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16
CHAPTER 2
LABORATORY POLLUTION TESTING OF POLYMERIC
INSULATORS
2.1 INTRODUCTION
The test on polymeric insulators can be classified as follows: for
specifications, for prediction of life expectancy of the insulator in service, to
determine the likely performance of the insulator under pollution conditions.
Among these tests the last one is the most important for reliable power
transmission. Normally the pollution test can be performed in two ways. One
is natural pollution and the other one is artificial pollution test. In natural
pollution test the insulator is energized at working voltage and exposed to
naturally heavily contaminated site, and its performance is monitored. This is
a useful and realistic method but has disadvantages such as, testing in high
severity of pollution sometimes gives false results, high cost and test takes
several months. An artificial pollution test has been developed by, simulating
one of the natural weather conditions causing pollution flashover. More than
ten different systems for artificial pollution testing are currently in practice
which are reported in Looms (1997). Artificial pollution tests are divided into
two types as clean-fog and salt-fog tests. Clean fog test reflects natural
conditions where pollution occurs through a combination of conductive (e.g.,
salts) and non conductive, airborne pollutants (e.g., soil, mineral etc.), mainly
in inland industrial regions. The salt-fog test simulates natural conditions
where pollutants are predominantly conductive (e.g sea salts), mainly in
17
coastal areas. In this chapter clean-fog pollution test procedures, artificial
pollution testing setup, LC current measurement system, and 11kV silicone
rubber insulator tests are discussed.
2.2 POLLUTION
Outdoor insulators are normally exposed to pollution from a variety
of sources. In some areas pollution has emerged to pose a serious threat to
power system insulation. The pollution may be classified as marine pollution
and inland pollution. Most of the insulator pollution near coastal areas is due
to airborne particles of sea salt. Small water droplets are absolved from the
tips of the ocean waves during rainy weather conditions. These small droplets
are driven away by winds. If the relative humidity is low, the water
evaporates completely leaving a small, more or less dry, crystalline, salt
particle. These salt particles are then deposited and trapped on the surface of
the insulator. Insulators in areas extremely close to the sea can be exposed to
direct salt water spray during the periods of the strong winds. The sun light
dries the surface and forms salt layer on the surface of the insulator. The
deposition of sea salt on the surface is the function of wind velocity and
distance.
The insulator in inland is mainly affected due to soil dust, fertilizer
deposits, industrial emissions, fly ash, bird waste, construction activities, etc.
wind drives these airborne particles onto the insulator surface. The wear of
the vehicles tires on the highways produces a slick, like carbon deposit on the
insulator surface. The powder used on the highway polishing during winter
plays an important role on the insulator surface pollution. The rate at which
the pollution deposits on the surface of the insulator depends on the shape of
the insulator, size and density of the particles and velocity of the airflow.
However, the part of the pollutant on insulator surface is naturally cleaned by
18
heavy wind and rain. The continuous deposition and cleaning produces a
seasonal variation of the pollution on the surface of the insulator. After long
time (months, years) the deposits are stabilized and a thin solid layer will
cover the insulator surface. The table 2.1 summarizes the typical sources of
insulator pollution.
Table 2.1 Typical Source of Insulator Pollution
Location Pollutant
Coastal areas and salt industries Salt
Cement industries, construction sites and rock
quarries
Cement
Fertilizer plants and frequent use of fertilizers
in cultivated fields
Fertilizers
Mining and mineral processing industries Metallic
Coal mining , coal handling plants/thermal
plants and coal burning/brick kilns areas
Coal
Wild fire, industrial burning and agriculture
burning
Fly-ash and smokes
The dry pollution will not be a problem on the insulator surface.
The wet pollution is creating a major problem on the insulator surface.
