Insulating Materials for Electrical Power Equipment
There are number of materials which are used either separately or as a combination of composite products to form an insulation system for electrical power equipment. The basic materials selected for insulation systems are selected based on their ability to withstand varied electrical, mechanical,
and thermal stresses during the life of the equipment. Listed below is a partial summary of the materials and products used for insulating electrical power equipment.
Rigid Laminates Sheet, Rod, and Tube
• Canvas-based phenolic laminate
• Paper-based phenolic laminate
• Glass melamine laminate
• Glass silicone laminate
• Glass epoxy laminate
• Cogetherm mica-based laminate
• Mica epoxy laminate
• Transite HT and NAD-11 high-temperature cement boards
Glass Polyester Products
• Glass polyester sheet
• Glass polyester channels and angles
• Glass polyester stand-off insulators
• Glass polyester rods
• Specialty glass polyester
Flexible Laminates and Films
• Diamond-coated kraft paper
• Vulcanized fiber sheets, rods, and tubes
• Kraft pressboard products
• COPACO rag paper
• Quin-T family of flexible laminates
• Melinex polyester film
• Mylar polyester film
• Dacron–Mylar–Dacron
• Kapton polyimide film
• Nomex aramid paper
• Nomex–Polyester–Nomex
• Rag Mylar and Rag–Mylar–Rag
Over the years organic insulating materials have been replaced with inor- ganic materials and this progression is still continuing. There are number of organic and inorganic insulating materials that are used in electric power equipment. Although this listing discussed in this section is somewhat long, it is not all encompassing because the choices of available insulating materials are many. The use of the trade names in the listing given here does not represent a particular brand preference but are included for clarity. The characteristics of various insulating materials are discussed as follows.
Cotton: Cotton has been used extensively in electrical insulation because of its low cost, strength, elasticity, flexibility, and adaptability to size requirements and manufacturing process. However, cotton has a tendency to absorb moisture and limited thermal capability. Cotton is always used with varnish or resin impregnation to obtain good dielectric strength and moisture resistance. The use of cotton is restricted to 105°C (Class A insulation system) because temperatures higher than class A cause decomposition of the cotton fibers with resulting embrittlement and loss of mechanical strength.
Cotton fabrics: There are many varieties of treated fabrics that are funda- mental Class A insulating materials. Untreated cotton fabrics that have been thoroughly dried are used for oil transformer insulation. These fabrics are quickly impregnated when the transformer is filled with oil, thereby providing excellent physical and dielectric properties. Two most commonly used fabrics are tan-varnish treated cloths and black-varnish treated cloths.
Paper: Many types of paper are used in insulating electrical equipment. These varieties include Japanese tissues, cotton rags, manila (hemp), the kraft (wood pulp), jute, fishpaper (gray cotton rag), fuller board, and the like. Rag and kraft paper often called transformer paper is used to separate windings in a transformer or in applications where there are no sharp edges that might poke through the relatively weak paper. Fishpaper is usually vulcanized and often laminated with Mylar giving it excellent resistance to tear and puncture. The paper–Mylar laminates resist soldering heat better since paper does not melt and the Mylar resist moisture best. Papers made with temperature resistance nylon and/or glass weaves have excellent electrical properties and good temperature resistance. Paper posses similar properties as cotton cloth, but because of its structure it has higher dielectric strength than cotton. The thermal stability and moisture absorption properties of paper are similar to cotton; however, paper does not possess the high mechanical strength of cloth. Untreated paper has little insulating value because of its extreme ten- dency to absorb moisture.
Asbestos: Recent restriction placed by Environmental Protection Agency (EPA) and OSHA has limited the use of the asbestos as an insulating mate- rial. It is used in the form of asbestos paper or asbestos tape, asbestos mill board or asbestos lumber for electrical insulating purposes. In such forms, it may contain 10%–20% wood pulp and glue to give it strength. Asbestos is generally heat resistance, but if heated excessively asbestos loses its hygro- scopic moisture and therefore becomes brittle. Asbestos absorbs moisture from the surrounding atmosphere which makes it a less effective insulator.
Glass: Glass insulation comes in a wide variety of forms including solid glass, fiber tapes, fiberglass sheets and mats, woven tubing and cloth, and various composites. The most common form of glass used in electrical equip- ment is fiberglass. Fiberglass is available as fiberglass yarns for insulating materials. Today, fiberglass cloths and tapes are widely used in high- temperature applications. Two types of fiberglass yarns commonly used for electrical insulation are continuous filament and staple fiber yarns. The con- tinuous filament yarns have the appearance of natural silk or linen whereas the staple fiber yarns exhibit a considerable degree of fuzziness similar to wool yarns. Fiberglass has advantages of high thermal endurance, high chem- ical resistance, high moisture resistance, good tensile strength, and good thermal conductivity when impregnated or coated with varnish or resin. Glass fibers exhibit poor abrasion resistance therefore fibers must be lubri- cated before they are woven into cloth. Varnish treatment of glass cloths greatly increases the abrasion resistance. Untreated glass cloth is used pri- marily as a spacing insulation and when the cloth is impregnated and coated with an insulating resin or varnish it becomes an effective dielectric barrier. Among the many uses of fiberglass are covering of wire, binder tapes for coils, and backing for mica tapes. It is also used as ground insulation and insulation for stator coil connections, leads, ring supports, etc. when treated with oleor- resinous and other varnishes. Fiberglass, varnish-treated fiberglass, and nonwoven forms are thermally stable, resistance to solvent depending on the type of varnish used. It is more secure spacer insulation than cotton or paper.
