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WG C4.303

Cigré WG C4.303

Guide for the selection of insulators with respect to contamination conditions

Chris Engelbrecht: Convener WG C4.303

WG C4.303

Topics:The selection of insulators with respect to

polluted conditions

• Present practise

• Vision of the future– Cigré guidelines– Revised IEC 60815

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WG C4.303

Mechanical Electrical

Ultimate failing loadCantilever loadEtc.

LIWL (kV)SIWL (kV)Wet a.c. (kV)

Creepage(mm)

IEC 60060Test methods

IEC 60507 Pollution tests

IEC 60071Ins. Co-ord.

IEC 60815Polluted ins.

Specification of Insulators

Guidance Testing

Present practise I

WG C4.303

Present practise II1986 IEC 815 Published:

– Much debate

– Mostly based on small posts

– Only porcelain and glass

– Guideline comprised• Simple site severity classification

• Simple table of creepage distance

• Correction for diameter

• Profile limitations

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WG C4.303

Site assessment by example descriptions

Example Description of Typical Environment> 50 km from any sea, desert, or open dry land

> 10 km from man-made pollution sources or within a shorter distance, but:

• the prevailing wind is not directly from these pollution sources

• and/or subjected to regular monthly rain washing

10-50 km from the sea, a desert, or open dry land

5-10 km from man-made pollution sources or within a shorter distance, but:

• the prevailing wind is not directly from these pollution sources

• and/or subjected to regular monthly rain washing

3-10 km from the sea, a desert, or open dry land

1-5 km from man-made pollution sources or within a shorter distance, but:

• the prevailing wind is not directly from these pollution sources

• and/or subjected to regular monthly rain washing

or further away, but:

• a dense fog (or drizzle) often occurs after a long dry pollution accumulation season (several weeks or months)• and/or heavy rains with a high conductivity occurs

• and/or there is a high NSDD level, typically between 5 and 10 times the ESDD level

Within 3 km of the sea, a desert, or open dry land

Within 1 km of man-made pollution sources or with a greater distance, but:

• a dense fog (or drizzle) often occurs after a long dry pollution accumulation season (several weeks or months) • and/or there is a high NSDD level, typically between 5 and 10 times the ESDD

Within the same distance of pollution sources as specified for “Heavy” areas and:

• directly subjected to sea-spray or dense saline fog

• or directly subjected to contaminants with high conductivity, or cement type dust with high density, and withfrequent wetting by fog or drizzle•Desert areas with fast accumulation of sand and salt, and regular condensation

•Areas with extreme levels of NSDD, more than 10 times the level of ESDD

Very heavy

Very Light

Light

Medium

Heavy

WG C4.303

Creepage Distance• Shortest distance along the insulating surface [mm]

• Up to now

– Specific creepage distance [mm/kV]

– Phase to phase voltage [Uh for equipment]

• In future

– Unified Specific creepage distance [mm/kV]

– Voltage across the insulator [norm. Uh /√3]

• Why this change

– Not all insulators are phase to ground

• Capacitor banks, phase to phase insulation etc

– Direct comparison with Laboratory testing

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WG C4.303

Past IEC 60815 RecommendationsSalt –Fog ESDD Specific Creepage Unified Specific

[g/l] [mg/cm2] [mm/kVpp] Creepage [mm/kVpg]Light 5 – 14 0.03 – 0.06 15 – 20 16 28Medium 14 – 40 0.10 – 0.20 24 – 25 20 35Heavy 40 – 112 0.30 – 0.60 > 36 25 43Very Heavy > 112 > 0.60 31 54

Category Layer conductivity [µS]

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800 1000

Average Diameter [mm]

Cor

rect

ion

fact

or [K

d]

� Site classification

� Selection of creepage

� Correction for diameter

WG C4.303

What’s wrong with this?

Let us look at past experience….

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WG C4.303

Salt Fog Test (W ithstand Salinity: kg/m3)0.7 2 3 5 7 20 30 50 70 200 3001 10 100

W et Contaminant Test (W ithstand Layer conductivity: µS)1.5 2 3 4 6 15 20 30 40 60 15010 100

Clean Fog Test (W ithstand SDD: mg/cm2)

0.0065 0.02 0.04 0.065 0.2 0.4 0.65 20.01 0.1 1

Uni

fied

Spe

cific

Cre

epag

e D

ista

nce

(US

CD

: mm

/kV

)

15

20

25

30

35

40

45

50

55

60

Range of experimental results

Average curve

Commonly used Creepagedistance requirement

IEC 815 and Line insulators

Generally works well

However:• Does not cover all

insulator shapes• Breaks down at

high pollution levels

WG C4.303

10

100

1 10 100 1000

Pollution severity [Salt-fog - g/l]

Un

ifie

d S

pec

ific

Cre

epag

e d

ista

nce

[m

m/k

V]

IEC 815 and Equipment insulators

• Not as good as for line insulators• Important to correct for diameter

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Why is this so?

