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FachgebietHochspannungstechnik

Overvoltage Protection and Insulation Coordination / Chapter 6 - 1 -

Procedure for Insulation Coordination in Four Steps

[IEC 60071-1]

Flow chart acc. to IEC 60071-1(Figure 1)


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Procedure for Insulation Coordination in Four Steps

Determination of the coordination withstand voltages Ucw

The coordination withstand voltages are the lowest values of withstand voltages ofeach overvoltage class, for which the expected low failure rate of the equipment is notexceeded over its full lifetime.

Derived from the representative overvoltages Urp by the coordination factor Kc.

[IEC 60071-1]

Typical forGermany:0.1% per year 1 failure in1000 years


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Insulation Strength Characteristics

Factors influencing the dielectric strength of the insulation:

magnitude, shape, duration and polarity of the applied voltage electric field distribution in the insulation

homogeneous or non-homogeneous electric field electrodes adjacent to the considered gap and their potential type of insulation

gaseous liquid solid combination of two or all of them impurity content and the presence of local inhomogeneities

physical state of the insulation temperature pressure

other ambient conditions mechanical stress

history of the insulation (aging, damage) chemical effects

conductor surface effects

Factors influencing the dielectric strength of the insulation:

magnitude, shape, duration and polarity of the applied voltage electric field distribution in the insulation

homogeneous or non-homogeneous electric field electrodes adjacent to the considered gap and their potential type of insulation

gaseous liquid solid

combination of two or all of them impurity content and the presence of local inhomogeneities

physical state of the insulation temperature

pressure

other ambient conditions mechanical stress history of the insulation (aging, damage)

chemical effects conductor surface effects


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Standard atmospheric conditions acc. to IEC 60060-1

Temperature: 20 C

Pressure: 1013 hPa

Absolute humidity: 11 g/m3

Temperature: 20 C

Pressure: 1013 hPa

Absolute humidity: 11 g/m3


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Topics to be covered in the following:

Insulators under polluted conditions

Probability of flashover (Normal and Weibull distributions)

Behavior of parallel insulation

Coordination procedure: deterministic and statistical approach

Correction with altitude of installation

Clearances in air; "gap factors"


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Pre-conditionsPre-conditions

Performance of Insulators under Pollution

Surface layers

dust carbon black salt (coastal areas) chemicals (industry, rural areas: fertilizers)

dust carbon black salt (coastal areas) chemicals (industry, rural areas: fertilizers)

no problem in dry condition

after long rain periods: only moderate effect on flashover performance

most critical:

Humidification after a long dry periodHumidification after a long dry period

typical time of the day for insulator flashovers: morning hours (dew!)


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Development of pollution flashoverDevelopment of pollution flashover


dry zone by inhomogeneity of the layer

enlargement of the dry zone by heating of the zoneedges (increased current density)

dry band

flashover of the dry band

enlargement of the dry band by arc heating

(max. temperature at foot points)


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Development of pollution flashoverDevelopment of pollution flashover


Voltage distribution

a) with dry bands b) dry bands bridgedby partial arcs

voltage bridged

voltage drop increased

for further details see HVT 2!


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Layer conductivity K is the most important parameter!Layer conductivity K is the most important parameter!

K= ds ... specific layer conductivity

ds... thickness of layer

K= 5 S "light to medium pollution"K= 10 S "medium to heavy pollution"K= 40 S "very heavy pollution"

Influence of layer conductivityInfluence of layer conductivity for details see IEC 60507


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Determination of layer conductivity frommeasured conductance and insulator geometry



2 dd

d d / 2

r s KG

l l r

= =

Measurement of conductance Gof the full insulator

shed core

creepage

distance lk

insulator length lV

Gl

=general:


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k

0

d

2

l

KG

l

r

=

k

0

d

2

l

lF

r=

G... conductance of total insulator surface

form factor"form factor"

K= FGK= FG

(IEC 60507*))

form factor to be determined by

graphical procedure, describedin IEC 60507



*) IEC 60507, 2nd Ed. 1991-04: "Artificial pollution tests onhigh-voltage insulators to be used on a.c. systems"


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Decrease in flashover voltage by conductive layersDecrease in flashover voltage by conductive layers


fo, rain

0.7 ... 0.9 fo, dry

Ufo, polluted 0.2 ... 0.3 Ufo,dry

An overhead line insulator must be desigend about five times as long asrequired to withstand operating stresses under dry conditions!

