Effect Of pollution on flashover voltage of pin insulators

(HIGH VOLTAGE PROJECT)

Electrical Engg.(Final Year) Group no.- 6 Project Supervisor Group Members Mr. Zafar Ahmad Akram Najfi (13EEB507) Md. Sharique Ahmad (13EEB470) MU Anas ( 13EEB055)

CONTENTS Insulator and its Design Considerations Types of Insulators Causes of Insulation failure Flashover Mechanism Pollution Tests of Insulators Sources of Insulator Contaminants Methods to Measure Contamination Methods to Prevent Flashover Practical Work References

What is an Insulator ?Overhead line insulators are used to insulate line conductor from each other and the supporting structure electrically.

DESIGN CONSIDERATIONS:

High permittivity so that it can withstand high electrical stresses. High mechanical strength to bear the conductor load. High resistance to temperature changes. Insulator material should not be porous.

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Types Of InsulatorsPin Type Insulators:- It is earliest developed overhead insulator, but still popularly used in power network up to 33 KV system. It can be one part, two parts or three parts type.Suspension Type Insulators:- A numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator.

Fig 1. Pin Insulator and suspension insulator

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Strain Insulators:- When there is a corner or sharp curve, the line is subjected to a greater tension. In order to relieve the line of excessive tension, strain insulators are used. Shackle Insulators:-They are frequently used for low voltage distribution lines and can be used either in a horizontal position or in vertical positions. They can be directly fixed to the pole or to the cross arm.

Fig 2. Strain and shackle insulator 3

Causes Of Insulation Failure

The electrical breakdown of insulator can occur either by flashover or by puncture.

In flashover an arc occur between the line conductor and the earth and the discharge jump across the air gap following the shortest distance.

In case of flashover the insulator will continue to act in its proper capacity.

In case of puncture, discharge occurs from conductor to the pin through the body of the insulator.

In case of puncture, the insulator is permanently destroyed due to excessive heat.

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Contamination Flashover Contamination flashover occurs when insulators become coated with a

wet conducting film containing dissolved salts. The wetting conditions responsible for flashover are fog, dew and

drizzle which form a conductive region on the insulator surface. This results in a drop in surface resistivity, which enables current to

conduct through the insulators. The surface layer is heated and causes an increase in the conductivity

and leakage current. This heating results in a local drying of the surface layer and so called

dry bands occur.

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Partial arc occurs across the dry bands on such contaminated insulators.

The partial discharges increase. Finally the partial discharges are connected in series and a complete

flashover occurs unless corrective maintenance is done.

6Fig.3 Flashover of insulator

7Fig.4 Flashover of insulator

Pollution TestsNatural Pollution Test

This method developed in Britain in 1936 before the development of artificial-pollution test methods.

It consists of energizing the outdoor insulators in a polluted area at its normal service voltage for a long time and record its behaviour.

The limitations of this method are:- 1) It takes long time. 2) Insensitive method. 3) The results are comparative.

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Artificial Pollution Test

Salt Fog Method A simple and direct method for assessing the withstand characteristics

of insulators when surrounded by a saline atmosphere. A cleaned insulator is subjected to a salt water fog while it is energized

at constant voltage. The fog is produced by array of nozzles, on opposite sides of the

insulator. A plain salt solution is used for the test with values usually from 2.5 to

226 kg/m3. The highest salinity(in kg/m3) at which at least three 1-hour tests out of

four are passed is called the withstand salinity and is regarded as the criterion of performance.

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Advantages Of Salt Fog Method

The method itself is quite simple. Control of parameters like fog salinity, nozzle diameters, air pressure,

flow of solution, etc is very easy to achieve. The test results are repeatable and satisfactorily reproducible.

However, due to the poor representability of the salt-fog test, it is desirable to modify the standard Salt-Fog test or to define a new pollution for coastal conditions.

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Insulator ContaminantsContaminants Sources of contaminants

Salt 1. Coastal areas/ Salt Industries2. Highways with deposit of snow where salt is used to melt the snow.

Cement 1. Cement plant2. Construction sites3. Rocks quarries

Smog 1. Automobile emissions at highway crossing2. Diesel engine emissions at railway crossing/yards

Fertilizers 1. Fertilizers plants2. Frequent use of fertilizers in cultivated fields

Metallic 1. Mining handling processes2. Mineral handling processes

Coal 1. Coal mining2. Coal handling plants/ thermal plants3. Coal burning/brick kilns areas

Volcanic ash Volcanic activity areas

Chemical Wide variety of chemical processes/process industries, oil refineries 1

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Methods to Measure Contamination

There are two methods of measuring contamination:-

1). Equivalent salt deposit density Method The density of equivalent salt deposit density(ESDD) equals an amount of

NaCl which, dissolved in water, will change water’s conductivity to the level equal to the solution of polluted deposits gathered from insulator surface.

The severity of the pollution is characterized by the equivalent salt deposit density.

ESDD is measured by periodically washing down the pollution from selected insulators with distilled water and carefully collecting the water.

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The conductivity of the collected water is measured and equivalent amount of salts, which produces the same conductivity, is calculated.

The obtained mg value of salt is divided by the cleaned area of this insulator to obtain the ESDD value.

