Polymer Compounds Used In High Voltage Insulators

Transcription

1 Polymer Compounds Used In High Voltage Insulators by R. Allen Bernstorf Randall K. Niedermier David. S. Winkler Principal Engineer - Manager Polymer Chemist Insulators Polymer Development of Hubbell Power Systems The Ohio Brass Company POWER SYSTEMS, INC.

2 POLYMER COMPOUNDS USED IN HIGH VOLTAGE INSULATORS Introduction Since their introduction in the early 1970s, polymer insulators have been increasingly accepted by utilities as suitable replacements for porcelain and glass insulators. This paper presents details to the non-chemist on the types of polymers used in compounds for high voltage insulation. The polymers that will be discussed are ethylene propylene polymers, which include ethylene propylene rubber (EPR) and ethylene propylene diene monomer (EPDM), silicone rubber (PDMSO) and an alloy of EPDM and PDMSO. Polymer Composition A polymer is composed of very large molecules. Each molecule contains atoms arranged one after another in a chain-like manner. The chain-like arrangement repeats in regular cycles, so the structure can be written as certain segments which repeat n times [1]. Each polymer is commonly named after the raw material used to make it. For example: H H H H [ ] n C = C C C H H H H n H H HCH HCH H H [ ] n C = C C C H H H H n Ethylene Polyethylene Propylene Polypropylene H H HCH HCH H H H H H H [ ] [ ] n C= C + mc = C C C C C m H H H H H H H H n Ethylene Propylene Ethylene Propylene Rubber Chemical Structures [2]

3 H H H HCH HCH HCH [ ] ncl Si Cl + 2H O n HO Si OH + 2HCL Si O 2 n HCH HCH HCH H H H Dimethyldicloro Dimethylsilane diol Polydimethyl Siloxane siloxane The word polymer is Greek for many parts, thus polyethylene translates to many ethylenes. The n and m are a measure of the molecular weight of the polymer. Typically, when n and m are small (low molecular weight) the polymer exhibits low physical properties and in some cases it may be a liquid. As n and m increases (molecular weight increases), the polymer s physical properties are improved [3]. As mentioned above, polymers are very large molecules, where in the case of EPR, the n and m can be as high as 1,000,000 [4]. In the case of silicone rubber n is in the range of 3,000 to 10,000 [5]. EPR is normally manufactured in a continuous process in large reactors. The monomers, ethylene, propylene and sometimes a diene (in the case of EPDM), along with the catalyst are metered into the reactor and the polymerization takes place almost instantly at room temperature. After polymerization, the polymer is cooled, dewatered, dried, cooled again and compressed into bales [6]. The EPR polymer is now ready for compounding (#). In comparison, silicone rubber is usually manufactured from the direct process of methyl chloride and elemental silicon. One of the by products of this process is not stable and condenses with the evolution of water to form a mixture of polydimethylsiloxanes of low molecular weight. Once separated, they are polymerized to higher molecular weight in the presence of catalysts [7]. Now the silicone rubber is ready for compounding (#). (#) - Once the polymer has been manufactured, it needs to be compounded into a suitable material for use in high voltage insulators. Polymer compounding is the science of mixing the polymers discussed above, with other chemicals to produce a polymer compound which has specific properties for an application. Examples of polymer compounds could be chewing gum, a garden hose, tires, solid rocket fuel, and of course, electrical insulators. Polymer compounding will be discussed in more detail in a later section.

