An insulator is a mechanical support. After the mechanical aspects of a design have been finalized, electrical characteristics are added. If the insulator doesn’t keep your line in the air, all electrical characteristics in the world mean nothing. Mechanical characteristics are so important to the functioning of an insulator that they’re the one commonality in the marking of all insulator. Another question to take in account is the consequences of a mechanical failure: Loss of leakage distance only or cable fall. This depend of the design of the insulator. IEC stablish the mechanical residual test methods and acceptance criteria for glass or porcelain string insulators with the dielectric part breakage.
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The use must determine the maximum loading that the line will ever apply to its insulator, including conductor weight, hardware weight, ice and wing loading, and other overload factors.
Suspension insulators are rated in terms of their Specified Mechanical Load (SML). Most manufacturers recommend that you never load the insulator to more than 50% of the Specified Mechanical Load (SML). The SML rating is a guaranteed minimum ultimate strength rating. Each batch of insulators produced is sampled for mechanical strength and all samples must meet or exceed the stated SML value and the statistical criteria. The routine test load is the proof load applied to each unit and also the maximum load that the insulator should ever see in service.
IEC -1 & IEC stablish the mechanical test methods and acceptance criteria for Ceramic or glass insulators units and Composite insulators respectively.
The electrical characteristics of the insulator are imparted by the air around it. This is principally defined by the arcing distance of the insulator defined as “shortest distance in the air external to the insulator between the metallic parts which normally have the operating voltage between them”.
The impulse withstand/flashover characteristics and dry power frequency characteristics are all based upon the dry arcing distance.
Some would argue that the wet power frequency withstand/ flashover characteristics are determined by leakage distance, but that argument only holds within a narrow band of leakage distances. Leakage distance plays a role, but only as a contributing factor.
IEC -1 recommend the withstand voltages associated with the highest voltage for the equipment
Last edition of series IEC TS has developed new techniques for the selection and dimensioning of high voltage insulators. It enables, after an established process, to determinate the most efficient insulation. The technical specification recommends three approaches to select suitable insulators based on system requirement and environmental conditions:
The applicability of each approach depends on available data, time and economics involved in the project.
Some of the parameters required for these approaches are mentioned on the following sub-chapter:
Figure 1 shows the relation between site pollution severity (SPS) class and RUSCD (reference unified specific creepage distance) for insulators. The bars are preferred values representative of a minimum requirement for each class and are given for use with approach 3 (measured and design) of the IEC/TS -1.
If the site pollution severities (SPS) are available, it is recommended to take a RUSCD (reference unified specific creepage distance) which corresponds to the position of the site pollution severity (SPS) measurements within the class by following the curve in Figure 1.
Basic USCD (mm/kV*)
* r.m.s. value of the highest operating
voltage across the insulator.
For Type A pollution (inland, desert or industrially polluted areas), SPS is calculated from ESDD/NSDD values.
For Type B pollution (coastal areas where salt water or conductive fog is deposited onto the insulator surface), SPS is calculated from SES (site equivalent salinity)
Different types of insulator and even different positions of the same insulator type may accumulate pollution at different rates in the same environment. In addition, variations in the nature of the pollutant may make some shapes of insulator more effective than others.
IEC TS -2 – Table 1 below shows a brief summary of the principal advantages and disadvantages of the main profile types with respect to pollution performance.
Antipollution profiles
IEC TS -2 – Tables 2 & 3 give simple merit values for ceramic and glass insulator profiles. Table 2 give the profile suitability, relative to standard profile assuming the same creepage distance per unit or string, Table 3 assuming the same insulating length.
Below tables show a brief summary of the principal advantages and disadvantages of the main profile types with respect to pollution performance.
Type A pollution: Solid pollution with non-soluble components.
Type B pollution: Liquid electrolytes with very little or no non-soluble components.
Also IEC TS -2 give profiles parameters to take in account:
IEC TS -3 – Chapters 8 & 9 give recommendations for polymer insulators profiles and parameter to take in account:
Pollution tests on glass and porcelain insulators on a laboratory can be carried out for two main objectives:
IEC prescribe procedures for artificial pollution tests applicable to ceramic and glass insulators for overhead lines.
