ASTM D3756-97(2010)
(Test Method)Standard Test Method for Evaluation of Resistance to Electrical Breakdown by Treeing in Solid Dielectric Materials Using Diverging Fields
Standard Test Method for Evaluation of Resistance to Electrical Breakdown by Treeing in Solid Dielectric Materials Using Diverging Fields
SIGNIFICANCE AND USE
This is a laboratory test designed to simulate the effects of (1) the presence of rough interfaces between conductor or semiconductive screen and primary insulation in an insulation system, (2) the presence of foreign particles (contaminants) in an insulation system, and (3) the presence of small voids or cavities within the insulation.
This test method provides comparative data. The degree of correlation with actual performance in service has not been established.
SCOPE
1.1 This test method covers the evaluation and comparison of the resistance of solid organic dielectric materials to the initiation or growth, or both, of tubular tree-like channels resulting from partial discharge (corona) and molecular decomposition that occur in the region of very high, diverging electric fields. ,
1.2 This test method is primarily for use at a power frequency of 50 or 60 Hz.
1.3 The test may be carried out at room temperature or temperatures above or below room temperature. The temperature should 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 after molding. The resistance to tree initiation is measured by the double-needle characteristic voltage, which is only applicable to non-opaque materials so that tree can be observed optically. The resistance to tree initiation and growth is reported by the double-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 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 responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D3756 − 97 (Reapproved 2010)
Standard Test Method for
Evaluation of Resistance to Electrical Breakdown by Treeing
in Solid Dielectric Materials Using Diverging Fields
This standard is issued under the fixed designation D3756; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
When failure occurs in solid organic dielectrics that are subjected to very high, continuous, and
nonuniform electrical gradients, it generally occurs by a mechanism called treeing. Materials of
different molecular structures have different degrees of resistance to failure by treeing, and this
resistance can sometimes be increased by the addition of other materials in low concentration.
Treesthatgrowbyamoleculardegradationmechanismresultingfrompartialdischarge(corona)are
called electrical trees to distinguish them from water and electrochemical trees which are quite
different.
This test method makes use of two opposing 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 be
tested. Because of the shape of the electrodes this is often called a needle test. This test provides a
statistical estimate of electrical treeing initiation and propagation of solid dielectric materials in high,
diverging electrical fields.
1. Scope 1.4 This test method can be used for any solid material into
which needles can be cast, molded, or inserted with heat after
1.1 This test method covers the evaluation and comparison
molding. The resistance to tree initiation is measured by the
of the resistance of solid organic dielectric materials to the
double-needle characteristic voltage, which is only applicable
initiation or growth, or both, of tubular tree-like channels
to non-opaque materials so that tree can be observed optically.
resultingfrompartialdischarge(corona)andmoleculardecom-
The resistance to tree initiation and growth is reported by the
positionthatoccurintheregionofveryhigh,divergingelectric
3,4 double-needle voltage life, which is applicable to both opaque
fields.
and non-opaque materials.
1.2 This test method is primarily for use at a power
1.5 The values stated in SI units are to be regarded as the
frequency of 50 or 60 Hz.
standard.
1.3 The test may be carried out at room temperature or
1.6 This standard does not purport to address all of the
temperatures above or below room temperature. The tempera-
safety concerns, if any, associated with its use. It is the
ture should not exceed the softening or melting point of the
responsibility of the user of this standard to establish appro-
sample material.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
This test method is under the jurisdiction of ASTM Committee D09 on
Electrical and Electronic Insulating Materials and is the direct responsibility of
2. Referenced Documents
Subcommittee D09.12 on Electrical Tests.
Current edition approved Oct. 1, 2010. Published October 2010. Originally
2.1 ASTM Standards:
approved in 1990. Last previous edition approved in 2004 as D3756–97(2004).
D149Test Method for Dielectric Breakdown Voltage and
DOI: 10.1520/D3756-97R10.
DielectricStrengthofSolidElectricalInsulatingMaterials
Symposium on Engineering Dielectrics, ASTM STP 783, ASTM, 1982, and
Symposium on Engineering Dielectrics, ASTM STP 926, ASTM, 1986.
at Commercial Power Frequencies
W. D.Wilkens, Chapter 7, “Statistical Methods for the Evaluation of Electrical
D1711Terminology Relating to Electrical Insulation
InsulatingSystems,” Engineering Dielectrics, Vol IIB, Electrical Properties of Solid
Insulating Materials, Measurement Techniques, R. Bartnikas, Editor, ASTM STP
926, ASTM, Philadelphia, 1987.
