Insulators

INSULATORS

An electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it nearly impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.

A perfect insulator does not exist, because even insulators contain small numbers of mobile charges (charge carriers) which can carry current. In addition, all insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms. This is known as the breakdown voltage of an insulator. Some materials such as glass, paper and Teflon, which have high resistivity, are very good electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at normally used voltages, and thus are employed as insulation for electrical wiring and cables. Examples include rubber-like polymers and most plastics.

Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation. The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.

PHYSICS OF CONDUCTION IN SOLIDS

Electrical insulation is the absence of electrical conduction. Electronic band theory (a branch of physics) says that a charge flows if states are available into which electrons can be excited. This allows electrons to gain energy and thereby move through a conductor such as a metal. If no such states are available, the material is an insulator.

Most (though not all, see Mott insulator) insulators have a large band gap. This occurs because the "valence" band containing the highest energy electrons is full, and a large energy gap separates this band from the next band above it. There is always some voltage (called the breakdown voltage) that gives electrons enough energy to be excited into this band. Once this voltage is exceeded the material ceases being an insulator, and charge begins to pass through it. However, it is usually accompanied by physical or chemical changes that permanently degrade the material's insulating properties.

Materials that lack electron conduction are insulators if they lack other mobile charges as well. For example, if a liquid or gas contains ions, then the ions can be made to flow as an electric current, and the material is a conductor. Electrolytes and plasmas contain ions and act as conductors whether or not electron flow is involved.

Breakdown

is the minimum voltage that causes a portion of an insulator to become electrically conductive.

For diodes, the breakdown voltage is the minimum reverse voltage that makes the diode conduct in reverse. Some devices (such as TRIACs) also have a forward breakdown voltage.

When subjected to a high enough voltage, insulators suffer from the phenomenon of electrical breakdown. When the electric field applied across an insulating substance exceeds in any location the threshold breakdown field for that substance, the insulator suddenly becomes a conductor, causing a large increase in current, an electric arc through the substance. Electrical breakdown occurs when the electric field in the material is strong enough to accelerate free charge carriers (electrons and ions, which are always present at low concentrations) to a high enough velocity to knock electrons from atoms when they strike them, ionizing the atoms. These freed electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in a chain reaction. Rapidly the insulator becomes filled with mobile charge carriers, and its resistance drops to a low level. In a solid, the breakdown voltage is proportional to the band gap energy. The air in a region around a high-voltage conductor can break down and ionise without a catastrophic increase in current; this is called "corona discharge". However, if the region of air breakdown extends to another conductor at a different voltage it creates a conductive path between them, and a large current flows through the air, creating an electric arc. Even a vacuum can suffer a sort of breakdown, but in this case the breakdown or vacuum arc involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself. In case of some insulators, the conduction may take place at a very high temperature as then the energy acquired by the valence electrons is sufficient to take them into conduction band.

Flashover Voltage

• The voltage at which flashover occurs.
• The voltage at which a disruptive discharge occurs between conductors, such as electrodes, separated by an insulating material. This discharge may occur around or along the surface of said insulator. Also called sparkover voltage (

TELEGRAPH AND POWER TRANSMISSION INSULATORS

Overhead conductors for high-voltage electric power transmission are bare, and are insulated by the surrounding air. Conductors for lower voltages in distribution may have some insulation but are often bare as well. Insulating supports calledinsulators are required at the points where they are supported by utility poles or transmission towers. Insulators are also required where the wire enters buildings or electrical devices, such as transformers or circuit breakers, to insulate the wire from the case. These hollow insulators with a conductor inside them are called bushings.

Material

Insulators used for high-voltage power transmission are made from glass, porcelain orcomposite polymer materials. Porcelain insulators are made from clay, quartz or alumina andfeldspar, and are covered with a smooth glaze to shed water. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain has a dielectric strength of about 4–10 kV/mm. Glass has a higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains. Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to ceramic materials.

