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Ageing Tests of Samples of Glass-Epoxy Core Rods in Composite Insulators Subjected to High Direct Current (DC) Voltage in a Thermal Chamber

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In this article, we presented the results of the tests performed on three sets of samples of glass-reinforced epoxy (GRE) core rods used in alternating current (AC) composite insulators with silicone rubber housing. The objective of this examination was to test the aging resistance of the rod material when exposed to direct current (DC) high voltage. We hypothesized that the long-term effects of the electrostatic field on the GRE core rod material would lead to a gradual degradation of its mechanical properties caused by ionic current flow. Further, we hypothesized that reducing the mechanical strength of the GRE core rod would lead to the breakage of the insulator. The first group of samples was used for reference. The samples from the second group were subjected to a temperature of about 50 °C for 6000 h. The third group of samples were aged by temperature and DC high voltage for the same time. The samples were examined using the 3-point bending test, micro-hardness measurement and microscopic analysis. No recordable degradation effects were found. Long-term temperature impact and, above all, the combined action of temperature and DC high voltage did not reduce the mechanical parameters or change the microstructure of the GRE material.
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energies
Article
Ageing Tests of Samples of Glass-Epoxy Core Rods in
Composite Insulators Subjected to High Direct
Current (DC) Voltage in a Thermal Chamber
Krzysztof Wieczorek 1,* , Przemysław Ranachowski 2, Zbigniew Ranachowski 2and
Piotr Papli ´nski 3
1Department of Electrical Engineering Fundamentals, Wroclaw University of Science and Technology,
50-370 Wroclaw, Poland
2
Department of Experimental Mechanics, Institute of Fundamental Technological Research Polish Academy
of Sciences, 02-106 Warsaw, Poland; pranach@ippt.gov.pl (P.R.); zranach@ippt.pan.pl (Z.R.)
3Environmental Impact and Overvoltage Protection Laboratory, Institute of Power
Engineering-Research Institute, 01-330 Warsaw, Poland; piotr.paplinski@ien.com.pl
*Correspondence: Krzysztof.Wieczorek@pwr.edu.pl
Received: 1 November 2020; Accepted: 16 December 2020; Published: 20 December 2020


Abstract:
In this article, we presented the results of the tests performed on three sets of samples of
glass-reinforced epoxy (GRE) core rods used in alternating current (AC) composite insulators with
silicone rubber housing. The objective of this examination was to test the aging resistance of the rod
material when exposed to direct current (DC) high voltage. We hypothesized that the long-term
eects of the electrostatic field on the GRE core rod material would lead to a gradual degradation of
its mechanical properties caused by ionic current flow. Further, we hypothesized that reducing the
mechanical strength of the GRE core rod would lead to the breakage of the insulator. The first group of
samples was used for reference. The samples from the second group were subjected to a temperature
of about 50
C for 6000 h. The third group of samples were aged by temperature and DC high voltage
for the same time. The samples were examined using the 3-point bending test, micro-hardness
measurement and microscopic analysis. No recordable degradation eects were found. Long-term
temperature impact and, above all, the combined action of temperature and DC high voltage did not
reduce the mechanical parameters or change the microstructure of the GRE material.
Keywords:
DC high voltage; composite insulator; glass-reinforced epoxy core; 3-point bending test;
mechanical strength; micro-hardness
1. Introduction
Progress in the construction of high voltage power converter systems and the dynamic development
of electricity generation systems from so-called renewable sources have resulted in an increasing
interest in the transmission of electricity through high voltage direct current (HVDC) transmission
lines [
1
,
2
]. The high cost of converter stations—from alternating current (AC) to direct current (DC)
and vice versa—are compensated by a significant reduction in loss of energy transmitted over long
distances via HVDC transmission lines, especially when compared to systems that operate at AC
voltages and have lower construction costs [3].
Current high voltage lines are more often equipped with modern composite insulators. The main
advantage of this is surface hydrophobicity. In polluted environments, this property makes it impossible
to create water paths that conduct leakage currents for housing composite insulators. Silicone elastomer
insulator housings have the ability to hydrophobize surface pollution and regenerate temporarily in
lost surface properties. Compared to ceramic and glass insulators, they are significantly lighter and,
Energies 2020,13, 6724; doi:10.3390/en13246724 www.mdpi.com/journal/energies
Energies 2020,13, 6724 2 of 13
in many countries, cheaper. However, the use of HVDC for the transmission of electricity unfortunately
raises some technical problems. The constant electric field, forced by the HVDC line, has a significant
impact on the integrity of dielectric materials usually produced for AC applications [
4
]. Compared to
systems operating at alternating voltages, electrostatic phenomena may cause changes in degradation
processes, electrical strength [
5
,
6
] and even a four-fold increase in the accumulation of surface soiling [
7
].
Under such conditions, when the leakage current flows through the polluted surface without passing
through the zero curve of voltage and current, the ignition of non-extinguishing concentrated surface
discharges may occur. This can lead to degradation of the composite housing. The ageing process in
the presence of HVDC results in an increased accumulation of spatial charge in areas where material is
more degraded, thus contributing to a stronger distortion of the electric field [
8
]. This phenomenon
may cause intensification of partial discharges. In addition, if the glass-reinforced epoxy (GRE) core
material is exposed to the long-term electrostatic field, then the ionic current flow may cause gradual
degradation of mechanical properties [
9
]. This process could be activated at increased temperatures.
Electrolysis of the carrying material (glass fibers) could then lead to the breakage of such insulators
and, consequently, to serious failures.
This experiment aimed to test the mechanical strength of GRE core rod samples after 6000 h of
aging at a temperature of about 50 C and in the presence of HVDC.
2. Materials and Methods
In the ageing comparative tests, we used samples of GRE material cut from the carrying rod of
a typical high voltage composite insulator for AC lines. Fibers in the rod were made of ECR-glass
(ECR—type of glass electrical chemical reinforced) [
10
,
11
]. Apart from silicon (SiO
2,
), these types of
fibers contain calcium from CaCO
3
(as flux and stabilizer) and aluminum from Al
2
0
3
(to improve
chemical resistance), which are used in the glass composition. Typical ECR-glass contains over
58% SiO
2
, about 22% CaO and less than 12% Al
2
0
3
, as well as smaller amounts of other additives.
The presence of mobile sodium cations (Na+) was excluded.
The samples had a diameter of 24.0 mm and a length of 120.0 mm. The first group, marked with the
letter A, were fresh reference samples and reflected the material’s initial structure. The samples of the
second series, marked with the letter B, were subjected to a temperature of about
50 C±2C
for 6000 h.
This temperature was selected following the measurements of the insulator housing temperature made
on a sunny, cloudless day with an ambient temperature of about 27
C. Measured temperature values
under actual insulator operation conditions reached about 46 C, as shown in Figure 1.
