Laboratory Investigation on Hydrophobicity and Tracking performance of Field Aged Composite Insulators

Laboratory Investigation on Hydrophobicity and Tracking performance of Field Aged Composite Insulators N. Mavrikakis Faculty of Engineering Aristotle University of Thessaloniki nmavrika@auth.gr K. Siderakis Technological Educational Institute of Crete ksiderakis@staff.teicrete.gr P. N. Mikropoulos Faculty of Engineering Aristotle University of Thessaloniki pnm@eng.auth.gr Abstract- Evaluation of the field performance of composite insulators is essential for maintaining the desired levels of reliability in power networks. Field performance of composite insulators depends on many factors, including the insulator design, material quality and service conditions experienced. The most critical part of the composite insulator is its external housing, usually made of silicone rubber having the advantage of recovering its hydrophobic properties in polluted conditions. Thus, condition assessment of the hydrophobic properties of the housing material especially under erosion and tracking processes is of major importance. In this study performance evaluation of the housing material of 150 kv field-aged silicone rubber insulators is carried out through physical, electrical and material analysis techniques. The hydrophobic properties of the housing material were found degraded to a different extent between field-aged insulators due to differences in material structure and pollution conditions. Hydrophobicity can be better assessed if the relevant diagnostic techniques are also applied after inclined plane tests. Index Terms- Ageing, Composite, Erosion, Hydrophobicity, Insulator, Silicone rubber, Tracking I. INTRODUCTION Insulators are one of the most important components affecting reliability of high voltage power networks. Ceramic insulators, made of either porcelain or glass, are increasingly replaced by composite insulators, commonly made of silicone rubber, due to their superior performance in polluted conditions. However, this advantage comes together with a comparably weak structure, vulnerable in ageing mechanisms mostly related to partial discharge activity. Thus, evaluation of the field performance of composite insulators is essential for maintaining the desired levels of reliability in power networks. The field performance of composite insulators depends on many factors, including insulator design, material quality and service conditions experienced. In the present study results on performance evaluation of the housing material of field-aged silicone rubber (SIR) insulators are presented. Insulators were sampled from the 150 kv transmission network of Crete, which being exposed to sea influence suffers from severe pollution. Over 50% of the insulators in the 150 kv transmission network of Crete are SIR insulators, operating from 2 up to 20 years. Hydrophobicity classification and structural analysis of the housing material of the field-aged insulators were performed before and after inclined plane tests; the latter were conducted as specified by IEC 60587 [1] so as to evaluate the resistance against tracking and erosion. The hydrophobic properties of the housing material were found degraded to a different extent between field-aged insulators due to differences in material structure and pollution conditions. II. CONDITION ASSESSMENT PROCEDURE Condition assessment of the housing material of composite insulators is conducted by physical, electrical and material analysis techniques [2] - [6]. In this study, the following diagnostic procedure was adopted in order to evaluate the hydrophobic properties and the resistance against tracking and erosion of field-aged silicone rubber (SIR) insulators: i. Insulator sampling. ii. Visual inspection according to STRI [7] and EPRI [8] visual inspection guides. iii. Pollution measurements according to IEC 60815 [9], to classify insulator samples based on pollution severity. iv. Hydrophobicity assessment according to IEC 62073 [10]. v. Fourier Transform Infrared (FTIR) spectroscopy, providing information on material structural bonding. vi. Inclined plane test (IPT) according to IEC 60587 [1], to evaluate the tracking and erosion resistance of the housing material. vii. Hydrophobicity assessment and FTIR spectroscopy are conducted repeated after the IPT, so as to evaluate the hydrophobic properties of the SIR insulators as affected by accelerated ageing. Composite insulators aged in service were sampled from the 150 kv transmission network of Crete. Insulators A and B (Fig. 1) were removed after seventeen years in service without having experienced any failures. There was limited data concerning the chemical structure of the housing material of the insulator samples; however, FTIR spectroscopy showed silicone rubber and alumina trihydrate (ATH) fillers as structural compounds. Insulators A and B were operated at coastal overhead transmission lines, thus, exposed to severe environmental stress conditions, including high UV radiation and salt deposition. Fig. 1 shows the