Moisture in the form of fog, mist, drizzle, light rain on the insulator surface
wet the pollution layer, dissolving the salts and any soluble electrolytes to
produce a thin conducting layer on the insulator surface. The mass of the
pollution on the insulator surface is normally non-conducting but moisture
will intermittently render it conductive. The conductivity of the resulting thin
conductive layer depends on the amount of moisture as well as the chemical
composition of the pollutant. The severity of the pollution is characterized by
the Equivalent Salt Deposit Density (ESDD). ESDD is measured by
19
periodically washing down the pollution from selected insulators with
distilled water. The conductivity of the washed water is measured and the
equivalent amount of salt is determined. The ESDD value is obtained by the
measured mg value of salt, dividing it by the washed area of the insulator
surface. The ESDD value is calculated using the formula derived by IEC
60507.
)20(120 bVs (2.1)
where 20s is the layer conductivity at temperature of 20oC(in S), V is the
measured volume conductivity at a solution temperature of oC (in S/m). b is
a factor depending on in the Table 2.2.
Table 2.2 Correction Factor b Value (IEC)
oC) b
5 0.0315
10 0.02815
20 0.02277
30 0.01905
The salinity, Sa in (kg/m3) of the solution at 20
oC is calculated by the equation
(2.2) and ESDD (mg/cm2) is given by the equation (2.3)
03.1
20 )7.5( SaS (2.2)
A
VSESDD a
(2.3)
where V is the volume of the washed water (in cm3), and A is the area of the
cleaned surface (in cm2). The typical range of values of inland pollution levels
provided by IEEE std 4-1995 is shown in Table 2.3.
20
Table 2.3 Typical Ranges of Inland Pollution Severity
Severity ESDD(mg/cm2)
Very light 0-0.03
Light 0.03-0.06
Moderate 0.06-0.1
Heavy >0.1
2.3 LEAKAGE CURRENT AND ARCING FORMATION
The voltage applied to a silicone rubber insulator and wetting of the
contamination layer starts the flow of leakage current. The sequences of
events leading to long arc may be described in the following steps:
i. Deposit of pollution on the insulator surface.
ii. The surface is moistened by fog or light misty rain etc, which
makes the layer conductive and leakage current starts flowing.
iii. Due to the leakage current flow the surface layer is heated and
causes an increase in the conductivity and the leakage current
magnitude.
iv. The heating results in local drying of the surface layer and
leads to the development of dry bands on the insulator surface.
The dry bands modify the voltage distribution along the
surface.
v. If the electric field strength in the dry band region exceeds the
withstanding value, localized partial arc is initiated on the
insulator surface and dry band will be spanned by discharge.
vi. The partial discharges increase with a number of streamer
discharges and glows across those dry bands with the highest
potential gradient. These discharges are also causing audible
noise.
21
vii. Finally the partial streamer discharges are connected in series
and form a long arc in on the insulator surface. This long arc is
the precursor of the flashover on the insulator.
2.4 EXPERIMENTAL TECHNIQUE
Silicone Rubber (SiR) polymeric insulators rated for 11kV were
collected from Goldstone company,(photograph is shown in Fgure 2.1), India
for the experiment. The insulator parameters are tabulated in Table2.4. The
insulator samples are shown in Figure 2.2 and their details are given in Table
2.5. The samples were prepared as per the IEC standard. An investigation of
the insulators that were in the field for a long time and exposed to various
pollutants indicated that the pollution layer was dispersed along the insulator
surface. In this work, the salt and cement were taken as pollutants. For the
slat, the pollution slurry to be applied on the insulator surface was obtained by
mixing 40g of kaolin in one liter of distilled water. Kaolin acts as the binder
in the slurry. For the cement, the pollution slurry is obtained by mixing
cement in one litre of distilled water.
Table2.4 Parameters of the Insulator Samples
Parameter Value
Creepage distance 330mm
Shed diameter 90mm
No. of sheds 3
Distance between two shed 48mm
Wet power frequency withstand voltage 35kV
Tensile load 70Nm
22
Figure 2.1 Photograph of 11 kV Insulator
Table 2.5 Insulator Samples Details
Insulator group No. of samples Degree of ageing
New and clean 1 -
New and artificially polluted
by salt
4 NaCl with koiline at
different concentration
New and artificially polluted
by industry pollutant
4 Cement at different
concentration
Initially the concentration of NaCl salt and cement was varied to 10g/m3,
30g/m3,50g/m
3 and 100g/m
3. Using a brush uniform coating of pollution was
applied on the insulator surface then the pollution severity index ESDD level
was calculated as per the IEC standard.