Energies 2020, 13, x FOR PEER REVIEW 2 of 14
regenerate temporarily in lost surface properties. Compared to ceramic and glass insulators, they are
significantly lighter and, in many countries, cheaper. However, the use of HVDC for the
transmission of electricity unfortunately raises some technical problems. The constant electric field,
forced by the HVDC line, has a significant impact on the integrity of dielectric materials usually
produced for AC applications [4]. Compared to systems operating at alternating voltages,
electrostatic phenomena may cause changes in degradation processes, electrical strength [5,6] and
even a four-fold increase in the accumulation of surface soiling [7]. Under such conditions, when the
leakage current flows through the polluted surface without passing through the zero curve of
voltage and current, the ignition of non-extinguishing concentrated surface discharges may occur.
This can lead to degradation of the composite housing. The ageing process in the presence of HVDC
results in an increased accumulation of spatial charge in areas where material is more degraded,
thus contributing to a stronger distortion of the electric field [8]. This phenomenon may cause
intensification of partial discharges. In addition, if the glass-reinforced epoxy (GRE) core material is
exposed to the long-term electrostatic field, then the ionic current flow may cause gradual
degradation of mechanical properties [9]. This process could be activated at increased temperatures.
Electrolysis of the carrying material (glass fibers) could then lead to the breakage of such insulators
and, consequently, to serious failures.
This experiment aimed to test the mechanical strength of GRE core rod samples after 6000 h of
aging at a temperature of about 50 °C and in the presence of HVDC.
2. Materials and Methods
In the ageing comparative tests, we used samples of GRE material cut from the carrying rod of a
typical high voltage composite insulator for AC lines. Fibers in the rod were made of ECR-glass
(ECR – type of glass electrical chemical reinforced) [10,11]. Apart from silicon (SiO2,), these types of
fibers contain calcium from CaCO3 (as flux and stabilizer) and aluminum from Al203 (to improve
chemical resistance), which are used in the glass composition. Typical ECR-glass contains over 58%
SiO2, about 22% CaO and less than 12% Al203, as well as smaller amounts of other additives. The
presence of mobile sodium cations (Na+) was excluded.
The samples had a diameter of 24.0 mm and a length of 120.0 mm. The first group, marked with
the letter A, were fresh reference samples and reflected the material’s initial structure. The samples
of the second series, marked with the letter B, were subjected to a temperature of about 50 °C ± 2 °C
for 6000 h. This temperature was selected following the measurements of the insulator housing
temperature made on a sunny, cloudless day with an ambient temperature of about 27 °C. Measured
temperature values under actual insulator operation conditions reached about 46 °C, as shown in
Figure 1.
Figure 1. The surface temperature of the composite insulator housing measured with a thermal
imaging camera.
The samples of the third series, marked with the letter C, with electrodes applied to their front
surfaces, were subjected for 6000 h to a temperature of about 50 °C ± 2 °C and a DC voltage of 20 kV.
The average value of the voltage distribution along the main axis of the typical insulator was about 1
kV/cm. In this research, it was applied twice as high as the electric field strength, i.e., 2 kV/cm.
Figure 1.
The surface temperature of the composite insulator housing measured with a thermal
imaging camera.
The samples of the third series, marked with the letter C, with electrodes applied to their front
surfaces, were subjected for 6000 h to a temperature of about 50
C
±
2
C and a DC voltage of 20 kV.
The average value of the voltage distribution along the main axis of the typical insulator was about
1 kV/cm. In this research, it was applied twice as high as the electric field strength, i.e., 2 kV/cm.
Increasing the voltage was supposed to accelerate the aging process. Figure 2shows one sample from
each of the three series.
Energies 2020,13, 6724 3 of 13
Energies 2020, 13, x FOR PEER REVIEW 3 of 14
Increasing the voltage was supposed to accelerate the aging process. Figure 2 shows one sample
from each of the three series.
Figure 2. Glass-reinforced epoxy (GRE) material samples of the high voltage alternating current
(HVAC) composite insulator carrying rod. From the left: reference sample—group A; thermally aged
sample—group B; and DC and thermally aged sample—group C, with visible electrodes attached to
the sample’s front surfaces.
The samples were arranged in a special stand and placed in a heating chamber. Figure 3 shows
the samples in the heating chamber (photograph taken with a fluke thermal imaging camera).
Figure 3. Samples placed in the heating chamber.
Samples from all three groups (both reference samples, thermally aged samples and those
thermally and voltage aged) were tested using the 3-point bending test. For this purpose, a testing
machine (INSTRON 1343) was used, which was extended with controllers and software by MTS.
(MTS–Mathematisch Technische Software-Entwicklung GmbH, Berlin, Germany). Special
adaptation of the support system was necessary as the tested samples had a cylindrical shape with a
diameter of 24.0 mm. Therefore, in the steel rollers on which the samples were based, semi-circular
notches with a radius of 12.0 mm and a depth of 10 mm were made at 100.0 mm of spacing. This is
illustrated in Figure 4. A relatively low crosshead speed of 0.1 mm/min was set. This corresponded
approximately to a force increase of 20 N/s.
The measuring system recorded the force acting on the sample, which was then converted to
stress, according to the relation [12]:
,
8
3
d
lF
f
(1)
where:
σf—is the bending stress, in MPa;
F—force loading the sample, recorded by the measuring system, in N;
l—distance between supports in the measuring system, equal to 100 mm;
d—diameter of the cylindrical sample, equal to 24 mm.
Figure 2.
Glass-reinforced epoxy (GRE) material samples of the high voltage alternating current
(HVAC) composite insulator carrying rod. From the left: reference sample—group A; thermally aged
sample—group B; and DC and thermally aged sample—group C, with visible electrodes attached to
the sample’s front surfaces.
The samples were arranged in a special stand and placed in a heating chamber. Figure 3shows
the samples in the heating chamber (photograph taken with a fluke thermal imaging camera).
d
Figure 3. Samples placed in the heating chamber.
Samples from all three groups (both reference samples, thermally aged samples and those
thermally and voltage aged) were tested using the 3-point bending test. For this purpose, a testing
machine (INSTRON 1343) was used, which was extended with controllers and software by MTS.
(MTS–Mathematisch Technische Software-Entwicklung GmbH, Berlin, Germany). Special adaptation
of the support system was necessary as the tested samples had a cylindrical shape with a diameter of
24.0 mm. Therefore, in the steel rollers on which the samples were based, semi-circular notches with
a radius of 12.0 mm and a depth of 10 mm were made at 100.0 mm of spacing. This is illustrated in
Figure 4. A relatively low crosshead speed of 0.1 mm/min was set. This corresponded approximately
to a force increase of 20 N/s.