observed for insulator A than B. Actually, according to the pollution severity classification for composite insulators suggested by IEC [9], the pollution levels for insulators A and B are heavy and medium, respectively (Fig. 2). The higher pollution levels of insulator A can be attributed to the fact that it was subjected to, besides marine, industrial pollution. TABLE I ESDD AND NSDD MEASUREMENTS OF FIELD INSULATORS Insulator A B Temperature, o C 17.7 16.8 Collecting pollutants area, cm 2 448.428 318.645 Salinity, kg/m 3 0.1096 0.0424 ESDD, mg/cm 2 0.0978 0.0532 NSDD, mg/cm 2 0.1668 0.0951 Fig.1: Meteorological data (1955-1997) for the region of installation of the line composite insulators. monthly variation in weather conditions for the city of Heraklion over the period of 1955-1997; these climatic data, obtained from the Hellenic National Meteorological Service [11], can be considered as typical for the region of installation of the evaluated line composite insulators. III. CONDITION ASSESSMENT RESULTS A. Visual inspection Increased levels of contamination were observed by visual inspection for both field aged insulators. Pollution was more notable in the insulator upper sheds, near to the suspension tower end. After cleaning the polymer housing, thin surface scratches and partial surface discolouration were detected especially for insulator A. In addition, local oxidation of metal end fittings was detected. These findings do not impose a need for immediate replacement of the field insulators according to STRI [7] and EPRI [8] guides. It must be noted, however, that, due to intense dust accumulation on the housing of the insulators, it was not possible to draw reliable conclusions on the degradation severity classification of each insulator sample. B. Pollution measurements Both insulators were operated at Crete s northern coastal line (Fig. 1), along which large amounts of sea salt are carried by strong onshore winds and deposited on equipment. Thus, to determine the amount and type of pollution deposition, equivalent salt deposit density (ESDD) and non soluble deposit density (NSDD) measurements were performed according to IEC 60815 [9]. These measurements were conducted by the Hellenic Electricity Distribution Network Operator immediately after insulator samples collection. As shown in Table I, higher ESDD and NSDD values were Insulator A Insulator B Fig. 2: Pollution severity classification of field composite insulators [9] C. Hydrophobicity measurements Hydrophobicity measurement of insulators was conducted in accordance to IEC 62073 [10], implementing methods C (spray method) and method A (contact angle method). Using the spray method, based on the appearance of the insulator surface after water mist exposure, wettability classes (WC) 6 and 2 were determined for insulators A and B respectively (Fig. 3). It must be noted that these results were obtained without removing surface pollutants prior to water spraying. Contact angle measurements were performed with the aid of the KSV-CAM 101 instrument [12]. Water droplets about 10 μl in volume were applied with a syringe on nine samples taken from different areas of the housing of each insulator (Fig. 4), the latter cleaned with distilled water. Average static contact angles of 84 and 103 degrees were measured for insulators A and B, respectively. Fig. 3: Insulators A (left) and B (right) after water mist exposure; middle area of insulators

0.9 0.8 0.7 C_new A_field-aged B_field-aged Si-O-Si stretching 0.6 Static contact angle: 95 o ; high voltage Static contact angle: 97 o ; ground area area of insulator A of insulator B 140 Insulator A Insulator B Absorbance 0.5 0.4 0.3 0.2 0.1 ATH C-H stretching in CH 3 Si-CH 3 symmetric deformation Contact Angle ( o ) 120 100 80 60 40 20 0 Top Bottom Sheath Top Bottom Sheath Top Bottom Sheath High voltage area Middle area Ground area Fig.4: Static contact angle variation along the housing of insulators A and B From the wettability classification and contact angle measurements (Figs. 3, 4) it is evident that the hydrophobic properties were certainly more degraded for insulator A than B. For insulator B the measured values of WC and contact angle, the latter similar to that of PDMS (~101 o ) [13], indicate that the hydrophobicity transfer mechanism was still functioning; this, however, must be confirmed through tests on hydrophobicity transfer property according to Cigre [14]. D. FTIR analysis Fourier Transform Infrared (FTIR) spectroscopy is a useful technique for identifying chemical bonds in a molecule by producing an infrared absorption spectrum. The infrared absorptions of the bonds containing silicon are quite strong. In general, the spectra of siloxanes are simple