Performance of Silicone Rubber Based Materials for

Chalmers University of Technology

Performance of Silicone Rubber Based Materials for Applications

in Outdoor Insulation

S.M. Gubanski Chalmers University of Technology

Gothenburg, Sweden


• 150 years of experience • Introduction of suspension and cap&pin

insulators (beginning of 20th century) • Extensive development aiming at

improving pollution performance (through 1920s to 1950s)

• Improving the quality of materials and manufacturing technologies - lifetime of 50+ years

• Some of the earlier ideas re-appear

History of HV insulators


Outdoor insulators • Many types of HV insulators are available

Cap & pin insulators Pin insulators Transformer bushings Station post insulators

Switchgear bushings Insulators for special applications

Composite line insulators

Composite apparatus insulators


Desired material properties

• Should withstand high electric stresses

• Should survive in environments with dust, uv, rain etc.

• Should have enough mechanical strength to bear tensile, compressive and cantilever forces

• Should exhibit good surface properties (chemical and physical stability, washability, hydrophobicity, etc)


Pollution mitigation techniques Replacement

Replacing insulators best suited to the ambient condition with

Increased creepage distance Improved profile Superior material characteristics (hydrophobicity)

Cleaning

Cleaning the insulator surface by

Hand washing Spray washing Live washing Dry cleaning


Silicon greasing

Creates hydrophobic surface, which inhibits formation of wet surface and encapsulates dirt particles

High labor intensive task

Silicon rubber coating

Creates a hydrophobic surface

Not sticky as silicon grease, provides surface similar to a composite insulator and lasts longer

Shed extenders

Increase the creepage distance

Might create problems if surface characteristics differ from the main insulator surface


• First introduced in 1959 by GE - made of epoxy - problems due to tracking and erosion • Manufactured by others through the 1960s &

1970s - epoxy, Teflon, silicone, EPR, polymer

alloys - end fittings glued, wedged, or crimped

- GFR rods (E and ECR glass) • Matured technology today (mainly silicones)

Non-ceramic (polymeric) insulators - NCIs


• Increased pollution performance (40) • Damage resistance (34) • Easy to transport and installation (35) • High mechanical strength and low weight (27) • Low cost (26) • Aesthetics (16) • Line upgrading (11) • Compact lines (27) • Other reasons (9)

Reasons for using NCIs - CIGRE (74)


• one piece shed structure • track-resistant polymeric housing • housing fully bonded to core and

hardware (sealing against moisture) • crimped end fittings

Current designs


• Circuit breakers • Instrument transformers • Surge arresters • Bushings • Cable terminations

HV components with NCI’s


• Current experience - high quality - used up to 1000 kV • Transmission systems - very popular from 69 to 345 kV - increasing use > 345 kV • Distribution systems - confidence is high - NCIs are often specified rather than porcelain

Current use of NCIs


Need for extremely high reliability !!!!

Development areas: • External insulation: - transmission lines - converter stations

- cable terminations • Converter transformers, barrier system • Wall bushings - electrical, mechanical • Transformer bushings - thermal, electrical • Seismic/mechanical questions (very high structures

in DC-yards)


• Corona damage • Ageing of housing • Stress corrosion fracture of GFR rods • Reliable diagnostics • Stability of hydrophobic properties • Biological contaminations • Installation damage

Current concerns


Impacts of material formulation in coastal environment

Aging mechanisms Leakage current

Corona discharges

Influence of creepage distances Flashover performance

Accelerated aging

Material formulation ATH added

Extra silicone oil added



Periodical field-site evaluation

Leakage current measurement •Statistical evaluation of peak current data •Wave-shape analysis of the current

Visual scrutiny •Erosion marks •Hydrophobicity


LC path erosion on DC, after 2 years

ATH(-)/ AK350(-)

ATH(+)/ AK350(-)

ATH(-)/ AK350(+)

ATH(+)/ AK350(+)


Number of LC pulses > 8 mA

0

200

400

600

800

1000

1200

DC ACATH(-)/AK350(-) ATH(-)/AK350(+)ATH(+)/AK350(-) ATH(+)/AK350(+)


Aging modes

Corona discharges

Spot discharges

Pollution build up

Cold discharges from electrodes and water droplets

Oxidative crosslinking

Pollution build up

Short duration localized, hot discharges between wet spots

Thermal depolymerization


Ageing mechanism

• Short LC pulses - short duration localized heating

• Thermal stress verified by FTIR • High content of D3 in pollution layers • Crater formations as seen with SEM • Weak oxidation, thus, no silica-like

layer formed

Regeneration of LMW siloxanes!