You need to look at the flashover mechanism….

WG C4.303

MechanismUnit Gets Contaminated:- Dry Contamination non-conductive

Unit becomes wet by condensation / absorption:-Wet Contamination conductive –current flows- Corona Occurs due to E-field Redistribution

I

V

I

V

Dry Bands Form due to Localized Heating-Where current density is high, e.g. close to pin- Dry Bands can be quenched by high wetting

Arcs bridge Dry Bands - Dry bands grow due to heating at arc roots- Arcs extinguish if dry band too large- If wetting critical entire unit flashes

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WG C4.303

Voltage

SHAPE DIMENSIONS

Pollution

Type (Solubility)

Washing

Wetting

Surfaceconductivity

Form factorHC

Wetting Intensity

FlashoverLengthCreepageDiameter

WG C4.303

Conclusion

The performance of an insulator

is the result of a complex

interaction between the insulator

and its operating environment.

Every site is an exception: Consider fundamentals

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WG C4.303

Insulation coordination:

8

7

6

5

4

3

2

1

300 500 700 900 1100 1300

Maximum system Voltage, kV

Insu

latio

n di

stan

ce, m

1.8 p.u2.6 p.u

PollutionSlow-frontLightning

PollutionSlow-frontLightning

AC Systems

Pollution based on glass or porcelain

WG C4.303

CIGRÉ Guidelines:• Polluted insulators: A review of current

knowledgeTechnical brochure 158, June 2000.

• Polluted insulators: Guidelines for selection and dimensioning– Part 1: General principles and the a.c. case

Technical brochure 361

– Part 2: The d.c. caseStill being worked on

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WG C4.303

Cigré Review of current Knowledge• Technical brochure 158 (June 2000)

– 9 Chapters + Annexes: 185 Pages, 382 references• Introduction• Pollution flashover process• Insulator characteristics• Environmental impact• Pollution monitoring• Testing procedures• Insulator selection and dimensioning• Palliatives and mitigation measures• Thermal effects on metal oxide arresters

WG C4.303

Cigré AC Guidelines

• Technical brochure 361 (June 2008)– General guidelines in Body

• Outline of method• Simplified statistical with correction factors

– Detail technical information in Annex• Worked examples• General descriptions of typical environments• Site pollution severity assessment• Insulator characteristics and correction factors• Laboratory test method for polymeric insulators

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WG C4.303

Old insulators

WG C4.303

Observations: No Activity

Leakage current < 1 mABack

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Observations: Corona

Leakage current < 10 mABack

WG C4.303

Observations: Pulsed scintillation

Leakage current ≈ 10-50 mABack

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WG C4.303

Observations: Continuous scintillation

Leakage current ≈ 40-70 mABack

WG C4.303

Observations: Pulsed dry-band arcs

Leakage current ≈ 60-100 mABack

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WG C4.303

Observations: Intense dry-band arcing

Leakage current > 100 mABack

WG C4.303

Pollution catch:

Low-Velocityturbulence

weak vortices

vortices

winddirection

Low-Velocityturbulence weak vortex

vortex

winddirection

Function of the aerodynamic shape

Back

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WG C4.303

Protected creepage

Protectedareas

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WG C4.303

Classification of pollution

�Active Pollution(Form a conductive layer)

– Conductive pollution

– High solubility salts � NaCl, MgCl, NaSO4, etc

– Low solubility salts� Gypsum, Fly ash, Cement

� Inert Pollution(Influence conductive layer)

– Hydrophilic pollution� Kaolin, clay

– Hydrophobic Pollution� Silicone grease

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WG C4.303

The Form Factor

• ESDD Pollution density– surface conductivity ( )

• Resistance is given by

• or ( )Rdx

D xinss

Sk

= ∫1 1

0σ π

R K

K

inss

f

f

=1

σ:Form factor

σs

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WG C4.303

Hydrophobic properties

1 2 3

4 5 6

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