thus substituent molecular groups as methyl can be identified [15]. Characteristic absorption bands for chemical bonds of the housing material of SIR insulators are listed in Table II. FTIR spectrum of insulators was obtained by using the IRPrestige-21 Shimadzu measuring system [16]. Fig. 5 shows the FTIR spectra for the field-aged insulators; this figure also includes the FTIR spectrum of a new SIR insulator (insulator C) used as reference. Lower levels of absorbance for all TABLE II SIR ABSORPTION BANDS Bond Wavenumber (cm -1 ) Si-O 1100-1000 [17], [18] Si-CH 3 1240-1280[17] C-H in CH 3 2960-2962[17] ATH 3435, (3300-3650) [19] 0.0 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm -1 ) Fig. 5: FTIR spectra of field-aged and new SIR insulators chemical bonds are observed for insulator A than B; this indicates depolymerisation of insulator A top surface areas due to ageing [20]. On the other hand, there was no significant difference in the FTIR spectra between the fieldaged insulator B and new insulator C. It is important that the FTIR spectrum of insulator A shows reduced absorbance specifically for bonds that are related to the hydrophobic properties of PDMS, namely Si-CH 3 and C- H (Fig. 5). Actually, the ratios of integrated absorbance areas between certain bonds may indicate the hydrophobicity degradation level of the housing material [21]. Combining the hydrophobicity measurement results with those shown in Table III, it can be deduced that the higher the ratio of Si- O/Si-CH 3 the more degraded the hydrophobic properties of housing material are. Thus, the ratio of integrated absorbance areas of Si-O to Si-CH 3 can be indicative of the level of hydrophobicity degradation of SIR housing material; this must be substantiated by FTIR spectroscopy of a greater number of aged insulators. TABLE III INTEGRATED ABSORBANCE AREAS OF BONDS OF SIR INSULATORS Insulator Si-O Si-CH 3 Ratio C, new 59.19 3.76 15.74 A, field-aged 21.21 0.75 28.28 B, field-aged 68.39 3.11 21.99 IV. INCLINED PLANE TEST A. Experimental arrangement The inclined plane test (IPT) method, as specified in IEC 60587 [1], was used to assess the performance of the housing material of the field aged SIR insulators against tracking and erosion. A schematic diagram of the test arrangement is shown in Fig. 6. From each insulator five samples were formed by cutting the silicone rubber sheds in hemicyclic shape. The area of the samples where partial discharges were taken place during the IPT had a thickness of 4-5mm. The samples were washed with distilled water in order to clean their field polluted surfaces by using an ultrasonic cleaner for 15 minutes and then they were mounted carefully at an angle

5100 51 0 5 511 0 5115 5120 12 10 8 6 4 2 0 Leakage current - rms value (ma) 12 10 8 6 4 2 Insulator A Insulator B Fig. 6: Inclined plane test arrangement of 45 o from the horizontal. A contaminant solution, consisting of natrium chloride of 2.53 ms/cm conductivity at 23 o C and a small amount of non-ionic agent TRITON X-100, was fed from the top electrode with a constant flow rate of 0.6 ml/min. After establishing a uniform flow of contaminant over the surface of the test samples, by using eight layers of filter paper between the top electrodes and the samples, a test power frequency voltage of 4.5 kv (rms) was applied for 6 hours in the five samples simultaneously; leakage current never exceeded the value of 60 ma (rms) for 2 sec [1]. The electrodes were made of stainless steel grade 304. In addition a 33 kω, 300 W resistor with 5% tolerance was connected in series with each sample at the high voltage side. Leakage current was measured through Hall sensors connected in series with the ground electrodes (Fig. 6). Current waveforms were acquired, with a sampling rate of 2 ks/sec, using a data acquisition system PCI 6143 of National Instruments. Operation and control of the measurement system were accomplished using the LabView software. B. Experimental results Fig. 7 shows the variation of the leakage current (one second rms values) during 20 s of the IPT procedure for the field-aged SIR insulator samples. These leakage current values are typical for the accelerated ageing of the evaluated insulators since similar values were recorded until the end of the test. Continuous leakage current activity is observed for insulator A during the IPT, resulting from the establishment of a conductive path along the surface of the insulator. This is not the case for insulator B, continuous flow of leakage current is not observed; leakage current is interrupted several times during the IPT (Fig. 7). Thus, leakage current behavior during IPT could provide useful information on degradation severity of the housing material of aged insulators. Table IV shows the average and maximum one second rms values of the leakage current measured for all SIR insulator samples. These currents, being