4 Comparison of Polymers in Use Today EPR EPRs are among the best weathering resistant synthetic polymers [8]. Because of EPR s saturated polymer backbone (*) it has excellent electrical, chemical and mechanical properties with superior aging and color stability. In general all EPRs have outstanding resistance to heat, oxygen, ozone and sunlight. In fact, the resistance of EPDM rubber to sunlight is rated equal to that of silicone rubber [9], [10], [11]. There are a variety of EPM and EPDM polymers available today, each offering different combinations of characteristics. The selection of a particular polymer has to be made on the basis of a thorough evaluation of the characteristics and their significance for the intended application. Silicone Rubber As shown on the previous page, silicone rubber contains a repeating silicon-oxygen backbone and has organic methyl groups attached to a significant proportion of the silicon atoms by siliconcarbon bonds [12]. Silicone rubber is classified as an organo-silicon compound [13]. This is due to the very important bond between carbon (organic) and silicon (inorganic) [14]. Silicone rubber is not inorganic. Because of the silicon-oxygen backbone, silicone rubber is resistant to sunlight, heat and is flexible over a wide range of temperatures. But, unlike the EPR backbone, the silicon-oxygen bond is susceptible to heterolytic cleavage, i.e., attack by acids and bases [15]. Silicone rubber has low physical properties compared to EPR based materials [16]. Silicone rubber is a hydrophobic material (repels water). This is due to the organic groups attached to the silicon atom. Silicone rubber is also over 10 times more permeable to moisture than EP rubber. As is the case for EPRs, many different silicone polymers are commercially available, but relatively few are suitable for high voltage applications. Extensive evaluation, analysis and testing are required to ensure an appropriate match of material characteristics with application needs. EPDM/Silicone Alloy Recognizing the only beneficial characteristic of silicone rubber - hydrophobicity - but desiring a compound which had the excellent mechanical properties that EPDM can provide, Ohio Brass developed an alloy of EPDM and silicone which combines the best properties of these two materials. This compound is called ESP. (*) Backbone refers to the C-C chain found in EPR materials and the Si-O found in silicone.

5 Compounding In the field of polymer technology, rubber compounding is a complex subject. Compounding is part art, part science. The compounder s ability to select and combine polymers and additives to obtain a mixture that will develop the necessary physical and chemical properties can take years to master [17]. A practical compound formulation can consists of 10 or more ingredients. Each ingredient has a specific function and each has an impact on properties and processibility. The remainder of this section will be dedicated to brief descriptions of the ingredients and their functions. Elastomers As discussed in previous details, the most important step in compounding is the selection of the elastomer, or base polymer. The selection of an elastomer should be based on the properties desired and processibility of that elastomer. Vulcanizing Agents Coagents Vulcanizing agents are ingredients used to cause a chemical reaction, resulting in the cross-linking of elastomer molecules. Through chemical cross-linking, an elastomeric compound is converted from a soft, tacky material to a stiff, temperature-stable material. There are many types of vulcanizing or curing agents used. Organic peroxides are the most widely used for high voltage insulation. Coagents protect the already cross-linked bonds between the polymer and vulcanizing agent from being torn apart. This assures that bonds are not broken as fast as new bonds are generated. There are two types of coagents: type I and type II. Type I coagents increase the cure rate (speed) and cure state (stiffness) of the compound. Type II coagents increase the cure state but do not effect cure rate. Antidegradants Antidegradants are used to retard the deterioration of a rubber compound initiated by oxygen, ozone, heat and light. In the selection of antidegradants the following factors must be considered: 1. type of protection desired 2. chemical activity 3. persistence (volatility and extractability) 4. discoloration and staining

6 Processing Aids Fillers Processing aids are added to a rubber compound to help mold flow and release, as well as aid in the mixing of the compound. Fillers are used to reinforce the base elastomer which can increase the physical properties or impart certain processing characteristics. There are two types of fillers: reinforcing and extending. The reinforcing type can improve tensile strength, modulus, tear strength and abrasion resistance of a compound. An extending filler is a loading or non-reinforcing material. It may be used to impart some desirable property. Alumina trihydrate (ATH) is used in nearly all insulator compounds to impart a high resistance to electrical tracking and inflammability. Coupling Agents A coupling agent provides a chemical bond between the filler and the elastomer. The coupling agent is a bridge between the ATH and the polymer. This can greatly improve the electrical properties, modulus and tensile strength. Plasticizers and Softeners Plasticizers and softeners are used to either aid mixing, modify viscosity or provide flexibility at low temperatures. Many ingredients in this group may also be considered as processing aids. Special Purpose Materials Special purpose materials are used for a specific purpose which is not normally required in the majority of rubber compounds. Antifungal agents, blowing agents, colorants, re-odorants, the Ohio Brass silicone material and materials which assist the compatibilization of silicone to the matrix are examples of special purpose materials. Formulation Testing/Material Characterization Polymer insulating materials are typically formulated to provide performance characteristics equivalent to, or better than that of porcelain. Because of the different physical properties of porcelain and polymers, a comparison is usually made only on short term electrical characteristics of complete insulators - e.g. wet and dry 60 Hz flashover, critical impulse flashover. However, many physical properties of a polymer material must be understood and properly controlled to give the material the characteristics necessary to serve a long life as a high voltage insulating polymer. Once a compound has been formulated to meet the desired criteria, various types of design tests are conducted. These tests ensure the formulation exhibits the required characteristics for use in high voltage insulators.