The measuring system recorded the force acting on the sample, which was then converted to
stress, according to the relation [12]:
σf=
8×F×l
π×d3, (1)
where:
σf—is the bending stress, in MPa;
F—force loading the sample, recorded by the measuring system, in N;
l—distance between supports in the measuring system, equal to 100 mm;
d—diameter of the cylindrical sample, equal to 24 mm.
Energies 2020,13, 6724 4 of 13
Taking into account the fixed values occurring in relation (1):
σf=0.0184 ×F(2)
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Taking into account the fixed values occurring in relation (1):
F
f0184.0
(2)
Figure 4. The sample of a group A from HVAC composite insulator carrying rod in a mechanical
system for strength testing via the 3-point bending test. The notches in bottom supporting rollers are
visible.
In addition to the 3-point bending test, we performed a micro-hardness examination of the
sample material. It constituted an important supplement to the results of the optical method of the
material testing. It also made it possible to independently assess the material homogeneity and
cohesion. The measurements were made using the Vickers method (with a typical micro-hardness
measurer) at 1 kG load of the indenter. We used a semi-automatic mode of measuring the imprint
diameter. It should be emphasized that, apart from the obtained average values, the scatter of results
(which proves the homogeneity of the microstructure of the material) provides important
information.
3. Results
3.1. Mechanical Strength Tests
The mechanical characteristics of the 3-point bending test for all 13 tested samples showed a
very high similarity (Figures 5–7). Small differences occurred individually for particular samples,
but there were no differences in the characteristics of the samples that would be typical for groups A,
B or C. For a stress that usually slightly exceeds 300 MPa (294–345 MPa for individual samples), the
displacement linearly increased when stress increased. The slope of characteristics was identical for
all tested samples. A slight non-linearity at the beginning of the characteristics resulted from the
arrangement of the samples in the clamping system. When the stress reached about 300 MPa, the
samples broke axially. Long cracks were formed that extended symmetrically from the middle of the
samples and were present on the upper and lower side of the specimens, as illustrated in Figures 8
and 9. These cracks usually did not reach the ends of the samples. The formation of cracks,
accompanied by a well audible crackle, was reflected by a clear fault, sometimes even two faults, on
the samples’ mechanical characteristics. This effect occurred for all tested samples and only the
length of axial cracks differed. The formation of these cracks can be considered as a critical point.
Sample stiffness was clearly reduced and, therefore, the slope of the further part of the
characteristics already showed some differences for individual samples. There were also deviations
from the straightforward course of the characteristics. Additional faults, visible on the characteristics
Figure 4.
The sample of a group A from HVAC composite insulator carrying rod in a mechanical system
for strength testing via the 3-point bending test. The notches in bottom supporting rollers are visible.
In addition to the 3-point bending test, we performed a micro-hardness examination of the sample
material. It constituted an important supplement to the results of the optical method of the material
testing. It also made it possible to independently assess the material homogeneity and cohesion.
The measurements were made using the Vickers method (with a typical micro-hardness measurer) at
1 kG load of the indenter. We used a semi-automatic mode of measuring the imprint diameter. It should
be emphasized that, apart from the obtained average values, the scatter of results (which proves the
homogeneity of the microstructure of the material) provides important information.
3. Results
3.1. Mechanical Strength Tests
The mechanical characteristics of the 3-point bending test for all 13 tested samples showed a
very high similarity (Figures 57). Small dierences occurred individually for particular samples,
but there were no dierences in the characteristics of the samples that would be typical for groups
A, B or C. For a stress that usually slightly exceeds 300 MPa (294–345 MPa for individual samples),
the displacement linearly increased when stress increased. The slope of characteristics was identical
for all tested samples. A slight non-linearity at the beginning of the characteristics resulted from
the arrangement of the samples in the clamping system. When the stress reached about 300 MPa,
the samples broke axially. Long cracks were formed that extended symmetrically from the middle of the
samples and were present on the upper and lower side of the specimens, as illustrated in Figures 8and 9.
These cracks usually did not reach the ends of the samples. The formation of cracks, accompanied
by a well audible crackle, was reflected by a clear fault, sometimes even two faults, on the samples’
mechanical characteristics. This eect occurred for all tested samples and only the length of axial
cracks diered. The formation of these cracks can be considered as a critical point. Sample stiness was
clearly reduced and, therefore, the slope of the further part of the characteristics already showed some
dierences for individual samples. There were also deviations from the straightforward course of the
characteristics. Additional faults, visible on the characteristics of individual samples, corresponded to
Energies 2020,13, 6724 5 of 13
the formation of consecutive cracks and the enlargement of existing ones. This was largely random,
hence the significant dierences in the characteristics of the dierent shapes. However, it should be
emphasized that there was no apparent link between these discrepancies and the group to which the
samples belonged.
Energies 2020, 13, x FOR PEER REVIEW 5 of 14
of individual samples, corresponded to the formation of consecutive cracks and the enlargement of
existing ones. This was largely random, hence the significant differences in the characteristics of the
different shapes. However, it should be emphasized that there was no apparent link between these
discrepancies and the group to which the samples belonged.
Figure 5. Mechanical characteristics of the 3-point bending test for reference samples–group A.
Notwithstanding any differences in sample characteristics above the critical point (indicating
the load where the breaking of the reinforcement initiates), samples exhibited high repeatability of
the maximum stress value. When this value was reached, faults were produced, often with a large
decrease in stress. In addition to audible cracklings, this indicates the formation of subsequent cracks
that substantially reduced sample rigidity. It should also be noted that the samples did not deflect
during the test. The recorded displacement of the crosshead (on the order of a few millimeters)
caused the test samples to significantly indent, as illustrated in Figures 8 and 10. Additionally, at
higher force values, steel components of the measuring system underwent deflection. After taking
the samples out of the clamping system, they did not show the slightest bend. However, most of
them had cracks on flat side surfaces, as seen in Figure 11. In Tables 1–3, the values of maximum
force and stress were collected for all tested samples. Table 4 shows averaged values of maximum
stress, together with standard deviation, for all three groups of samples.
Figure 6. Mechanical characteristics of the 3-point bending test for samples subjected to
temperature—group B.
Figure 5. Mechanical characteristics of the 3-point bending test for reference samples–group A.
Energies 2020, 13, x FOR PEER REVIEW 5 of 14
of individual samples, corresponded to the formation of consecutive cracks and the enlargement of
existing ones. This was largely random, hence the significant differences in the characteristics of the
different shapes. However, it should be emphasized that there was no apparent link between these
discrepancies and the group to which the samples belonged.
Figure 5. Mechanical characteristics of the 3-point bending test for reference samples–group A.
