IEC 507-2 article.pdf

4.111.S25

INVESTIGATIONS ON RAIN FOG POLLUTION TEST

D.Devendranath Pradeep M.Nirgude G.V.Rao RSShivakumara Aradhya

A.P. Sudheer* N.S.Mohan Rao

Channakeshava

UHV Research Laboratory

Central Power Research Institute

P.B.No.9, Uppal

Hyderabad, India - 500 039

*Dept. of Electrical Engg.

JNTU, Kakinada

Andhra Pradesh

India

Abstract

flashovers are more predominant since the flashover

occurs at the operating voltages or even at lower

values of system voltages. Thus, overcoming an

outage caused by pollution has become a crucial

thing in the power system. There are various factors

which signify the importance of the pollution

problem. First of all, the fast industrialization and

secondly the increase in the number of the thermal

power stations to meet the load requirement. The

industrial users are now demanding a higher quality

of electrical power supply. Besides, the pollution

phenomena is more severe with the increase in the

overhead transmission system voltages.

In the solid layer test method it is mentioned

that a warm or cold water may be used, instead of

steam for wetting the pollution layer. So an attempt

is made to perform the solid layer test with the

wetting action obtained from tap water, called the

Rain Fog test. An important aspect of this method is

that the existing spraying system of salt fog method

is used for the development. This will simulate the

actual conditions of the pollution layer wetted with

slight rain or drizzle or mist.

In the Rain Fog method, the insulator is

coated with a pollution whose layer conductivity

simulates the site condition and tap water fog is

applied to the insulator to simulate the actual rain,

dew or drizzle. The withstand capability of the

insulator string is tested as specified in IEC -

507,199 1. To standardize the Rain Fog test facility,

two tests are conducted viz. wetting action test (layer

conductance test) and reference test on nine disc

insulator string. These two tests are identical to the

tests necessary to standardize the solid layer test

facility as specified in IEC - 507. Rain fog test is a

combination of salt fog test as well as solid layer test

facility .

In the standards it is mentioned that warm

water or cold water may be used instead of steam for

wetting the pollution layer. So an attempt is made to

perform the solid layer test with wetting action

obtained from tap water, called the Rain Fog test.

This will simulate the actual conditions of the

pollution layer wetted with slight rain or drizzle or

mist. The power consumed in doing the test is less

when compared with the solid layer test as the time

taken for the saturation of wetting action curve is

less. As per IEC 507, for conducting solid layer test,

ambient temperature in the chamber at the start of the

test should not be less than 5

and test object should be in thermal equilibrium with

the ambient. But in tropical countries like India the

ambient temperature is more than 30 ' in sunmer. In

this case rain fog test is more helpful.

In the Rain Fog method two types of tests

are conducted :

nor grater than 30

1.0 Introduction

The increase in demand & losses lead power

system engineers to go for higher system voltages.

The increase in the system voltages have created a

number of problems such as the overvoltages

generated in the system due to nature, switching,

system faults, etc. These over voltages and faults

cause an outage in the part of the system or in the

whole system. So the overhead transmission lines are

designed based on the lightning overvoltages,

switching over voltages. However, pollution caused

High Voltage Engineering Symposium, 22-27 August 1999

Conference Publication No. 467, 0 IEE, 1999

1. Layer Conductance test ( wetting rate test)

2. Reference test

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4.112.S25

1.1 Layer Conductance Test

2.1 Layer Conductance Test:

In this method, the cleaned test specimen is

coated with the pollutant of required salinity by

dipping in the slurry and Rain fog is sprayed on

specimen. The voltage applied depends on the

creepage length of the total string. IEC-507 standard

specifies a voltage of > 700 Vrms per meter of the

overall creepage distance and measuring the current

flowing through the wet layer. The layer

conductance measurement is repeated on the

insulator during its wetting, with the aim of

detemiining the maximum value reached. The layer

conductivity is calculated by multiplying the layer

conductance measured on the unenergised insulator

by form factor. A graph is drawn between the time

and per unit leakage current. The curve is compared

with the standard curve given in IEC-507 standard.

