Conference Record of the 1990 IEEE International Symposium on Electrical Insulation, Toronto, Canada, June 3-6, 1990

INFLUENCE OF POLLUTION ON H.V. INSULATORS

by 0.E.Gouda , Cairo University ,Faculty of Engineering , Electrical Engineering Dept. , Cairo Egypt.

Abstract: fn this paper the factors affecting the pollution layer conductivity are reported and discussed. This paper deals also with tests to simulate the pollution conditions in desert and near cement factories. The tests were carried out using the cap-and- pin type of insulators which are extremely used in the over head networks in Egypt and neighbouring countries.

ID INTRODUCTION Plashover on high voltage insulators caused by the combined action of pollution and moisture has been known as early as the construction of outdoor transmission syst- ems. During dry weather , pollutants are deposited on insulators surfaces. When the the surfaces become moist, some pollutants are dissolved and the surfaces start to conduct electrically /1/ , /2/. In Egypt the insulators are exposed to series pollu- tion by dirt and chemicals in industrial areas and by salt deposits near the coast. They are exposed at the same time to lack of rain-falls and frequent dew early morning hours. In desert areas the pollution conditions are serious and diffe- rent from the conventional marine and indu- strial pollution / /. The performance of insulation of elecZrical installation depends mainly on the surface layer conduc- tivity of the pollution layer. Experience in operation of the electrical installati- ons has shown that the ability of insulat- ors to withstand over-voltage differ from one place to another according to the areas where they are constructed. Heavy pollution density and high humidity are the most sever conditions for flashover of polluted insulators.

The insulators are exposed to different types of natural pollution such as: a-desert pollution:-chemical analysis of the desert-contaminant showed that the soluble salt amount to 18°/u by weight, mostlx consistin ides CaSO 1 O 0 $ O , NaCl 3 O / O and KC1 0.5O7O / 3 ? .

b-Industrial pollution:-It was found that the contaminant layer formed on the insulators in industrial areas contents chemical compounds of smokes , fumes of the factories ana cement dust / 4/.

c-Marine pollution:- The wind coming from the sea carries salt particles are preci- pitated on insulator surfaces. The layer oy this contaminant way depends upon the distance between the sea shore and the insulators location and the speed of wind

in the

of sulphates and chlor-

The process of wetting of the polluted

surfaces of transmission line insulators has a great effect on the conductivities and flashover voltages of the transmission line insuators. In &apt sometimes a brief rain shower falls. Otherwise with sharp drop of temperathe during the night,dew may form. Also wken the relative humidity approacnes 1000/0.

The tests were carried*out by using two testing transformers. They were available to be fed from the same control circuit,one of the transformers is rated at 40 kV and the equivalent R/X = 0.15 and its short circuit current = 7 Amp. just complying with the IEC/ 5 / . The other transformer is rat- ed at 250 kV, its R/X =0.2 and its short circuit current is 1.5 Amp. . Hollow alumin- um conductors of 1.5 cm diameter were used to connect the elements of the circuit in the high voltage side for reducing the corona effects. The input voltage was cont- rolled by means of a 28 kVA induction voltage could be varied from 0 to 400 volt.

A fog chamber complying with the IEC was filled with dense fog. Relative humidity 100°/o was used in testing the insulators according to IEC. The vapour density is the saturation density at both points.

regulator whose out put voltage

According to the IEC specification / 5 /,the composition of the polluting suspension is 1000 gm Kieselguhr , 10 gm highly dispersed silicon dioxide for each 1000 ml water. To this is added a suitable amount of salt N a C l for control of the resulting pollution layer conductivity. As alternatives to the import- ed Kieselguhr cement , gypsum and lime were tried as iocal materials for holding the moisture in the pollution layer. From the author's experience / 6 /, / 7 powder proved to be the most appropriate to simulate Egyptian pollution. Chemical analy- ses of samples of natural contaminants on local insulators showed contents of Calcium oxide of up to 4Oo/O / 8 /, / 9 / e

The method recommended by the IEC /5 / in forming the contamination layer was employed i.e ; the solid layer method . For measuring the pollution layer conductivity 1 kV,A.C., 50 Hz was applied to the insulator under test. The pollution layer was applied to the insulator by dipping it into the suspension. The insulators were properly cleaned and then dipped in the suspension several times

/ lime

CH2727-6/90/0000-195 $1.00 Q 1990 IEEE

separated by 24 hours at leas&. They were left to dry suspended in the room temperat- ure. The layer conductivity in each test was calculated according to the relation

K= IF mFF / 5 /.