7 Comparison of Polymer Compounds in Use Today The materials discussed in the following testing section are commercially available rubber compounds designed for high voltage insulators. The EPM compound is the original Ohio Brass compound. The EPM/Silicone alloy is the original Ohio Brass with silicone added. ESP is the current Ohio Brass compound which is an EPDM with silicone added. Again, the other compounds listed are commercially available for any insulator manufacturer to purchase. Design Tests While there are several standardized (e.g. ASTM, ANSI, CEA CSAN, IEC) tests to evaluate insulating materials, few of these are particularly suited to proper assessment of the characteristics required for outdoor high voltage insulation. As a consequence, Ohio Brass developed specialized tests for evaluating materials for its polymer insulator products. A polymer compound s suitability for long term use in high voltage insulators is assessed by means of design tests. By necessity, given that an insulator s lifetime is measured in decades, design tests involve accelerated aging or conditioning. Good polymer compounds used for high voltage insulation should be tested for the ability to resist tracking, erosion, corona, oxidation and ultra-violet (UV) radiation exposure. Tracking Test The tracking resistance of a compound is a measure of its ability to withstand intense leakage currents combined with dry band arcing. The dry band arcing generates ozone, high temperature and UV radiation exposure on the polymer surface. Test conditions vary among laboratories, but typically samples of the compound are wetted with a conducting solution and then energized in a circuit with a controlled current. Samples are then evaluated in terms of the number of cycles or time to failure. Acceptable performance is dependent upon materials and test methods. Good compounds will survive for tens of thousands of cycles. A polymer compound must exhibit excellent resistance to tracking and erosion. Polymers degrade by erosion and tracking due to the heat generated by leakage currents and dry band arcing. Since each polymer differs in terms of tracking resistance, one of the ways to differentiate various compounds is to subject them to a tracking test. The Ohio Brass tracking test was designed to evaluate glaze compounds for porcelain insulators. It was used next for evaluating the tracking and erosion resistance of polymeric insulating materials. This test evaluates the relative ability of an insulating material to withstand electrical discharges on the surface which are similar to those that may occur in service under the influence of dirt, moisture and conducting salts condensed from the atmosphere. Samples are mounted on a 30 degree incline with electrodes attached on either side. The electrodes are positioned 35 mm apart. The samples are cyclically sprayed with a conductive solution

8 and then energized. Each cycle is 90 seconds. The conductive liquid has a resistivity of about 400 ohm-cm and is formulated to leave no residue on the sample s surface. A voltage of 10 kv is applied with a controlled current of 20 ma. The leakage current and subsequent dry band arcing dry the surface of the sample. Test results obtained from the tracking test provide a means of comparatively ranking the tracking resistance of materials. Failure is judged by one of three criteria: 1. carbonization or tracking of the sample s surface; 2. the sample remains conductive at the end of the 90 second cycle; or 3. erosion creates a hole in the sample. MATERIAL CYCLES COMMENTS Porcelain 50,000 No Failure EPM 50,000 No Failure EPM/Silicone Alloy 22,700 Failure ESP 50,000 No Failure Silicone Rubber 1 16,000 Failure EPDM 1 1,800 Failure Table 1. Tracking Test Results Silicone Rubber 1 and EPDM 1 are commercially available insulating compounds. Ultra Violet Polymer insulating compounds are exposed to UV radiation not only from sunlight, but also from corona and dry band arcing. Resistance to degradation resulting from ultra violet (UV) exposure is an important factor in determining the service life of a polymer. The energy from sunlight that is destructive to polymers is between 320 and 270 nanometers. This destructive energy constitutes less than five percent of the total radiation reaching the surface of the planet. The absorption of this UV radiation results in mechanical and chemical degradation of the polymer structure which can affect the dielectric and weathering properties of that polymer. The rate at which the degradation occurs is dependent upon the intensity and wavelength of the radiation. These factors vary with season, time of day, elevation and latitude. Acceleration of the effects occurs in the presence of moisture on the polymer s surface. Polymer compounds for use in outdoor environments should, therefore, be evaluated in the combined presence of UV radiation and high humidity. An accelerated evaluation can be performed in a Weatherometer or QUV tester. Ohio Brass believes the QUV test to be the most meaningful test for outdoor insulation. Through comparisons with tests performed at the Desert Sunshine Exposure Test (DSET) site in Arizona, the aging acceleration of the QUV test has been assessed as 8:1; that is one hour of QUV test is equivalent to eight hours of exposure in the Arizona desert, which is considered one of the most severe natural UV environments in North America.

