The 20th International Symposium on High Voltage Engineering, Buenos Aires, Argentina, August 27 – September 01, 2017
ASSESSMENT OF FIELD AGED COMPOSITE INSULATORS
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B.Sravanthi , K.A.Aravind , B.Yashodhara , Pradeep M Nirgude ,
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K.Sandhya , V.Kamaraju and A.V.R.S Sarma
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Stanley College of Engineering and Technology for Women, Hyderabad, India
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CPRI, Hyderabad, India
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Mahaveer Institute of Science and Technology, Hyderabad, India
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Osmania University, Hyderabad, India
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Email: <aravind@cpri.in>
Abstract: The performance of composite insulators in transmission and distribution lines
is a key factor for reliability of power systems. Significant improvement has been
achieved in the composite insulators technology in past 50 years and hence the usage.
Various laboratory testing facilities and procedures has been developed to confirm with
the withstand levels and performance of composite insulators before installation. But
none of them provides the life expectancy and other degradation issues to be faced inservice. Hence, an investigation or assessment is necessary to address such issues after
installation. Such assessment provides the necessary information for maintenance and
replacement and also the reliability and efficiency of insulators. For this study, 25 kV
composite insulators selected from three different pollution regions (coal, cement and
marine) are removed from service. The samples removed from the service are of 4 to 6
years aged and affected with different contaminants which include cement, coal and
marine pollution. This paper presents the various tests that are carried out to assess the
performance of the field aged samples and also reports the methods including visual
observation to assess the appearance of the field aged samples with relative
performance of similar composite insulators with same arcing distance and creepage
distance studied in comparison to various contaminants. The determination of pollution
severity by equivalent salt deposit density (ESDD) and non-soluble deposit density
(NSDD) methods, the classification of hydrophobicity using STRI Guide and material
diagnostic techniques such as Scanning Electron Microscopy and Energy Dispersive XRay Analysis to determine degradation are presented. the results obtained demonstrate
that the hydrophobicity of composite insulator has decreased with service and variation of
silicone content in different contaminated samples in comparison to virgin sample are
signs of ageing. The results of different tests are compared with virgin sample and field
aged samples behaviour is assessed
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INTRODUCTION
The performance of composite insulators
in transmission and distribution lines is a key factor
for reliability of power systems. Significant
improvement has been achieved in the composite
insulators technology in past 50 years and hence
the usage. The porcelain and glass insulators were
replaced by composite insulators due to their
superior performance characteristics [1,2].
Indian Railways has replaced most of the
porcelain insulators with the composite insulators.
Nearly 28% of the Indian Railway lines reported
the failure of porcelain insulators subjected to
heavy pollution and vandalism. Since the
population of composite insulators have increased
in recent years, a performance check is necessary
for further improvement in design characteristics
which gives major exposure of composite
insulators to meet the future pollution scenario.
The continuous service and ageing of
insulators may lead to significant deterioration in
their pollution withstand characteristics and may
result in flashover of insulators. The properties of
composite insulators tend to change with time
because of long time exposure to environmental
stresses, mechanical loads and electrical
discharges in the form of arcing or corona. Such a
reduction in the electrical and mechanical
properties is termed as ageing. The composite
insulators tend to lose one of their most important
properties of hydrophobicity when they are
continuously subjected to various extreme levels of
contamination. Hence, periodic assessment of
these insulators is to be performed in order to
check with their withstand characteristics to meet
required standards. When these insulators are
exposed near cement, industrial, agricultural, coal
and marine areas, the pollutants deposited on
insulating surfaces greatly reduce the surface
resistivity and the flashover of the insulator may
occur.
For this study, 25 kV composite insulators selected
from three different pollution regions are removed
from service. The samples removed from the
service are of 4 to 6 years aged and affected with
different contaminants which include cement, coal
and marine pollution. This paper presents the
various tests that are carried out to assess the
performance of the field aged samples, which are
collected from south central railway region (SCR)
of India and also reports the methods including
visual observation to assess the appearance of the
field aged samples with relative performance of
similar composite insulators with same arcing
distance and creepage distance studied in
comparison to various contaminants. The results
show that there are no major signs of cracks to be
reported.
The determination of pollution severity by ESDD
and NSDD methods is performed, the classification
of hydrophobicity using STRI Guide and material
diagnostic techniques such as Scanning Electron
Microscopy (SEM) and Energy Dispersive X-Ray
Analysis (EDX) to determine degradation are
presented and the results obtained demonstrate
that the hydrophobicity of composite insulator is
decreased with service and variation of silicon
content in different contaminated samples in
comparison to virgin sample are signs of ageing. A
limited clean fog test is also conducted to study the
leakage current variation of contaminated samples.
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(a) S2-Coal
b) S3-Cement
(c) S5-Virgin
(d) S6-Marine
Figure 1: Hydrophobicity of in service samples
VISUAL OBSERVATION
Visual observation technique is used for assessing
the state of composite insulators[3]. It is an
intensive scanning of structural parts of in service
composite insulators.