Where K is the surface layer conductivity

and FF is the form factor

I is the maximum leakage c m e n t J" is the applied voltage

111. EXPERLUNTAL WORK AND ANALYSIS OF THE RMULTS

A-FACTORS APFECTING THE CONDUCTIVITY OF THE P O m O N LAY ER :

Effect Of Contaminant Material On The Pollution Layer Conductivity: The conductivity of the pollution layer and inturn the flashover voltage depends on the pollution layer type. In this part of the paper the industrial pollution near cement factories and the desert pollution are simulated. The pollution layer conduc- tivity of each layer is measured.

iSimulation of industrial pollution:- Portland cement which is produced by the factories is different from the cement dust can escape through the main chimney . F o r this reason the tests by previous investig- ators using portland cement in forming the pollution layer are not sufficient to simu- late the polluted insulators of H.V,T.L near cement factories ho /, / 11 /. The chemical analysis of cement dust that is collected from industrial area located south of Cairo is given in table I . This cement dust was used to prepare a solution used in polluting the insulators under tests. The pollution layer was applied to insulator by dipping it into the suspension< The surface layer conductivity was measured using the IEC method . The pollution surfa- ce density of th tested insulators varied from .0625 gm/c$ up to .31 @7n/cm2.Figurel gives the relation between the surface density of the pollution layer and the surface layer conductivitya As it was expe- cted increasing the pollution layer density increases the surface layer conductivity. The reason is that, increasing the cement dust density means increasing the major carriers of salts given in table I. The ionic mobility and degree of dissociation increase with increasing the amount of water that could be absorbed by polluted layer on the insulator surface under test,

iiSimulation of desert pollution:- Natural sand with grain size less than 66

m contains sulphates and chlorides :CaSO4 10 O / O - N a C l 3 O / O and KC1 0.5 O / O / 3 /is -- co1l'ectZ.b located in the desert. The sand was unifor- mly distributed on the unit under test by mechanical ways. The d nsity o f sand was varied from .03 gm/cz up to .31 gm/cm2 The conductivity was measured by using IEC

from the surfaces of insulators

method. The relation between the surface layer conductivity and the pollution layer density is given in figure1 From this figure it is clear that the conductivity in case of desert pollution simulation is higher than the conductivity of insulators polluted by cement dust . The reason i s m y be due to the foundation of NaCl in the d-esert pollution and its ability to absorb the water vapour.

Table I Chemical Analysis Of The Cement Dust: 7

ement I Percentage Chemical &l

9-29 - 10.35 2.66 - 2.79 1.62 - 1.44 18.80 - 30.38 1.49 - 2.17 12.96 - 7.24 6.12 - 4.68 2.13 - 1.87 5.31 - 6.58

.on

I1

I

c7 $1 L 0.1 0.2 0 .3 0.4

2 gnJ cm Figure 1 Surface layer density versus surrace layer conductivity

Effect Of Wetting Method Of Poilu@ Tnsulator On The Polluted Layer

%%%%%%fen.t methods are used in this paper to wet the insulators to simulate. the different weather conductions: In method A , the insulators were wetted b$ fog only according to i E C . In method B the insulators were wetted by dew only. To generate dew in the Laboratory the insula- tors were left In a refrigerator at about 4 OC for 24 hours, then they were hanged inside the fog chamber of temperature about 25 OC . The relative humidity inside the fog chamber was controlled to be 60-80 O / O during the conductivity measurements. Finally in the last method , method C, the insulators were wetted by simultaneous fog and dew, In thi8 method the insulators were left in a refrigerator at 4 O C for 24 hours then, the insulators were hanged inside the f o g chamber with vapour in the saturation

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density at both points in fog chafnber. In the last method the amount of water which was absorbed by the layer of the polluted

insulator under test is more than that in both methods A and B , for this reason the conductivity of the polluted layer is higher in case C, see figure 2 . This is because increasing of water absorbation increases the solvability and ionizability by several times. For the same reason the conductivity of the polluted layer wetted by method B is higher than wetted by method A.From figure 2 it is evidant that the relation between the surface layer conduct- ivity and salt concentration in the polluted solution for the tested range (0-100 gm/ liter) is approximately linear. This relat- ion can be written when wetting the tested insulators by methods A , B and C as follows : K = 0.3 S in case of wetting by method A K = 0.2 S in case of wetting by method E X = 0.45 S in case of wetting by method C,

Where K is the polLuted surface conductivi- ty and S is the salt concentration.

: A c1 4

rl +a

$50

2 a

us 4

L Salt Concentration

A-IEC test B-Tests to simulate dew

CzTeste to simulate dew

100 &liter

only

+ fog -

z - Actual Curve --- Approximated Curve v

h L

," c. 6C'

c, d

- Actual Curve --- Approximated Curve

L , 2 , 3 and 4 corresponding t0 the number of insulator dippings

Concentration of salt in the solution gm/liter

A-IEC fog teat BSimulating dew only CSimulating dew + fog Figure 2 lhlation Between Salt Concentration and Surface Layer Conduct i v i tg .