Synthetic textiles and films: Numerous synthetic fiber textiles are in use as electrical insulation. These are either continuous monofilaments of resins, or short fibers made of resins that are spun into threads and woven into fabrics. They have to be coated or impregnated with varnish to become effective dielectric barriers. The thermal ability of these materials lies between cellu- lose fabrics and glass fabrics and depends on the type of resin coating or impregnating material used more than their own characteristics. Examples of synthetic fiber cloth and mats are Dacron, other polyesters, aromatic nylons, such as Nomex, Kelvar, and others. They have thermal stability, sol- vent resistance, and lack of fusibility. Several polyester films exhibit excellent electrical and physical properties, such as Mylar which enjoys widespread use in a variety of insulation systems.
Polyester films: They are used at temperatures above 105°C–125°C and have fair solvent resistance. Some specially formulated polyester varieties are used for service up to 180°C. The common uses of polyester films such as Mylar and others are slot liners, layer insulation in transformers, capacitors, and as laminated backing of paper insulation.
Aromatic polyimide (Kapton, Nomex, and others): It is used at temperatures of 180°C–220°C. It has excellent resistance to solvents and has good heat resis- tance and superb mechanical and electrical properties. Nomex is a Dupont aromatic polyamide with a temperature rating above 220°C and has high voltage breakdown strength.
Polyolefins: Polypropylene and polyethylene are two known polyolefins that are available with ultrahigh molecular grade matching the strength of steel. Polypropylene is used for insulation not to exceed temperatures above 105°C whereas polyethylene film has limited use such as class O insulation system since it softens at temperatures higher than 70°C.
Polycarbonate: Common trade names for it are Lexan and Merlon and are used for insulation system rated at temperatures 105°C and below. It has excellent electrical insulating properties and has good oil resistance but poor solvent resistance.
Polysulfone: This is another thermoplastic that include polyetherimide, polyamide, and polyphenylene with trade names like Noryl, Udel, Vespel, and Torlon. These materials are used at temperatures from 105°C to 130°C and have good oil resistance but not chlorinated solvent resistance.
Polytetrafluoroethylene (Teflon): It is an excellent high-temperature insulation with excellent electrical insulating properties and is used at temperatures of 220°C and higher. Teflon tubing and wire insulation is available in a variety of colors and typically feels slippery. It has good resistance to moisture.
Nylon: Nylon has good resistance to abrasion, chemicals, and high voltages and is often used to fashion electromechanical components. Nylon is extruded and cast and is filled with a variety of other materials to improve weathering, impact resistance, coefficient of friction, and stiffness.
Phenolics: Phenolic laminated sheets are usually brown or black and have excellent mechanical properties. Phenolics are commonly used in the manu- facture of switches and similar components because it is easily machined and provides excellent insulation. Phenolic laminates are widely used for terminal boards, connectors, boxes, and components.
Polyvinylcloride (PVC): PVC is perhaps the most common insulating mate- rial. Most wiring is insulated with PVC including house wiring. Irradiated PVC has superior strength and resistance to heat. PVC tapes and tubing are also quite common. Electrical and electronic housings are commonly molded from PVC.
Acrylic: Lucite and Plexiglas are trade names for acrylic which has wide- spread use where toughness and transparency are required.
Beryllium oxide: A hard white ceramic-like material used as an electrical insulator where high thermal conductivity is required. Beryllium oxide is highly toxic in powder form and should never be filed or sanded and conse- quently has fallen out of common use. Power semiconductor heat sinks can still be found with beryllium oxide spacers for electrical insulation.
Ceramic: Ceramics are used to fabricate insulators, components, and circuit boards. The good electrical insulating properties are complemented by the high thermal conductivity.
Melamine: Melamine laminated with woven glass makes a very hard laminate with good dimensional stability and arc resistance. It is used in combination with mica to form rigid fiber laminates.
Mica: Mica sheets or stove mica is used for electrical insulation where high temperatures are encountered. Two kinds of mica, Muscovite (white or India mica) and phlogopite (amber mica) are generally used for insulating pur- poses. Mica and reconstituted mica paper are inorganic and infusible. Mica has high dielectric strength, high insulation resistance, low dielectric loss, good mechanical strength, good dielectric constant (specific inductive capacity), and good heat conductivity. Puncture resistance is good but the edges of the mica should be flush against a flat surface to prevent flaking. Mica finds uses in composite tapes and sheets which are useful up to 600°C with excellent corona resistance. Sheets and rods of mica bonded with glass can tolerate extreme temperatures, radiation, high voltage, and moisture. It is also available as mica paper where tiny mica flakes are made into paper like structure and reinforced with fiber, glass, or polyester.
