Surface Erosion Studies on Polymer Insulators Used for HighVoltage Transmission

B. S. Reddy, A. R. Verma, and D. S. PrasadDepartment of Electrical Engineering, Indian Institute of Science, India

Abstract—Polymeric/silicon rubber insulators are being widely used in high voltage transmission and distribution systems due totheir advantageous over the conventional porcelain or glass insulators. The polymeric insulators being organic in nature experiencesurface degradation from environment as well as tracking, erosion, corona, dry-band arcing etc, due to continuous electric stress.The present work reports the results of tracking and erosion study conducted on commercially used silicon rubber insulators fordifferent acidic contamination conditions. A comparison is made for both high temperature vulcanized rubber (HTV) and liquidsilicon rubber LSR) insulating materials used for high voltage transmission.

Keywords—Surface erosion, tracking, polymer insulators, contamination, FTIR, SEM

I. INTRODUCTION

With the increase in power demand, there is a necessityfor the efficient, reliable power generation and transmissionsystems. Outdoor insulators play an important role in thehigh voltage power transmission and distribution systems. Thepolymer/silicon rubber insulators have exhibited some betterfeatures in comparison with the conventional porcelain or glassinsulators, particularly due to (i) better pollution performance(ii) light weight (iii) hydrophobicity recovery etc [1], [2].But the long term service performance of these polymerinsulators is not yet completely understood by the utilities andmanufacturers. CIGRE working group D1.14 [3] has identifiedresistance to tracking and erosion as one of the importantparameter to be evaluating the shed materials. The inclinedplane test (IPT) as per IEC 60587 [4] is used to access theresistance to tracking and erosion of the polymeric materials.Some of the important works on the tracking and erosionare reported in [5]–[8]. Recently these insulators are beingused for DC applications, hence to study performance of thesilicone rubber insulators using IPT under DC stress is gainingimportance.

However, with the ever increase in the pollution level; itnecessitates to understand the performance of the polymericinsulators under different acidic environment conditions. Someof the earlier works in this regard are reported in [9], [10].

In the present work two commercially available siliconerubber insulator samples are examined for the resistanceagainst the tracking and erosion using solution of differentacidic contamination. It is believed that the study of this kindwill help in better understanding the material performanceunder extreme contaminated environmental conditions.

Corresponding author: B. Subba Reddye-mail address: reddy@ee.iisc.ernet.in

Presented at the 3rd ISNPEDADM 2015 (New electrical technologies forenvironment), in October 2015

II. EXPERIMENTAL INVESTIGATIONS

A. Samples used for experimentation

The silicon rubber (SIR) materials obtained from the insu-lator manufacturers are used for present investigations. Twotypes of samples namely high temperature vulcanized (HTV)silicone rubber, and Liquid Silicone Rubber (LSR) samples areused for the study. The HTV SIR is a polydimethylsiloxane(PDMS) with 50% of aluminatrihydrate (ATH) added as fillermaterial. Liquid silicone rubber is made up of two compo-nents, called as component A and component B, mixed inthe ratio 1:1. Silica is added as a filler material and platinumis added as a curing agent. Test samples are cut as per thestandard size for the IPT (50 cm×120 cm×6 mm). All thesamples are cleaned using the iso-propane alcohol before theexperimentation.

B. Contamination solution for experimentation

As per IEC Std [4], the inclined plane experimentation hasto be conducted with the ammonium chloride (NH4Cl) as thecontaminant. In the present study the acidic contamination isprepared with the salt concentration of the solution adoptedfrom the acidic rain data recorded in Mexico [11], [12]. Theconstituents of the acidic solution are given in Table I. ThepH of the solution is adjusted to a required value by addingthe concentrated nitric acid to the solution.

TABLE ICOMPOSITION FOR ACID RAIN OF PH 3.3 [11], [12]

Ingredients Quantities (mg/l)

NH4Cl 235NaCl 170KCl 5.9

MgSO4 35.2CaSO4 29.4

Reddy et al. 121

(a) Block diagram of the experimental setup

(b) Schematic of the experimental arrangement.(1) 50 kV HVAC Source, (2) Resistor bank, (3) IP setup, (4)

Dropping arrangement.

Fig. 1. Experimental setup.

C. Experimental arrangement

The block diagram and the schematic of the experimentalarrangement is shown in Fig. 1(a) and (b). The test setupconsists of a 50 Hz HVAC source of rating 50 kV, 50 mA, inseries with the resistor bank comprising of resistors of 1 kΩ,11 kΩ, 22 kΩ and 33 kΩ, a suitable resistor is adopted ac-cordingly for the selected voltage range. The discharges duringthe experiments are monitored using the Rigol digital storageoscilloscope (100 MHz and 1 GS/s). The online Fast FourierTransformation (FFT) is performed to obtain the frequencycomponent of the discharge current. Contamination flow isadjusted at the rate of 1 ml/min using a dripping arrangement.The experiments are conducted at a voltage magnitude of3.5 kVrms for all the samples and the time adopted for theexperimentation is of three hours duration.