Recently, some electric utilities have begun converting to polymer composite materials for some types of insulators. These are typically composed of a central rod made of fibre reinforced plastic and an outer weathershed made of silicone rubber or ethylene propylene diene monomer rubber (EPDM). Composite insulators are less costly, lighter in weight, and have excellent hydrophobiccapability. This combination makes them ideal for service in polluted areas. However, these materials do not yet have the long-term proven service life of glass and porcelain.

10 kV ceramic insulator, showing sheds

Power lines with ceramic insulators in California, USA

Design

The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways:

·         A puncture arc is a breakdown and conduction of the material of the insulator, causing an electric arc through the interior of the insulator. The heat resulting from the arc usually damages the insulator irreparably. Puncture voltage is the voltage across the insulator (when installed in its normal manner) that causes a puncture arc.

·         A flashover arc is a breakdown and conduction of the air around or along the surface of the insulator, causing an arc along the outside of the insulator. They are usually designed to withstand this without damage. Flashover voltage is the voltage that causes a flash-over arc.

Most high voltage insulators are designed with a lower flashover voltage than puncture voltage, so they flash over before they puncture, to avoid damage.

Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be reduced by more than 50% when the insulator is wet. High voltage insulators for outdoor use are shaped to maximise the length of the leakage path along the surface from one end to the other, called the creepage length, to minimise these leakage currents. To accomplish this the surface is moulded into a series of corrugations or concentric disc shapes. These usually include one or more sheds; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in wet weather. Minimum creepage distances are 20–25 mm/kV, but must be increased in high pollution or airborne sea-salt areas.

Types of insulators

These are the common classes of insulator:

·         Pin type insulator - As the name suggests, the pin type insulator is mounted on a pin on the cross-arm on the pole. There is a groove on the upper end of the insulator. The conductor passes through this groove and is tied to the insulator with annealed wire of the same material as the conductor. Pin type insulators are used for transmission and distribution of communications, and electric power at voltages up to 33 kV. Insulators made for operating voltages between 33kV and 69kV tend to be very bulky and have become uneconomical in recent years.

·         Post insulator - A type of insulator in the 1930s that is more compact than traditional pin-type insulators and which has rapidly replaced many pin-type insulators on lines up to 69kV and in some configurations, can be made for operation at up to 115kV.

·         Suspension insulator - For voltages greater than 33 kV, it is a usual practice to use suspension type insulators, consisting of a number of glass or porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the top end is secured to the cross-arm of the tower. The number of disc units used depends on the voltage.

·         Strain insulator - A dead end or anchor pole or tower is used where a straight section of line ends, or angles off in another direction. These poles must withstand the lateral (horizontal) tension of the long straight section of wire. In order to support this lateral load, strain insulators are used. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strings of cap-and-pin (suspension) insulators are used, attached to the crossarm in a horizontal direction. When the tension load in lines is exceedingly high, such as at long river spans, two or more strings are used in parallel.

·         Shackle insulator - In early days, the shackle insulators were used as strain insulators. But now a day, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.

·         Bushing - enables one or several conductors to pass through a partition such as a wall or a tank, and insulates the conductors from it.

·         Line post insulator

·         Station post insulator

·         Cut-out

Cap and pin insulator string (the vertical string of discs) on a 275 kV suspension pylon.

Suspended glass disc insulator unit used in cap and pin insulator strings for high voltage transmission lines

 Cap and pin insulators

Higher voltage transmission lines usually use modular cap and pin insulator designs (pictures, left). The wires are suspended from a 'string' of identical disc-shaped insulators that attach to each other with metal clevis pin or ball and socket links. The advantage of this design is that insulator strings with different breakdown voltages, for use with different line voltages, can be constructed by using different numbers of the basic units. Also, if one of the insulator units in the string breaks, it can be replaced without discarding the entire string.

Each unit is constructed of a ceramic or glass disc with a metal cap and pin cemented to opposite sides. In order to make defective units obvious, glass units are designed with Class B[clarification needed] construction, so that an overvoltage causes a puncture arc through the glass instead of a flashover. The glass is heat-treated so it shatters, making the damaged unit visible. However the mechanical strength of the unit is unchanged, so the insulator string stays together.

