Surface Degradation of Outdoor Polymeric Insulators Resulting from Electrical and Environmental Stress

Transcription

1 98-E-HVS-437 Surface Degradation of Outdoor Polymeric Insulators Resulting from Electrical and Environmental Stress Morteza Ehsani 1, Gholam Reza Bakhshandeh 1, Jalil Morshedian 1, Hossein Borsi 2, Ernst Gockenbach 2 1 Iran Polymer and Petrochemical Institute, Tehran, Iran, 2University of Hannover, Institute of Electric Power System (Schering-institute), Germany Key Words: Insulator, Tracking, Hydrophobicity, Silicone Rubber, EPDM, Erosion Abstract: The use of polymer material in outdoor insulation applications has been steadily increasing over time. This paper reports on experimental investigation on electrical and surface properties of new polymeric alloy compared with silicone rubber (SIR), ethylene-propylene diene monomer (EPDM), and alloy of SIR-EPDM. In this study a few kinds of a ternary blend of silicone rubber - ethylene-propylene diene monomer -TP (Thermoplastic polymer) with many different proportions were prepared and consequently the surface and electrical (tracking and erosion resistance) properties of them have been studied. The results of tests showed that the surface properties such as water repellency and electrical properties such as tracking and erosion resistance in different condition (UV aging, water salinity aging, and humidity salt spray stress) are improved compared to known polymeric insulators. Introduction The Electrical insulator is very important component in the electrical power system such as sub-station and distribution & transmission lines. Insulators are the devices which are used on electricity supply networks to support, separate or contain conductors at 1 high voltage. All insulators have dual functions, mechanical and electrical, which commonly present opposing demands to the designer. The most serious complicating factor is the impossibility, in practice, of providing an ideally nonconductive element. All insulators have external surfaces which will become contaminated to some extent in service. The contamination will carry leakage current; the surface layer, on a typically polluted insulator, will contain inert mineral matter, electronic conductive dusts like carbon or metal oxides, soluble salts and water. This layer will behave as a highly variable and nonlinear resistor, in most cases unstable in the presence of electric field. The leakage current which it carries will give rise to heat, electrochemical products of electrolysis and electrical discharge. Secondary consequence will range from electrochemical erosion through discharge ablation to complete by passing of the electrical insulation by flashover. Leakage current and its consequences largely administrate the design of an insulator, especially one which is to be used outdoors in atmospheric wetting and pollution. The result of the interrelation between electrical, mechanical and environmental variables has demonstrated to be the unsuitability of

2 completely designing an outdoor insulator on purely theoretical bases [1]. Outdoor insulating bodies have traditionally been made out of glass or porcelain materials. Both glass and porcelain insulators had their start when local potteries began making telegraph insulators in the 185's and 186's [1]. The electric and mechanical stresses which telegraph insulators were required to withstand were evidently negligible in comparison with those from power line duty. From around 1915, the porcelain insulator virtually replaced by glass all electrical distribution, even at low voltages, as the superiority was demonstrated in both insulation quality and strength. The new demands soon disclosed serious shortcomings in both the materials and designs of insulator. However, there is a long time experience with glass and porcelain insulators, but they suffer from some problems such as: puncture breakage, erosion, pin erosion, long term mechanical & electrical strength reduction, coupling hardware corrosion, weight, pollution performance. Figure 1 shows the reasons of default of porcelain insulators on distribution lines in Iran. It can be seen that more failures are related to puncture interface in porcelains [2]. It can be seen also from Figure 2 that most reason of failure in glass insulators is due to mechanical problem [2]. 16% 5% 1% 1% 77% Interfacial puncture 77 % Mechanical failure 16 % Pollution flashover 5 % Lightning flashover 1 % Other causes 1 % Figure 1. Reasons for failure of porcelain insulators on distribution lines in Iran Since the 196s, non-ceramic insulators were advanced and its improvements in think up and producing have made them interesting to public service [3]. The use of non-ceramic insulation systems is increasingly capturing a large share of the market. In the U.S.A polymer insulators represent approximately 6-7% of all new installations of HV insulators and their share of the market continues to grow [4]. In the Ireland 75% of all upgraded 2 kv lines will employ composite insulators in place of glass [3]. In 1997, Hydro-Quebec in Canada installed on a 16 Km section of 735 kv transmission line 282 composite insulators made by three different manufactures in equal proportions. The history of polymeric insulators began in the 194s when organic insulating materials were used to manufacture high voltage indoor electrical insulators from epoxy resins [5]. These materials were light weight, impact resistance, and could be used to form large complex parts. 2% 2% 1% 1% 94% Mechanical failure 94 % Interface punctute 2 % Pollution flashover 2 % Lightning flashover 1 % Other reasons 1 % Figure 2. Statistics on the failure of glass insulators on distribution lines in Iran. Different polymers were used in the manufacture of composite polymeric insulators. At the beginning, non-ceramic insulators contained ethylene propylene rubbers (EPR) which were made by different companies such as Ceraver of France (1975), Ohio Brass of U.S.A (1976), Sedivar of U.S.A (1977) and Lap of U.S.A (198). At 1976, Rosenthal Co and Reliable Co of the U.S.A in 1983 presented silicone rubber (SIR) [5]. The suppliers have developed an alloy consisting of EPDM with silicone rubber. Ohio Brass (1986) produced a blend of ethylene propylene (EPR) and silicone which was subsequently changed to ethylene propylene diene monomer (EPDM) and SIR compound in The alloy of EPDM and SIR may make it possible to combine the properties of two materials [4]. It was reported that one company has produced commercially with the alloys of 2