III. RESULTS AND DISCUSSION

A. Results of experimentation

The inclined plane experiment primarily simulates the dryband arcing phenomenon observed during the contaminationcondition on the insulators in service. Fig. 2(a) shows the dis-charge current wave shape without any partial arcs or scintil-lation during the IPT experiment, the FFT of the current waveshape indicated only the fundamental component (50 Hz).During the scintillation period (due to the contamination flow)the current wave shape will be predominantly composed of theodd harmonics (3rd, 5th, 7th, etc.,) as evident from the Fig.2(b). The odd harmonics injected into the discharge currentsignifies the thermal heating [13]. There by the samples get

(a) Without dry band arcing

(b) With dry band arcing

Fig. 2. Current waveform and corresponding FFT.

(a) Severe burning near the ground electrode

(b) Samples after the experimentation

Fig. 3. Experimental results.

burnt near the junction of the bottom electrode (as shown inFig. 3(a) and (b)).

During experimentation a thermal imager of TESTO makewas used to monitor the discharge activity on the sample. Theinitial temperature distribution observed on the sample is asshown in Fig. 4. The temperature is found to be more near

122 International Journal of Plasma Environmental Science & Technology, Vol.10, No.2, DECEMBER 2016

(a)

(b)

Fig. 4. (a) Thermal image of the sample under initial stage of IPT treatment,(b) temperature profile along the line P1.

the bottom electrode and steadily decreases along the P1 (seeFig. 4(a)) in the direction of top electrode. During the burningstage the temperature rise was observed to be more than 350Cwhich will cause the erosion of the material.

The erosion results into weight loss, the material loss onthe sample was measured using a precision electronic weighbridge having a resolution of 0.1 mg. The percentage weightloss results are presented in Fig. 5(a) and (b), which show thatthe loss of weight is more in case of pH 2 and reduces withincrease in pH (decreasing acidity). It was observed for LSRsample more weight loss in comparison with that of HTVsamples. On an average the weight loss increased 10 timesmore for the LSR in comparison with the HTV silicone rubber.

B. Results of physico-chemical analysis

The treated samples have been further analysed usingthe physico-chemical tehniques including fourier transforminfrared (FTIR) spectroscopy, scanning electron microscopy(SEM) and the energy dispersive X-ray analysis (EDAX). TheFTIR conducted on the treated LSR samples are presented inthe Fig. 6. The FTIR is performed using the Perkin Elmermake FIR/ MIR spectrometer fronteir model with attunuatedtotal reflection (ATR) equipped with the diamond crystal.Each sample has been scanned for 16 times and the averagespectrum is reported. The following bonds are of interestin evaluating the silicon rubber (PDMS) materials, C−Hsymmetric stretching in CH3 at 2962 cm−1, CH3 asymmetricstretching at 1260 cm−1, Si−O−Si symmetric stretching at

(a)

(b)

Fig. 5. Percentage weight loss during IPT. (a) for HTV SIR and (b) for LSRsamples.

Fig. 6. FTIR spectrum of fresh and treated LSR samples.

1008 cm−1 and Si−C symmetric stretching at 788 cm−1 [14].

It may be seen from Fig. 6, the peaks corresponding to theSi−O−Si bond (1008 cm−1 wave number) are getting reducedfor the treated samples indicating depolymerization. It is alsoobserved with the increase in acidity the bond breakage ismore intense. Thus the increase in acidity is having adverseeffect on the performence of the polymeric insulator. It isthought a similar phenomena is likely to occur in the field.

SEM analysis is carried out using FEI make FEG Quanta200 machine, which is equipped EDAX detector. Each samplehas been gold coated with a thickness of 10 nm using goldsputtering technique, before conducting SEM.

The results of the SEM studies are presented in Fig. 7(a) and(b) which show the SEM image of the fresh and the treatedHTV sample. Similarly EDAX has also been conducted onthe fresh and the treated samples. Fig. 7(c) and (d) representsthe EDAX results showing the X-ray count from different

Reddy et al. 123

Fig. 7. SEM and EDAX images.

elements on the surface. It may be observed that in comparisonto the fresh sample, the X-ray count for the aluminum (Al)has increased in case of treated samples. The reason could beattributed to the liberation of the aluminum from the ATH dueto the excessive burning and the traces of aluminum may haveentered into the insulator surface, hence EDAX results showmore aluminum.

IV. CONCLUSION

Following conclusions are drawn from the present study.

• The increasing acidity adversely affects the performanceof the silicone rubber insulator samples.

• The performance of the HTV silicone rubber insulator isfound to be better than the liquid silicone rubber samples.

• The percentage weight loss is found to be more for theLSR in comparison with that of HTV silicone rubber.

• The increase in the acidity of the contamination resultsin higher de-polymerization (evident from the results ofFTIR studies).

• Experimentation on the performance of polymeric sam-ples to DC stress is in progress.

ACKNOWLEDGMENT

The authors are grateful to the authorities of Indian Instituteof Science, Bangalore, India for permission to publish thispaper.

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