Download Investigation on Surface Degradation of Modified Plane Tracking Method

Investigation on Surface Degradation of Modified
Polymeric Electrical Insulation Tested via Inclined
Plane Tracking Method
Amnart Suksri and Kittipong Tonmitr
Department of Electrical Engineering, Khon Kaen University, 40002 Thailand,
E-mail: [email protected]., [email protected]
ABSTRACT
This research work is carried out to investigate
the surface degradation of modified polymeric electrical
insulator when tested under inclined plane tracking
method. The test method is based on the IEC587 standard
for testing of insulation under severe ambient conditions.
Polymeric insulators with different compositions were
investigated for the ability to resist the surface tracking
phenomena. It is found that, the merit of time to initiate
the tracks on modified polymeric insulator surface
depends upon the type of fillers as well as the
composition ratio used.
-The flashover process on insulator surfaces
-Creating conduction path for leakage current
-Leading to spark over and complete breakdown
Normally, the conducting path may results form the
humidity in an atmosphere. In most cases, where
polymeric insulator that have been installed nearby the
sea or in the heavily polluted vicinity will be affected at
most.
Top electrode
Bottom electrode
Keywords: Surface Tracking, IEC587
1. INTRODUCTION
High voltage insulation in the last century has
been replaced with the new polymeric materials due to
their better dielectric properties, light weight, and cost
when compared to the porcelain and glass insulators.
Although porcelain and glass were the preferred materials
for insulators, their high surface energy rendered them
wettable when exposed to environmental pollution,
causing an increase in leakage current thereby leading to
a complete breakdown of the insulation system. Up to
present, there is an increasing interest in the improvement
of the high voltage insulation system in order to
withstand the high levels of voltages and in the same time
to minimize the lowest amount of leakage current. This
paper investigates the degradation of surfaces of modified
polymeric electrical insulator based on epoxy resin tested
according to the IEC587 standard[1].
2. SURFACE TRACKING OF AN INSULATOR
Surface tracking is a very peculiar phenomenon. It
occurs on the surface of an insulator because of the
creepage discharge activity resulting from surface
contamination. Several factors affect this activity such as
surface field intensity, surface current magnitude and the
surface roughness condition as well as levels of
contamination. Once tracking has occurred, it is an
irreversible process, leading to a complete breakdown on
solid insulator[3,5]. The process of tracking may lead to:
Tracking area
Fig.1 Surface Tracking of Insulator
Figure 1 shows the result of surface tracking
evidence from one of several samples. The degradation of
a polymeric insulator normally resulted from the
localized heating effect as well as the flashover/spark
over leading to the carbonization path on its surfaces.
Polymeric insulation is normally based on carbon
contents so that the carbonaceous dust has finally become
a good conductor, providing pathway for heavy current to
flow.
Once the track is initiated there will be a large
amount of current flowing through, at this very point, the
acceleration process will occur if the surface is partially
wetted or covered with contamination. The consequences
of tracking process may lead itself to the erosion
processes. Carbonization process alone may not lead to
the complete breakdown on insulator surfaces but with
the insulator having rough surface, the degradation will
rapidly continues and spread out even faster. Finally, the
degradation will be enhanced further by several factors
such as UV radiation, chemical reaction, and oxides
resulting from surface discharges.
3. TEST EQUIPMENTS AND METHODS
3.1 Experimental setup
All samples have been prepared by using polyester
resin[2]. All the test samples have dimensions of 120
mm*50 mm*9 mm. Mount the specimens, with the flat
test surface on the underside, at an angle 45o from the
horizontal as shown in Figure 2.
The details of components used in the experimental
setup as shown in figure 3 are:
extremely high value at this very instant and led to the
flashover on its surface.
The heating of large amount of current flowing
through the surfaces immediately destroys the insulator.
If time is permitted long enough, the polymeric insulator
will eventually lead to catastrophic failure.
1.AC supply 220 volts
2.Breaker switch
3.AC voltage variable transformer
4.HV Transformer 220/6,000 V 100mA
5.Protection resistor
6.Specimen insulator
7.Standard Resistor
8.Osciloscope HP 54520C 500MHz
9. DAQ Card & Computer PC
3.2 Test method and procedure[4]
Mount the specimens, with the flat test surface on the
underside, at an angle 45o from the horizontal as show in
Figure 2. Start introducing the contaminant (ammonium
chloride 0.1%) into the filter-paper pad allowing the
contaminant to wet the paper thoroughly. Adjust the
contaminant flow rate and calibrate to give a flow rate as
0.60 ml/min. Observe the flow for the least 10 min and
ensure that the contaminant flows steadily down the face
of the test specimen between the electrodes.
