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DC vs. AC Insulators

Gary Sibilant

,IEEE HVDC Lines Working Group

San Diego, CA

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Purpose of Overhead Transmission

z Electrical properties

so a e energze con uc or

electrically from groundedstructures under all

Wetting, contamination,

lightning, temporaryovervolta es switchinsurges, etc.

z Mechanical properties

o attach su ort conducto

to grounded structures underall conditions

Ever da loads

Wind and ice loading, etc.

Disconnect operations

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nsua or a era

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Design Differences: What Drives AC

an nsua on es gn8 8

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2

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RequiredS

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0

1

300 400 500 600 700 800 900 1000 1100 1200

0

1

300 400 500 600 700 800

0

1

300 400 500 600 700 800 900 1000 1100 1200

0

1

300 400 500 600 700 800

0

1

300 400 500 600 700 800 900 1000 1100 1200

0

1

300 400 500 600 700 800

Maximum System Voltage (kV) Maximum System Voltage (kV)Maximum System Voltage (kV) Maximum System Voltage (kV)Maximum System Voltage (kV) Maximum System Voltage (kV)

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Performance Differences

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er ormance erences

z Contamination Accumulation andPerformance

z Flashover / Arcin

z Mechanical

z Corrosion

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Performance Differences:

on amna on ccumua onz Constant E-field

z

Electrostatic Catch

z Lon er Leaka e

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Effects of Contamination on HVDC

nsua ors: ong erm gng ec s

Service experience

has been good

areas of highcontamination has

been noted

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Effects of Contamination on HVDC

nsua ors: on amna on epos on

z

Aerodynamic Action Insulator shape

z Precipitation by Gravity

Polluted industrial areas (with dust laden air)

Mining (especially open cast mines)

, .

Roads (road salt during winter, and soot from automobiles)

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Performance Differences:

as over rc ng

z Continuous current

Insulator

drawings

NGK-LockeErosion due toarcing has also

z DC fog type neededbeen observed

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Performance Differences:

as over rc ng

z Single disk flashovers

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HVDC Insulator Flashover

arac ers csz Polarity of the applied voltage

Negative lower for disk typeinsulators

eg g e erence or ongrod, post and equipment

insulators.

z Linearit of flashover stren th

with the length of the insulator

ro en s s a

energized end

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nsua ors: orros on

z Porcelain Insulators

In service failureshave beenrelated tocorrosion of thezinc-alloy sleeve

of the pin Solved using a

pure zinc sleeveand coating the

area ofpin/cementcontact with a

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Performance Differences

ec anca : ass

suffered fromspontaneousshattering

z High ResistivityToughened Glass

(HRTG)

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z S ecific leaka e len th reduced

(depends on number of shattered disks

Source: