2007 International Conference on Solid Dielectrics, Winchester, UK, July 8-13, 2007

Long-term Accelerated Multistress Aging of Composite Outdoor Polymeric Insulators

B.Venkatesulu* and M. Joy ThomasHigh Voltage Laboratory

Department of Electrical EngineeringIndian Institute of Science, Bangalore, India.

* Email: venkatesulughve.iisc.ernet.in

Abstract: Polymeric outdoor insulators are beingincreasingly used for electrical power transmission anddistribution in the recent years. One of the current topicsof interest for the power transmission community is theaging of such outdoor polymeric insulators. A fewresearch groups are carrying out aging studies at roomtemperature with wet period as an integral part ofmultistress aging cycle as specified by IEC standards.However, aging effect due to dry conditions alone atelevated temperatures and electric stress in the presenceof radiation environment has probably not beenexplored. It is interesting to study and understand theinsulator performance under dry conditions where wetperiods are either rare or absent and to estimate theextent of aging caused by multiple stresses.

This paper deals with the long-term acceleratedmultistress aging on full-scale 11 kV distribution classcomposite silicone rubber insulators. In order to assessthe long-term synergistic effect of electric stress,temperature and UV radiation on insulators, they aresubjected to accelerated aging in a specially designedmultistress-aging chamber for 3800 hours. All thestresses are applied at an accelerated level. Using a dataacquisition system developed for the work, leakagecurrent has been monitored in LabVIEW environment.Chemical changes due to degradations have beenstudied using Energy Dispersive X-Ray analysis,Scanning Electron Microscope and Fourier transformInfrared Spectroscopy. Periodically different parameterslike low molecular weight (LMW) molecular content,hydrophobicity, leakage current and surfacemorphology were monitored. The aging study is underprogress and only intermediate results are presented inthis paper.

INTRODUCTION

insulators in service experience degradation due toelectrical stress, mechanical stress and environmentalstresses like temperature, pollution, UV radiation, fogand rain.

It is well known that under wet conditions dryband arcing occurs in service resulting in loss ofhydrophobicity subsequently leading to flashover ifdischarges persists or repeatedly occur. It is reportedthat heat generated due to discharges deteriorateshydrophobicity. Hence, heat generated due to dischargesand long-term effects of ambient temperature are someof the essential aging factors, which can decide long-term performance, reliability and the life of insulators.Multistress accelerated aging of insulators has generatedlot of interest among researchers as it can simulatereasonably well, conditions as seen by the insulator inservice.

It is known that the average electric field ondistribution line insulator is significantly less ascompared to transmission line insulators, which impliesreduced occurrence of dry band arcing. Hence,temperature and other weathering conditions can bemore aggressive aging factors than the effect due to dryband discharges in case of distribution class polymericinsulators. Few researchers have conducted aging testson distribution polymeric insulators. But, multistressaging on full-scale insulator has probably not beenconducted simulating tropical atmosphere to assess thedamage due to weathering factors and electric stress. Itis of interest to know the behaviour and to assess thelong-term performance of insulation and to see how dryconditions alone can age the insulator.

EXPERIMENTAL SET UP

AGING CHAMBER

Composite polymeric insulators have generated a lot ofinterest among electric utilities and are replacingconventional overhead transmission and distributionline insulators over the last few decades. Polymericinsulators offers several advantages like light weight,better vandalism resistance, better contaminationperformance, easy installation and transportation.However, being organic in nature they are vulnerable toaging due to weathering. Aging is manifested in termsof discolouration, chalking, cracking, strength loss,erosion, and reduction in contamination flashovervoltage due to reduced hydrophobicity. Polymeric

1-4244-0750-8/07/$20.00 ©2007 IEEE.

A multistress-aging metallic chamber of 0.56 m x 0.56m x 0.71 m was designed and developed for the study.The chamber is capable of subjecting the 11 kVinsulators to accelerated electrical stress, controlledthermal stress and different levels ofUV radiation eithersingly or simultaneously. Eight UV fluorescent tubesand a UV lamp are arranged as shown in Fig. 1. Thespectrum of UV sources was in UV-A region and closeto the Sun light spectrum as ascertained with HR-400,UV spectro photometer made by Ocean optics. UVintensity on the insulators was measured with UV lightmeter (Lutron make) and it was found to be 1.1mW cm2 + 15 00 on four insulators (Sample codes-

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303 kO

Fig. 1: Test set up for the aging studies

AS02, AS03, AS05 and AS08), 0.85 + 15 0O mW/cm2on ASOI and AS06, 2 + 15 0 mW/cm2 on insulatorAS07 and 3 mW/cm2 on insulator AS04 so that differentaccelerated intensities are available in the same run .

There is a small variation ofUV intensities on differentweather sheds of the same insulator.

DATA ACQUISITION (DAQ) SYSTEM

A PC based eight-channel data acquisition card ofNational Instruments make, with 16- bit resolution and250 kS/sec sampling rate for each channel is employedto acquire the leakage current. LabVIEW software andassociated driver software is used for the DAQ system.Leakage current is passed through 200 Q shunt resistorsand drop across the resistors is fed to DAQ system. Onechannel has been used as reference channel to determinethe zero crossover of the applied voltage.

