1、Designation: D3756 97 (Reapproved 2010)Standard 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 the yea
2、r 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 dielectrics that a
3、re 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 of other
4、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 cylindrical elec
5、trodes, 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 propagati
6、on 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 partial discharge (corona) an
7、d 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. The tempera-ture should not e
8、xceed 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 voltage, which is only applic
9、ableto 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 as thestandard.1.6 This standa
10、rd 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. Referenced Documents2.1 AST
11、M 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 D09 onElectrical and Electroni
12、c 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/D3756-97R10.2Symposium on Engineering Die
13、lectrics, 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 SolidInsulating Materials, Measurement
14、 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 Electrical Behavior, R. Bartnikas and R.
15、 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 Summary page onthe ASTM website.Co
16、pyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1D1928 Practice for Preparation of Compression-MoldedPolyethylene Test Sheets and Test Specimens (Withdrawn2001)6D2275 Test Method for Voltage Endurance of Solid Electri-cal Insulating Mate
17、rials Subjected to Partial Discharges(Corona) on the Surface2.2 Other Document:ANSI/IEEE 930-1987 IEEE Guide for the StatisticalAnaly-sis of Electrical Insulation Voltage Endurance Data73. Terminology3.1 Definitions:3.1.1 partial discharge, nrefer to D1711.3.2 Definitions of Terms Specific to This S
18、tandard: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 observable diele
19、ctricdamage at the point of the sharp electrode in half of thespecimens.3.2.2 median voltage life (t50), nthe time, determined froma Weibull plot, when 50 % failure occurs from a group of 10identical specimens subjected to the same voltage stress.4. Summary of Method4.1 In this test method, specimen
20、s 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 the specimens
21、 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 interfaces between
22、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 degreeof correlation
23、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 a controllabl
24、e 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 to a controlle
25、d 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 preparation ofco
26、mpression 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 can hold at leas
27、t 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 Fig. 4.Prepar
28、e 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 determination(i
29、.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 for cold-water
30、cooling of the platens. Use a6The last approved version of this historical standard is referenced onwww.astm.org.7Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036. FIG. 1 Machine Setup for Needle SharpeningD3756 97 (2010)2positive pressure mol
31、d, which may be of either the picture-frame type or the milled-cavity type. Use parting sheets ofcellophane, polyester film, or aluminum foil between the moldsurfaces and the resin. The choice of parting sheet dependssomewhat on the molding temperature, although aluminumsheets, washed with alcohol a
32、nd thoroughly dried, are pre-ferred. The size of the mold is not critical, 200 200 mm isrecommended. For peroxide-crosslinkable materials, the typi-cal compression-molding conditions should follow the materialmanufacturers recommendation of temperature, time and pres-sure. The by-products of peroxid
33、e 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 laboratory conditions, 23C and 50
34、% relativehumidity, and protect them from dirt and atmospheric contami-nation until 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 a jig, such as the one sho
35、wn 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 withmethyl ethyl ketone; then carefu
36、lly 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 cover withsmall C-clamps a
37、t 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.FIG. 2 Chase for Preparation of Specimens Contai
38、ning Elec-trodesFIG. 3 Needle Insertion JigFIG. 4 Finished SpecimenD3756 97 (2010)38.3.2 When the specimens and needles are mounted and theneedles are adjusted into proper position, place the whole jiginto a circulating air oven at 105C, for low-density polyeth-ylene or crosslinked materials, for a
39、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. Repeat at 5-min intervals. M
40、ake 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 for voltage life tests. Use
41、 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 with a microscope,if poss
42、ible, 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 25 6 mm thick and containssix pointed and six hemispherical electrodes. Cut six squarespecimens from this block. Se
43、e 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 cut in the backup plate accomm
44、odate 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 section. Replace plateAand tigh
45、ten 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 spacingbetween the opposing electrodes is ob
46、tained. 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 200 250-mm polished plates on thelower platen of the molding press. Cover this with a sheet ofpolyester film or
47、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 the cavity in themold. Then lay a s
48、econd 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-mm2individualspecimens.8.4.8 Check each specimen un
49、der 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 electricallyconnected to it be properly desig