The contaminant shall flow to fill the edge of the top
electrode and not from either side or the top of the filter
paper. Switch on and raise the voltage to one of the
preferred test voltage at 4 kV. The leakage current was
measured through a standard resistor of 100 Ω and
collected by a computer via DAQ card. Meanwhile, a
stopwatch is to record the time to track so that there will
be a record of precise time of each different specimen.
4. TEST RESULTS
Fig.2 Test assembly, schematic and test specimen
Fig.3 Schematic of test circuit diagram
Table 1. Tracking time of each material for a track length
of 3/4-length specimen under power frequency test
No.
Si
Results obtained from the inclined plane test are
shown in the table 1. The measurement of the leakage
current waveforms and tracking time during the test has
also been recorded. An analysis was to determine the
amount of leakage current amplitude at fundamental
frequency (50 Hz). It is found that there were a lot of high
frequencies components riding on top of the power
frequency.
The high frequency effects may have developed the
rate of repetitive discharges on tracking activity. It is also
found that the surface of an insulator has heavily large
amount of deposited carbon dust.
During discharge
activity, the flow of contaminant (NH4Cl) on insulator
surface will dries out once the discharge has occurred. It
can be said that the surface resistance has reached to the
1
2
3
4
5
6
3
Time to
Specimen Compositions
track
C
CaCo3 Resin
(min)
2
12
0.56
2
2
12
2.03
3
7
3.38
2
8
7.56
4
25
3.15
7
0.45
From table 1, it can be observed that specimens with the
CaCo3 8:2 compositions exhibits the longest period of
time in the region of 7 minutes and over to start the
tracking activity under the power frequency surface
tracking test. The sampling leakage current was measured
through a standard resistor and the typical waveform is
shown in the figure 4 and figure 5.
modified polymeric samples. Each of them is based on
epoxy resin but differs in terms of fillers type. Also, each
of the 6 types sample were consisted of 3 additional
samples so that total of 18 samples in all. The weight of
all samples was measured and recorded before dip them
all into the distilled water for a period of one week. After
the dipping period, all samples were recovered and weigh
again. The changes in weight gain and weight loss is
shown in the table 2.
Peak Leakage Current Waveform of Carbon:Resin 2:12
200
150
I(t)Leakage Current (mA)
100
50
0
-50
-100
Table 2. The changes in weight gain and weight loss
-150
-200
0
10
20
30
Time (sec)
40
50
60
Fig.4 Leakage current waveform of Carbon:Resin 2:12
Type
1
2
Leakage Current Waveform of Carbon:CaCo3:Resin 2:2:12
200
3
150
4
I(t)Leakage Current (mA)
100
5
50
0
6
-50
-100
-150
-200
0
20
40
60
80
Time (sec)
100
120
140
Fig.5 Leakage current waveform of C: CaCo3:Resin
5. MODIFIED STANDARD TEST
The degradation on surfaces of modified polymeric
electrical insulation were further investigated by alter test
conditions and the test configuration of the test rig by
means of additional rectification action so that the power
deliver to the surfaces of material is changed to direct
current[6] instead of alternating type as tested earlier.
+
Tranformer
+
V
N1
N2
Sampling
Fig.6 Modified standard test with Diode Bridge rectifier
The high voltage diode used in this experiment is the
ESJC13 type with the rating of 12kV/350 mA capacity.
Due to the current limit of 100 mA, the testing voltage
was done at 6 kV DC fixed so that the series resistor was
selected with the value of 59.9 kΩ to protect the power
transformer. For this experiment, there are 6 types of
Compositions
(by weight 100
%)
Resin + Caso4
8:2
Resin + Kaolin
8:2
Resin + ZnO
8:2
Resin + Caco3
8:2
Resin + Caso4
8:1
Resin + Kaolin
8:1
Weight
before
(g)
39.25
Weight
after
(g)
39.50
% of
Change
+ 0.6369
41.75
41.25
- 1.1976
39.25
40.00
+ 1.9108
41.25
41.00
- 0.6060
40.00
40.25
+ 0.6250
40.75
40.00
- 1.8404
From the table 2, it can be observed that some of the
sample has loss its weight and some of them have gained
the weight. This is due to the water ingress, which has
entered into the specimen body so that the weight has
increased. On the other hand, some of the sample has loss
its weight because of the filler compositions used was
soluble in water such as Kaolin and CaCo3. After the
determination of weight from dipping into the water, the
entire sample was left to dry under room temperature and
put into the surface tracking rig. The time to track was
recorded and shown in the table 3.
