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Technical information

Voltage grading of EXLIM and PEXLIM surge arresters

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Surge arrester performance

The performance of a zinc oxide surgearrester is dened by its protectivelevels, its temporary overvoltage andenergy discharge capabilities and thelong term-stability of the zinc oxidematerial.

The temporary overvoltage andenergy handling capabilities are closelyrelated to the temperature of the zincoxide blocks during normal operation.

This temperature depends on the powerlosses, which increase rapidly with volt-age stress, due to the block materialsnon-linear voltage-current characteris-tics. Therefore the most essential param-eters to minimize the temperature

during normal operation are inherentlylow power losses in the zinc oxidematerial, together with a linear voltagedistribution along the block column.

The voltage distribution along theblock column depends on the capaci-

tance of the zinc oxide blocks and theinuence of stray capacitances. The straycapacitances are strongly dependent onthe height of the block column.

Short arresters - up to about onemeter in height - usually have a suf-

ciently linear voltage distribution alongthe block column, as the self-capaci-tance of the zinc oxide blocks is rela-tively high.

For taller arresters, the inuence ofstray capacitances makes the voltagedistribution less linear.

If no measures are taken to preventan uneven voltage distribution on a tallarrester, the local voltage stress at the

top will reach the knee-point of thevoltage-current characteristic of the zincoxide material. This leads to a localizedincrease in the power losses, with high

temperatures in the block column asconsequence.

Electric eld calculations

EXLIM and PEXLIM

arresters equipped

with grading rings

Figure 1:

Single-phase electric

feld calculation

The solution to this problem is theintroduction of additional voltage grad-ing equipment. For EXLIM and PEXLIMsurge arresters the addtional voltagegrading is obtained by using externalgrading rings. When designing surgearrestesrs, grading rings are usuallyselected and positioned with help of

two-dimensional, axis-symmetric, single-phase electric eld calculations.

Figure1 shows equipotential lines asthe result of such a calculation for anEXLIM Q360-DH420 arrester, positionedtwo meters above ground.

An important matter, which oftenremains unconsidered, is the inuenceon the voltage distribution by adjacentsurge arresters in a three-phase instal-

lation. Three-dimensional, three-phaseeld calculations according to the

ABB2

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boundary element method show, thatthe voltage stress at the top of anarrester may be up to eight percenthigher compared to values calculatedby normal single-phase eld calculation

methods.The inuence from adjacent phases

on voltage distribution has been cal-culated for several typical three-phaseinstallations ranging from 145 kV to 800kV system voltage. In order to simulatethe maximum possible three-phase inu-ence on the voltage distribution, thesecalculations were performed using theminimum phase spacing for each volt-

age level and the arresters were placedin the vicinity of a grounded wall.

Figure 2 shows the set-up used forcalculations on a three-phase 245 kVarrester installation.

2,0 m

2,5 m

1,6 m3,0 m

Figure 2:

Typical 245kV surge

arrester installation

Verication of long-term stability

Both IEC and ANSI standards require themaximum voltage stress to be taken intoconsideration in accelerated ageing testson zinc oxide blocks.

However, it is not possible to deter-mine the correct voltage stress to beused in these tests without proper cal-culations of the maximum voltage stressoccuring in practical three-phase instal-

lations. If no such calculations havebeen performed, the tests should there-fore be carried out with a voltage stresscorresponding to the knee-point of thevoltage-current characteristics, i.e. at thereference voltage.

The grading arrangements for EXLIM

and PEXLIM surge arresters are basedon complete electrical eld calculationsfor each arrester design - at the maxi-mum continuous operating voltage andwith the maximum possible three-phaseinuence taken into account. This guar-antees that the voltage stress remainsbelow the critical level at all pointsalong the block column. This maximum

voltage stress level is used in acceler-ated ageing tests on the zinc oxideblocks for ABBs arresters. Thus, the long-term stability of the zinc oxide blocks isveried at the highest possible voltagestress found in any installation under

normal service conditions.

ABB 3

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ABB Power Technology Products ABHigh Voltage ProductsSurge ArrestersS-771 80 Ludvika, Sweden

Tel: +46 (0)240 78 20 00Fax: +46 (0)240 179 83

www.abb.com

2353en

ed.

2

/2001-08