Cigre_WG_C4303_0

10/03/2010

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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Present practise I Specification of Insulators Mechanical

Electrical Guidance

Ultimate failing load Cantilever load Etc.

IEC 60071 Ins. Co-ord. IEC 60815 Polluted ins.

Testing

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

IEC 60060 Test methods IEC 60507 Pollution tests

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Present practise II 1986 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 WG C4.303

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Site assessment by example descriptions Very Light

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

Light

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

Medium

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

Heavy

Very heavy

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 with frequent 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

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 WG C4.303

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Past IEC 60815 Recommendations Category

Salt –Fog ESDD Layer Specific Creepage Unified Specific 2 conductivity [ S] [g/l] [mm/kV ] Creepage [mm/kVpg] µ [mg/cm ] pp Light 5 – 14 0.03 – 0.06 15 – 20 16 28 Medium 14 – 40 0.10 – 0.20 24 – 25 20 35 Heavy 40 – 112 0.30 – 0.60 > 36 25 43 Very Heavy > 112 > 0.60 31 54 1.4







Site classification Selection of creepage

Correction factor [Kd]

1.2 1 0.8 0.6 0.4 0.2




Correction for diameter

0 0

200

400

600

800

1000

Average Diameter [mm]

WG C4.303

What’s wrong with this?

Let us look at past experience….

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Unified Specific Creepage Distance (USCD: mm/kV)

IEC 815 and Line insulators

Generally works well However: • Does not cover all insulator shapes • Breaks down at high pollution levels

60 55 50 45 40

Commonly used Creepage distance requirement

35 30

25

Average curve Range of experimental results

20

15 0.0065 0.01

0.02

0.04

0.065 0.1

0.2

0.4

0.65

2

1

Clean Fog Test (W ithstand SDD: mg/cm 2 ) 0.7

2

1

3

5

7

10

20

30

50 70 100

200 300

3

Salt Fog Test (W ithstand Salinity: kg/m ) 1.5

2

3

4

6

10

15

20

30

40

60

100

150

W et Contaminant Test (W ithstand Layer conductivity: µS)

WG C4.303

IEC 815 and Equipment insulators Unified Specific Creepage distance [mm/kV]

100

10 1

10

100

1000

Pollution severity [Salt-fog - g/l]

• Not as good as for line insulators • Important to correct for diameter WG C4.303

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Why is this so?

You need to look at the flashover mechanism…. WG C4.303

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

I V

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 WG C4.303

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SHAPE

DIMENSIONS Voltage

Pollution Length Creepage Diameter

Type (Solubility)

Washing Wetting

Flashover

Form factor HC Wetting Intensity

Surface conductivity

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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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Insulation coordination: AC Systems Insulation distance, m

8

1.8 p.u

2.6 p.u

7 6 5 4 3

Pollution Slow-front Lightning

2 1 300

500

700

900

1100

Pollution Slow-front Lightning 1300

Maximum system Voltage, kV Pollution based on glass or porcelain

WG C4.303

CIGRÉ Guidelines: • Polluted insulators: A review of current knowledge Technical 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. case Still being worked on WG C4.303

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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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Old insulators

WG C4.303

Observations: No Activity

Back

Leakage current < 1 mA

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Observations: Corona

Back

Leakage current < 10 mA

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Observations: Pulsed scintillation

Back

Leakage current ≈ 10-50 mA

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Observations: Continuous scintillation

Back

Leakage current ≈ 40-70 mA

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Observations: Pulsed dry-band arcs

Back

Leakage current ≈ 60-100 mA

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Observations: Intense dry-band arcing

Leakage current > 100 mA

Back WG C4.303

Pollution catch: Function of the aerodynamic shape weak vortices LowVelocity turbulence

vortices weak vortex vortex

wind direction wind direction LowVelocity turbulence

Back WG C4.303

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Protected creepage

Protected areas Back WG C4.303

Classification of pollution  Active Pollution (Form a conductive layer)

– Conductive pollution – High solubility salts 

NaCl, MgCl, NaSO4, etc

– Low solubility salts 

 Inert Pollution (Influence conductive layer)

– Hydrophilic pollution 

Kaolin, clay

– Hydrophobic Pollution 

Silicone grease

Gypsum, Fly ash, Cement

Back WG C4.303

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The Form Factor • ESDD

Pollution density – surface conductivity ( σs ) • Resistance is given by S 1 1 k dx Rins = σs π ∫0 D( x) • or 1 Rins = K f

σs

K f :Form factor

Back WG C4.303

Hydrophobic properties 1

2

3

4

5

6

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