Rigid fiber laminates: They are made of layers of cloth (glass, cotton, poly- ester, etc.) or paper with resin (phenolic, melamine, polyester, and epoxy) impregnation. They are supplied as thermosetting, thermoforming, and postforming materials used as insulators.
Micarta: It is rigid fiber laminate made of cloth, paper, or wood saturated with either a synthetic or organic resin, and then compressed under heat. This process makes the resin permanently hard and therefore Micarta becomes impermeable to heat, pressure, and solvents. Originally, Micarta was devel- oped as an insulator but today it has many applications. Micarta is mechanically strong, rigid, and nonmagnetic. It is less susceptible to mois- ture and most acids. Micarta is used for insulating washers, controller panels and cams, slot wedges, brush rigging, bus bar supports, insulating barriers, and transformer insulation.
Synthetic resins: Synthetic resins are used extensively in varnish manu- facture. The polyester and epoxy types are representative of heat hardening resins. Varnishes containing such resins are thermosetting and will cure by heat alone and do not require oxygen. Other synthetic resins are phenolic resins suitable for molding and bonding; alkyd type resins are being substi- tuted for the old black varnishes and compounds; melamine resins are used for making laminates and molded compounds; and vinyl resins are used in the compounding of plastics and rubber substitutes.
Varnish: Insulating varnishes are of great importance in the mainte- nance of electrical equipment and apparatus. An insulating varnish is a chemical compound of synthetic resins or varnish gums and drying oils, having high dielectric strength and other properties that protect electri- cal equipment. Varnishes provide important insulating and protective functions, which are
• Protect the insulation and equipment against moisture
• Electrically and thermally enhance other insulating materials
• Add mechanical strength to other components of the insulation
• Minimize the accumulation of dust and contaminants, and improve heat dissipation by filling voids
• Enhance and increase the life of insulating materials
Adhesive–coated tapes: Many of the insulating films and fabrics described above can be obtained and used with adhesive backing that are usually ther- moset or heat curable.
Rubber: Natural or Buna S rubber is not normally used for insulation these days because it is affected by ozone and has poor thermal stability. On the other hand, butyl and ethylene propylene rubbers are ozone resistance and more thermally stable. These are used for molded parts, cable insulation, and lead insulation in motors.
Silicone rubber: A variety of silicone foam rubbers are available as an insulating material. Silicone rubbers exhibit characteristics of superb chemical resistance, high-temperature performance, good thermal and electrical resis- tance, long-term resiliency, and easy fabrication. Liquid silicone rubbers are available in electrical grades for conformal coating, potting, and gluing. Silicone rubbers have excellent thermal stability and ozone resistance, but only fair mechanical strength and abrasion resistance.
Silicone/fiberglass: Glass cloth impregnated with a silicone resin binder makes an excellent laminate with good dielectric loss when dry.
Insulation Temperature Ratings
An insulation system is an assembly of insulating materials in association with the conductors and supporting structural parts of an electrical equip- ment and apparatus. Insulation systems for electrical equipment may be classified as solid, liquid, air and vacuum, and gases. The liquid insula- tion system comprise of mineral oil, silicone, and other less-flammable fluids. The gas that is primarily used for electrical insulation is SF6 gas known as sulfur hexafluoride gas. The liquids and gases used as insulat- ing medium in electrical system and equipment are covered in Chapter 4. The air and vacuum insulation system has been used from the very begin- ning and its characteristics are well documented and understood. The ability of insulating materials or an insulation system to perform its intended function is impacted by other aging factors. The major aging factors are electrical stresses, mechanical stresses, environmental stresses, and thermal stresses. Mechanical stresses imposed upon the system and its supporting structure by vibration and differential thermal expansion may become of increasing importance as the size of the apparatus increases. Electrical stresses will be more significant with high-voltage
apparatus or with equipment exposed to voltage transients. Environmental stresses will have an impact depending on the presence of moisture, dirt, chemicals, radiation, or other contaminants. Thermal stresses depend upon environmental conditions (high ambient), loading, and ability to dissipate heat. All such factors should be taken into account when select- ing insulating materials and/or insulation systems. To help the user, IEEE Standard 1-2001, “IEEE recommended practice—General principles for temperature limits in the rating of electrical equipment and for the evalu- ation of electrical insulation,” has established the temperature rating for solid insulation systems of electrical equipment and apparatus. The IEEE Standard 1-2001 takes into account these factors in establishing the stan- dards of temperature limits for particular classes of apparatus. Thus, for temperature rating purposes insulation systems are divided into classes according to the thermal endurance of the system.
According to IEEE Standard 1-2000, insulation system classes may be designated by letters and may be defined as assemblies of electrical insulating materials in association with equipment parts. These systems may be assigned temperature rating based on service experience or on an accepted test proce- dure that can demonstrate an equivalent life expectancy. The thermal classi- fication of electrical insulating systems established by IEEE Standard 1-2000 is given in Table 1.2.