23
Figure 2.2 Insulator Samples for Testing
24
The procedure for calculating the ESDD is as follows: The deposits are
collected by a small brush from the insulator surface of 1cm x 1cm area and
mixed with one liter of distilled water to get a solution of the defined area.
The conductivity of each collected salt solution is measured using
conductivity meter and at the same time solution temperature is also recorded.
The surface conductivity, salinity and ESDD was calculated based the
equation (2.1),(2.2) and (2.3) respectively. The calculated values are shown in
the Table 2.6.
Table 2.6 Layer Conductivity and ESDD Values
NaCl quantity (g) 20V (s/m) 20s Sa(kg/m3) ESDD
10
6.5 2.2 13.5 0.06
30 9.2 3.1 59.05 0.08
50 11.6 4.7 74.87 0.12
100 22.3 9.2 146.67 0.25
2.5 EXPERIMENTAL SET-UP
The artificial pollution tests are conducted in a laboratory fog
chamber. The fog chamber is a (1.5 m x 1.5 m x 1.5 m) wood frame structure.
Fog is generated by ultrasonic nebulizer with different humidity levels. The
humidity is measured by wall-mounted Hygrotherm instrument. It has the
following features of Combined temperature and humidity measurement
instrument with integrated sensor, integrated data hold and switchable for
measurements in °C and °F. Voltage is supplied from a 10kVA,50 Hz,
230/100kV testing transformer. The test polymeric insulator samples are
suspended vertically from a wooden brace on the ceiling of the chamber,
25
about 2.5 ft from the chamber wall. Figure 2.3 shows a Photograph of the
experimental setup.
For protection of personnel, a contact switch with a relay is
provided which will de-energizes the transformer whenever the door to the
enclosure is opened. A red light lamp visible illuminates the room when the
transformer is energized and danger signs are posted on the wire mesh to
warn against unauthorized entry into the fog chamber.
Figure2.3 Photograph of Experimental Setup
Figure 2.4 Schematic Diagram of the Experimental Setup
26
2.5.1 Leakage Current Measurement System
The leakage current is monitored and measured by converting the
istor is fed in to the analog inputs of a high
sampling rate data acquisition system (DAQ)(National Instruments, 1.25
MSa/sec) by a coaxial cable. The analog signal is converted into digital signal
by ADC in DAQ with sampling frequency of 5kHz and stored in personal
computer (PC) through LabVIEW software for further processing. In order to
access the reliability of the DAQ system, a 1GSa/s Digital storage
oscilloscope (DSO) also provides visual display of the leakage current. The
DAQ was protected by back-to-back zener diode as a protection unit (PU).
Figure 2.4 shows a schematic diagram of the experimental setup
LabVIEWTM
is a powerful, simple, and flexible development
system that meets all the requirements for data acquisition, data analysis, cost
and time required for development. The system consists of a PC running on
Windows Xp, equipped with a USB port and LabViewTM
software. The
software automatically performs data acquisition and analysis and data is
stored in the hard drive. This ensures that the data could also be analyzed
further using software applications like MATLABTM
. The Technical
specifications for NI card are displayed in Table2.7 and photograph is shown
in Figure 2.5.
27
Table 2.7 Technical Specifications for NI Card
Parameter Specifications
Number of channels 32
Maximum sampling rate 1.25MS/s
ADC Resolution 16 bit
Timing resolution 50 ns
FIFO buffer size 4096 samples
Band width 1.5 MHz
Input voltage range + or -10V
Input coupling DC
Input impedance
Input bias current + or 100pA
Maximum inrush current 500 mA
Figure 2.5 Photograph of NI Card
28
2.5.2 Test Procedures
The insulators used for the pollution performance tests were
silicone rubber insulator. The artificial pollution tests are conducted as per
IEC 60507 clean fog test procedure. The tests are conducted in such a way as
to reproduce the natural conditions.