The test duration is 90 min to 120 min. The test is

repeated several times for checking repeatability of

the values.

Two IEEE standard disc insulators are used

and are coated with slurry coating. The solution is

prepared with 40 g Kaolin + 1000 ml demineralised

water + sufficient amount of laboratory grade salt

(Nacl) to get a volume conductivity of 28 mS/cm i.e

2.8 S/m or an equivalent layer conductance of 8 US

or an SDD of 0.07 mg/cm2 . The flow of water is

adjusted to 100 ml/min from each nozzle. The

pressure of the compressed air is adjusted to 7

kglcn? . The connection diagram is shown in Figs.

1,2,3. The number of nozzles opened are 7. The

maximum length of nozzle column is 3.6 ni.

The string is placed in the top position as

shown in Fig 1. A supply voltage of 500 V is applied

at every five minutes and leakage current is

measured. Thus the test is conducted upto 90 min. A

graph is drawn between the time in minutes and per

unit leakage current. The graph is compared with the

standard graph in IEC 507.The test is repeated twice

for checking the repeatability.

1.2 Reference Test .

The cleaned specimen is coated with pollution

coating depending on the required salinity, by

dipping. Rain fog is sprayed on to the specimen. The

voltage applied depends on the profile,

contamination degree and overall creepage as

specified in IEC-507 standard. The test duration is

one hour. It is conducted for three times, out of

which three times the specimen -must withstand. If

one of them fails the test is conducted for fourth

time. If it passes, the specimen is deemed to have

passed the test otherwise, failed. The object of this

test is to confirm the specified withstand degree of

pollution at specified voltage.

2.0 Test Procedures 121

The test specimen is cleaned and dried. The

slurry coating is applied and dried. The number of

nozzles to be opened are chosen such that the wetting

action is uniform The length of nozzle columns is

chosen depending on .the length of the string of

insulators. Two standard disc insulators are taken for

the test. These insulators are placed at different

heights with respect to the lowest nozzle. of the

nozzle column. This setup is used to conduct the

layer conductance test. The test is performed for a

-132 kV system ( 9 disc Insulators ). For the reference

test, nine disc insulators are used, coated with the

suspension and number of nozzles opened were same

as in the case of layer conductance test. Alternate

nozzles are opened on both sides and also flowmeters

are adjusted to control the wetting rate.

Now the string is put in the middle position

( Fig.2 ) and the test is repeated.

Same procedure is repeated for the bottom position

also. (Fig.3 ).

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4.1 13.S25

3 times,out of which 3 must withstand . If the test

specimen passes two times and fails once, the test is

conducted for the 4" time. If it withstands it has

passed the test, otherwise it fails to withstand the

particular severity.

2.2.2 Precautions

2.1.1 Precautions

a) The slurry coating should be uniform on

the top and bottom surfaces of the insulator.

b). The insulator string is erected after

ensuring the flow of solution ,air pressure etc. are as

per standards.

c). The minimum clearances between any

part of the insulator and any earthed object other than

the structure which supports the insulator and the

columns of the nozzles, when used shall be not less

than 0.5 m / 100 kV of the test voltage and in any

case not less than 1.5 m.

Before conducting the next test, the

previously conducted test specimen must be earthed

before handling.

e). The supply must be adjusted continuously

up to the specified value to compensate for the

decrease in voltage due to scintillations.

9. The supply must be readily switched off

when there is a pollution flashover during the test to

protect the equipment. Overload tripping device can

be used for the protection of the equipment.

a. The slurry coating is such that it is more

uniform (as specified in IEC 507 standard) on the top

and bottom surfaces of the insulators.

b. The metal-parts i.e cap and pin a:: 1w-y

clean so that the contacts have very less resistance.

c.The discharge from all the nozzles must

be uniform.

d).

2.2 Reference Test

For the reference test, 9 IEEE standard disc

insulators ( shown in Fig.4 ) are used.

3.0 Results

Layer Conductance Test:

.. .