Notwithstanding any differences in sample characteristics above the critical point (indicating
the load where the breaking of the reinforcement initiates), samples exhibited high repeatability of
the maximum stress value. When this value was reached, faults were produced, often with a large
decrease in stress. In addition to audible cracklings, this indicates the formation of subsequent cracks
that substantially reduced sample rigidity. It should also be noted that the samples did not deflect
during the test. The recorded displacement of the crosshead (on the order of a few millimeters)
caused the test samples to significantly indent, as illustrated in Figures 8 and 10. Additionally, at
higher force values, steel components of the measuring system underwent deflection. After taking
the samples out of the clamping system, they did not show the slightest bend. However, most of
them had cracks on flat side surfaces, as seen in Figure 11. In Tables 1–3, the values of maximum
force and stress were collected for all tested samples. Table 4 shows averaged values of maximum
stress, together with standard deviation, for all three groups of samples.
Figure 6. Mechanical characteristics of the 3-point bending test for samples subjected to
temperature—group B.
Figure 6.
Mechanical characteristics of the 3-point bending test for samples subjected to
temperature—group B.
Energies 2020, 13, x FOR PEER REVIEW 6 of 14
Figure 7. Mechanical characteristics of the 3-point bending test for samples subjected to high direct
current (DC) voltage and temperature–group C.
Figure 8. Long axial crack on top of sample A1, with an indentation caused by a crosshead of the
strength testing machine.
Figure 9. Long axial crack in the lower part of sample C1.
Figure 10. Indentation at the point of operation of the crosshead of the strength testing machine, long
axial crack and cracks on the side surface of sample C3.
Figure 7.
Mechanical characteristics of the 3-point bending test for samples subjected to high direct
current (DC) voltage and temperature–group C.
Energies 2020,13, 6724 6 of 13
Energies 2020, 13, x FOR PEER REVIEW 6 of 14
Figure 7. Mechanical characteristics of the 3-point bending test for samples subjected to high direct
current (DC) voltage and temperature–group C.
Figure 8. Long axial crack on top of sample A1, with an indentation caused by a crosshead of the
strength testing machine.
Figure 9. Long axial crack in the lower part of sample C1.
Figure 10. Indentation at the point of operation of the crosshead of the strength testing machine, long
axial crack and cracks on the side surface of sample C3.
Figure 8.
Long axial crack on top of sample A1, with an indentation caused by a crosshead of the
strength testing machine.
Energies 2020, 13, x FOR PEER REVIEW 6 of 14
Figure 7. Mechanical characteristics of the 3-point bending test for samples subjected to high direct
current (DC) voltage and temperature–group C.
Figure 8. Long axial crack on top of sample A1, with an indentation caused by a crosshead of the
strength testing machine.
Figure 9. Long axial crack in the lower part of sample C1.
Figure 10. Indentation at the point of operation of the crosshead of the strength testing machine, long
axial crack and cracks on the side surface of sample C3.
Figure 9. Long axial crack in the lower part of sample C1.
Notwithstanding any dierences in sample characteristics above the critical point (indicating
the load where the breaking of the reinforcement initiates), samples exhibited high repeatability of
the maximum stress value. When this value was reached, faults were produced, often with a large
decrease in stress. In addition to audible cracklings, this indicates the formation of subsequent cracks
that substantially reduced sample rigidity. It should also be noted that the samples did not deflect
during the test. The recorded displacement of the crosshead (on the order of a few millimeters) caused
the test samples to significantly indent, as illustrated in Figures 8and 10. Additionally, at higher force
values, steel components of the measuring system underwent deflection. After taking the samples out
of the clamping system, they did not show the slightest bend. However, most of them had cracks on
flat side surfaces, as seen in Figure 11. In Tables 13, the values of maximum force and stress were
collected for all tested samples. Table 4shows averaged values of maximum stress, together with
standard deviation, for all three groups of samples.
Energies 2020, 13, x FOR PEER REVIEW 6 of 14
Figure 7. Mechanical characteristics of the 3-point bending test for samples subjected to high direct
current (DC) voltage and temperature–group C.
Figure 8. Long axial crack on top of sample A1, with an indentation caused by a crosshead of the
strength testing machine.
Figure 9. Long axial crack in the lower part of sample C1.
Figure 10. Indentation at the point of operation of the crosshead of the strength testing machine, long
axial crack and cracks on the side surface of sample C3.
Figure 10.
Indentation at the point of operation of the crosshead of the strength testing machine,
long axial crack and cracks on the side surface of sample C3.
Energies 2020,13, 6724 7 of 13
Energies 2020, 13, x FOR PEER REVIEW 7 of 14
Figure 11. Cracks on flat side surface of sample C4.
Table 1. Maximum force and stress values recorded for reference samples—group A.
Sample Designation A1 A2 A3 A4 A5
Maximum force (kN) 32.33 30.85 30.82 31.38 30.00
Maximum stress (MPa) 595 568 567 577 552
Table 2. Maximum force and stress values recorded for samples subjected to temperature—group B.
Sample Designation B1 B2 B3 B4
Maximum force (kN) 31.40 30.14 30.06 32.29
Maximum stress (MPa) 578 555 553 594
Table 3. Maximum force and stress values recorded for samples subjected to high DC voltage and
temperature—group C.
Sample Designation C1 C2 C3 C4
Maximum force (kN) 31.14 31.44 31.33 31.28
Maximum stress (MPa) 555 578 576 575
Table 4. Average values of maximum stress, including standard deviation, for the samples of all
three groups.
Group of Samples A B C
Average value maximum stress (MPa) 572 ± 14.1 570 ± 17.1 571 ± 9.7
All recorded mechanical characteristics of the 3-point bending test were collected (Figure 12).
The figures and tables illustrate the reproducibility of the results obtained from the 3-point bending
test.
Figure 11. Cracks on flat side surface of sample C4.
Table 1. Maximum force and stress values recorded for reference samples—group A.
Sample Designation A1 A2 A3 A4 A5
Maximum force (kN) 32.33 30.85 30.82 31.38 30.00
Maximum stress (MPa) 595 568 567 577 552
Table 2. Maximum force and stress values recorded for samples subjected to temperature—group B.
Sample Designation B1 B2 B3 B4
Maximum force (kN) 31.40 30.14 30.06 32.29
Maximum stress (MPa) 578 555 553 594
Table 3.
Maximum force and stress values recorded for samples subjected to high DC voltage and
temperature—group C.
Sample Designation C1 C2 C3 C4
Maximum force (kN) 31.14 31.44 31.33 31.28
Maximum stress (MPa) 555 578 576 575
Table 4.
Average values of maximum stress, including standard deviation, for the samples of all
three groups.