Clean-Fog Tests: Before commencing the tests, the insulator
surfaces were cleaned by washing with isopropyl alcohol and rinsing with
distilled water, in order to remove any trace of dirt and grease. To reproduce
saline pollution typical of coastal areas, a pollution layer consisting of NaCl
and 40g of kaolin mixed with 1 litre of deionized water was applied to the
surface of insulator. The concentration of NaCl salt was varied to give
Equivalent Salt Deposit Density (ESDD) in mg/cm2 to 0.03 (very lightly
polluted), 0.06 (lightly polluted),0.08 (moderately polluted),0.12 (highly polluted)
and 0.25 (very high pollution, which is not normally experienced in service).
Laboratory tests were carried out in the following test conditions:
i. Silicone rubber insulator at clean surface condition, with
different relative humidity levels.
ii. Silicone rubber insulator at a constant pollution level of 0.06
ESDD, with different relative humidity conditions.
iii. Silicone rubber insulator at different pollution levels varying
from 0.01 ESDD to 0.25 ESDD, at constant 100 % relative
humidity conditions.
The insulator samples were applied with AC voltage of 11kVrms
2.6 TEST RESULTS AND OBSERVATION
The measured waveform contains dynamic state of LC of the
insulator. So it is used to study the effect of LC on the insulator surface.
29
During each test, the LC wave form was recorded in the PC. The recorded
waveform was analyzed using MATLAB.
2.6.1 Test Results of Clean Insulator
Initially a clean insulator was tested inside the fog chamber with an
applied voltage of 11 kVrms. Figure 2.6 shows the typical waveform of LC
obtained for clean insulator at different Relative Humidity (RH) conditions.
There is no audible and visual discharges observed under this test condition.
Even at100% RH only negligible amount of the LC (LC < 0.5 RMS mA)
flows on the surface of the insulator.
Figure 2.6 Typical LC Waveforms of Silicone Rubber Insulator at
Clean and Dry Under Various RH Conditions
30
2.6.2 Test Results of Polluted Insulator
Outdoor insulators located in coastal areas are mostly affected by
flashover due to the deposition of NaCl salt particles. Insulators located in
cement industry are affected by the cement particles. Hence the polymeric
insulator performance is analyzed for both NaCl and cement pollutant. Figure
2.7 shows the typical LC waveform measured at 0.06 ESDD salt pollution
with different RH. During 60% to 70% RH the wave forms look like a clean
insulator waveform. The audible discharge emerged at
Figure 2.7 Typical LC Waveforms of Silicone Rubber Insulator at 0.06
ESDD Pollution Under Various RH Conditions
31
80% and 90% RH conditions. The LC magnitude increased from 0.7 mA to
2mA at the time when audible discharge appeared. The light visible discharge
is observed at 100% RH condition. The LC magnitude increases from 0.7 mA
to 5 mA at visible discharge period.
Figure 2.8 Typical LC Waveforms of Silicone Rubber Insulator at 0.08
ESDD Pollution Under Various RH Conditions
The Figure 2.8 shows the typical LC waveform measured at 0.08
ESDD salt pollution with different RH. The LC waveform at 60% RH
conditions is similar to clean and dry conditions. The audible discharge
32
emerged at 70% RH condition and LC magnitude increased to 2 mA. The
Light visible discharge is observed during 80% and 90% RH conditions and
the LC magnitude increased to 5mA. The medium discharge emerged at
100% RH condition and LC magnitude increased to 5 mA.