9 The negative effects of UV radiation for a polymer include: 1. crazing, checking or cracking of the surface 2. loss of hydrophobicity 3. discoloration. The QUV test is an accelerated weathering test performed in conformance to ASTM G53. The QUV test alternates UV radiation exposure and condensation during each cycle. A cycle consists of 8 hours of condensation and 16 hours of UV exposure. The QUV test simulates the effects of sunlight by means of fluorescent UV lamps positioned within inches of the test specimen. A water reservoir at the bottom of the test chamber is heated to produce vapor. The hot vapor keeps the chamber at 100% relative humidity. The water vapor condenses on the cooler surfaces of the test specimens. The combination of condensation with high intensity UV radiation results in an accelerated exposure test. Under accelerated testing methods, like ASTM G53 (QUV), a good polymer compound should be capable of sustaining 10,000 hours of exposure without crazing, cracking or displaying any loss of hydrophobicity. However, loss of hydrophobicity does not indicate the end of life for a polymer compound. The test results obtained from the QUV test provide a means of comparatively ranking the UV resistance of materials. Such results are listed in Table 2. MATERIAL TIME, HRS COMMENTS EPM 8,000 Loss of Hydrophobicity EPM/Silicone Alloy 8,500 Loss of Hydrophobicity ESP 36,500 Still Hydrophobic* EPDM 1 1,000 Checking of Surface EPDM 2 4,000 Loss of Hydrophobicity Silicone Rubber 1 39,000 Still Hydrophobic* Table 2. QUV Test Results *indicates ongoing test EPDM 1, EPDM 2 and silicone rubber 1 are commercially available insulating compounds from different manufactures. Corona Corona discharges form at the surface of an insulator when the electric field intensity on the surface exceeds the breakdown strength of air, which is about 15 kv/cm. Corona generation is dependent on atmospheric conditions such as air density, humidity, and geometry of the insulator. The effects of corona are radio interference, TV interference, noise generation, ozone production, and energy loss. Corona accelerates the aging of polymers, by generating ozone and UV light. The UV light produced is of short wavelengths and includes the spectra of light damaging to polymers. The electric discharge subjects the insulator to severe electrical strain and chemical degradation. Continued degradation may render the polymer unusable. A polymer must be able to withstand this chemical

10 degradation throughout its service lifetime. Electrical insulation that may be subject to corona must be made from a properly compounded EPR or silicone rubber. The presence of corona combines UV and heat with a high level of ozone. The Ohio Brass corona cutting chamber combines this with mechanical stress to accelerate the degradation of a polymer. Polymer insulator samples are subject to a mechanical stress of approximately 300,000 microstrain by bending the sample over a grounded electrode. Corona is continuously generated by applying 12 kv to a needlelike electrode placed 1 mm above the strained surface of the sample. Under a combination of stress and continuous surface corona, an unaged polymer should be capable of surviving for at least 1,000 hours without cracking, splitting, cutting or electrical failure. The test can be conducted surface corona, an unaged polymer should be capable of surviving for at least 1,000 hours without cracking, splitting, cutting or electrical failure. The test can be conducted under dry air or with controlled humidity. The test is usually run until failure of the sample. The cumulative time to failure is recorded. This test is used for relative comparison. The test results are listed in Table 3. MATERIAL TIME, HRS COMMENTS EPM 404 Failure EPM/Silicone Alloy 1,290 Failure ESP 3,250 Test Terminated EPDM 2 2,780 Test Terminated Silicone Rubber Failure Silicone Rubber 2 1,650 Test Terminated Table 3. Corona Cutting Test Results Silicone rubber 1, silicone rubber 2 and EPDM 2 are commercially available insulating compounds from different manufactures. Oxidative Stability Anti-oxidants are incorporated into a polymer compound to inhibit the attack of oxygen and ozone on the compound s chemical composition. The effectiveness of an antioxidant is very important in electrical insulation applications. An oxidative stability test measures the time to deplete the anti-oxidants within a material under controlled conditions. Oxidative stability is measured by using the thermal analysis technique of differential scanning calorimetry (DSC). The DSC measures the amount of heat flowing into (endothermic) or out of (exothermic) the sample as a function of the material s temperature. Oxidation is a highly exothermic process and is readily studied by DSC techniques. A test sample is rapidly heated in a nitrogen atmosphere to the test temperature of 200 C. The atmosphere is then changed to oxygen and the temperature is maintained until the sample begins to oxidize. When the sample oxidizes or decomposes, an exothermic reaction is registered. This indicates all of the antioxidant has been consumed and the rubber sample is no longer protected. Good compounds typically exhibit oxidation in times greater than 400 minutes in oxygen at 200 C. Typical results are listed in table 4.