Where, σ: is the layer conductivity at lab temperature(in mS) V: volume for solution conductivity measurement at lab temperature (in ml) T: temperature of the insulator surface A: area of cleansed surface

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2). Directional dust deposit gauge method

This method was first chosen by research institute of electricity supply commission (Eskom) in 1974 to examine insulator pollution.

DDG includes four vertical split pipes and a pot below each pipe to gather pollution.

The pipes are placed along the four geographical directions, north, south, east and west.

DDG pollution index, is the average of four conductivities (µS/cm) obtained from the four direction for a thirty day month in 500 cc normalized washing water.

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Conductivity in north direction, Average conductivity = (+ where :are normalized conductivity in north, south, east and west C= conductivity (µS/cm)V= volume of distilled water (ml) N= no. of days when the insulator has been under investigation

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Fig 4. Directional dust deposit gauge

Methods to Prevent Flashover1).Periodic hand wiping Insulators can be prevented from flashover if the surface are kept clean

by hand wiping. This method is efficient but is also tedious, time consuming and

expensive process which requires equipment outage. Hand wiping is normally used on insulator where high pressure washing

is either impractical due to inaccessibility or ineffective due to hardness of surface deposit.

Material such as wet or paraffin-soaked cloth, brushes, or similar cleaning material may be needed for insulator with hard caked deposit.

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2). Periodic washing either energized or de-energized

De-energized cleaning of salty deposits is most effectively done by washing, but hard industrial deposits can be removed by caustic soda solution or grit blasting.

Grit blasting can be done by using the material such as crushed coconut oil shell, which will not damage the glaze but will remove deposit such as compacted cement dust.

To avoid frequent outages, live washing is often preferred to clean un-energized insulators.

Manual jet washing is done from portable installations and automatically spray washing from fixed nozzles around the insulators.

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3).Periodic coating, either energized or de-energized with grease compound

Coating the insulator surface with a water repellent material such as silicon grease which prevents formation of continuous water films.

Coating insulators with grease compounds has been used satisfactorily as a means of preventing contamination flashover for 20 years.

This method is expensive and requires periodic maintenance of removing and recoating, the frequency of such recoating, depending upon the degree of contamination and the weather conditions.

The grease can be applied to de-energized insulators by a number of methods — by hand using rubber gloves, pads or brushes, or by air spray equipment.

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Practical WorkIn order to investigate the influence of different contaminants and other factors on performance of pin insulators, artificial pollution testing will be carried out in steps as discussed below.STEPS:-1. We will choose 3 salts, NaCl, KCl, CaCl2 as contaminants for this

experiment.2. The surface of the specimen insulator is washed with detergent to

remove repellence.3. Make solutions of three different concentrations of each salt of

normality N, N/2, N/4.4. To see the variation of the flashover voltage with the normality of

contaminants, contamination will be carried out from one flange to the other flanges and flashover voltages will be noted accordingly.

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5. Temperature, pressure and humidity in the atmosphere at the time of testing is to be noted to take into account the influence of weather condition.

6. Now taking NaCl of normality N, we will contaminate the upper flange (1) by spraying the salt solution on it. The insulator is left in the sunlight to dry.

7. The flashover voltage (FOV) of the insulator is noted in this condition.8. After washing the insulator again, the middle flange (2) is contaminated

with the same salt and same concentration and FOV is noted.9. Repeat the same procedure for bottom flange (3).10. After getting these three FOV values, now contaminate flanges in pair

like upper-middle (1-2), middle-bottom (2-3) , and bottom-upper (3-1) and note down their flashover voltages respectively.

11. Now contaminate all the three flanges (1-2-3) simultaneously to get the 7th reading of NaCl of concentration N. The readings of NaCl of normality N is thus completed.

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12. Now we will take NaCl of N/2 concentration and repeat the steps numbered from 6-11 and note down the FOV values to get second set of readings.

13. Third set of readings can be found out by taking NaCl (N/4 normality) and following the same procedures as above.

14. After taking all these readings, the graph is to be plotted between FOV against varying normality for NaCl and seven different sets of flange contamination giving us 7 graphs for each salt.

15. Now, we will take KCl of concentrations N, N/2, N/4 one by one and follow, the same procedures as was for NaCl (step 6-14). The same will be done with CaCl2 .

16. The combined graph for the three salts of same concentration, say N, can be plotted by keeping flashover voltage on Y-axis and flange position on X-axis. Thus we will get three graphs for three different concentrations.

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REFERENCES B. Onyemaechi, M. J. Mbunwe, and M. Kingsley, "The Performance

Characteristics Of Low Voltage Insulator," IOSR Journal Of Electrical and Electronics Engineering, vol. 7, no. 4, pp. 43–47, Aug. 2013.

M. T. Gencoglu and M. Cebeci, "Investigation of pollution flashover on high voltage insulators using artificial neural network," Expert Systems with Applications: An International Journal, vol. 36, no. 4, pp. 7338–7345, May 2009.

A. A. Khan and E. Husain, "Porcelain Insulator Performance under Different Condition of Installation around Aligarh," International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering, vol. 7, no. 7, pp. 933–937, 2013.

A. Husain, Electrical Power System, 5th ed. CBS Publishers & Distributors, 2010.

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