3 EPDM and SIR over 2.5 million distribution,.1 million transmission class line post insulators and.4 million suspension insulators which are currently installed in power systems in different parts of the world [4]. This gives a clear indication of a world acceptance of this blend of materials. In this paper, the results of the investigations in electrical and surface properties will be presented. The effect of type of polymer and UV and water salinity on the tracking and erosion of the polymeric insulator systems for outdoor high voltage will be discussed. Experimental Materials All the materials used for this work were commercial products and they were used as received without further treatments and all experiments were performed at Schering Institute: a) Silicone rubber (SIR), b) Ethylene propylene diene rubber (EPDM) with Diene (ethylene norbornene) content 5.7 %; ethylene content 55.5 %; Moneny viscosity ML (1+8) at (125 C) 82; Density.86, c) Thermoplastic polymer with Melt flow index (MFI) = 2 g /1 min. d) The di cumyl peroxide (DCP) 98 %; Table 1. General description of materials evaluated Material (phr) Sample SIR EPDM TP DCP A B Mixing and Molding Several formulations containing silicone rubber, TP and EPDM were prepared (Table 1). Silicone rubber was blended with EPDM and TP in a Haake internal mixer, model Sys9, Germany, for 1 min at a rotor speed of 1 rpm for preparation alloys of SIR-EPDM-LDPE. The individual elastomers and the blends were compound with peroxide in a roll mill at room temperature. Vulcanization has been performed with a hydraulically operated press at 17 C and 15 bar for 1 min. Contact angle Measurements Hydrophobicity of a surface is intimately related to the so-called contact angle. Consider an ideal axisymmetric drop of water resting on an ideal flat homogenous horizontal solid surface; the contact angle is the angle θ which is formed by the air-water interface of the drop and the solid surface at the threephase line of contact. The most commonly used method for contact angle measurements of surface hydrophobicity is the sessile drop technique. A droplet of a purified liquid (distillated water) is placed on a surface using a syringe. The resulting angle between the droplet is measured, generally using a goniometer or a charge coupled camera device (CCD) fitted onto a microscope (Figure 3). A drop shape analysis system G1 (Kruss-USA) was used for hydrophobicity studies. C D E F G H phr: parts per hundred parts of rubbers Figure 3. Schematic set-up of the contact angle measurement 3

