ASTM D3756-97
(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
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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An American National Standard
Designation: D 3756 – 97
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 D 3756; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) 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.
Trees that grow by a molecular degradation mechanism resulting from partial discharge (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 molding. The resistance to tree initiation is measured by the
double-needle characteristic voltage, which is only applicable
1.1 This test method covers the evaluation and comparison
to non-opaque materials so that tree can be observed optically.
of the resistance of solid organic dielectric materials to the
The resistance to tree initiation and growth is reported by the
initiation or growth, or both, of tubular tree-like channels
double-needle voltage life, which is applicable to both opaque
resulting from partial discharge (corona) and molecular decom-
and non-opaque materials.
position that occur in the region of very high, diverging electric
3,4
1.5 The values stated in SI units are to be regarded as the
fields.
standard.
1.2 This test method is primarily for use at a power
1.6 This standard does not purport to address all of the
frequency of 50 or 60 Hz.
safety concerns, if any, associated with its use. It is the
1.3 The test may be carried out at room temperature or
responsibility of the user of this standard to establish appro-
temperatures above or below room temperature. The tempera-
priate safety and health practices and determine the applica-
ture should not exceed the softening or melting point of the
bility of regulatory limitations prior to use.
sample material.
1.4 This test method can be used for any solid material into
2. Referenced Documents
which needles can be cast, molded or inserted with heat after
2.1 ASTM Standards:
D 149 Test Method for Dielectric Breakdown Voltage and
This test method is under the jurisdiction of ASTM Committee D-9 on
Dielectric Strength of Solid Electrical Insulating Materials
Electrical and Electronic Insulating Materials and is the direct responsibility of
at Commercial Power Frequencies
Subcommittee D09.12 on Electrical Tests.
D 1711 Terminology Relating to Electrical Insulation
Current edition approved Sept. 10, 1997. Published February 1998. Originally
e1
published as D 3756 – 90(95) . Last previous edition D 3756 – 96.
D 1928 Practice for Preparation of Compression-Molded
Symposium on Engineering Dielectrics, ASTM STP 783, ASTM, 1982, and 6
Test Sheets and Test Specimens
Symposium on Engineering Dielectrics, ASTM STP 926, ASTM, 1986.
3 D 2275 Test Method for Voltage Endurance of Solid Elec-
W. D. Wilkens, Chapter 7, “Statistical Methods for the Evaluation of Electrical
trical Insulating Materials Subjected to Partial Discharges
Insulating Systems,” Engineering Dielectrics, Vol IIB, Electrical Properties of Solid
Insulating Materials, Measurement Techniques, R. Bartnikas, Editor, ASTM STP
(Corona) on the Surface
926, ASTM, Philadelphia, 1987.
R. 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. M. Eichorn,
Annual Book of ASTM Standards, Vol 10.01.
Editors, ASTM STP 783, ASTM Philadelphia, 1983.
Annual Book of ASTM Standards, Vol 08.01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 3756
2.2 Other Document:
ANSI/IEEE 930-1987 IEEE Guide for the Statistical
Analysis of Electrical Insulation Voltage Endurance Data
3. Terminology
3.1 Definitions:
3.1.1 partial discharge, n—refer to D 1711.
3.2 Definition of Terms Specific to This Standard:
3.2.1 characteristic voltage or DNCV (double-needle char-
acteristic voltage), n—that voltage which, when applied for 1
h between the ends of two thin cylindrical electrodes (one
sharpened to a point, the other with a hemispherical end) in a
group of replicate specimens, produces observable dielectric
damage at the point of the sharp electrode in half of the
specimens.
3.2.2 median voltage life (t ), n—the time, determined
from a Weibull plot, when 50 % failure occurs from a group of
10 identical specimens subjected to the same voltage stress.
FIG. 1 Machine Setup for Needle Sharpening
4. Summary of Method
6.5 Specimen Molding Chase, for single-step preparation of
4.1 In this test method, specimens are prepared and needles
compression molded specimens containing needles. A typical
inserted to serve as electrodes. Voltage is applied to the needles
chase is shown in Fig. 2.
and continued for1hinthe double-needle characteristic
6.6 Needle-Insertion Jig—A jig is required for slow, con-
voltage test or until electrical breakdown occurs in the double-
trolled insertion of electrodes. Fig. 3 shows a specimen
needle voltage life test. Results are expressed as the voltage at
insertion jig.
which half of the specimens show dielectric damage in 1 h, or
6.7 Test Chamber—Any temperature-controlled test cham-
the median time to failure of a group of specimens subjected to
ber or enclosure, which can hold at least ten specimens and
a given continuous voltage, at a selected or predetermined
maintain uniform temperature, is appropriate for this test.
temperature.
7. Sampling
5. Significance and Use
7.1 Sample so that the specimens tested will represent the
5.1 This is a laboratory test designed to simulate the effects
entire lot.
of (1) the presence of rough interfaces between conductor or
semiconductive screen and primary insulation in an insulation
8. Test Specimens
system, (2) the presence of foreign particles (contaminants) in
8.1 Test specimens are approximately 25-mm blocks, 6 mm
an insulation system, and (3) the presence of small voids or
thick, containing two needle-like electrodes as shown in Fig. 4.
cavities within the insulation.
Prepare by compression molding, extrusion, or cutting from
5.2 This test method provides comparative data. The degree
finished pieces. The tips of the electrodes are separated by 12.0
of correlation with actual performance in service has not been
established.
6. Apparatus
6.1 Power Supply—A high 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 D 149.
6.2 Current—Sensitive Individual Specimen Disconnect—
When ten specimens are tested to failure for the voltage life
test, use a disconnect circuit for each.