IEC'/5/. It is observed also that increasing is lesser as the number of dippings increa- ses inwhich the insulator layer reaches to saturation.

u

40 x 2 30 a 0

k 2 20

c:: 10

C

A B F

0 1 1 1 1 2

1 2 3 4 Number of Insulator dippings

Figure 3 Rblation Between Number of Inaulator diopings (Increasing the Pollution Layer Density) and Surface Layer Conductivity .

c. ~olutf8n ~oAfi8ctivit$~~ms/cm

Figure 4 Reference Layer CoDductivity Versus Solution Conductivity

Effect Of Pollution Layer Density On The Gurface Layer Conductivity: bcreasing the number of insulator dippin- gs resulting in increasing the surface layer conductivity for the same NaCl conce- ntration, see figure 3 . It is clear also that the rate of increasing in conductivity decreases as the number of dippings increa-. ses. This may be because the polluted layer reaches to the saturation after dipping the insulator three times. This phenmenon is clear in method of wetting B more than in

amount of water that absorbed by the pollu- ted layer of the insulator under test.From 4 it is observed that the relation between the reference layer conductivity inpS versus the solution conductivity in m s / c m is linear and this is in agreement with the

both methods A and C due to variation in the B-HIGH VOLTAGE TESTS AND DISCUSSION

Test Procedure :- The investigated insulators were covered with a homogenous layer. The degree of pollution was evaluated by the weight of

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cement aust per unit area of the insulator surface. The pollution surface density of the polluted insulators under test varied from 0.062 &cd up to 0.31 gm/c&. The insulators were wetted by the three differ- ent methods of wttings mentioned before to simulate IEC fog test, dew formation and simultaneous dew and fog formation, The applied voltage was 50°/0 ,of the expected value, then it is increased until flashover occured. Table I1 gives the average value of the flashover voltage of more than 10 tests in case o f the three different test procedures.

Table I1 :The Average Value Of The Flashov- er Voltage:

Type of test Surface layer density gm/cm2 0.0625 0.18 0 . 3 1

Simulating dew 22 17 8

Fog + dew 14 7 5

Analysis Of Leakage Current:- To obtain enough data about the performa- nce of the polluted insulators near cement factories, the leakage current was observed by a digital instrument, under different test procedures of wetting the insulators. Test procedure to simulate dew only, to simulate IEC fog test and to simulate dew+ fog. The applied voltage was percentage of flashover voltage.Figure 5 gives the number of leakage current pulses per minute with amplitudes exceeding 100 , 70 , 40 and 10 mA under each of the given voltages. The cement dust density was 0.31 gm/cmz.The results given in this figure simulate the condition of fog + dew, From these results it is clear that the number of pulses of leakage current3100 mA ,270 mA and740 mB increase with increasing the percentage of flashover voltage. But it is not the case when the leakage current pulses ?/ 10 mA. This is because the ioule heating given from the leakage current pulses with ampli- tudes exceeding 40 mA in case of high volt- ages dries the insulators surface. This is followed by few number of pulses which were less amplitudes and visa versa, when the applied voltage is low the number of leakage current pulses with amplitudes exc- eeding 40 mA are small , and the surface o f the polluted insulators still wet, more joule heating is required to be dried and more number of pulses with low amplitudes should produce.

IV -CONCLUSIONS

Prom the work carried out in this paper it is concluded that : 1- The pollution layer conductivity depends

on the contamination material , wetting method and the pollution layer density.

2-A procedure is suggested to simulate the conditions of the polluted high voltage insulators near cement dust factories.

. a c1 5 f .5 B s o t t 6 5 1

3 a 301

I , Percentage of

2c LO 60 80 100 the -flashover Figure 5 Number of leakagev8?&mt pulses versus percentage o f the flashover voltage. ( 5 kV ).

These tests were carried out on polluted insulators by cement dust collected from that which escaped from the chiimney of the factories.

V-REFERENCSS

/l/M.Kawai and D.ivl.Milon Tests on salt contaminated insulators in artificial and natural wet conditions"1EEE , 1969.

/2/G.Revery and E.Stoltetf Tests on insulat- ors under natural conditions of contami- nation",Cigre ,210 , 1960. A s s ad, id0 hamed Y. Awad" Compare t ive t es t ed in contaminatedinsulators with reference to desert condition". Electricity corpo- ration U.A.R. ,1972 session 28 hug/6 Sep

/4/lvl.lfi.Khalifa and A.El-Morshedy "A study of wet flashover characteristics of pol- luted insulators" Conf. on progress in cables and overhead lines for 220 kV and .above, IEE ,No. 176, 1979.

/5/IEC publication No.507 , 1975 Geneve IIArtificial pollution tests on high voltage insulators to be used in A.C. systems.

/6/H.,d.El-Ghazalyt~A suggested artificial pollution test for H.V. Insulator in Egyptian desert environment"M.Sc .Thesis Cairo University.

/7/Zeen-El-Abdeen1!A study of leakage curre- nt on H.V. polluted insulators" M.Sc. thesis , Cairo university, 1987-

/8/Int- Report "Pollution flashover of high voltage insulatorsn FRCU 81014 ,Report No. 1 , Cairo University.

/9/Int. Report "Pollution flashover of high voltage insulatorsfq FRCU 81014 ,Report No. 3 , Cairo University.

/10/N0R.Nssseem"Tests to simulate the natur- al pollution conditions of H.V. outdoor inSU1atOrSn M.Sc. Thesis Cairo Universi- ty , 1988.

/11/J.J and others "Field and laboratory tests of contaminated insulators for for the design of state electricity commission of victoria's 500 kV system" IEEE ,Trans. PAS , vol. 87-1986.

/3/F.A-lfi*Rizk YAhmed A. gl-sarky, &tared A .

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