Standard disc insulator units are 25 centimetres (9.8 in) in diameter and 15 cm (6 in) long, can support a load of 80-120 kN (18-27 klbf), have a dry flashover voltage of about 72 kV, and are rated at an operating voltage of 10-12 kV. However, the flashover voltage of a string is less than the sum of its component discs, because the electric field is not distributed evenly across the string but is strongest at the disc nearest to the conductor, which flashes over first. Metal grading rings are sometimes added around the disc at the high voltage end, to reduce the electric field across that disc and improve flashover voltage.

In very high voltage lines the insulator may be surrounded by corona rings.[6] These typically consist of toruses of aluminium (most commonly) or copper tubing attached to the line. They are designed to reduce the electric field at the point where the insulator is attached to the line, to prevent corona discharge, which results in power losses.

 

Typical number of disc insulator units for standard line voltages Line voltage (kV) Discs 34.5 3 46 4 69 5 92 7 115 8 138 9 161 11 196 13 230 15 287 19 345 22 360 23

  

History

The first electrical systems to make use of insulators were telegraph lines; direct attachment of wires to wooden poles was found to give very poor results, especially during damp weather.

The first glass insulators used in large quantities had an unthreaded pinhole. These pieces of glass were positioned on a tapered wooden pin, vertically extending upwards from the pole's crossarm (commonly only two insulators to a pole and maybe one on top of the pole itself). Natural contraction and expansion of the wires tied to these "threadless insulators" resulted in insulators unseating from their pins, requiring manual reseating.

Amongst the first to produce ceramic insulators were companies in the United Kingdom, with Stiff and Doulton using stoneware from the mid-1840s, Joseph Bourne (later renamed Denby) producing them from around 1860 and Bullers from 1868. Utility patent number 48,906 was granted to Louis A. Cauvet on 25 July 1865 for a process to produce insulators with a threaded pinhole: pin-type insulators still have threaded pinholes.

The invention of suspension-type insulators made high-voltage power transmission possible. As transmission line voltages reached and passed 60,000 volts, the insulators required become very large and heavy, with insulators made for a safety margin of 88,000 volts being about the practical limit for manufacturing and installation. Suspension insulators, on the other hand, can be connected into strings as long as required for the line's voltage.

A large variety of telephone, telegraph and power insulators have been made; some people collect them, both for their historic interest and for the aesthetic quality of many insulator designs and finishes. One collectors organisation is the US National Insulator Association, which has over 9,000 members.

  

HIGH VOLTAGE INSULATOR

The purpose of the insulator or insulation is to insulate the electrically charged part of any equipment or machine from another charged part or uncharged metal part. At lower utilization voltage the insulation also completely covers the live conductor and acts as a barrier and keeps the live conductors unreachable from human being or animals. In case of the high voltage overhead transmission and distribution the transmission towers or poles support the lines, and insulators are used to insulate the live conductor from the transmission towers. The insulators used in transmission and distribution system are also required to carry large tensional or compressive load.

Here our brief discussion will be restricted to high voltage insulators used in transmission lines andsubstations.

The HV/EHV insulators are broadly divided into two types based on the material used. One is ceramic and the other is polymer (composite) insulator. In Fig-A is shown the sketch of a porcelain disc insulator unit and in Fig-B is shown a glass disc insulator.

Traditionally ceramic insulators of porcelain are used in both transmission and distribution lines.

Now polymer or composite insulators are increasingly used in high voltage transmission systems. The polymer insulators have a fibre rod surrounded by outer sheath of some polymer. Due to the hydrophobic nature of the polymer insulator surface, dry areas are formed between wet areas resulting in discontinuities in wet creepage path. This phenomenon helps improve the performance of the polymer insulator in polluted and coastal areas. The polymer insulators has one great advantage that it is quite lighter in comparison to porcelain insulators. It is reported that the polymeric insulator surface degrade faster in comparison to porcelain insulator. One important disadvantage with porcelain insulator is that the porcelain insulators can bear large compressive force but less tensional force. The porcelain insulators surface is hydrophilic in nature, which means affinity for water. Polymer insulators age faster than ceramic insulators.

Below are few definitions in relation to insulator that one should know which are required here to understand some concepts. 

