Phillips DegradationModesCompositeInsulators

8/12/2019 Phillips DegradationModesCompositeInsulators

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Degradation & Failure Modes

of Composite Insulators

Dr. Andrew Phillips

Electric Power Research [email protected]
mailto:[email protected]:[email protected]


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Stresses & Degradation Modes

Insulator is subjected to a range of stresses from:During Manufacture Removal

\

Stresses Include Environmental Stresses Electrical Stresses Mechanical Stresses Biological


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Manufacturing Stresses Mechanical Stresses

Over crimping

Mechanical Stresses Rubber due to heating and cooling

Rod due to heating and coolingMishandling

Routine Test Load (50% of SML)

Packaging Stresses

Affect Housing, End Fitting Sealand Core


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Storing Transporting and Installing

Mechanical Stresses

Abrasion

Bending

Torquing

Compression

Impact

Affect Housing, End Fitting Seal & Core


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In-service Mechanical Stresses Loads

Everyday load

Extreme Load

Suspension

Tension Loads only Post

Tension, Cantilever and Compression

Phase spacersTension, Bending and Compression


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Environmental Stresses Ultraviolet Light

Housing End Fitting Seal Ambient Temperature

Housing End Fitting Seal Core

Precipitation Corrosion Contamination

React with polymer materials Housing End Fitting Seal

Together with electrical stresses Flashovers Material Degradation


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Biological Stresses

Animals Bird

Rodents

Humans (gunshot)

Fungus

Insects

Bacteria


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Electrical Stresses - Transients Types

Lightning

Switching

Temporary Overvoltages

Impact of Transients if Flashover occursPower arcs terminate

End fittings

Corona Rings

Heating from power arc candegrade

Housing

Seal


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Electrical Stresses Power Frequency

Low Contamination

Dry Discharges - Corona

Wet Discharges

Non-uniform Wetting Corona

Spot Discharges

High ContaminationLeakage Currents and Dry Band Arcing


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Dry Corona Activity

Results in Radio interference, audio noise

Degradation

Housing

End Fitting Seal


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Degradation Due Dry Corona Activity


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Degradation of SIR Insulators due to Dry Corona Activity

Field

Field

Evidence of continual corona on the end fitting

Field

Field


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Non- Uniform Wetting Discharge Activity

Wetting Discharges degradeWeathershed system

End Fitting seal

Water Drop Corona = 80

Increases E-field

above Corona Threshold Inter Water Patch Discharges

Discharges between patches of waterE-fieldNormalized

1

2.83

E-fieldNo

rmalized

1

3

H20

Metal

1V

0V


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Wetting Corona Aging Mechanism

Corona generates

UV light

Heat

Gaseous by-products

03(Ozone), NO2, etc

NO2+ H2O = HNO3(Nitric Acid)

EPRI tests: Wett ing on NCI lowers pH to 3.4

after 15 min. of wetting corona activity


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Recommendations for E-field Magnitudes

EPRI / STRI Recommendations for Sheath

- E-field should not exceed 0.42kV/mm for

10mm (1/4)- E-field should not exceed 0.35kV/mm on seal

surfaces (testing underway to confirm/revise)

Recommendations for Metal end Fittings- E-field should 1.7 to 2kV/mm


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Degradation Due Wetting Discharges

Localized Loss of Hydrophobicity Localized Whitening

Localized WhiteningCrazing

SIR

EPDM


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Degradation Due Wetting Discharges

Cracking along mold lines Cracking at mold injection points

Cracking of sheathCracking of sheds

SIR

EPDM


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Degradation due to Contamination

Activity

Hydrophyllic

Hydrophyllic Rubber

(EP)

Rubber that has lost

Hydrophobic Properties

(Silicone Rubber)

Hydrophobic

Silicone Rubber


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Degradation of Hydrophyllic

by Contamination Activity

Insulator become contaminated

Non-conductive When Dry

Wet Contamination Results in

Conductive Layer

Leakage Currents Flow

Dry band forms blocking

leakage current

Arc Bridges Dry Band

Arc may

Extinguish

Grow

Move around

Wet Electrolyte

e.g. Salt and Water

Dry Band

Insualtor

Energized

End

Grounded

End

Arc


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Degradation of Hydrophobic Insulators by

Contamination Activity

Insulator become contaminated Low Molecular Weight Silicone Molecules

penetrate the contamination layer

transferring hydrophibic property

Insulators become wet Contamination does not become conductive

due to hydrophobic properties of surface

Any conductive contamination patches are isolated

from one another by hydrophobic properties

No leakage currents, No dry band arcing, No degradation

If Hydrophobicity is lost degradation will occur as for

hydrophyllic insulators degradation then depends on rubber

SIR Surface

Low Molecular Weight

Silicone Molecules

Contamination LayerContamination Layer


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Contamination Activity (ionic)

Degradation of Material due to leakage currents and dry band arcs

Crazing Tracking(minor)

Erosions Tracking

Erosions Tracking


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Multiples Stresses

Mechanical

Electrical

Environmental

Biological

Primary Aging Mode is

Discharge Activity


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Brittle Fracture

Water reached rod

Acids formDischarge activity

ContaminantsAcid rain

Corrosion

Fibers cut bystresscorrosion cutting


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Brittle Fracture - Features

Clean cut fracture planes

Broomstick

Axial delaminations

Other types

BroomstickFracture Plane

Axial

Delamination

Fracture Plane


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Failure of Rod Due

to Discharge Activity Internal Discharge Activity

Water ingress

Internal Defects

Discharge activity degrades rod

Chemically

Ionic wind

UV

Temperature

Rod fails under load


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Flashunder

Internal Discharge ActivityWater ingress

Internal Defects

Conductive pathThrough rod itself

On rod surface

NCI cannot holdvoltage - flashover

Power arc bursts through rubber


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End-fitting Attachment

& Mechanical Failure Under crimping - pull out

Over Crimping Cracked rod May break

with time

Degradation of Epoxy (Epoxy Wedge Only) Overheating of Rod in Manufacturing

Mishandling Storing Transporting Installing


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Contamination Flashover Insulator becomes severely contaminated due to

local environment

Flashover occurs under critical wetting conditions

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Failure Database : per year

364 failures

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Failure Mode

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Failure Database: Age of Failures

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Failure Database: Year of Installation

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Questions?

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