6.3 Electrodes—The critical electrode is a round steel rod, 1
mm in diameter, sharpened at one end to a controlled radius of
3 6 1 μm and an included angle of 30 6 1 degrees.
6.4 Needle-Grinding Lathe, for preparation of the sharpened
electrodes to a controlled-point sharpness and included angle.
A typical lathe and grinder combination are shown in Fig. 1.
Available from American National Standards Institute, 11 W. 42nd St., 13th FIG. 2 Chase for Preparation of Specimens Containing
Floor, New York, NY 10036. Electrodes
D 3756
FIG. 3 Needle Insertion Jig
6 0.5 mm for characteristic voltage determination and 6.5 6
0.5 mm for voltage life determination. The number of test
specimens is at least 24 for characteristic voltage determination
(i.e. six specimens for each of at least four different testing
voltages) and 10 for voltage life determination.
8.2 Preparation of Thermoplastic and Crosslinked
Specimens—Compression-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, which may be of either the picture-
frame type or the milled-cavity type. Use parting sheets of
cellophane, polyester film, or aluminum foil between the mold
surfaces and the resin. The choice of parting sheet depends
somewhat on the molding temperature, although aluminum
sheets, washed with alcohol and thoroughly dried, are pre-
ferred. The size of the mold is not critical, 200 3 200 mm is
recommended. For peroxide-crosslinkable materials, the typi-
cal compression-molding conditions should follow the material
manufacturers recommendation of temperature, time and pres-
FIG. 4 Finished Specimen
sure. The by-products of peroxide decomposition should be
removed before testing by use of a vacuum oven at elevated
temperatures (80°C for 7 days for XLPE using dicumyl shims into the needle slots above the needles, and use a cover
peroxide). plate to close the top of the needle slots. Secure this cover with
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 humid- enter straight into the specimen. Place twelve electrodes with
ity, and protect them from dirt and atmospheric contamination hemispheric ends in the slots on the opposite sides of the
until used. specimens in the same manner.
8.3 Insertion of Needles—Insert the needle electrodes into 8.3.2 When the specimens and needles are mounted and the
the specimen blocks slowly and carefully to avoid orientation needles are adjusted into proper position, place the whole jig
strains, formation of cavities, and damage to the sharp points. into a circulating air oven at 105°C, for low-density polyeth-
Use a jig, such as the one shown in Fig. 3, to ensure that every ylene or crosslinked materials, for a 1-h preheating period. For
needle will be inserted under identical conditions. other partially crystalline polymeric materials use a tempera-
8.3.1 Make the insertions as follows: Place twelve specimen ture approximately 10°C below the thermodynamic first-order
blocks in the slots provided for them and lightly clamp into transition point. After preheating, simultaneously advance the
place. Inspect the twelve sharpened needles, after cleaning with needles 1.30 mm by making one full turn of the large lead
methyl ethyl ketone; then carefully place them into the needle screw. Repeat at 5-min intervals. Make sufficient turns to
slots on one side so their tips just touch the specimens. Use the accomplish the insertion. Five turns are normally required for
individual adjusting screws for positioning the needles. Slip the characteristic voltage test and seven for the voltage life test.
D 3756
An electrode gap of 12.0 6 0.5 mm is commonly used for 10. Procedure
characteristic voltage determination and 6.5 6 0.5 mm is
10.1 Warning—Lethal voltages are a potential hazard dur-
preferred for voltage life tests. Use constant gaps for compari-
ing the performance of this test. It is essential that the test
son of materials by this test.
apparatus and all associated equipment that may be electrically
8.3.3 When the insertions are complete, leave the jigs
connected to it be properly designed and installed for safe
untouched for at least 30 min in the oven for stress relief or
operation. Solidly ground all electrically conductive parts
annealing. Remove the jigs from the oven and cool naturally to
which are possible for a person to contact during the test.
room temperature. Examine each specimen with a microscope,
Provide means for use at the completion of any test to ground
if possible, to ensure that the needle point was not damaged
any parts which were at high voltage during the test or have the
during insertion.
potential for acquiring an induced charge during the test or
8.4 Preparation of Specimens with Molded-In Electrodes—
retaining a charge even after disconnection of the voltage
Condition as specified in Section 9.
source. Thoroughly instruct all operators as to the correct
8.4.1 The molding is 150 3 25 3 6 mm thick and contains
procedures for performing tests safely. When making high-
six pointed and six hemispherical electrodes. Cut six square
voltage tests, particularly in compressed gas, oil, water or
specimens from this block. See Fig. 2.
aqueous solution, it is possible for the energy released at
8.4.2 A compression mold that can be used for preparation breakdown to be sufficient to result in fire, explosion, or
of the six specimen plaques consists of a chase, in two layers,
rupture of the test chamber. Design test equipment, test
grooved to clamp the electrodes during molding. It is sur-
chambers, and test specimens so as to minimize the possibility
rounded laterally by a steel backup plate to prevent distortion
of such occurrences and to eliminate the possibility of personal
of the chase. Slits cut in the backup plate accommodate the
injury. If the potential for fire exists, have fire suppression
ends of the needles. The top and bottom are polished stainless
equipment available.
steel plates.
10.2 Characteristic Voltage Test:
8.4.3 To mount the needles, remove the top clamp plates A
10.2.1 Electrical Stressing of Specimens— Prepare 24
and B. Lay the hemispherical electrodes carefully in place in specimens of one sample for double-needle characteristic
the grooves so the ends project approximately 11 mm into the
voltage determination. Carefully wash all lateral surfaces of the
hollow section. Replace plate A and tighten the screws. Follow test blocks with alcohol. Place six of the specimens in
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