Behaviour of ZnO surge arrester in pollution

BEHAVIOUR OF ZINC OXIDE SURGE ARRESTER

UNDER POLLUTION

DEPARTMENT OF ELECTRICAL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY

BANARAS HINDU UNIVERSITY

VARANASI-221005 INDIA

submitted by:Mukesh Kumar Tanwar (11104EN067)

Akhilesh Kumar Singh (11104EN064)

CONTENTS

1. Introduction

2. Test arrangement and measuring equipment

3. Pollution and dry band formation

4. Results of pollution test with standard test procedure

5. Conclusion

6. References

Introduction

Here we presents results of pollution tests with A.C. voltages on a

zinc oxide arrester. Zinc oxide surge arresters are exposed to heavy

thermal and electrical stresses if the porcelain housings of the

arrester units are polluted together with other environmental stresses

such as temporary over voltages or high temperatures this can lead

to thermal runaway of the metal oxide arrester (MOA).

What is surge arrester ?

A surge arrester is a device to protect electrical equipment from over-voltage

transients caused by external (lightning) or internal (switching) events. This class

of device is used to protect equipment in power transmission and distribution

systems.

What is varistor ?

A varistor is an electronic component with a nonlinear current–voltage

characteristic, and is therefore also known as a voltage-dependent resistor

(VDR). At low voltage it has a high electrical resistance which decreases as the

voltage is raised.

Test arrangement and measuring equipment

The varistor column is mounted

inside of the porcelain housing

and is fixed to the housing using

different constructions depending

on the manufacturer. In most

industrial designs there is an air

insulation between the porcelain

housing and the arrester column.

Pollution and dry band formation

To investigate the influence of dry band formation on the

stresses of the zinc oxide material a special test has been

carried out with a single arrester unit. First the arrester housing

was polluted and dried. Afterwards at a certain location on the

porcelain housing a dry zone of about 10 % of the total creep

age length was formed artificially which was just large enough

to prevent arcing across the dry zone.

Impulsive Currents Caused by Internal Discharges

In case of only one dry zone at the bottom flange of the arrester

housing there is a high radial electric field strength between the

pollution layer and the varistor elements. This electric field is

caused by the full applied test voltage only reduced by a small

voltage drop along the outside pollution layer and the voltage

drop along a few bottom varistor element.

Results of pollution test with standard test procedure

The thermal preformation of a ZnO surge arrester in polluted

condition studied in:

1. Laboratory Test

a. Salt Fog Method

b. Solid Layer Method

c. Partial Wetting Method

d. Slurry Method

2. Field Test

a. Salt Fog Method

The test procedure consisted of energizing the arrester at a

voltage of 245/√3 kV for a predetermined time in a salt fog

atmosphere, unless a flash over occurred in this period. The

main characteristics of the salt fog method is the possibility

of high thermal stress on the arrester. The thermal stress on

the one unit is maintained on the same unit during the entire

test period.

b. Solid layer method

The solid layer tests were performed with an increased

steam input rate. The solid layer test gives a considerably

lower external charge accumulation compared to the other

test method. This is due to the washing effect of the steam

fog, which limits the duration of the pollution activity.

c. Partial wetting method

The partial wetting test differs considerably from the

other test methods. The location of temperature rise in the

upper half of the arrester is predetermined by the wetting

procedure, and a very high temperature rise may appear

in a very short time.

d. Slurry Method

In contrast to the partial wetting test, the slurry test implies that

the pollutant (Slurry) is applied to the complete arrester and not

only to the lower half. The main characteristic of slurry test is

that the temperature rise may appear any arrester any unit with

about the same probability. The external charge depends on the

number of test cycles in included in the test. The temperature rise

varies from one cycle to another.

2. Field Test

The pollution test on surge arrester at Martigues (France) and

Lista (Norway) were carried out in areas of marine and

industrial pollution. The results from the field test can be used to

select a laboratory test method which is able to regenerate the

actual thermal stress imposed on ZnO surge arrester in naturally

polluted condition. The field tests show that:

Conti…

a. Significant temperature rise may appear in any unit

of a multi-unit arrester.

b. The influence of the specific creep age distance on the

accumulated external charge in negligible within the

range studied.

c. The thermal stress on the arrester is not mainly

caused by larger surface leakage currents during short

periods.

Conclusion

A comparison of test results from the field test and the

laboratory test shows that the salt fog method and the slurry

method are both capable of regenerating the thermal stress

imposed on surge arresters in naturally polluted conditions.

Conti…

Dry band formation on polluted metal oxide surge arresters at A.C.

voltages may result in:

a. Rise of the internal A.C. current in the arrester column up to

four times higher than under clean condition.

b. Discharges inside of the arrester due to a high radial electric field

strength.

c. High local temperatures at the varistor column near to the dry

band depending on the size and position the dry band

Future work

Modelling and Simulation of the Surge Arresters using IES

COULOMB, a dedicated software for 3D-Field Solution of any

3D Model.

The result obtained by the simulation will be verified by taking

some experimental data if available from the internet or doing

by ourselves.

References

1. Behavior of ZnO surge arresters under pollution, IEEE

transactions on power delivery vol. 6, No. 2, April 1991.

2. Thermal stress on ZnO surge arresters in polluted

conditions part-1 and part-2, IEEE.

Thank you……!!