Group of Samples A B C
Average value maximum stress (MPa)
572 ±14.1 570 ±17.1 571 ±9.7
3.2. Microscopic Examination of Samples
Microscopic examinations were carried out on the flat side surfaces of the samples, which were
randomly selected from all three groups: A, B and C. For this purpose, fragments of the material were
cut out of the selected samples. Further, we made so-called metallographic micro-sections, which were
also used in micro-hardness measurements.
The metallographic micro-sections were prepared on a Struers LaboPol-2 polishing machine.
The surface of the samples were grinded using SiC abrasive papers, and then polished using Struers
DiaPro diamond suspension with the grain diameters of 3
µ
m and 1
µ
m. The final polishing was carried
out on a colloidal SiO
2
suspension, with a grain size of 0.04
µ
m (Struers OP-S suspension). After each
grinding and polishing step, the samples were washed in an ultrasonic washer in ethyl alcohol.
All recorded mechanical characteristics of the 3-point bending test were collected (Figure 12).
The figures and tables illustrate the reproducibility of the results obtained from the 3-point bending test.
Energies 2020,13, 6724 8 of 13
Energies 2020, 13, x FOR PEER REVIEW 8 of 14
Figure 12. Mechanical characteristics of the 3-point bending test of all tested samples.
3.2. Microscopic Examination of Samples
Microscopic examinations were carried out on the flat side surfaces of the samples, which were
randomly selected from all three groups: A, B and C. For this purpose, fragments of the material
were cut out of the selected samples. Further, we made so-called metallographic micro-sections,
which were also used in micro-hardness measurements.
The metallographic micro-sections were prepared on a Struers LaboPol-2 polishing machine.
The surface of the samples were grinded using SiC abrasive papers, and then polished using Struers
DiaPro diamond suspension with the grain diameters of 3 µm and 1 µm. The final polishing was
carried out on a colloidal SiO2 suspension, with a grain size of 0.04 µm (Struers OP-S suspension).
After each grinding and polishing step, the samples were washed in an ultrasonic washer in ethyl
alcohol.
The analysis of microscopic images concluded that the microstructure of the tested material was
characterized by high homogeneity. Images from different places on the tested surface, both from
the same and different samples, showed no differences. The tight arrangement of fibers and binders
(in the form of epoxy resin occupying about 1/3 surface) was analogous to all of the tested
observation fields. The glass fiber diameter was also the same (several micrometers) with a small
size dispersion. It should be emphasized that there were no differences for individual samples from
groups A, B and C. Neither the long-term subjection to a temperature of about 50 °C nor the
combined action of temperature and DC voltage of 20 kV caused any observable effects in the
material’s microstructure, which had been found in earlier studies [9]. Microstructure images of the
sample material from all three groups are presented in Figures 13–15. There were few small dark
chippings in the microstructure elements (i.e., fiber fragments and, less often, binder). They were
created during the preparation of the surface of the samples.
After appropriate reformatting of the 8-bit grey scale images and processing of these images
with an optical microscope, the areas with the glass fibers and epoxy resin binder were depicted and
distinguished more clearly, allowing for more accurate measurements. Image analysis was carried
out with a Clemex computer-based analyzer. The blue phase was made up of glass fibers against a
dark organic phase. The binary mask that was applied to the image, as shown in Figure 15, allowed
for the quantitative measurement of glass fibers and resin content in the material and facilitated the
determination of fiber diameters. Figure 16 shows the size distribution of glass fiber diameters
obtained by averaging three microscopic images (A, B and C, one from each sample group).
Figure 12. Mechanical characteristics of the 3-point bending test of all tested samples.
The analysis of microscopic images concluded that the microstructure of the tested material was
characterized by high homogeneity. Images from dierent places on the tested surface, both from
the same and dierent samples, showed no dierences. The tight arrangement of fibers and binders
(in the form of epoxy resin occupying about 1/3 surface) was analogous to all of the tested observation
fields. The glass fiber diameter was also the same (several micrometers) with a small size dispersion.
It should be emphasized that there were no dierences for individual samples from groups A,
B and C
.
Neither the long-term subjection to a temperature of about 50
C nor the combined action of temperature
and DC voltage of 20 kV caused any observable eects in the material’s microstructure, which had
been found in earlier studies [
9
]. Microstructure images of the sample material from all three groups
are presented in Figures 1315. There were few small dark chippings in the microstructure elements
(i.e., fiber fragments and, less often, binder). They were created during the preparation of the surface
of the samples.
Energies 2020, 13, x FOR PEER REVIEW 9 of 14
Figure 13. Microstructure image of sample A2 on its flat side surface, 200×. The ECR glass fibers in
section and slightly darker epoxy resin are visible. Dark chipping of fiber fragments and less often of
binder are few.
Figure 14. Microstructure image of sample B2 on its flat side surface 100×. Few dark chipping of fiber
fragments and binder are visible.
On all tested micro-sections of samples A2, B2 and C3, glass fibers represented, on average,
67.0%. The differences for individual observation fields did not exceed 2.1%. The average value of
the glass fiber diameter was 14.9 mm. The whole distribution was between 11.0–18.5 mm. However,
over 90% of the fibers had a diameter in a narrow range, from 13.0 to 16.5 mm. The distribution was
clearly multimodal, with fibers with a diameter of 13.5–16.0 mm dominating.
The microstructure of the tested GRE material of the HVAC composite insulator carrying rod
was clearly assessed as entirely appropriate, compact and homogeneous. Chipping created during
the grinding and polishing process of test surfaces (mainly small fragments of glass fibers) did not
exceed 3% of the surface. Tightly arranged glass fibers constituted 2/3 of the material by volume. The
areas with only the resin visible in the micro-sections were not numerous and the size did not exceed
several fibers. The ECR glass fibers used were uniform. Further, their diameter distribution was
quite narrow and they did not raise any objections.
Figure 13.
Microstructure image of sample A2 on its flat side surface, 200
×
. The ECR glass fibers in
section and slightly darker epoxy resin are visible. Dark chipping of fiber fragments and less often of
binder are few.
After appropriate reformatting of the 8-bit grey scale images and processing of these images
with an optical microscope, the areas with the glass fibers and epoxy resin binder were depicted and
distinguished more clearly, allowing for more accurate measurements. Image analysis was carried out
with a Clemex computer-based analyzer. The blue phase was made up of glass fibers against a dark
organic phase. The binary mask that was applied to the image, as shown in Figure 15, allowed for
Energies 2020,13, 6724 9 of 13
the quantitative measurement of glass fibers and resin content in the material and facilitated the
determination of fiber diameters. Figure 16 shows the size distribution of glass fiber diameters obtained
by averaging three microscopic images (A, B and C, one from each sample group).
