Properties of traditional insulating material, SF6, Vacuum, Air, Insulating oil, Ceramics, PVC, PTFF, PMMC, Fiber glass, Polyethylene
Insulators are mounted on the cross arms and the line conductors are attached to the insulators so as to provide the conductors proper insulation and clearances between the conductors and metal work.
Ø Insulators provide clearances against the highest voltage in worst atmospheric condition to which the line is likely to be subjected.
Ø Insulators prevent short circuiting between the different phase conductors.
Ø Insulators provide mechanical support for the line conductors.
Properties of insulating material
§ High mechanical strength so as to bear the load like wind force, ice loading.
§ High relative Permitivity so as to provide high dielectric strength
§ High insulation resistance in order to prevent leakage of currents to earth.
§ High ratio of rupture strength to flash over voltage.
§ Ability to withstand large temperature variation.
Material used should not be porous and should be impervious to the fluids and gases with the
atmosphere. It should be free from internal impurities and cracks etc.
SF6 (sulphur hexa floride) is a gaseous insulation having high dielectric strength and low liqui- friction temperature; it can be used over a wide range of operating condition.
It has good arc quenching properties and therefore, widely used as an insulating as well as arc quenching medium in high voltage apparatus such as high voltage cables, current and voltage transformer, circuit breaker and metal enclosed substations.
Dielectric strength- 73KV/cm to 30 KV/cm
Under vacuum conditions the dielectric loss is low and dielectric strength is high. Hence vacuum or low gas pressure finds application in electronic equipment including capacitors, circuit breaker and electrostatic separator.
Air is free gift from god. It has got application everywhere in spite of our consent. It is used as insulation in transmission line. It has high insulation resistance but droppps due to moisture. When using air, there is very less dielectric loss. At high voltage corona effect takes place.
Dielectric strength- 30 KV/cm
Insulating oil mineral insulating oils are hydrocarbons refined from crude petroleum deposit from the ground. It is of hydrocarbon of group CnH2n+2.
Transformer oil is very commonly used as insulating oil for dissipation the heat by convection from winding and core to the cooling surface and for maintaining the insulation of windings. Insulating oil is used in circuit breaker and all those equipments where rate of heat production is high or heat dissipation is required at fast rate.
Dielectric strength- 300KV/cm to 500 KV/cm
Ceramics are inorganic materials produced by consolidating minerals into monolithic bodies. By high temperature heat treatment. Ceramics can be divided-
Low Permitivity ® er <12 as insulator
High Permitivity ® er >12 in capacitors and transducers
CERAMICS ARE DIVIDED:
Safe temperature (0C)
Water Absorption (PPM)
Dielectric strength (KV/MM)
Ceramics are chemically inert to all alkalis and most of the acids. They are corrosion proof and resistant to contamination. They are used in the fabrication of high current vacuum interrupter and a variant of electrical and ceramic components.
Alumina is used for suspension type insulators and station type post insulators.
High permit ivies ceramics are:
MgTio3-Magnesium Meta titamate
CaTio3 – Calcium Titanate
SrTio3 -Stantium Titanate
BaTio3 –Barium Titanate
Epoxy resins are thermosetting types of insulating materials; they possess excellent dielectric and mechanical properties. They can be easily cast into desired shapes even at room temperature. They are very versatile and their basic properties can be modified by using fillers.
Epoxy resin can be formed into an insulator of any desired shape for almost any type of high voltage application. Insulators, bushings, apparatus etc can be made out of epoxy resin. It can be used for encapsulation of electronic components, generator windings and transformers. It is used for bonding of very diverse materials such as porcelain, wood, metals, plastic etc. It is very important adhesive used for sealing of high vacuum joints.
Glass fiber rein forced epoxy are widely used for overload transmission line insulators, bushing and bus bar insulators.
PVC= polyvinyl chloride.
PVC is obtained from polymerization of gaseous vinyl chloride. Vinyl chloride has the following molecular structure
Since the vinyl chloride molecular posses a dipole moment its loss tangent is higher. PVC is hard and brittle. Its resistance to moisture, acid, alkalis, oils and alcohols is quite satisfactory. PVC is widely used for making various plastic and rubber like articles for insulating telephone and other wires for protecting electric and frost resistant, plasticisers liquid with high boiling points are added.
It is resistant to the action of sunlight, water and oxygen.
Poly –tetra- fluro-ethylene is obtained through the polymerization of tetra fluro ethylene and has a linear molecular structure
F F F
½I ½ ½
- C- C –C -
½ ½ ½
F F F
PTFE possesses exceptionally valuable properties. Some of these properties are extreme heat resistance to almost all chemical reagents, non-hygroscopic and excellent insulating properties over a wide range of temperature and frequency. Its volume resistivity is more than 1010W-cm. Its Permitivity is 1-9-2-2 and loss tangent is 0.00001 to 0.0003. Its electrical strength drops sharply when subjected to a continuous ac voltage PTFE has a low radiation resistance. The application of PTFE is as capacitors dielectrics, insulators for all kind of windings, as conductor insulation of cables and wires as heat resistant material.
Plastics (polymeric materials) used for electrical insulation purpose consists of long chain macromolecules with repeating monomer units. A polymer is named by putting the prefix poly in front of the monomer.
PMMC stand for poly-methyl-meta-crylate plastics are very widely used as insulating materials because of their excellent dielectric properties.
Dielectric strength- 170-280KV/cm
Volume resistivity -1016W-meter
Tand (50 Hz to106 Hz) -0.0002
It is made from material, which is a free alkali metal oxide, which may form a surface coating that may attack the glass silicates. Glass does not absorb moisture volumetrically, but may attract
It by capillary action between the fine filaments. Tapes and clothes woven from continuous filament yarns of glass have a high resistivity, thermal conductivity and tensile strength and from a good class B insulation.
The dielectric strength of glass varies from 3000 5000 KV/Cm decreases with increase with temperature, reaching half the value at 1000C. Glass is used as a cover and for internal support in electric bulb, electronic valve, mercury arc switches, X-ray, equipment capacitors and as insulator in telephone.
Polyethylene is a thermoplastic material that combines unusual electrical process, high resistance to moisture and chemical, easy process ability and low cost. It has got high resistivity and good dielectric properties at high frequencies and therefore, is widely used for power and coaxial cables, telephone cables multi-conductor control cable, TV lead in wire etc.
Electrical properties of polyethylene
Dielectric strength- KV/cm
Dielectric constant 50 Hz to106 Hz
Tand (50 Hz to106 Hz
Classification of solid insulation materials
Poly vinyl chloride
Cross linked Polyethylene