Creepage Length -The creepage length is the shortest distance between two metallic end fittings of insulator along the surface of insulator . In the string of insulators for creepage length calculation the metallic portion between two consecutive insulator discs is not taken into account.

The corrugation below the insulator is for the purpose of obtaining longer creepage path between the pin and cap. The corrugation  increases the creepage length so consequently increasing resistance to the insulator leakage current. The leakage current that flows through the surface of  insulators should be as little as possible.

The creepage distance required  in clean air may be 15 mm per kiloVolt (line voltage). In the polluted air depending on the level of pollution of air the required creepage distance increases.

Flashover distance - It is the shortest distance through air between the electrodes of the insulator. For a pin type insulator shown  in Fig-C the double headed red arrow line is flashover distance.

Flashover voltage - The voltage at which the air around insulator breaks down and flashover takes place shorting the insulator.

Puncture voltage - The voltage at which the insulator breaks down and current flows through the inside of insulator. 

An insulator may fail due to excessive electrical stress, excessive thermal and mechanical stress or degradation due to environmental chemical action of surface of the insulator. The electrical failure can happen between conductor and earth through air or through the volume of insulating material. In one case due to excessive electric stress the insulator may fail when a flashover takes place through the air between the conductor and tower. In other case the insulator may be punctured through the volume. The insulating materials say porcelain has high dielectric strength in comparison to air. The insulators are designed so that it will flashover before it gets punctured. Failure due to flashover is generally temporary and self restoring. But failure due to insulation puncture is permanent and the insulator is damaged and required to be replaced. An insulator which have internal defects like voids and impurities, reduces the electrical strength of the insulator.

The flashover may results in damage of insulator glaze which can be repaired. In polluted regions contaminants deposit on the surface of the insulator that results in reduction of the flashover voltage of the insulator in wet condition. For example if the power frequency flashover voltage of a 33 kV pin insulator is 95 kV in dry then in wet condition the flashover voltage may be reduced to below 80 kV. Insulators are designed to withstand flashover voltage. In this example you can observe that even in the wet condition the flashover voltage (80 kV) is more than twice the insulator working voltage (33 kV).

The other important electrical parameters of insulator are Electromechanical failing load, lightning withstand voltage and switching impulse withstand voltage etc.. HV Line insulator requirement is based upon the creepage length. The switching impulse withstand voltage is particularly more important in case of Extra High Voltage (more than 300 kV) and Ultra High Voltage lines.

Insulators of different design are available for different applications some cases are outlined below.

SUSPENSION INSULATOR

The suspension insulators are used to support conductors in high voltage transmission lines. The suspension insulators string used in transmission lines are obtained by joining several disc insulator units. according to the type of hardware fittings, usually two varieties of disc insulators are used in HV transmission line. These are cap and pin type and ball and socket type. A porcelain cap and pin disc insulator is shown in Fig-A. Also in Fig-B is shown a glass disc insulator. In the porcelain insulator the somewhat umbrella like upper  part called skirt is glazed and smoothened so that when it rains the dust and salt deposited on it are easily washed away. The contaminants cannot easily penetrate the glazed surface. When it rains the lower corrugated part does not wet and remains dry. This dry portion is the effective creepage length in wet condition.

In the transmission line a string of disc insulators are formed by fitting the pin of one disc to the cap of next disc. Simply by adding more  numbers of discs in the string the insulator string is used for higher voltage. Moreover when one disc is damaged only that particular disc is replaced not the whole string.

Pin Type Insulator

The pin type insulators are suitable for use in low and high voltage distribution systems. Actually in distribution lines you will hardly find any other type of insulators. Pin type insulators are not usually used above 33 kV as the insulator size will become large and costly and unfeasible. See the figure-C for a pin type insulator.

Post Insulator

The post type insulators are mostly used in high and extra high voltage substations. In the substation Post type insulators are used for supporting equipments and Bus conductors. See Figure-D for a post type insulator.

The post insulator is manufactured as single unit from porcelain or composite material. The post insulators are also required to have sufficient bending strength and torsional strength. Both porcelain and polymeric post type insulators are used in practice.