4 5
R. M. Eichorn, Chapter 4, “Treeing in Solid Organic Dielectric Materials,” For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Engineering Dielectrics, Vol IIA, Electrical Properties of Solid Insulating Materi- contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
als: Molecular Structure and Electrical Behavior, R. Bartnikas and R. M. Eichorn, Standards volume information, refer to the standard’s Document Summary page on
Editors, ASTM STP 783, ASTM Philadelphia, 1983. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3756 − 97 (2010)
D1928Practice for Preparation of Compression-Molded 6.2 Current—Sensitive Individual Specimen Disconnect—
PolyethyleneTest Sheets andTest Specimens (Withdrawn When ten specimens are tested to failure for the voltage life
2001) test, use a disconnect circuit for each.
D2275Test Method for Voltage Endurance of Solid Electri-
6.3 Electrodes—Thecriticalelectrodeisaroundsteelrod,1
cal Insulating Materials Subjected to Partial Discharges
mm in diameter, sharpened at one end to a controlled radius of
(Corona) on the Surface
3 6 1 µm and an included angle of 30 6 1 degrees.
2.2 Other Document:
6.4 Needle-Grinding Lathe,forpreparationofthesharpened
ANSI/IEEE930-1987 IEEEGuidefortheStatisticalAnaly-
electrodes to a controlled-point sharpness and included angle.
sis of Electrical Insulation Voltage Endurance Data
A typical lathe and grinder combination are shown in Fig. 1.
3. Terminology
6.5 Specimen Molding Chase, for single-step preparation of
3.1 Definitions: compression molded specimens containing needles. A typical
3.1.1 partial discharge, n—refer to D1711. chase is shown in Fig. 2.
3.2 Definitions of Terms Specific to This Standard: 6.6 Needle-Insertion Jig—A jig is required for slow, con-
3.2.1 characteristic voltage or DNCV (double-needle char- trolled insertion of electrodes. Fig. 3 shows a specimen
acteristic voltage), n—that voltage which, when applied for 1 insertion jig.
h between the ends of two thin cylindrical electrodes (one
6.7 Test Chamber—Any temperature-controlled test cham-
sharpened to a point, the other with a hemispherical end) in a
ber or enclosure, which can hold at least ten specimens and
group of replicate specimens, produces observable dielectric
maintain uniform temperature, is appropriate for this test.
damage at the point of the sharp electrode in half of the
specimens.
7. Sampling
3.2.2 medianvoltagelife(t ),n—thetime,determinedfrom
7.1 Sample so that the specimens tested will represent the
a Weibull plot, when 50% failure occurs from a group of 10
entire lot.
identical specimens subjected to the same voltage stress.
8. Test Specimens
4. Summary of Method
8.1 Testspecimensareapproximately25-mm blocks,6mm
4.1 In this test method, specimens are prepared and needles
thick,containingtwoneedle-likeelectrodesasshowninFig.4.
insertedtoserveaselectrodes.Voltageisappliedtotheneedles
Prepare by compression molding, extrusion, or cutting from
and continued for1hinthe double-needle characteristic
finishedpieces.Thetipsoftheelectrodesareseparatedby12.0
voltage test or until electrical breakdown occurs in the double-
6 0.5 mm for characteristic voltage determination and 6.5 6
needle voltage life test. Results are expressed as the voltage at
0.5 mm for voltage life determination. The number of test
which half of the specimens show dielectric damage in 1 h, or
specimensisatleast24forcharacteristicvoltagedetermination
themediantimetofailureofagroupofspecimenssubjectedto
(i.e. six specimens for each of at least four different testing
a given continuous voltage, at a selected or predetermined
voltages) and 10 for voltage life determination.
temperature.
8.2 Preparation of Thermoplastic and Crosslinked
Specimens—Compression-mold plaques, 6-mm thick, of the
5. Significance and Use
sample material, in a steam or electrically heated hydraulic
5.1 This is a laboratory test designed to simulate the effects
press equipped for cold-water cooling of the platens. Use a
of (1) the presence of rough interfaces between conductor or
semiconductive screen and primary insulation in an insulation
system, (2) the presence of foreign particles (contaminants) in
an insulation system, and (3) the presence of small voids or
cavities within the insulation.
5.2 Thistestmethodprovidescomparativedata.Thedegree
of correlation with actual performance in service has not been
established.
6. Apparatus
6.1 Power Supply—Ahigh voltage supply having a sinusoi-
dal voltage output at a power frequency equipped with con-
tinuous voltage control and an adjustable protective automatic
circuit-breaking device that operates at a controllable current
level. See Test Method D149.