higher for the field-aged insulators than the new one, are highest for insulator A, which showed strongly degraded hydrophobic properties. Also, as can be deduced for Fig. 5 and Table IV, mass loss was lesser for insulators with higher quantity of ATH fillers. This could 0 5100 5105 5110 5115 5120 Time (seconds) Fig. 7: Leakage current (LC) measurements of field aged insulators during 20 seconds of the inclined test procedure TABLE IV INCLINED PLANE TEST RESULTS Insulator Leakage current Leakage current Average Mass Average RMS (ma) Maximum RMS (ma) Loss (mg) C, new 4.1 31.6 348 A, field-aged 8.2 45.6 52 B, field-aged 4.6 36.5 132 be associated with the increased heat conductivity due to higher amount of ATH fillers resulting in lesser mass loss [22]-[24]. To better assess the hydrophobic properties of the SIR insulators, static contact angle and FTIR measurements were performed for all insulator samples after the IPT procedure. A significant reduction in the contact angle, up to 50%, was observed for all specimens (Figs. 4 and 8). From the comparison of the FTIR spectra of the field-aged insulators obtained before and after the IPT procedure (Fig. 9) it is evident that there is a marked reduction in absorbance for all chemical bonds of housing material caused by the accelerated ageing. It is important that the latter affects also significantly the ratio of integrated absorbance areas between chemical bonds, as evinced in Table V. The hydrophobic properties of insulator B were not influenced by the accelerated ageing, as suggested by the rather constant ratio of Si-O/Si-CH 3 ; the main effect of accelerated ageing on this insulator was the marked reduction in the amount of ATH fillers. On the contrary, the relatively poor hydrophobic properties of insulator A were further degraded, as indicated by the ~ 40% increase in the ratio of Si-O/Si-CH 3. Static contact angle: 52 o ; high voltage Static contact angle: 74 o ; ground area area of insulator A of insulator B Fig. 8: Static contact angle measurements after the IPT

TABLE V RATIOS OF INTEGRATED ABSORBANCE AREAS OF BONDS OF SIR INSULATORS Insulator Si-O/ATH Si-O/Si-CH 3 A-field aged 1.30 28.28 A-field/Lab. aged 1.46 40.57 B-field aged 5.31 21.99 B-field/Lab. aged 15.43 22.01 Absorbance 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 A_field-aged A_after IPT B_field-aged B_after IPT 0.0 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm -1 ) Fig. 9: FTIR spectra of field and laboratory aged SIR insulators V. CONCLUSIONS Condition assessment of the housing material of two 150 kv field-aged silicone rubber (SIR) insulators has been made through physical, electrical and material analysis techniques. Pollution severity measurements, hydrophobicity classification and structural analysis of the housing material were performed. Following, the resistance of the housing material to tracking and erosion was evaluated through the inclined plane test procedure. The hydrophobic properties of the SIR housing material were found more degraded for the field-aged insulator that had experienced more severe pollution and had a higher amount of ATH fillers. This insulator also showed higher degradation in hydrophobic properties, higher leakage current values but lesser mass loss caused by inclined plane testing. Thus, the combination of physical and electrical analysis methods may provide a better assessment of the housing material degradation severity of field-aged SIR insulators. In addition to wettability classification and contact angle measurements, the ratio of absorbance peak integrations between chemical bonds obtained thought FTIR spectroscopy may assist in evaluating the hydrophobicity degradation of the housing material. The hydrophobic properties of housing material could be better assessed if hydrophobicity measurements are also performed after accelerated ageing through inclined plane tests. Certainly, further work is needed, involving additional diagnostic techniques, for the better evaluation of the housing material of field-aged composite insulators. ACKNOWLEDGEMENTS This work was partly supported by the Polydiagno research project (project code 11SYN-7-1503) which is implemented through the Operational Program Competitiveness and Enterpreneurship. Action Cooperation 2011 and is cofinanced by the European Union and Greek national funds (National Strategic Reference Framework 2007-2013). The material analysis measurements were supported by the Center of Materials Technology and Photoionics of the Technological Educational Institute of Crete. REFERENCES [1] International Standard for Electrical insulating materials used under severe ambient conditions Test methods for evaluating resistance to tracking and erosion, IEC 60587, Third Edition, May 2007 [2] R. Hackam, Outdoor HV composite insulators, IEEE Trans.on Dielec. and Elec. 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