Thermal depolymerization dominant


Conclusions • Thermal

depolymerization dominated the aging

• Short-pulsed localized discharges provided the thermal stress

• The higher ATH content increased the electrical activities on the surfaces but provided better protection against discharge damage

• About 75 phr ATH required for sufficient thermal protection

• LMW siloxanes was regenerated during aging

• Less surface degradation on the insulators compared with the material samples


Department of Materials and Manufacturing Technology

Examples of insulators with growth - I

Suspension insulator installed in South Africa

Distribution type insulator installed in Sri Lanka

Distribution type insulator installed in Tanzania



Examples of insulators with growth - II

Hollow core insulators installed in Anneberg test station, Kungsbacka, Sweden

Distribution type insulator installed in Småland, Sweden



Algae

•Utilizes photosynthesis

•Can be found almost anywhere



Fungi

•No photosynthesis

•Mycelia facilitates colonization of non-solvable materials

•Secretes extracellular enzymes to break polymer chains

•Hyphae may create great turgor pressure



Biological deterioration

Process

Effect

Polymer

Fouling Degradation of leaching components

Change in surface properties

Loss of stability

Loss of stability

Conductivity Swelling

Change in appearance

Biotic degradation

Hydration Penetration

Color

Biofilm Enzymes Radicals



Prevention/removal of growth

Use biocides (substances that kills or inhibit reproduction of microorganisms) - Biomass will remain on the treated surface, facilitating recolonization Add additives that functions as biocides - Should diffuse into the surface but not be washed out… Removal of organic contamination (cleaning) - The properties of the insulator surface is often restored



Additives effecting growth

Addition of the flame retardant zincborate, ZnBO3

Effective against fungi

Very effective against algae



Influence of growth on insulator performance

Generally, the impact of biological growth on electrical performance of composite insulators is rather low. Biofilms has an ability of retaining water on the insulator surface, resulting in increased leakage current levels under wet conditions. However, since the conductivity of growth is low, the observed current amplitudes are probably uncritical. Presence of growth may also alter hydrophobic properties, sometimes even mask hydrophobicity completely of covered regions. Due to this, wet flashover voltage levels have been found to be reduced by up to approximately 30%.


Image analysis - Example

Insulator with growth Detected edges and identified rims (ellipses)

Result of segmentation

Shed model

Covered area



Laser-induced fluorescence (LIF)

Principle of measurements

Excitation by light from pulsed laser

Recording of spectrum of fluorescence light

Fluorescence spectra contain information about energy levels of the studied surface

Possible to detect surface changes and pollutants

Basic idea:


LIF spectroscopy - Example

Measurements on insulator installed in Tanzania and a clean reference.


Remote imaging LIF measurements - I

Truck with laser system

Studied insulators

Exciting laser pulse, 355nm, 4-5ns

Fluorescence light

Folding mirror


Remote imaging LIF measurements - II

Photograph taken from close distance

Mean fluorescence intensity 400-800 nm

Mean fluorescence intensity 670-700 nm


Among the numerous properties that are of interest, resistance to corona and ozone exposure belongs to the group of highly desired properties

Methodology for evaluating resistance to

corona and ozone


Aim of performed work • Development of corona ageing test • Evaluation of resistance to AC

corona/ozone exposure (CIGRE RRT) • Comparison between effects of AC and

DC corona/ozone treatments


Testing Methodology

Components of test arrangement


Corona discharge characterization

AC

+DC

-DC


Effects on properties of material bulk and surface


• Mechanical properties – tensile strength and elongation at break


• SEM observations (e.g. LSR)

Initial AC corona treated


Summary • AC corona/ozone exposure – higher discharge intensity,

higher doses of ozone, more severe surface oxidation • DC corona/ozone exposure – less pronounced influences • Surface resistivity – most affected parameter • Mechanical properties – weakly affected • Surface oxidation – revealed by FTIR & XPS, in line with

ozone concentration • Hydrophilic groups – detected by FTIR, determine

hydrophobicity dynamics • Materials’ behavior – similarly, but distinguishably • Positive indication for HVDC installations