4 Tracking and Erosion Resistance The tracking resistance is evaluated according to IEC 6587 method [6]. The setup according to IEC 6587 tracking and erosion test is shown in Figure 4. The contamination electrolyte used in this study was a.1% ammonium chloride solution (NH 4 CL) and.2% Isooctylphenoxypolyethoxyethanol (a non-ionic wetting agent) in de ionized water (Triton X- 1). The conductivity of the contaminant was measured with a conductivity meter (Schott Geräte CG 858). It was 253 µs / cm at 23 C. The samples were slab shaped (12 cm x 5 cm x 6 mm). The distance between the top and the bottom electrodes was 5 cm. An AC load of 3 kv was applied over the 5 cm distance, which created average electric field strength of 6 V / cm. The flow rate of the contaminant was controlled with a peristatic pump, at.3 (ml/min). The tracking time was defined as the time of which the leakage current is exceeded 6 ma and sustained for a period of 2 sec. ~ 22V AC Variable Ratio Transformer High Voltage Transformer 22 kω Contamination Feed Top Electrode Bottom Electrode Sample Figure 4. Experimental set-up of the tracking and erosion test Results and Discussion Hydrophobicity Studies Hydrophobicity can be estimated using the contact angle variation. If the contact angle of the material increases above 9, it indicates that the material is hydrophobic. Table 2 shows the contact angle of the samples in the virgin state and after salt solution spray aging (5% NaCL solution in 5 Hrs at 3 C). Salt fog spray aging was performed according to ASTM B117 at 95-98% humidity. It can be seen also in Table 2 that contact angle of samples varies after UV aging. The used UV carbon arc lamp as a light source has a wave length between 3 and 4 nm which is more than 9% of the whole spectrum. The test condition was maintained as 5 ± 5 %RH and 3 ± 3 C, and samples were subjected to UV light for 12 Hrs. It is generally assumed that 2 hours under these testing conditions is equivalent to one year of actual outdoor exposure, considering that only the UV wave length in the range 3-4 nm causes deterioration of polymers [7]. The thickness of samples for the hydrophobicity test and aging in both salt spray and in UV light was 1 mm. It can be seen that contact angle of (H) is higher for silicone rubber than EPDM and their blend (5 /5), both before and after UV aging. The reduction in contact angle after aging means that the methyl groups, responsible for the hydrophobic nature of silicone rubber were destroyed by the photon energy of short wavelength. It is known that hydroxyl and carboxyl groups are produced on silicone and EPDM surfaces by UV irradiation. As a result, the wet ability increases with the duration of UV radiation. Table 2. Contact angle of virgin and aged samples Sample C a 1 C a 2 C a 3 s A B C D E F G H C a1: Contact angle in virgin in degree C a2: Contact angle after salt spray fog aging C a 3 : Contact angle after UV aging 4

5 Contact angle, Degrees A B C H UV aging, h Figure 5. Variation of contact angle with duration of UV aging for test samples It can be seen from Figure 5 that reduction of contact angle for H is slower than for silicone rubber and EPDM and their blend (5/5) after UV aging. These results mean that the methyl group having the hydrophobic property in silicone rubber was damaged by the photon energy of short wavelength. It was well known (the result of FTIR measurement) that hydroxyl and carboxyl groups were made on silicone surface by UV irradiation. The hydroxyl and carboxyl groups were formed, when Si-CH 3 and C-H were broken by photon energy and then bonds are formed at this broken site. Therefore the polar groups were formed more and more with the duration of UV radiation. As a result, the wettability gradually increases with the duration of UV radiation. The decrease of hydrophobicity for EPDM (B) after UV aging can be the result from formation of polar oxidation products, principally alcohols and hydroperoxides. The main failure reason of EPDM insulators has been attributed to loss of hydrophobicity. This eventually causes the surface of the insulator to become conductive as it is wet by water, resulting in flashover and irreparable damage. substances which form electrolytes and give rise to surface leakage currents under moist conditions such as fog, rain, dew or mist. In regions of high current density differential evaporation of the moisture from the surface layer produces local voltage gradients (electrical stresses) greater than the breakdown stresses for the surrounding atmosphere (air). The discharges propagate across dried regions, or bands. This mechanism is known as dry band formation. These types of electrical discharges may occur in service under the influence of dirt, moisture, and deposited conductive salts. Discharge inception under the above conditions is the consequence of several sequential processes: a) The coating of the insulator with ionic deposits b) Formation of an electrolyte by the absorption of moisture c) Surface leakage currents and localized drying of the electrolyte (dry band formation). Processes (a) and (b) are dependent on the environment, and may even be combined as in the case of salt storms in coastal regions, whereas (c) is dependent on the rate of formation of the ionic conduction on the surface. The degrading effect of these processes is simulated by the tracking test. Tracking and erosion are serious degradation modes for non-ceramic insulators that reduce their insulating properties and mechanical strength considerably. Table 3- Tracking test conditions Applied voltage Flow rate contamination Type of contamination 3.5 kv.3 ml/min NH 4 CL Conductivity 253 µs/cm Tracking and Erosion Resistance Evaluation When subjected to polluted atmospheres, insulators attain deposits of airborne solid contaminants, usually containing some ionic Surface electrical discharging ultimately results from build up of leakage current on insulating devices during wet contaminated conditions. In this situation, the polymer 5