11 MATERIAL TIME, MINUTES COMMENTS EPM Failure EPM/Silicone Alloy 400 No Failure, End of Test ESP 400 No Failure, End of Test EPDM 1 20 Failure EPDM No Failure, End of Test Silicone Rubber No Failure, End of Test Table 4. Oxidative Stability Test Results EPDM 1, EPDM 2 and Silicone Rubber 1 are commercially available insulating compounds from different manufactures. Ohio Brass first developed these test techniques in the mid 1960s for evaluating the characteristics of polymer insulating compounds, and over the intervening years has continued to improve the methods. Twenty-five years of experience have indicated that materials which perform well in these tests will offer decades of satisfactory service in field use. Summary This paper has presented details on the types of polymers used for high voltage insulators. There are many factors which must be considered while designing a polymer compound for use as high voltage insulation.

12 References 1. The Vanderbilt Rubber Handbook, pp. 2. The Vanderbilt Company, Inc., Connecticut, (1990). 2. Fundamentals of Polymer Science, P.C. Painter, M.M. Coleman, pp Technomic Publishing Company, Inc. (1994). 3. The Vanderbilt Rubber Handbook, pp. 3. The Vanderbilt Company, Inc., Connecticut, (1990). 4. Science and Technology of Rubber, F.R Eirich, pp Academic Press, New York, (1978). 5. Rubber Technology, M. Morton, pp Van Nostrand Reinhold Company, New York, (1973). 6. The Vanderbilt Rubber Handbook, pp The Vanderbilt Company, Inc., Connecticut, (1990). 7. Rubber Technology, M. Morton, pp Van Nostrand Reinhold Company., New York, (1973). 8. The Vanderbilt Rubber Handbook, pp The Vanderbilt Company, Inc., Connecticut, (1990). 9. Royalene EPDM Rubber Handbook, Comparative Property Chart, Uniroyal Chemical Company, Connecticut, (1988). 10. Basic Compounding and Processing of Rubber, pp. 6. Rubber Division, American Chemical Society,Inc., The University of Akron, Akron, OH 44325, (1985). 11. Compounders Pocket Book - Chemicals for the Rubber Industry, pp Monsanto Rubber Chemicals Division, Akron, OH 44313, (1981). 12. Concise Encyclopedia of Polymer Science and Engineering, pp John Wiley and Sons, Inc., New York, (1990). 13,14. Silicone Chemistry, J.L. Speier, M.J. Hunter, Reprinted from International Science and Technology, New York, (1963). 15. Concise Encyclopedia of Polymer Science and Engineering, pp John Wiley and Sons, Inc., New York, (1990). 16. Basic Compounding and Processing of Rubber, pp. 6. Rubber Division, American Chemical Society,Inc., The University of Akron, Akron, OH 44325, (1985). 17. Basic Compounding and Processing of Rubber, pp. 6. Rubber Division, American Chemical Society,Inc., The University of Akron, Akron, OH 44325, (1985). POWER SYSTEMS, INC. 210 N. Allen Street, Centralia, MO Fax NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice. Copyright 2004 Hubbell 210 North Allen Street Centralia, MO 65240