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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

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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

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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

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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.

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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.

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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

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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.

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Figure 2.2 Insulator Samples for Testing

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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,

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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

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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.

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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

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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.

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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

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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

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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

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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

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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

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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.

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(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

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(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

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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

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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

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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

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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

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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.

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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.