6 material can undergo various chemical reactions leading to deterioration of mechanical and electrical properties. One method of degradation is the formation of a carbonaceous conducting path on the surface of the insulation, due to heat generated by discharges. These start locally across the dry bands formed on the surface of a sample, when sufficient contamination is accumulated in the surface regions between the electrodes. The carbonized conduction path finally forms over the short circuited path. Another mode of damage is the progressive loss of material because of the formation of degradation products due to a localized reaction, resulting in erosion of the insulting surface. The mode of degradation may vary according to the chemical composition of the polymer, type of contamination, and the type of the discharge activity. Table 3 and 4 show the condition of tracking test and the tracking and erosion resistance of polymers, respectively. It can be seen from Table 4 that silicone rubber has a poor tracking resistance and has high erosion losses compared to other polymers. It can be seen from Table 4 that with increase TP content in ternary blends, time of tracking is increased and erosion is decreased. It is also found that TP improved tracking resistance and surface discharge properties of Natural rubber compounds [8]. thermal degradation of HTV-silicone rubber [9]. Silicone rubber subjected to thermal decomposition is known to form cyclic dimethylsiloxane (DMS) together with a small amount of linear DMS ( 1%wt). The molecular units of most formed cyclic DMS ( 9%wt) are D3 to D6 [9, 1-12]. It has been shown already that electrical discharges increase the number of cyclic silicone oligomers of 4 to 6 unites (D4 to D6) on the surface of HTV silicone rubber [13]. Cyclic silicone oligomers of low unite would increase at the surface with the duration of the inclined plane tracking and erosion test. Table 5 shows that low unit silicone oligomers possess low boiling temperatures, with those of D4 to D6 ranging from 173 C to 245 C [13]. When dry-band arcing is concentrated in one point, formed the oligomers D4 to D6 are volatilized due to their low flash points (Table 5). Therefore, the increase of tracking resistance of ternary blends can be the result of reduction of LMW silicone ologomers in theirs. Table 5. Boiling points (B.P [ C]) and flash points (F.P [ C]) of silicone oligomers of low molar mass Type chemical structure F.P B.P D3 (Si O (CH 3 ) 2 ) D4 (Si O (CH 3 ) 2 ) D5 (Si O (CH 3 ) 2 ) Table 4. Tracking time, weight loss and maximum erosion depth for samples Sample T (min) Weight loss % Maximum erosion depth (mm) A 4 3 B >6 Fire - C D E F G H Tracking time, min. D6 (Si O (CH 3 ) 2 ) Virgin samples 3 Water Salinity 25 UV aging A B(Firing) C H Gas Chromatography-Mass spectrometer (MS) shows the evolved gases during the 6 Samples Figure 6. Comparison of tracking time in different condition

7 M ax erosion depth, mm Weight loss % Virgin Water salinity UV aged A B (Firing) C H Samples 3.3 Virgin Water Salinity UV aged A B (Firing) C H Figure 8. Maximum erosion depth for samples in different condition Samples.5.13 Figure 7. Loss of weight samples after tracking test in different condition The tracking resistance of polymer insulating materials can be offered by environmental conditions such as UV radiation, moisture, and acid rain. The influence of water salinity and UV radiation on tracking resistance of samples also studied. Figures 6-8 show the tracking resistance and erosion parameters after UV and water salinity aging. The samples were immersed in water salinity (5% NaCL) for 15 Hrs. The polymers have also been exposed to UV for 17 Hrs at 3 C. Figures 6 and 7 show a comparison of tracking time and erosion parameters in different condition for sample A, B, C, and H. It can be seen that sample H has good tracking and erosion resistance compared to silicone rubber, EPDM and blend of silicone- EPDM..7 Although, tracking resistance for silicone rubber improved by UV aging, but it should be noted that the reduction of hydrophobicity during UV aging causes accumulation pollution on surface of polymer and flashover. It can be seen from Figures 6 that the tracking and erosion resistance have changed after water salinity aging for the case of silicone rubber. This can be the result of the absorption of water during water salinity aging. This reduction is clearly caused by an expansion force during the boiling of absorbed water. Furthermore, if water is injected into the bulk, ions or electrons in contaminated electrolyte can migrate into the interior via absorbed water, and a current develops through the bulk. The development of such current causes the boiling of absorbed water and, finally, promotes mechanical and chemical erosion. It seems that NaCL in salt water permeates into the samples and acts as ionic carrier, causing on increase in current density. Conclusion New polymeric alloy for outdoor use of high voltage insulator has been introduced and its electrical, mechanical, thermal and surface properties are compared to already known outdoor composite insulation in different conditions. Contact angle measurements have shown that new alloy is more hydrophobic compared to silicone rubber, EPDM and blend of silicone-epdm after previous UV radiation stress. Its hydrophobicity is also comparable with blend of silicone - EPDM after thermal aging. The hydrophobicity of samples were found to decrease with UV and thermal aging which was due to the formation of polar groups (such as carbonyl groups) and damaged of hydrophobic groups (such as methyl groups) on surface of polymers The results of the contact angle measurement show that the water repellency 7

