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Chalmers University of Technology
Performance of Silicone Rubber Based Materials for Applications
in Outdoor Insulation
S.M. Gubanski Chalmers University of Technology
Gothenburg, Sweden
Chalmers University of Technology
• 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
Chalmers University of Technology
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
Chalmers University of Technology
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)
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
• 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
Chalmers University of Technology
• 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)
Chalmers University of Technology
• one piece shed structure • track-resistant polymeric housing • housing fully bonded to core and
hardware (sealing against moisture) • crimped end fittings
Current designs
Chalmers University of Technology
• Circuit breakers • Instrument transformers • Surge arresters • Bushings • Cable terminations
HV components with NCI’s
Chalmers University of Technology
• 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
Chalmers University of Technology
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)
Chalmers University of Technology
• Corona damage • Ageing of housing • Stress corrosion fracture of GFR rods • Reliable diagnostics • Stability of hydrophobic properties • Biological contaminations • Installation damage
Current concerns
Chalmers University of Technology
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
Chalmers University of Technology
Chalmers University of Technology
Periodical field-site evaluation
Leakage current measurement •Statistical evaluation of peak current data •Wave-shape analysis of the current
Visual scrutiny •Erosion marks •Hydrophobicity
Chalmers University of Technology
LC path erosion on DC, after 2 years
ATH(-)/ AK350(-)
ATH(+)/ AK350(-)
ATH(-)/ AK350(+)
ATH(+)/ AK350(+)
Chalmers University of Technology
Number of LC pulses > 8 mA
0
200
400
600
800
1000
1200
DC ACATH(-)/AK350(-) ATH(-)/AK350(+)ATH(+)/AK350(-) ATH(+)/AK350(+)
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
Department of Materials and Manufacturing Technology
Examples of insulators with growth - II
Hollow core insulators installed in Anneberg test station, Kungsbacka, Sweden
Distribution type insulator installed in Småland, Sweden
Chalmers University of Technology
Department of Materials and Manufacturing Technology
Algae
•Utilizes photosynthesis
•Can be found almost anywhere
Chalmers University of Technology
Department of Materials and Manufacturing Technology
Fungi
•No photosynthesis
•Mycelia facilitates colonization of non-solvable materials
•Secretes extracellular enzymes to break polymer chains
•Hyphae may create great turgor pressure
Chalmers University of Technology
Department of Materials and Manufacturing Technology
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
Chalmers University of Technology
Department of Materials and Manufacturing Technology
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
Chalmers University of Technology
Department of Materials and Manufacturing Technology
Additives effecting growth
Addition of the flame retardant zincborate, ZnBO3
Effective against fungi
Very effective against algae
Department of Materials and Manufacturing Technology
Chalmers University of Technology
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%.
Chalmers University of Technology
Image analysis - Example
Insulator with growth Detected edges and identified rims (ellipses)
Result of segmentation
Shed model
Covered area
Department of Materials and Manufacturing Technology
Chalmers University of Technology
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:
Chalmers University of Technology
LIF spectroscopy - Example
Measurements on insulator installed in Tanzania and a clean reference.
Chalmers University of Technology
Remote imaging LIF measurements - I
Truck with laser system
Studied insulators
Exciting laser pulse, 355nm, 4-5ns
Fluorescence light
Folding mirror
Chalmers University of Technology
Remote imaging LIF measurements - II
Photograph taken from close distance
Mean fluorescence intensity 400-800 nm
Mean fluorescence intensity 670-700 nm
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
Testing Methodology
Components of test arrangement
Chalmers University of Technology
Corona discharge characterization
AC
+DC
-DC
Chalmers University of Technology
Effects on properties of material bulk and surface
Chalmers University of Technology
• Mechanical properties – tensile strength and elongation at break
Chalmers University of Technology
• SEM observations (e.g. LSR)
Initial AC corona treated
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
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
Chalmers University of Technology
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.
Chalmers University of Technology
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.
Chalmers University of Technology
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.
Chalmers University of Technology
Can one further improve insulator performance? Does it mean improving the hydrophobic properties?
Let us learn from nature!
Chalmers University of Technology
Hydrophobic plants
Chalmers University of Technology
Treated
Untreated
Model of high voltage insulator
Chalmers University of Technology
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