TEST SAMPLE

Commercially available and nominally identical full-scale 11 kV composite silicone rubber insulators areused for the aging test. Each test insulator has got threeidentical weather sheds. Its leakage distance is 305 mm,shed spacing is 51 mm, dry arc distance is 134 mm andshed, sheath and FRP rod diameters were 90, 23 and17.5 mm respectively.

AGING METHODOLGY

Eight samples were continuously subjected to voltagestress of 9.1 kV, temperature of 88 °C, UV intensities of0.85 mW/cm2, 1.1 mW/cm2, 2 mW/cm2 and 3 mW/cm2in the same run as explained earlier.

CHEMICAL ANALYTICAL STUDIES

Energy dispersive X-ray (EDX) and ScanningElectron Microscopy (SEM) studies

An EDX technique is used to estimate the aging statusof the insulator surface. The elemental compositions ofthe surface of the aged as well as new samples give anidea on the level of aging. SEM studies reveal thephysical changes on the insulator surface likeroughness, micron cracks, erosion and chalking etc.Scanning Electron Microscope machine (Model FEISirion) equipped with EDX probe has been used forboth the studies. The samples used were small pieces of4 mm x 4 mm x 3 mm cut from the weather sheds.Sample surfaces were gold-coated using ion-sputteringmachine of Joel make (Model JFC- IOBE). Anaccelerated voltage of 20 kV is employed for theprobing. As the probing areas are of micron size,different sizes of areas were probed to ensure theconsistency in results and conclusion made. It wasnoticed that at 2500X magnification, consistent resultswere obtained.

Fourier Transform Infra-Red (FT-IR) Spectroscopy:

FT-IR spectroscopy has been used to identify thechanges in functional groups and molecular structure onthe surface with aging.

A Perkin Elmer Spectrum GX FT-IRspectrometer with ATR attachment was employed toobtain IR spectra. The new and aged samples werethoroughly cleaned with isopropyl alcohol anddesiccated before the test. Flat samples cut from theweather shed of about 20 mm x 9 mm x 3 mm wereplaced on the Ga-As crystal and spectra obtained.

PHYSICO-CHEMICAL PROPERTY

Extraction of Low molecular weight (LMW)molecules:

Low molecular weight molecules are responsible for thehydrophobic property of the surface. Leakage currentand subsequently flashover are much dependent onLMW molecules content on the surface. Hence, this isan important property, which imparts superiorperformance for polymeric material.

Silicone rubber samples 20 mm x 9 mm x 3mm were cut from the weather sheds of new, AS02,AS04 and AS07 samples. They were immersed in n-hexane to extract LMW molecules for about 100 hoursand afterwards removed from n-hexane and kept outsidefor about 30 hours to allow the n-hexane to getevaporated [1]. The weight of the sample before andafter immersion gives the extracted LMW moleculescontent. High precision digital balance with readabilityof 0.1 mg has been used for weighing the samples.

Contact angle:

Contact angle reflects the state of hydrophobicity of thesurface. The surface free energy or hydrophobicitychanges due to structural changes, chalking, cracking,and roughness of the surface with respect to aging. It is

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one of the vital parameters to assess the level of aging.Contact angle measurements need to be analysed withsufficient care as factors like drop volume, the way it isplaced on the sample surface, time elapsed after placingthe drop and the measurement taken, ambient humidityand liquid used for the study etc. affects the magnitudeof the contact angle. As the contact angle is verysensitive to the size of the droplet placed on the surface,identical drop sizes were placed on the surface foranalysis.

Rame-hart Goniometer was employed tomeasure the contact angle, which has a tolerance of twodegrees. A sample cut from the weather shed of size 20mm x 9 mm x 3 mm was placed on the sample holder ofGoniometer. Sufficient care was taken to make thesurface reasonably flat as sample cut from weather shedwas like a tapered section. Using micro-pipette 10 ulsessile drop was placed on the sample surface bybringing pipette tip close to the surface. The contactangle is obtained through a graduated scale. Averagecontact angle magnitude reported is the average of atleast ten drops

PHYSICAL PROPERTY

Surface roughness

Surface roughness is a permanent change due to aging.The surface roughness parameter helps to assess thelevel of aging. Using Veeco 3D optical profiling systemmodel WYKO NT 1100 surface roughness 3D profileand 2D profiles were obtained

RESULTS AND DISCUSSIONS

to be more rough than the other samples. Due to spacelimitations only one aged sample image is provided as arepresentative image. The increased roughness wasconfirmed by surface roughness measurement using anoptical Profilometer. On samples AS02 and AS04, thesurface roughness increased by close to 75 00 and onAS07 increase in roughness is about 33 00 compared tothe new sample (Table 2). As the samples get agedsurface becomes more rough which was also noticed bythe earlier researchers [3].

a) New sample surface

b) Aged sample surface

Fig. 2: SEM images of new and aged insulatorsEDX Studies

Table 1 shows the EDX results of new and agedsamples. It is evident from the results that UV radiationat elevated temperatures in the presence of electricstress oxidises material surface significantly. After 3800h of aging, it can be seen that oxygen content in agedsample increased substantially compared to the newsample. Increased oxygen content can be due tooxidation caused by UV [2]. However, it is alsoobserved that the increase in oxygen content is not inproportion to the UV stress levels.