An overhead line insulator must be desigend about five times as long as

required to withstand operating stresses under dry conditions!

Um = 123 kV

L-E = 100 kV

d = 5 kV/cm l = 20 cm would be sufficient (dry!)

Actual length: ca. 1100 mm


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pCountermeasuresCountermeasures


ShedsSheds

s ... flashover or arcing distance

lk ... creepage distance

li ... insulator length

p ... shed overhang

t ... shed spacing

Terms ...


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Actual Situation for IEC 60815 (as per February 2009)


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Actual Situation for IEC 60815


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Example of a change in IEC 60815-1:2008 compared with 60815:1986


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31 mm/kV for very heavy" pollution severity (IV)25 mm/kV for heavy" pollution severity (III)20 mm/kV for "medium" pollution severity (II)

16 mm/kV for "light" pollution severity (I)

31 mm/kV for very heavy" pollution severity (IV)25 mm/kV for heavy" pollution severity (III)

20 mm/kV for "medium" pollution severity (II)16 mm/kV for "light" pollution severity (I)

CountermeasuresCountermeasures

Correction of these values necessary depending on insulator's average diameter Dm*)

Correction factor kD (derived from service experience):

*) for definition of Dm see IEC 60815

Dm (mm) kD< 300 1

300 - 500 1.1

> 500 1.2

Pollution performance gets worsewith increasing diameter!


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Pollution level Examples of typical environments

I - Light

- Areas without industries and with low density of houses equipped with heating plants- Areas with low density of industries or houses but subjected to frequent winds and/or

rainfall- Agricultural areas

1)

- Mountainous areasAll these areas shall be situated at least 10 km to 20 km from the sea and shall not be

exposed to winds directly from the sea2)

II - Medium

- Areas with industries not producing particularly polluting smoke and/or with averagedensity of houses equipped with heating plants- Areas with high density of houses and/or industries but subjected to frequent windsand/or rainfall

- Areas exposed to wind from the sea but not too close to the coast (at least severalkilometres distant) 2)

III - Heavy- Areas with high density of industries and suburbs of large cities with high density ofheating plants producing pollution- Areas close to the sea or in any case exposed to relatively strong winds from the sea

2)

IV - Very heavy

- Areas generally of moderate extent, subjected to conductive dusts and to industrial smokeproducing particularly thick conductive deposits- Areas generally of moderate extent, very close to the coast and exposed to sea-sprayor to very strong and polluting winds from the sea- Desert areas, characterized by no rain for long periods, exposed to strong windscarrying sand and salt, and subjected to regular condensation

1) Use of fertilizers by spraying, or the burning of crop residues, can lead to a higher pollution level due to dispersalby wind.

2) Distances from sea coast depend on the topography of the coastal area and on the extreme wind conditions.

IEC 60815:1986, Table 1IEC 60815:1986, Table 1


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Shed profilesShed profiles


Some typical shed profiles (from IEC 60815; explanation of the parameters see there).From left to right: normal shed profile, alternating shed profile, underrib sheds (fog profile),

cap-and-pin insulators

Some typical shed profiles (from IEC 60815; explanation of the parameters see there).From left to right: normal shed profile, alternating shed profile, underrib sheds (fog profile),cap-and-pin insulators

IEC 60815


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Recommendations of IEC 60815 - ExampleShed profilesShed profiles

c 30 mm

p1p2 15 mm

s/p1 0.65 (in case of plain, non-underripped sheds)

lx/dx < 5

C.F. 3.5 (pollution classes I + II) 4 (pollution classes III + IV)

C.F.= creepage factor = lt/stlt = total creepage distancest = arcing distance (arcing horns not considered)

IEC 60815


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Shed profilesShed profiles Example of user'sexperience

From:

Raouf Znaidi: "ServiceExperience and MaintenanceRequirements for Different Typesof Insulators in Tunisia", WorldCongress on Insulators, Arrestersand Bushings, Hong Kong, Nov.27-30, 2005


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Semi-conducting glazingSemi-conducting glazing


Idea: to avoid dry-band arcing by resistive bypass

No flashover due to bypass current

surface current

Drawback: stable semiconducting glazing difficult to produce


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Semi-conducting glazingSemi-conducting glazing


Under development: for composite insulators by coating filled with micro-varistors