Figure 2.9 Typical LC Waveforms of Silicone Rubber Insulator at 0.12
ESDD Pollution Under Various RH Conditions
The Figure 2.9 shows the typical LC waveforms measured at 0.12
ESDD salt pollution with different RH. The LC waveform at 60% RH
conditions is similar to clean and dry conditions and magnitude increases
lightly. The audible discharge appeared at 70% and 80% RH conditions and
LC magnitude increased to 3 mA. A very light arc appeared but it was not
33
completely visible and LC magnitude increased to 8mA. A heavy discharge
emerged at 100% RH and LC magnitude increased to 8 mA during the
discharge period.
Figure 2.10 Typical LC Waveforms of Silicone Rubber Insulator at 0.25
ESDD Pollution Under Various RH Conditions
The Figure 2.10 shows a typical LC waveform measured at 0.25
ESDD salt pollution with diverse RH conditions. The LC waveform at 60%
RH, is very similar to clean and dry conditions but the magnitude has
increased significantly. Audible discharge appeared at 70% and LC
magnitude is increased to 3 mA. The short duration discharge appeared at
80% RH conditions. It is sustained for maximum three cycles and magnitude
increased to 5mA. A photograph of this discharge captured by high speed
34
camera, is shown in Figure 2.11 (a). A long duration discharge and short
duration arc emerged at 90% RH conditions. The long duration discharge is
sustained for maximum 5 cycles and short duration arc is sustained for
maximum 2 cycles. The LC magnitude during long duration discharge
increased to 3mA and during short duration increased to 5mA. A photograph
of long duration discharge and short duration arcing is shown in Figure 2.11
(b). The long duration arc appeared at 100% RH condition and photograph of
this arc is shown in Figure 2.11 (c). During the arcing period the waveform
looked like sinusoidal and magnitude is increased to 8mA. The arc is sustained
for maximum 4 cycles and only once this kind of arc emerged during the complete
testing of the insulator samples. This kind of arc is the precursor of the flashover
and the LC current waveform is the pre-flashover LC waveform.
(a) (b) (c)
Figure 2.11 Photograph of (A) Short Duration Discharge Observed at
0.08 ESDD Pollution at 90% RH (B) Long Duration
Discharge and Short Arc Observed at 0.12 ESDD Pollution
at 97% RH (C) Long Arc Observed at 0.25 ESDD Pollution
at 100% RH
The peak value trend of LC of insulator for NaCl and cement
pollutant is investigated further and the frequency components of different LC
patterns are extracted by different frequency methods and it is analyzed
elaborately in chapter 3.
35
(a) at clean and dry condition
(b) at 0.06 ESDD pollution level
(c) at 0.08 ESDD pollution level
(d) at 0.12 ESDD pollution level
(e) at 0.25 ESDD pollution level
Figure 2.12 Trend Followed by Leakage Current (Peak) of Insulator at
Different NaCl Pollution Levels
36
(a) at 10 g/m3 pollution level
(b) at 30 g/m3 pollution level
(c) at 50 g/m3 pollution level
(d) at 100 g/m3 pollution level
Figure 2.13 Trend Followed by Leakage Current (Peak) of Insulator at
Different Cement Pollution Levels
37
In this analysis, voltage is applied continuously to the insulator
specimen at a constant pollution level with an increasing RH of the fog
chamber. Figures 2.12 and 2.13 show the trends followed by the leakage
current peak value of silicone rubber with an increase in relative humidity of
the fog chamber at different pollution conditions of NaCl and cement
pollution respectively. During each test, the maximum value of the peak
current and RMS current was calculated from the measured LC waveform.
The comparison of maximum value of peak and RMS current with respect to
NaCl pollution level and relative humidity are shown in Figures 2.14 and 2.15.
The comparison of maximum peak and RMS currents with respect to cement
pollution level and relative humidity are shown in Figures 2.16 and 2.17.