Table 1. EDX data after 3800 hours of ageingSample New AS02 AS04 AS07Element C 0 C 0 C 0 C 0Weight%o 27.2 23.4 24.8 29.8 31.8 26.6 29 26.6

SEM studies

It can be seen from the SEM images (fig. 2) that theaged sample surface became more rough than the newsample due to multistress aging. AS02 surface appears

a)00X

%.10

(1

Q)cQ

118 -116 -114 -112 -110:108-106-104-102-100-98-

a AS02b AS04c AS07d New

d~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

a

1000 2000Wave Number (cm-1)

4000

Fig. 3: FTIR spectra of new and aged insulators

FTIR studies

New sample FTIR spectra lies above all the otherspectra indicating that absorbance of that sample islarge. The intensity of absorption peaks of aged samplesat 2300 and 3700 cm- were less as compared to the new

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Table 2. Surface roughness and LMW moleculescontent after 3800 hours of ageing

Surface Wt. % of the % reduction inSample Srae 0ofte LMW contentroughness LMW moleculesCode w.r.t to new(um) content smlsampleNew 1.39 2.614AS02 2.44 1.948 9.296AS04 2.59 2.297 14.393AS07 1.85 2.291 8.522

sample. This may be due to the depolymerisation andscission [3]. Since the spectrum was obtained without aclamping device for ATR accessory there is a slightinconsistency in the spectra.

LMW molecular studies

From table 2, it is clear that as the sample gets aged, thecontent of LMW molecules gets reduced. There is amaximum reduction of about 14 00 on the surface ofAS02 and about 8 00 on the surface of AS07 ascompared to the surface of the new sample. It is verywell known that discharges on the wet surface ofinsulator consume volatile LMW molecules but fromthese results it is evident that even under dry conditions,multistress depletes considerable amount of LMWmolecules thus enhancing the wetting of the insulator.Subsequently life of the sample gets reduced due to theeffect of intensified surface discharge. Surface oxidationaffects the mobility ofLMW molecules from the bulk tothe surface [4].

It is also noticed (Table 3) that significantamount of reduction in the contact angle occurs on theaged samples which is in agreement with reduction inLMW molecular weight percentage.

insulators were recorded. A higher leakage current onAS07 could be due to processing or manufacturingvariations of that particular sample. Hence, leakagecurrents on dry surfaces may not show the actual agingcondition of the samples or the incipient aging processesat the molecular level. Also, monitoring of leakagecurrent for longer durations may indicate the quality ofthe sample.

120-

:' 100-

80-a) 60-0a) 40-` 20-a)

Ij 0-

0

AS02AS04AS07

1000 2000 3000Aging period ( h)

I

4000

Fig. 4: Leakage current of aged insulators

Conclusions

1. Substantial oxidation takes place on the surface ofthe polymeric sample at the current level of aging

2. An increase in the surface roughness and a moderatereduction in the contact angle have been observed

3. Multistress even under dry conditions can age thematerial considerably in the long run

4. Monitoring of leakage current under dry conditionsdoes not reflect the chemical changes on the surfaceat the molecular level

References

Table 3. Contact angle on the surfaces of a new andaged samples for a duration of 3800 hours measured

with a droplet size of 10 uL

Sample Max. Min. Average(deg) (deg) (deg)

New 107 104 105.2AS02 102 97 99.36AS04 102 97.5 99.03AS07 102 92 97.33

Leakage Current studies

Fig 4 shows that there is no appreciable change in theleakage current flow on the surface of differentinsulators. One of the reasons may be constantresistance offered by the dry surface in spite of thecurrent level of aging. Peak leakage currents of 6 pA,5.7 ptA and 110 pA were observed on samples AS02,AS04 and AS07 respectively. After about 2500 hours ofaging a step increase in leakage current in all the

[1] H. Deng, R. Hackam "Low-molecular weightSilicone fluid in RTV Silicone rubber coatings"IEEE Transactions on Dielectrics and ElectricalInsulation, Vol. 6, No. 1, pp 84-94, Feb.1999

[2] Chang-Su Huh, Bok-Hee, Youn Sang-Youb Lee"Degradation in Silicone rubber Used for OutdoorInsulator", Proc. of 6th Int. Conf. on Properties andApplications of Dielectric Materials, China, pp.367-370, June-2000

[3] R. S. Gorur, D.W. Gedach, R.S. Thallam, "AgingIn Outdoor Insulating Polymers Due To UV andHigh Temperature," Conference on ElectricalInsulation and Dielectric Phenomena, pp.68-73,Oct.1991.

[4] H. Hillborg, U. W. Gedde "Hydrophobicityrecovery of polydimethylsiloxane after exposure tocorona," Polymer, Vol. 39, No. 10, pp. 1991-1998,1998.

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