8 of polymers decrease with water salinity conditions The tracking resistance of silicone rubber decreased with water salinity aging which could be the result of diffusion of water. It was also shown that UV aging decreases the tracking resistance of EPDM and may be the result of loss of hydrophobicity at surface of EPDM. It was also found that UV aging improves the tracking and erosion resistance of silicone rubber. This can be the result of disappearing carbon and increasing of oxygen because of UV effect. This result means that oxygen binds to silicone polymers at the surface instead of carbon in methyl groups (CH 3 ). When carbon that results in the formation of conductive path decreases, therefore, tracking resistance increases. Acknowledgment The authors thank the companies Wacker, Exxon, Hercules for supplying raw materials. References [1] J.S.T Looms, Insulators for High Voltages, Peter Peregrinus Ltd, London, [2] A Khalilpour, Service experience and problems with insulators on distribution Line in Iran, INMR, Vol.11, No. 4, 23, pp [3] R Hackam, Outdoor High Voltage Composite Polymeric Insulators, IEEE Transactions on Dielectrics and Electrical Insulation, 1999, 6 (5); pp [4] R.S Bernstorf and T Zhao, Aging Testes of Polymeric Housing Materials for Non- Ceramic Insulators, IEEE Electrical Insulation Magazine, Vol.14, No.2, 1998, pp [5] J.F Hall, History and Bibliography of Polymeric Insulators for Outdoor Applications, IEEE Transactions. PD, Vol. 8, No. 1, 1993, pp [6] IEC Publication, The Methods for Evaluating Resistance to Tracking and 8 Erosion of Electrical Insulating Materials Under Severe Ambient Conditions, 1982, Second Edition. [7] S. Jeon, S. Hwang-bo, Accelerated Outdoor Degradation Characteristic of Polymer Concrete Insulator, proceeding of the 4 th ICPADM, Brisbane Australia, 1994, pp [8] M. A.M. Piah, A. Darus, Leakage current and surface discharge phenomena: effect on tracking and morphological properties of LLDPE-natural rubber compounds, Proceeding of 7 th International Conference on Properties and Applications of Dielectric Materials, Nagoya, Vol. 1, 1-5 June 23, pp [9] T.H Thomas, T.C Kendrick, Thermal Analysis of Polydimethylsiloxanes I J. Polymer Sci. A-2, Vol. 7, 1969, pp [1] H Homma, T Koroyagi, K Izumi, Evaluation of Surface Degradation of Silicone Rubber Using Gas Chromatography/Mass Spectroscopy, IEEE Transactions on Power Delivery, Vol. 15, No.2, 2, pp [11] W Patnode, D.F Wilcock, Methylpolysiloxanes, Journal of American Chemical Society, Vol. 68, 1964, pp [12] J. C Kleiner, C Weschler, Pyrolisis Gas Chromatographic-Mass Spectometric Idetification of Polymethylsiloxanes, J. Analytical Chemistry, Vol.52,No.8, 198, pp [13] S Kumagai; X Wan; N Yoshimura. IEEE Trans. Dielectrics and Electrical Insulation, Vol.6, No.5, 1999, pp