2 A compression mold that can be used for preparationof the six specimen plaques consists of a chase, in two layers,grooved to clamp the electrodes during molding. It is sur-rounded laterally by a steel backup plate to prevent distortionof the chase. Slits c
44、ut in the backup plate accommodate theends of the needles. The top and bottom are polished stainlesssteel plates.8.4.3 To mount the needles, remove the top clamp plates Aand B. Lay the hemispherical electrodes carefully in place inthe grooves so the ends project approximately 11 mm into thehollow se
45、ction. Replace plate A and tighten the screws. Followthe same procedure for the sharp electrodes under plate B,except do not tighten the screws. Position the sharp electrodesunder a microscope with a built-in optical scale. Carefullymanipulate each sharp electrode so that the specified spacingbetwee
46、n the opposing electrodes is obtained. Then tighten thescrews in plate B. Check to see that the spacing has beenmaintained. Care must be exercised throughout in order not tobend the needle points.8.4.4 Place one of the 200 3 250-mm polished plates on thelower platen of the molding press. Cover this
47、with a sheet ofpolyester film or aluminum foil, at least 0.1-mm thick, toprevent sticking of the resin in the polished plate. Lay the chasecentrally on this polyester sheet or aluminum foil. Evenlydistribute approximately 30 g of the molding material (forexample, polyethylene) to be tested within th
48、e cavity in themold. Then lay a second sheet of polyester film or foil and thesecond polished plate over the mold.8.4.5 Mold the specimen in accordance with PracticeD1928.8.4.6 Remove the specimens by removing the screws inboth plates A and B.8.4.7 Cut the 150-mm block into six 25-mm2individualspeci
49、mens.8.4.8 Check each specimen under a microscope with anoptical scale and use only if the electrode spacing is found tobe correct. If any contamination or voids are found within thetest region, reject the specimen.9. Conditioning9.1 After specimen preparation is complete, store all speci-mens for approximately 24 h at 23C and 50 % relativehumidity before testing.10. Procedure10.1 WarningLethal voltages are a potential hazard dur-ing the performance of this test. It is essential that the testapparatus and all associated equipment that may be electricallyconnecte