Combined corona-humidity degradation test

Hydrophobicity: advancing and receding angle

Mechanical test: tensile strength and elongation at break

FTIR: surface oxidation and formation of nitric acid

XPS: surface atomic composition SEM: surface topography

Corona treatment in dry condition

Humidification treatment

100 h

65 h

1 h

2 h

1 week 1 week

10 weeks

Corona treatment in dry condition

Humidification treatment

100 h

65 h

1 h

2 h

One cycle of long term corona and humidity exposure of housing materials


Volume and surface resistivity

Values of volume and surface resistivity of the SIR materials (initial and after treatment; calculated for the time of measurement equal to 4×104 s)

Samples Surface resistivity Effect of

treatment Volume resistivity Effect of

treatment Initial Treated Initial Treated

HTV rubber 3.16 × 1016 1.48 × 1012 ↓ 4.33

1.17 × 1013 2.11 × 1013 ↑0.256

LSR 3.55 × 1017 4.90 × 1012 ↓ 4.86

3.31 × 1013 2.44 × 1014 ↑0.868


Mechanical strength

Reference Treated Effect Reference Treated Effect

Samples TS(MPa) σTS TS(MPa) σTS - Eb(%) σEb Eb(%) σEb -

HTV rubber

3.43 0.16 3.30 0.06 ↓3.8 166.8 39.3 119.5 21.2 ↓28.4

LSR 7.22 0.27 5.78 0.24 ↓19.9 391.4 18.5 264.9 11.2 ↓32.3

Tensile strength (left) and elongation at break (right) distributions among specimens cut from treated samples

Test results of tensile strength and elongation at break of the reference samples of HTV rubber and LSR

3

4

5

6

7

0 1 2 3 4 5 6 7 8

Ten

sile

stre

ngth

(MPa

)

Sample number

LSR

HTV

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8

Elo

ngat

ion

at b

reat

k (%

)

Sample number

LSR

HTV


Contact angle measurements

Pictures of water droplets (10 µl) on the surface before (left) and after 100 h corona treatment (right) for test 2 Note : The picture of right side is taken immediately after 100 h corona treatment

Contact angles at various stages of corona/humidity treatment (note: the test round 0 indicates the reference value measured before the treatment). Note: This contact angles measurement were carried out after humidification of each rounds

20

40

60

80

100

120

140

-1 0 1 2 3 4 5 6 7 8 9 10

Con

tact

ang

le (º

)

Test rounds

θa θr

60708090

100110120130140

-1 0 1 2 3 4 5 6 7 8 9 10

Con

tact

ang

le (º

)

Test rounds

θa θr

HTV Rubber LSR


Surface inspection by scanning electron microscopy (SEM)

100 µm 100 µm

10 µm 10 µm

SEM images of surfaces of the treated samples

HTV LSR The width of the cracks were typically ~1 µm (marked with arrows) and 5-8µm for the HTV rubber and LSR, respectively.


SEM continue…

10 µm

364.7nmcrack

surface

volume

1.240 µm

surface

volume10 µm

crack

2 µm

364.7nmcracksurface

volume 10 µm

brittlesurface

volume

1.021 µm

Cross sections of freeze-fractured treated specimens

HTV LSR

The thickness was lower for the HTV rubber, in the order of 0.3 – 0.4 µm; whereas the thickness was around 1-1.2 µm for the LSR.


Conclusions The treatment yielded a strong reduction of surface resistivity of both the materials, though still

remaining within the limits set for safe functioning in outdoor applications.

The mechanical properties were not affected significantly, less for the HTV material.

The hydrophobic properties, after an initial decrease, recovered with the duration of the treatment.

The penetration depth of this process remained however limited to a depth of ~1 µm, indicating that a long term exposure of silicone rubber based materials to subsequent corona and humidity cycles does not create strong risks as regards material’s mechanical and electrical integrity.

Although, the difference in the roughness of the surfaces of both materials before the treatment was obvious, it became afterwards similar to each other.

The investigations aiming at detecting gases formed during the corona treatment as well as the later performed infrared analyses of the treated surfaces did not reveal any detectable evidences of acidic contaminations.


Can one further improve insulator performance? Does it mean improving the hydrophobic properties?

Let us learn from nature!


Hydrophobic plants


Treated

Untreated

Model of high voltage insulator


53

An RSA patent superhydrophobic, self-cleaning surfaces

one-pot formulation applied in one step, easily onto large convoluted surfaces

Greyling, C; PCT/ZA2008/000121

(Nano)3

Nano TiO2 photocatalyst

Nano-structured micron roughness 1μl water drop, 270 μm diameter

on rough surface of insulator

Date post:	12-Feb-2022
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