In this study, the visual appearance of insulator is
assessed, and the characteristics of samples are
shown in Table 1. This inspection reveals that
there were no signs of tracking and erosion on the
surface housing of the insulators. Traces of
electrical activities were observed on the
insulators. Contaminants were deposited on the
top and bottom surface and end fittings of the
samples and the surface of the samples were
found to become hard. When compared with virgin
sample, the colour of the aged contaminated
sample is changed. After 4-6 years of service,
contaminated composite insulators showed
satisfactory characteristics. Figure 1 shows the
picture of insulators after 4-5 years in service.
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DETERMINATION OF POLLUTION
SEVERITY
The surface contaminants on insulators are
composed of both water soluble and non-soluble
materials. The soluble component consists of
various types of salinity expressed as equivalent
salt deposit density (ESDD) and non-soluble part
of the pollutant expressed as non-soluble deposit
density (NSDD). The ESDD and NSDD were
measured according to IEC 60507 [4] and the
range of pollution is classified as light, medium,
high and very high as per IEC 60815 [5].
Observing the pollution distribution on the whole
insulator, the pollutants were found to be unevenly
distributed on the contaminated samples. The
ESDD and NSDD results shown in Table 2 and
Table 3 indicate that there is no excess of
contaminants deposited on these samples.
Table 2: ESDD Measurement
Table 1: Characteristics of the test insulators
ESDD
S2
S3
S5
S6
Voltage in kV
25
25
25
25
Arcing Distance
in mm
Creepage
Distance in mm
380
385
380
375
1180
1093
1224
1150
Parameter
Sample
Pollutant
ESDD
(mg/cm2)
(mg/cm2)
Larger
Shed
Smaller
Shed
Pollution
Range
S2
Coal
0.045
0.557
Medium
S3
Cement
0.205
0.217
High
S6
Marine
0.069
0.102
High
Table 3: NSDD Measurement
NSDD
NSDD
(mg/cm )
(mg/cm )
Smaller
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Sample
Pollutant
S2
Coal
S3
S6
4
Cement
Marine
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Larger
Shed
2
Pollution
Range
Shed
0.00003
0.0056
Light
0.0026
0.005
Light
0.06027
0.0057
Light
HYDROPHOBICITY MEASUREMENT
The degree of hydrophobicity, for the different
contaminated insulator surfaces is measured in
accordance with the STRI guide [6]. This guide
classifies the hydrophobicity of surfaces to seven
categories, HC1 to HC7. The HC1 refers to the
highest surface hydrophobicity while HC7
represents the lowest hydrophobicity [6].
Distilled water was sprayed several times on the
surface of the insulator. Figure 2 shows picture of
contaminated silicone rubber insulator samples.
Comparing the figures with STRI guide figures for
hydrophobicity, concludes that the hydrophobicity
of the samples after removal from service is
ranging between HC1 to HC4 shown in Table 4
and Figure 2. It was observed that the polluted
areas showed better hydrophobicity even after six
years of
service.
It
indicates excellent
hydrophobicity transfer to the pollution layer which
is a positive effect that can reduce the leakage
characteristics of the contaminated sample [7, 8].
Table 4: Hydrophobicity Classification
Sample
Hydrophobicity Class
S2-Coal
HC3
S3-Cement
HC4
S6-Marine
HC3
a). S2-HC3
(b).S3-HC4 (c). S6-HC3
Figure 2: Hydrophobicity test of samples
MATERIAL ANALYSIS
The changes in surface morphology and material
structure of contaminated samples were compared
with virgin sample, examined before and after
ageing [9]. The test is carried out at Defence
Metallurgical Research
Laboratory (DMRL)
Hyderabad.
5.1 Scanning Electron Microscopy (SEM): SEM
shows the molecular structural changes of the
surface of the silicone rubber. It provides detailed
high resolution images of the samples by rastering
a focussed electron beam across the surface and
detecting secondary or backscattered electron
signal [10]. The microscopic structures of top side
of silicone rubber weather sheds was observed in
SEM.
Virgin
S3 - Cement
S2 - Coal
S6 - Marine
Figure 3: Material test samples
Contaminated silicone rubber samples of size 2 ×
2 cm approximately as shown in Figure 3 are
chosen for material analysis. Figure 4 shows the
micrographs for virgin and aged sample at
1000X.The characterization of material structure,
surface defects and residues on polymers can be
assessed using SEM technique.
The overall observation from Figure 4 indicates
that there is no major degradation, like cracking; as
the virgin sample has a smooth, and less porous
surface, while the surface roughness and porosity
increases with ageing for aged contaminated
silicon rubber insulators [11].