Energies 2020, 13, x FOR PEER REVIEW 9 of 14
Figure 13. Microstructure image of sample A2 on its flat side surface, 200×. The ECR glass fibers in
section and slightly darker epoxy resin are visible. Dark chipping of fiber fragments and less often of
binder are few.
Figure 14. Microstructure image of sample B2 on its flat side surface 100×. Few dark chipping of fiber
fragments and binder are visible.
On all tested micro-sections of samples A2, B2 and C3, glass fibers represented, on average,
67.0%. The differences for individual observation fields did not exceed 2.1%. The average value of
the glass fiber diameter was 14.9 mm. The whole distribution was between 11.0–18.5 mm. However,
over 90% of the fibers had a diameter in a narrow range, from 13.0 to 16.5 mm. The distribution was
clearly multimodal, with fibers with a diameter of 13.5–16.0 mm dominating.
The microstructure of the tested GRE material of the HVAC composite insulator carrying rod
was clearly assessed as entirely appropriate, compact and homogeneous. Chipping created during
the grinding and polishing process of test surfaces (mainly small fragments of glass fibers) did not
exceed 3% of the surface. Tightly arranged glass fibers constituted 2/3 of the material by volume. The
areas with only the resin visible in the micro-sections were not numerous and the size did not exceed
several fibers. The ECR glass fibers used were uniform. Further, their diameter distribution was
quite narrow and they did not raise any objections.
Figure 14.
Microstructure image of sample B2 on its flat side surface 100
×
. Few dark chipping of fiber
fragments and binder are visible.
Energies 2020, 13, x FOR PEER REVIEW 10 of 14
(a) (b)
Figure 15. Microstructure image of sample C3 on its flat side surface, 200×. Few chipping of fiber
fragments and binder are visible (a) on the right side and (b) the same area with a colored binary
mask, which enables precise measurements to be taken.
Figure 16. Size distribution of glass fiber diameters in the GRE material for the HVAC composite
insulator carrying rod.
3.3. Micro-Hardness Measurements of Samples
Apart from the abovementioned microscopic tests, micro-hardness measurements were carried
out for all samples. They allowed to access the cohesiveness and homogeneity of the material, being
an important supplement to the results of other tests of the material. The measurements were carried
out using the Vickers method, with a Struers Dura Scan universal micro-hardness meter with 1 kG of
indenter load (HV1 – Hardness Vickers method, 1 means 1 kG ). The measurements were carried out
on the same side surfaces of the samples on which microscopic tests were performed. A
semi-automatic mode of measuring the imprint diameter was used. If different diameters were
obtained on the same imprint with generally small differences, their length was averaged (Figure
17). The following HV1International designation of Hardness Vickers method. HV1 means a
hardness of 1 kG values were obtained and averaged over 5 measurements on each sample:
Reference sample A2–165 ± 9;
Temperature-aged sample B2–169 ± 11;
Temperature- and voltage-aged sample C3–155 ± 8.
Figure 15.
Microstructure image of sample C3 on its flat side surface, 200
×
. Few chipping of fiber
fragments and binder are visible (
a
) on the right side and (
b
) the same area with a colored binary mask,
which enables precise measurements to be taken.
Energies 2020, 13, x FOR PEER REVIEW 10 of 14
(a) (b)
Figure 15. Microstructure image of sample C3 on its flat side surface, 200×. Few chipping of fiber
fragments and binder are visible (a) on the right side and (b) the same area with a colored binary
mask, which enables precise measurements to be taken.
Figure 16. Size distribution of glass fiber diameters in the GRE material for the HVAC composite
insulator carrying rod.
3.3. Micro-Hardness Measurements of Samples
Apart from the abovementioned microscopic tests, micro-hardness measurements were carried
out for all samples. They allowed to access the cohesiveness and homogeneity of the material, being
an important supplement to the results of other tests of the material. The measurements were carried
out using the Vickers method, with a Struers Dura Scan universal micro-hardness meter with 1 kG of
indenter load (HV1 – Hardness Vickers method, 1 means 1 kG ). The measurements were carried out
on the same side surfaces of the samples on which microscopic tests were performed. A
semi-automatic mode of measuring the imprint diameter was used. If different diameters were
obtained on the same imprint with generally small differences, their length was averaged (Figure
17). The following HV1International designation of Hardness Vickers method. HV1 means a
hardness of 1 kG values were obtained and averaged over 5 measurements on each sample:
Reference sample A2–165 ± 9;
Temperature-aged sample B2–169 ± 11;
Temperature- and voltage-aged sample C3–155 ± 8.
Figure 16.
Size distribution of glass fiber diameters in the GRE material for the HVAC composite
insulator carrying rod.
Energies 2020,13, 6724 10 of 13
On all tested micro-sections of samples A2, B2 and C3, glass fibers represented, on average, 67.0%.
The dierences for individual observation fields did not exceed 2.1%. The average value of the glass
fiber diameter was 14.9 mm. The whole distribution was between 11.0–18.5 mm. However, over 90%
of the fibers had a diameter in a narrow range, from 13.0 to 16.5 mm. The distribution was clearly
multimodal, with fibers with a diameter of 13.5–16.0 mm dominating.
The microstructure of the tested GRE material of the HVAC composite insulator carrying rod was
clearly assessed as entirely appropriate, compact and homogeneous. Chipping created during the
grinding and polishing process of test surfaces (mainly small fragments of glass fibers) did not exceed
3% of the surface. Tightly arranged glass fibers constituted 2/3 of the material by volume. The areas
with only the resin visible in the micro-sections were not numerous and the size did not exceed several
fibers. The ECR glass fibers used were uniform. Further, their diameter distribution was quite narrow
and they did not raise any objections.
3.3. Micro-Hardness Measurements of Samples
Apart from the abovementioned microscopic tests, micro-hardness measurements were carried
out for all samples. They allowed to access the cohesiveness and homogeneity of the material, being an
important supplement to the results of other tests of the material. The measurements were carried
out using the Vickers method, with a Struers Dura Scan universal micro-hardness meter with 1 kG of
indenter load (HV1 – Hardness Vickers method, 1 means 1 kG). The measurements were carried out on
the same side surfaces of the samples on which microscopic tests were performed. A semi-automatic
mode of measuring the imprint diameter was used. If dierent diameters were obtained on the same
imprint with generally small dierences, their length was averaged (Figure 17). The following HV1
(International designation of Hardness Vickers method. HV1 means a hardness of 1 kG) values were
obtained and averaged over 5 measurements on each sample:
Reference sample A2–165 ±9;
Temperature-aged sample B2–169 ±11;
Temperature- and voltage-aged sample C3–155 ±8.