Table 3. Time to track from modified DC test
Type
1
2
3
4
5
6
Compositions
(by weight 100 %)
Resin+ Calcium Sulphate (Caso4)
Resin+ Kaolin
Resin+ Zinc Oxide
Resin+ Calcium Carbonate(Caco3)
Resin+ Calcium Sulphate (Caso4)
Resin+ Kaolin
Time
(s)
8:2
8:2
8:2
8:2
8:1
8:1
5
1
23
9
4
1
Based on the data obtained from table 3, it can be seen
that the duration of time to track at ¾ length of the test
sample before the track will completely bridge both
electrodes is tremendous quicker than with the AC case.
However, there is an outstanding polymeric insulation,
which has filled with Zinc Oxide exhibits the resistant to
track although in the region of seconds but still shows a
better ability to withstand the DC testing voltage.
6.CONCLUSION
The degradation of polymeric insulator performance
has been investigated. The fillers used in this experiment
were chosen to improve the insulation properties so that it
can resist the tracking activity on its surfaces. However,
the addition of filler may results to non-linear electrical
properties and may have benefited only mechanical
improvement. However, a non-linear insulator could
enhance the tracking behaviour by reducing the resistance
of the dry band. When all of the applied voltage appears
over the dry band, the insulator will function in its high
electrical stress region and hence show a slightly
conducting behaviour, reducing the resistance of the dry
band and consequently reducing the voltage over the dry
band. The resulting discharges will be smaller in
magnitude and have less damaging effects on the insulator
surface. An insulator that has been filled with CaCo3 8:2
proportions exhibits the ability to resist the build up of
electrical track from an AC tracking test. Hence, the time
to initiate the track has been extended in the region of 7
minutes and over.
Also, it can be observed from the AC experiment that
the high frequency components is directly superimposed
upon leakage current regardless of any type of filler that
have been added to the resin based materials. These high
frequency components may have additional heating
effects on the surface so that the processes of erosion on
surface of insulation can starts abruptly.
On the other hand, the modified tracking test was
enabling us to identify the ability of individual insulations
to resist surface tracking activity. However, the process of
surface tracking under DC condition occurs very quickly
and resulted to catastrophic failure of an insulator. This
shows the severity of the DC voltage on polymeric
insulators under polluted condition. Therefore, it can be
concluded that the modified polymeric insulation, which
has included the filler compound, has a benefit of an
improvement on electrical property by which prolonging
the time to track and better mechanical property as an
additional benefit.
7. ACKONWLEDGMENT
The authors wish to express thanks to the HighVoltage Research Laboratory, department of Electrical
Engineering, Khon Kaen University, Thailand also to Mr.
Kongjak Boontan and others for experimental supports in
this research.
8. REFERENCES
[1] BS 5604:1986. IEC587: 1984, British Standard Test
Method For Insulator Tested Under Severe ambient
Condition.
[2] W.Tillar Shugg, “Handbook of Electrical and
Electronic Insulating Materials, Second Edition, The
institute of Electrical and Electronics Engineering,
Inc.”, New York, 1995
[3] M. Uğur, B. R. Varlow, “Analysing and Modelling the
2D surface tracking patterns of polymeric insulation
materials”, IEEE Transactions on Dielectrics and
Electrical Insulation, Vol. 5, No 6, pp 824-828, 1998.
[4] ASTM D2303, “Standard test method for liquid
contaminant, inclined plane tracking and Erosion of
Insulating Materials, Annual book of ASTM
standards”, Vol. 10.01, pp. 504-513, 1999.
[5] M. Kurtz, “Comparison of Tracking Test Methods”,
IEEE, Electrical Insulation, Vol. 6, No. 2, pp.76-81,
1971.
[6] R Sarathi and Uma Maheswar Rao, “Investigation of
surface modifications in ethylene propylene diene
monomer rubber due to tracking under a.c. and d.c.
voltages”, Bull. Mater. Sci., India, Vol. 24, No. 6,
December 2001, pp. 587-593