5.2 Energy Dispersive X-Ray (EDX): .EDX
analysis can be employed to study the percentage
of chemical elements present on the surface of the
insulator. EDX systems are attachments to SEM
instruments the composition of the insulator
surfaces of in service aged insulators were studied
using EDX technique. More specifically, the effects
of aging through environmental and electrical
stresses can be related to oxidation effects and
loss in hydrophobicity of outer polymeric material
[12].
a. Virgin Sample
maintained so that the layer conductivity reaches
its maximum value within 30 min from the start of
the fog generation. The test samples were
positioned at 1 m above the ground. The test
voltage is maintained for 100 minutes from the
start of the test [8].
b. Coal Sample
6.1 EXPERIMENTAL TEST SETUP:
c. Cement Sample d. Marine Sample
Figure 4: Micrographs of aged samples
The data generated by EDX analysis consists of
spectra showing peaks corresponding to the
elements making up the composition of the aged
and virgin sample as shown in Table 5. A distinct
difference between aged and virgin sample is
found. The aged contaminated and virgin sample
revealed the presence of oxygen (O), carbon(C),
silicon(Si),
aluminium
(Al),
magnesium
(Mg).Cement has got calcium (Ca), Sulphur (S)
and Potassium (K). An increase in carbon and
silicon concentrations is observed for the aged
contaminated samples compared to virgin one.
Table 5 shows the elemental analysis for aged and
virgin samples [11, 12].
The experimental test set-up consists of composite
insulators arranged in an artificial chamber as
shown in Figure 5. All the insulator samples were
suspended vertically inside the chamber. The test
voltage applied is 25 kVrms. The leakage current
was measured through a digital ammeter. The
continuous steam of de-ionized water is used to
generate fog in the test chamber. The steam
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generation rate is 0.486 kg/h/m . The conductivity
of de-ionized water used is 0.005 S/m. The
samples are exposed to steam generation and
leakage current values were recorded. The peak
value of leakage current on the test samples is
continuously monitored using by the leakage
current measuring system and the value is
recorded for every 60 sec for a period of 100
minutes [10].
Table 5: Elemental Analysis of samples
Atomic
percent
age
Virgin
O
Al
Si
Na
Ca
Mg
28.4
35.2
15.6
19
0
0
1.65
Coal
36.8
35.5
10.2
1.04
0
0.9
Cement
34.6
32.2
6.09
15
.5
16
1.47
6.46
1.49
Marine
26.3
33.6
12.5
27
.6
0
0
0
6
C
Figure 5: Clean Fog Test setup
CLEAN FOG TEST
The test is performed using the standard IEC
60507 in an artificial test chamber with scale down
parameters. The field aged samples and virgin
sample were subjected to a combined stress of
voltage and clean fog. The leakage currents were
recorded throughout the tests. The recorded
leakage current predicts the level of contamination
severity and results are compared with virgin
sample [8].
The test samples were wetted by means of fog
generators which provide a uniform fog distribution
over the whole length and all around the test
chamber. A Plastic tent, surrounding the test object
was used to limit the volume of the test chamber.
The flow rate of the fog input to the chamber was
Figure 6: Variation of leakage current during
clean-fog test
6.2 .RESULTS:
The insulators exposed to three regions were
chosen to observe the different contaminated site
conditions along the 25 kV railway lines of south
central region, India.
service test samples. The authors also thank CPRI
management for permitting to publish this paper.
A. Region 1 (Coal Pollution): This region gives
the leakage current level of 6 years aged coal
contaminated sample.
REFERENCES
B. Region 2 (Cement Pollution): This region
gives the leakage current level of 4 years aged
cement contaminated sample.
C. Region 3 (Marine Pollution): This region gives
the leakage current level of 4 years aged marine
contaminated sample.
Figure 6 shows the variation of leakage current of
all samples during clean-fog test. The leakage
current of contaminated samples is compared with
virgin sample. It is observed that the insulator
exposed to marine pollution has more leakage
current
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CONCLUSIONS
From the investigation of insulator samples of 4-6
years field aged, it is observed that –
(a). Physical observation of the field aged samples
did not show any significant changes from the
virgin sample
(b). The results of hydrophobicity test and pollution
severity of the pollutants deposited on the surface
of field aged samples show that pollution
performance of composite insulators is still
adequate. Also, the results obtained demonstrate
that the hydrophobicity of composite insulator was
found to decrease with service which is a sign of
ageing.
(c).
The
leakage
currents
for
different
contaminated samples were observed by Clean
Fog Test and Marine contaminant has got more
leakage current compared with the other samples.
Further research is focused on marine
contamination & Leakage Current study.
(d). Through experimental analysis it was found
that marine pollution is more severe but through
material analysis it was observed that coal
pollution is more severe. It is also observed that
coal severity cannot be proved through electrical
analysis. Hence, more experimental investigation
is required.
ACKNOWLEDGEMENTS
The authors wish to thank management, staff &
technicians of CPRI UHVRL, Hyderabad for giving
permission to conduct the experimental work. The
authors thank the South Central Railway officers
and staff for their co-operation in getting the in
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rd
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