Energies 2020, 13, x FOR PEER REVIEW 11 of 14
The obtained micro-hardness values were high and clearly proved the quality of the tested GRE
material of the HVAC composite insulator carrying rod. This confirmed the generally high
evaluation of homogeneity and cohesiveness of the tested material. The imprints were often of a
regular shape, allowing for automatic diameter measurements. In the case of less regular imprints, it
was necessary to correct the marker setting manually, as illustrated in Figure 17. The typical
phenomenon of relaxing the energy of load interaction through microcracks, especially running
from the apex of the imprint, was not observed. However, the fibers located inside the area of the
indenter operation often cracked. Images of typical imprints obtained on samples from three series
are shown in Figures 17–19.
Compared to the average hardness of the reference sample, the thermally aged sample showed
a slightly higher hardness (2.4%). The temperature and voltage-aged sample had a reduced hardness
(6.1%). The differences were small and comparable to the standard deviation values. Therefore, we
can conclude that the material hardness of the tested samples from all three series remain at a similar
level. There is no clear impact of ageing, neither as a result of temperature or the combined action of
temperature and voltage on the material hardness, especially given the fact that the mechanical
strength of all three series of tested samples remained at a similar level.
Figure 17. Image of an indenter imprint on the flat side surface of sample A2, 500×. In order to
measure it correctly, the position of the markers was manually corrected. The measured values of the
imprint diameter were averaged. There are no common microcracks running from the apex of the
imprint, but there are visible fiber cracks as a result of the indenter operation.
Figure 17.
Image of an indenter imprint on the flat side surface of sample A2, 500
×
. In order to measure
it correctly, the position of the markers was manually corrected. The measured values of the imprint
diameter were averaged. There are no common microcracks running from the apex of the imprint,
but there are visible fiber cracks as a result of the indenter operation.
Energies 2020,13, 6724 11 of 13
The obtained micro-hardness values were high and clearly proved the quality of the tested GRE
material of the HVAC composite insulator carrying rod. This confirmed the generally high evaluation
of homogeneity and cohesiveness of the tested material. The imprints were often of a regular shape,
allowing for automatic diameter measurements. In the case of less regular imprints, it was necessary
to correct the marker setting manually, as illustrated in Figure 17. The typical phenomenon of relaxing
the energy of load interaction through microcracks, especially running from the apex of the imprint,
was not observed. However, the fibers located inside the area of the indenter operation often cracked.
Images of typical imprints obtained on samples from three series are shown in Figures 1719.
Energies 2020, 13, x FOR PEER REVIEW 12 of 14
Figure 18. Image of an indenter imprint on the flat side surface of sample B2, 500×. Cracks and
chipping of fiber fragments and resin damage as a result of the indenter operation are clearly visible.
Figure 19. Image of an indenter imprint on the flat side surface of sample C3, 500×. Cracks and
chipping of fiber fragments and resin damage in the area of the indenter operation are visible.
4. Discussion
Numerous publications that have focused on the use of composite insulators in high DC voltage
lines have mainly examined their surface properties. Meanwhile, an important research issue that
has not been mentioned in the literature is the long-term mechanical strength of the GRE cores
exposed to high DC voltage. Long-term exposure to high DC voltage can lead to the development of
an ionic current in the GRE core. This process may reduce the mechanical strength and,
consequently, the insulator can break and the line would fall to the ground. This problem has been
noticed in the case of glass disc insulators [13]. However, our research showed that the mechanical
properties of the tested samples of the GRE cores did not deteriorate under the experiment’s
adopted conditions. Both the long-term subjection to a temperature of about 50 °C and the collective
action of temperature and DC voltage of 20 kV did not cause any observable and undesirable effects
in the microstructure of the material. A similar statement has been made in earlier studies [9]. The
analysis of the obtained research results is presented in the conclusion.
Figure 18.
Image of an indenter imprint on the flat side surface of sample B2, 500
×
. Cracks and
chipping of fiber fragments and resin damage as a result of the indenter operation are clearly visible.
Energies 2020, 13, x FOR PEER REVIEW 12 of 14
Figure 18. Image of an indenter imprint on the flat side surface of sample B2, 500×. Cracks and
chipping of fiber fragments and resin damage as a result of the indenter operation are clearly visible.
Figure 19. Image of an indenter imprint on the flat side surface of sample C3, 500×. Cracks and
chipping of fiber fragments and resin damage in the area of the indenter operation are visible.
4. Discussion
Numerous publications that have focused on the use of composite insulators in high DC voltage
lines have mainly examined their surface properties. Meanwhile, an important research issue that
has not been mentioned in the literature is the long-term mechanical strength of the GRE cores
exposed to high DC voltage. Long-term exposure to high DC voltage can lead to the development of
an ionic current in the GRE core. This process may reduce the mechanical strength and,
consequently, the insulator can break and the line would fall to the ground. This problem has been
noticed in the case of glass disc insulators [13]. However, our research showed that the mechanical
properties of the tested samples of the GRE cores did not deteriorate under the experiment’s
adopted conditions. Both the long-term subjection to a temperature of about 50 °C and the collective
action of temperature and DC voltage of 20 kV did not cause any observable and undesirable effects
in the microstructure of the material. A similar statement has been made in earlier studies [9]. The
analysis of the obtained research results is presented in the conclusion.
Figure 19.
Image of an indenter imprint on the flat side surface of sample C3, 500
×
. Cracks and
chipping of fiber fragments and resin damage in the area of the indenter operation are visible.
Compared to the average hardness of the reference sample, the thermally aged sample showed a
slightly higher hardness (2.4%). The temperature and voltage-aged sample had a reduced hardness
Energies 2020,13, 6724 12 of 13
(6.1%). The dierences were small and comparable to the standard deviation values. Therefore, we can
conclude that the material hardness of the tested samples from all three series remain at a similar
level. There is no clear impact of ageing, neither as a result of temperature or the combined action
of temperature and voltage on the material hardness, especially given the fact that the mechanical
strength of all three series of tested samples remained at a similar level.
4. Discussion
Numerous publications that have focused on the use of composite insulators in high DC voltage
lines have mainly examined their surface properties. Meanwhile, an important research issue that has
not been mentioned in the literature is the long-term mechanical strength of the GRE cores exposed to
high DC voltage. Long-term exposure to high DC voltage can lead to the development of an ionic current
in the GRE core. This process may reduce the mechanical strength and, consequently, the insulator
can break and the line would fall to the ground. This problem has been noticed in the case of glass disc
insulators [
13
]. However, our research showed that the mechanical properties of the tested samples
of the GRE cores did not deteriorate under the experiment’s adopted conditions. Both the long-term
subjection to a temperature of about 50
C and the collective action of temperature and DC voltage
of 20 kV did not cause any observable and undesirable eects in the microstructure of the material.
A similar statement has been made in earlier studies [
9
]. The analysis of the obtained research results
is presented in the conclusion.