Figure 2.14 Comparison of Maximum Peak Value of Leakage Current
at Constant NaCl Pollution With Respect to Relative
Humidity
38
Figure 2.15 Comparison of Maximum RMS Value of Leakage Current
at Constant NaCl Pollution With Respect to Relative
Humidity
Figure 2.16 Comparison of Maximum Peak Value of Leakage Current
at Constant Cement Pollution With Respect to Relative
Humidity
39
Figure 2.17 Comparison of Maximum RMS Value of Leakage Current
at Constant Cement Pollution With Respect to Relative
Humidity
Form these figures, it is noticed that,
i. The leakage current of clean and dry insulator maintained
almost constant with increase in relative humidity and the
maximum values of peak and RMS current are 0.75 mA and
0.58 mA respectively.
ii. The leakage current of insulator at 0.06 ESDD(light
pollution) pollution increased with an increase in relative
humidity. The short bursts of leakage current peak appeared
after 80% RH for NaCl pollutant, but there is no burst of
leakage current peak for cement pollution(10g/l) even at
very high relative humidity condition. The maximum values
of peak and RMS current are 4.2mA and 3.6 mA
respectively for NaCl pollutant at 100% RH. The maximum
40
values of peak and RMS currents are 0.79mA and 0.56 mA
respectively for cement pollutant at 100% RH.
iii. The leakage current of insulator at 0.08 ESDD(moderated
pollution) pollution increased with an increase in relative
humidity. The medium bursts of leakage current peak
appeared after 80% RH for NaCl pollutant, but there is no
burst of leakage current peak for cement pollution(30g/l)
even at very high relative humidity condition. The maximum
values of peak and RMS current are 6.8mA and 4.9 mA
respectively for NaCl pollution at 100% RH. The maximum
values of peak and RMS current are 0.83mA and 0.59 mA
respectively for cement pollution at 100% RH.
iv. The leakage current of insulator at 0.12 ESDD(heavy
pollution) pollution increased with increase in relative
humidity. The short bursts of leakage current peak appeared
during 70% to 80% RH, long burst of leakage current peak
emerged after during 80% to 100% for NaCl pollution. The
short bursts of leakage current peak appeared during 80% to
100% RH for cement pollution (50g/l). The maximum
values of peak and RMS current are 7.6mA and 5.4 mA
respectively for NaCl pollutant at 100% RH. The maximum
values of peak and RMS current are 3.3mA and 2.6 mA
respectively for cement pollution at 100% RH.
v. The leakage current of insulator at 0.25 ESDD(heavy
pollution) pollution rapidly increased with an increase in
relative humidity. The short bursts of leakage current peak
appeared during 70%-80% RH, medium burst of leakage
current peaks observed during 80-90% and long bursts of
41
leakage current peak emerged during 90% to 100% for
NaCl pollution. The short bursts of leakage current peaks
appeared during 80% to 100% RH condition for cement
pollution(100g/l). The maximum values of peak and RMS
currents are 10.06mA and 8.4 mA respectively for NaCl
pollutant at 100% RH condition. The maximum values of
peak and RMS currents are 8.3mA and 6.2 mA respectively
for cement pollution at 100% RH condition.
2.7 CONCLUSION
This chapter describes the Experiment technique, Experimental
setup, test procedure and test results of the polymeric insulator. The leakage
current wave form and trend of peak current on surface of the insulator are
measured during each test and recorded in PC. The maximum values of peak
and RMS currents of each test were calculated using MATLAB. From these
observations the following conclusions are made.
i. In clean and dry insulator surface, the leakage current flow
on the surface is almost negligible even at high relative
humidity, due to hydrophobic surface.
ii. There was no flashover even at very highly polluted
condition (0.25 ESDD which does not exist in practice).
iii. No rapid long arcs were observed and only once a long arc
was recorded at very high pollution during the entire test
period.
iv. The leakage currents of silicone rubber insulator did not
exceed 8mA and 10mA for NaCl and cement pollutants
respectively.
42
v. There is not much effect on the insulator surface, due to
cement pollution compared with NaCl pollution.
vi. Based on the trend of the peak values of leakage currents, it
is concluded that, there is more possibility of surface
degradation due to NaCl pollutant when compared to cement
pollution.