-varistors


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Composite insulatorsComposite insulators


introduced in the beginning of the 1970s today virtually" state of the art

problems": long time performance not yet clear,"brittle fracture", animal attacks

Shed material:

EPDM (Ethylene-Propylene-Diene-Monomer)

only in distribution

Silicone rubber (SIR)

FRP core

extruded SIR sheath

push-over SIR sheds

crimped-on metal end fitting


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From an EPRI Questionnaire in North America (publ. in 2003)


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compare this with the"bathtub curve"of failure no evidence for aging


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HydrophobicityHydrophobicity


One of the most important properties of composite insulators

with regard to pollution performance isHydrophobicityHydrophobicity


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Advancing angle

Receding angle= most important for characterization

of hydrophobicity

Receding angle= most important for characterization

of hydrophobicity

Properties change under the influence of electrical field actual research!


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IEC/TS 62073

Wettability classes*)

*) Based on the "STRI Guide" (of STRI, Ludvika/Sweden)


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Natural Test SitesNatural Test Sites

Weather Aging Tests for Polymeric Insulators *)

Example: Koeburg, RSA

Realistic test conditions, but no acceleration

factors

long test times necessary (several years)

"(In)famous" test sites:

Koeburg, RSA (Atlantic Ocean) Dungeness, UK (The Channel)

Martiguez, F (Mediterranean Sea)

*) NOTE: often the term NCI =non ceramic insulators isbeing used


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The "Tracking and Erosion Test" acc. to IEC 61109The "Tracking and Erosion Test" acc. to IEC 61109

Weather Aging Tests for Polymeric Insulators

Examples of test chambers


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Cyclic TestsCyclic Tests


Cyclic tests usually consist in applying, in addition to voltage stress, various stresses in a cyclic

manner:

- solar radiation simulation;- artificial rain;

- dry heat;

- damp heat (near saturation);

- high dampness at room temperature (saturation has to be obtained);

- salt fog at low concentration.Furthermore, temperature variations may cause some degree of mechanical stress, especially

at the level of insulator interfaces and also give rise to condensation phenomena, which are

repeated several times in the course of a cycle.

For power frequency test voltage, a test transformer shall be used. The test circuit when loaded

with a resistive current of 250 mA (r.m.s.) on the high voltage side shall experience a maximum

voltage drop of 5 %. The protection level shall be set at 1 A (r.m.s.).

Problem: no general agreement on one particular test!Problem: no general agreement on one particular test! Examples next slides:


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Practical testproblem: rainand solarradiation at thesame time!


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h f l l


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"EPRI" cycle: a year in service is considered to be represented by 10 days of summercycle and 11 days of winter cycle. A duration of 5040 h is required for the whole test,10 summer/winter cycles of 21 days each.

O h A i T f P l i I l


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Wheel Test acc. to IEC 62217Wheel Test acc. to IEC 62217

Other Aging Tests for Polymeric Insulators

The test specimens shall be cleaned with de-ionized water before starting the test. The testspecimens are mounted on the wheel as shown in Figure A.1 below. They go through fourpositions in one cycle. Each test specimen remains stationary for about 40 s in each of the

four positions. The 90 rotation from one position to the next takes about 8 s. In the first partof the cycle the insulator is dipped into a saline solution. The second part of the test cyclepermits the excess saline solution to drip off the specimen ensuring that the light wetting ofthe surface gives rise to sparking across dry bands that will form during the third part of thecycle. In that part the specimen is submitted to a power frequency voltage. In the last part ofthe cycle the surface of the specimen that had been heated by the dry band sparking isallowed to cool.

Electrical stress: The power frequency test voltage in kV is determined by dividing theactual creepage distance in millimetres by 28,6.

NaCl content of de-ionized water: 1,40 kg/m 0,06 kg/mAmbient temperature: 20 C 5 KTest duration: 30 000 cycles

The test is regarded as passed, if on both test specimens: no tracking occurs for composite insulators: erosion depth is less than 3 mm and does not reach the core; if applicable for resin insulators: erosion depth is less than 3 mm; no shed, housing or interface is punctured.

Extremely severe test!

Oth A i T t f P l i I l t


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Wheel TestWheel Test

Other Aging Tests for Polymeric Insulators

P f f I l t d P ll ti


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"Silicone Bonus""Silicone Bonus"


For NCIs with permanent (recovering) hydrophobic characteristics a "silicone bonus" may beapplied as a reduction factor of creepage distance (C.D.) compared with ceramic insulators:

Class 1 : 70 % 75 % of C.D. of ceramic insulators

Class 2 : 80 % of C.D. of ceramic insulators (not applicable in coastal areas!)