The insulators are coated-with the specified volume

conductivity i.e. 28 mS/cm. The total string

alongwith the dummy insulators, is hung in between

the nozzle columns. Number of nozzles, pressure of

compressed air, water conductivity etc. are same as

used in the layer conductance test. A voltage of 76

kV is applied , as per standard IEC 507, for IEEE

standard disc with an SDD of 0.07 mg/cm2 .

The wetting action of the Rain Fog test was

conducted on 2 nos. of IEEE standard disc insulators.

To startwith, the compressed air pressure is

maintained at 7 kg/cm2 and the discharge rate of tap

water varied and 3.6 m length of nozzle column

suitable for conducting pollution test on 220 kV

insulator was considered. It was found that for

discharge rates of 210 mumin, the wetting rate was

faster as compared to the standard wetting curve

given in IEC. Consequently, the discharge and

number of nozzles have been reduced. The result of

9 nos. of nozzles and discharge rate of 150 nillniin is

given . Here also the wetting rate is faster. The

discharge rate of tap water for nozzles is further

reduced.

It was found that, for an average water

discharge rate of 75 ml/min k 10 ml/min, the wetting

action of rain fog is comparable with that of standard

curve given in IEC. Number of tests have been

conducted to ensure the repeatability of the results.

It was found that the results are repeatable.

2.2.1 Test procedure

A test voltage of 76 kV is applied to the pin

of the lowest insulator string. The test arrangement is

shown in Fig 4. The voltage is applied for 1 hour to

check the withstand capability. This test is conducted

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4.114.S25

The position of the two disc insulators is

also varied so as to verify the effect of the position of

the insulator string . It was found that the discharge

rate of nozzles i.e 75 mVmin t 10 mVmin and using

the alternative nozzles. i.e nozzles seperated by 1.2

m each, the standard wetting action curve is obtained

at difrerent string positions. The comparison of the

results ( with the string at different positions ) with

the standard curve is shown in Figure 5.

Number of nozzles to be used will be alternate

unlike in salt fog method i.e nozzles spaced at 1.2

For the given discharge rate of 75 mYmin f 10

mVmin and position of the nozzles used

alternatively, the standard wetting action curve

could be obtained and the no. of tests conducted

to establish the same prove the repeatability.

The reference test conducted on 9 disc insulators

show that it withstood for a voltage of 76 kV for

a given SDD of 0.07 mg/cm2 as specified for

solid layer test.

The Rain fog test facility developed can at

present be used up a to system voltage of 132 kV.

The existing salt fog test nozzle column can be

used to conduct the test. In the case of the solid

layer test it is expensive to erect the tent, and

also for providing heaters to wet the pollution

layer. For higher system voltages the amount

required to be spent for steam fog test will be still

more. The test facility developed can be used to

conduct the rain fog test up to a system voltage of

220 kV.

Reference test:

The reference test was conducted on 9

standard disc insulators to validate the test procedure

developed. The insulator string withstood an SDD of

0.07 mgkm at a test voltage of 76 kV and at a

creepage distance of 20 dV

PER UNIT LAYER CONDUCTANCE

1.2

VS TIME

-4- TOP

+ Middle

+ Bottom

-8- standard I

Acknowledgements

The authors would like to thank the CPRI

authorities for permitting to publish this paper.

We also acknowledge the contributions of Mr.

A.Yellaiah, Miss G. Girija, A.Ramulu and Mr. N.

S. Parthasarathy in the form of discussion at

various stages of project.

-0.2

Test Time (min) 3

Fig.5 The comparison of the results with the

standard wetting action curve .

References

4.0 Conclusion: -

[ 11 IEC 507,1991. “ Artificial Pollution Test

on high voltage insulators to be used on AC

system”.

[2] A.P.Sudheer, “Development of Rain

Fog Test Facility “, M.Tech Thesis, JNTU,

Kakinada, India. 1997.

In order to obtain the correct wetting action

of the rain fog, the discharge rate of the tap water,

number of nozzles and their position is varied, the

following conclusions are drawn from the results of

the tests conducted on insulator to establish the a

Rain fog test facility:

1. The average water discharge rate of 75 mYmin f

10 mVmin is found to be appropriate to obtain the

proper wetting action.

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