The last approved version of this historical standard is referenced on
www.astm.org.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036. FIG. 1 Machine Setup for Needle Sharpening
D3756 − 97 (2010)
FIG. 2 Chase for Preparation of Specimens Containing Elec-
trodes
FIG. 3 Needle Insertion Jig
FIG. 4 Finished Specimen
positive pressure mold, which may be of either the picture-
frame type or the milled-cavity type. Use parting sheets of 8.3 Insertion of Needles—Insert the needle electrodes into
the specimen blocks slowly and carefully to avoid orientation
cellophane, polyester film, or aluminum foil between the mold
surfaces and the resin. The choice of parting sheet depends strains, formation of cavities, and damage to the sharp points.
somewhat on the molding temperature, although aluminum Use a jig, such as the one shown in Fig. 3, to ensure that every
sheets, washed with alcohol and thoroughly dried, are pre- needle will be inserted under identical conditions.
ferred. The size of the mold is not critical, 200×200 mm is 8.3.1 Maketheinsertionsasfollows:Placetwelvespecimen
recommended. For peroxide-crosslinkable materials, the typi- blocks in the slots provided for them and lightly clamp into
calcompression-moldingconditionsshouldfollowthematerial place.Inspectthetwelvesharpenedneedles,aftercleaningwith
manufacturers recommendation of temperature, time and pres- methyl ethyl ketone; then carefully place them into the needle
sure. The by-products of peroxide decomposition should be slotsononesidesotheirtipsjusttouchthespecimens.Usethe
removed before testing by use of a vacuum oven at elevated individual adjusting screws for positioning the needles. Slip
temperatures (80°C for 7 days for XLPE using dicumyl shims into the needle slots above the needles, and use a cover
peroxide). platetoclosethetopoftheneedleslots.Securethiscoverwith
8.2.1 After molding, cut the plaque into 25-mm square small C-clamps at each end. The purpose of the shims and
blocks with square and smooth edges. Store the squares under cover is to prevent the needle from cocking, and to force it to
standard laboratory conditions, 23°C and 50 % relative enter straight into the specimen. Place twelve electrodes with
humidity,andprotectthemfromdirtandatmosphericcontami- hemispheric ends in the slots on the opposite sides of the
nation until used. specimens in the same manner.
D3756 − 97 (2010)
8.3.2 When the specimens and needles are mounted and the 8.4.5 Mold the specimen in accordance with Practice
needles are adjusted into proper position, place the whole jig D1928.
into a circulating air oven at 105°C, for low-density polyeth- 8.4.6 Remove the specimens by removing the screws in
ylene or crosslinked materials, for a 1-h preheating period. For
both plates A and B.
other partially crystalline polymeric materials use a tempera-
8.4.7 Cut the 150-mm block into six 25-mm individual
ture approximately 10°C below the thermodynamic first-order
specimens.
transition point. After preheating, simultaneously advance the
8.4.8 Check each specimen under a microscope with an
needles 1.30 mm by making one full turn of the large lead
optical scale and use only if the electrode spacing is found to
screw. Repeat at 5-min intervals. Make sufficient turns to
be correct. If any contamination or voids are found within the
accomplish the insertion. Five turns are normally required for
test region, reject the specimen.
thecharacteristicvoltagetestandsevenforthevoltagelifetest.
An electrode gap of 12.0 6 0.5 mm is commonly used for
9. Conditioning
characteristic voltage determination and 6.5 6 0.5 mm is
9.1 After specimen preparation is complete, store all speci-
preferred for voltage life tests. Use constant gaps for compari-
mens for approximately 24 h at 23°C and 50% relative
son of materials by this test.
humidity before testing.
8.3.3 When the insertions are complete, leave the jigs
untouched for at least 30 min in the oven for stress relief or
10. Procedure
annealing.Removethejigsfromtheovenandcoolnaturallyto
10.1 Warning—Lethal voltages are a potential hazard dur-
roomtemperature.Examineeachspecimenwithamicroscope,
ing the performance of this test. It is essential that the test
if possible, to ensure that the needle point was not damaged
apparatusandallassociatedequipmentthatmaybeelectrically
during insertion.
connected to it be properly designed and installed for safe
8.4 Preparation of Specimens with Molded-In Electrodes—
operation. Solidly ground all electrically conductive parts
Condition as specified in Section 9.
which are possible for a person to contact during the test.
8.4.1 The molding is 150×25×6 mm thick and contains
Provide means for use at the completion of any test to ground
six pointed and six hemispherical electrodes. Cut six square
anypartswhichwereathighvoltageduringthetestorhavethe
specimens from this block. See Fig. 2.
potential for acquiring an induced charge during the test or
8.4.2 A compression mold that can be used for preparation retaining a charge even after disconnection of the voltage
of the six specimen plaques consists of a chase, in two layers, source. Thoroughly instruct all operators as to the correct
grooved to clamp the electrodes during molding. It is sur- procedures for performing tests safely. When making high-
rounded laterally by a steel backup plate to prevent distortion voltage tests, particularly in compressed gas, oil, water or
of the chase. Slits cut in the backup plate accommodate the aqueous solution, it is possible for the energy released at
breakdown to be sufficient to result in fire, explosion, or
ends of the needles. The top and bottom are polished stainless
steel plates. rupture of the test chamber. Design test equipment, test
chambers, and test specimens so as to minimize the possibility
8.4.3 To
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