Class 3 : same C.D. as for ceramic insulators

Class 4 : in general, application of NCI should be carefully checked for each

individual application

Class 1 : 70 % 75 % of C.D. of ceramic insulators

Class 2 : 80 % of C.D. of ceramic insulators (not applicable in coastal areas!)

Class 3 : same C.D. as for ceramic insulatorsClass 4 : in general, application of NCI should be carefully checked for each

individual application

P f m nc f P l m ic Insul t s


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Silicone rubber as insulator materialSilicone rubber as insulator material

Performance of Polymeric Insulators

Other problems ....

Moss, algea



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Other problems ....

Animal attack (parrots, cockattoos, termites)

Example: AustraliaExample: Australia




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Other problems ....

Animal attack (parrots, cockattoos, termites)

Example: AustraliaExample: Australia




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FachgebietHochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 6 - 54 -

Brittle fractureBrittle fracture


From:

M. Kuhl: "FRP Rods for Brittle Fracture Resistant CompositeInsulators",http://www.lappinsulator.com/downloadcenter/technical.asp

Countermeasures:

ECR glass (electro-chemical resistant)

quality of sealing at triple point field stress reduction by grading rings



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2 different methods:

Salt fog methodSalt fog method

Solid layer methodSolid layer method

Artificial pollution testsArtificial pollution tests

IEC standard 60507IEC standard 60507



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Salt fog methodSalt fog method


Test specimen energized at operating voltage under conductive salt fog exposureTest specimen energized at operating voltage under conductive salt fog exposure

Salt mass concentration between 2.5 kg/m3 und 224 kg/m3Salt mass concentration between 2.5 kg/m3 und 224 kg/m3

(1 kg/m3 corresponds to 1 g/l)

Classification by withstand salt mass concentrationClassification by withstand salt mass concentration

Test specimen must not flash over within a specified time of exposureTest specimen must not flash over within a specified time of exposure



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Solid layer methodSolid layer method


Test specimen is energized in a coldfog chamber and then exposed tohumidity

Test specimen is energized in a coldfog chamber and then exposed tohumidity

Layer conductivity between 3 S and 80 SLayer conductivity between 3 S and 80 S

Classification by withstand layer conductivity or withstand salt deposit densityClassification by withstand layer conductivity or withstand salt deposit density

Solid layer of specified conductivity is applied in wet condition and driedSolid layer of specified conductivity is applied in wet condition and dried

Test specimen is exposed tohumidity in a cold fog chamberand then energized

Test specimen is exposed tohumidity in a cold fog chamberand then energized

Test specimen must not flash over within a specified time of exposure

Test specimen must not flash over within a specified time of exposure

Salt Deposit Density (SDD) between 0.03 mg/cm2 and 0.60 mg/cm2Salt Deposit Density (SDD) between 0.03 mg/cm2 and 0.60 mg/cm2


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Radial Field Stress under PollutionRadial Field Stress under Pollution


MO column

Conductivelayer

Gas or solid

Solid

Uaxial,int

Uradial

Arises if there is an internal active part with a

given, constant axial voltage distribution; risk of

internal PD in case of internal gas volume

puncture in case of pure solid insulation

MO-Scheiben

Porzellangehuse-Innenwand

MO discs

porcelain housing, inner wall

Photo: PD in aporcelain housedsurge arrester



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Dielectric and Thermal EffectsDielectric and Thermal Effects

f m f

Internal partialdischarges

changes in internalatmosphere

risk of deterioration

of all internal parts

Internal partialdischarges

changes in internalatmosphere

risk of deterioration

of all internal parts

Risk of partialheating of internal

active elements

Risk of partialheating of internalactive elements

Risk of external flashoversRisk of external flashovers

Outer surface discharges

Risk of partial heating ofinternal active elements

Outer surface discharges

Risk of partial heating ofinternal active elements

Example: 800-kV surge arrester

Emerging Insulator Standards


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g g

From:

Claude de Tourreil: "New IECstandards: their Impact on futureSelection of CompositeInsulators", World Congress onInsulators, Arresters andBushings, Hong Kong, Nov. 27-30, 2005

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