5. Conclusions
The mechanical characteristics of the 3-point bending test for all tested samples (reference and
aged) showed a high similarity. Small dierences occurred individually for particular samples, but there
were no dierences in the characteristics of samples that would be typical for any of the sample
groups—reference A, or aged B and C.
Notwithstanding the relatively small dierences in sample characteristics, all tested samples
showed a high repeatability of the maximum stress value. The average maximum stress values for the
three sample groups (A, B and C) were almost identical.
The microstructure of the tested GRE material of the HVAC composite insulator carrying rod
should be assessed as entirely appropriate, compact and homogeneous.
Tightly arranged glass fibers constituted 2/3 of the material by volume. Areas with just resin were
not numerous and their size did not exceed several fibers. This is dicult to avoid in the production
process. The ECR glass fibers used were uniform; their diameter distribution was quite narrow and
did not raise any objections.
The obtained micro-hardness values were high and proved the quality of the tested material
of the HVAC composite insulator carrying rod. A small dispersion of results (
±
6.5%) should be
emphasized. This confirmed the overall good evaluation of homogeneity and cohesiveness of the
tested material. It can be concluded that the material hardness of the tested samples from all three series
(
A, B and C
) remained at a similar high level. Therefore, there was no clear impact of ageing neither as
a result of temperature nor as the combined action of temperature and voltage on the hardness of the
GRE material.
On the basis of the mechanical and microscopic tests presented above (performed on reference
samples and aged GRE samples of the HVAC composite insulator carrying rod) it was found that there
were no registrable degradation eects. Long-term (6000 h) interaction of temperature 50
C and the
combined action of temperature and DC voltage 20 kV, did not cause any decrease of GRE material
mechanical parameters or change in its microstructure. Thus, the test results presented in [
9
] were
fully confirmed.
Author Contributions:
Conceptualization: K.W. and P.R.; methodology: K.W., P.R., Z.R., and P.P.; validation:
K.W., P.R., Z.R., and P.P.; investigation: K.W. and P.R.; data curation: K.W. and P.R.; writing—original draft
Energies 2020,13, 6724 13 of 13
preparation: K.W., P.R., Z.R., P.P.; writing—review and editing: K.W., P.R., Z.R., and P.P. All authors have read and
agreed to the published version of this manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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In this paper, the most important results are presented and discussed from a multiyear interdisciplinary study directed toward the identification of the most suitable glass/polymer composite systems with the highest resistance to brittle fracture for high voltage composite insulator applications. Several unidirectional glass/polymer composite systems, commonly used in composite insulators, based either on E-glass or ECR-glass fibers embedded in either polyester, epoxy, or vinyl ester resins have been investigated for their resistance to stress corrosion cracking in nitric acid. The most important factors (fiber and resin types, surface fiber exposure, polymer fracture toughness, moisture absorption, interfacial properties, sandblasting) affecting the resistance of the composites to brittle fracture have been identified and thoroughly analyzed. It has been shown that the brittle fracture process of composite (nonceramic) insulators can be successfully eliminated, or at least dramatically reduced, by the proper chemical optimization of composite rod materials for their resistance to stress corrosion cracking.
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Space charge behavior in sheds removed from service aged 500 and 800 kV HVDC silicone rubber insulators and compared to a stored reference insulator is reported in this paper. The aim of this investigation is to evaluate the material aging. The investigations are made under electric field of ±20 kV/mm at room temperature using pulsed electro-acoustic technique. Based on the results, the electric field distribution and mobility of charge carriers, considering the contributions of de-trapping process from both shallow and deep traps, are deduced. It is concluded that space charge concentration in the aged material increases compared to that in the reference material and the dynamics of the charge decay changes. The understanding of this behavior provides a means of possibility assessing insulator material aging.
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This article presents the results of an examination performed on a set of samples of glass-epoxy core rods used in composite insulators with silicone rubber housings. The goal of the examination was to test the aging resistance of the core material when exposed to Direct Current (DC) high voltage. Long term exposure of a glass-epoxy core rod to DC high voltage may lead to the gradual degradation of its mechanical properties due to the ion migrations. Electrolysis of the core material (glass fiber) may cause electrical breakdown of the insulators and consequently lead to a major failure. After being aged for 6000 hours under DC high voltage, the samples were subjected to microscopic analysis. Their chemical composition was also examined using Raman spectroscopy and their dielectric losses and conductance in the broad range of frequencies were tested using dielectric spectroscopy.
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Overhead lines dominate the planet much more than underground cable system primarily because of its lower cost of transmission (as low as 10 % of underground transmission). Disc insulators form a critical component in isolating the line conductors from tower structures both in EHVAC and HVDC transmission lines. Over the years, many types of materials are chosen for use in the transmission lines right from porcelain, glass till composite insulators. In this paper, feasibility study has been made to utilize porcelain disc insulators used in EHVAC lines to HVDC lines while line conversion.
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High voltage equipment designed for DC applications is stressed differently from ac equipment. Most notably, the load conditions of the equipment play a major role in determining the electric field distribution in insulation under dc, due to the temperature dependence of conductivity. Additionally, polarization and space charge phenomena impose a time dependent behavior on the stress distribution. This has major consequences for the way in which degradation of the insulation under dc stress takes place. The nature of the stresses under dc may lead to accelerated degradation processes which could result in early breakdown of the insulation. This paper provides an overview of the stress conditions specific for dc application and highlights via which routes they may lead to failure. The stress distributions in converter transformers and dc cable systems will serve as typical examples.
Design and selection criteria for HVDC overhead transmission lines insulators
  • J George
  • Z Lodi
George, J.; Lodi, Z. Design and selection criteria for HVDC overhead transmission lines insulators. In Proceedings of the 2009 CIGRE Canada Conference on Power Systems, Toronto, ON, Canada, 4-6 October 2009.
Polymer matrix composites in high voltage transmission line application
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Kumosa, M.; Armentrout, D.; Burks, B.; Hoffman, J.; Kumosa, L.; Middleton, J.; Predecki, P. Polymer matrix composites in high voltage transmission line application. In Proceedings of the 18th International Conference on Composites Materials (ICCM), Jeju Island, Korea, 21-26 August 2011.
Surface Charge & DC Flashover Performance of Composite Insulators
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Gubanski, S. Surface Charge & DC Flashover Performance of Composite Insulators. INMR. 13 August 2016. Available online: https://www.inmr.com/surface-charge-flashover-performance-composite-insulators/ (accessed on 20 October 2020).
Composite Insulators for DC
  • A Pigini
Pigini, A. Composite Insulators for DC. INMR. 28 June 2018. Available online: https://www.inmr.com/ composite-insulators/ (accessed on 20 October 2020).