Two categories of pollution test methods are recommended for standards tests:
This standardized laboratory pollution test methods are not applicable for composite (polymeric) or RTV-coated insulators. A proposal for a test method for artificially polluted composite insulators is covered by the recent CIGRE TB 555: “Artificial Pollution Test for Polymer Insulators”.
In the case of naturally polluted insulators removed from the service a recent CIGRE TB 691 (WG D1.44), “Pollution Test of Naturally and Artificially contaminated insulators” summarizes the recent experience with so-called rapid flashover test methods:
Both tests can be applied for both ceramic (glass and porcelain) and composite insulators for both AC and DC.
The objective of these tests is a need for reliable diagnostic of naturally polluted insulators to evaluate their residual dielectric strength and also a general trend to make testing more cost-effective and time-efficient, even for the case of artificially polluted insulators.
A reduction of performance can be due to pollution in case of ceramic insulators or due to a combination of pollution and ageing in case of polymeric insulators. In both case cases the residual pollution strength should be quantified in terms of a flashover voltage, not a withstand voltage; because the withstand voltage does not provide the user by the information about the probability for flashover or standard deviation of flashover voltage.
Sometime, the combination of the many variable environmental parameters which influence an insulator’s behaviour over its lifetime are difficult to artificially simulate and, moreover, to accelerate. The validity of laboratory tests is thus often questions as the procedures adopted may not take into account significant factors which would be encountered in service or they may over-emphasise others.
The evaluation of insulator performance in naturally polluted outdoor test stations is becoming more important and popular. Although involving a longer test duration, and still requiring care in the correct interpretation of the test data, the results tend to be accepted with more confidence.
Outdoor test stations is also a good tool for new insulation technologies in which there is as yet no technical or normative specification for its testing or characterization.
CIGRE Technical Brochure No. 333, “Guide for the establishment of naturally polluted insulator testing stations” serve as a general guide for the establishment of natural test stations which will facilitate the overall comparison of various insulators designs, the explorations of particular aspects of insulator performance and/or the selections of the most appropriate insulation for a particular application. It is relates specifically to insulators intended for use under AC conditions but aspects are applicable to DC stations as well.
Typical test aims may be one or more of the following:
The severity of the pollution and the prevailing climate must be examined and should be representative of conditions found on the system. As is the case for laboratory tests, an over-acceleration of the ambient stresses can produce misleading results. Contamination severity assessment by means of ESSD and NSDD measurements and/or directional dust deposit gauges should be undertaken to ensure that an appropriate site is selected.
Insulator test stations have a wide range of size and sophistication and may be categorized as:
Photo 1: Permanent insulator research station at Martigues (France) and examples of Verescence La Granja Insulators tests
Photo 3: In-service insulator test structure
Leakage current activity (include number of flashovers experienced), climatic effects and pollution severity are usually monitored. In addition, the performance of the test samples should be judged by a regular inspection of the insulators: Close visual examination of the surface; an assessment of the hydrophobicity of the dielectric material and the viewing of electrical activity.
Insulators corrosion occurs if the surface of the insulators is polluted, and in the presence of humidity.
When the insulators surface is covered by deposit wet pollution, leakage currents start. Its amplitude is a function of the degree of pollution (the amount of soluble salts).
Polluted and wet insulators energized with AC voltage display a biased leakage current having a DC component that causes electrolytic corrosion of the pins.
Leakage currents effects are even more harmful when the frequency and duration of wet periods are high (humid tropical climate) and also when pollution finds a hygroscopic surface (Hence the importance of inert contaminants that absorb or retain humidity).
This corrosion is most important on DC voltage than AC voltage for same site, due to current unidirectional and electrostatic phenomena that cause the most important pollution deposition formation.
For insulators, dominant electrolytic corrosion effects add to those of atmospheric corrosion and those due to the formation of oxidising agents caused by the presence of arcs near fittings. These last ones can be initiated and maintained during periods of humidification and drying that precedes and succeeds in critical conditions or when the insulator is more humid. Protection field dispatcher accessories can be beneficial to limit these phenomena of humidification and drying periods which are a factor of acceleration of the insulator fittings corrosion for those pieces that are more electrically forced.
Corrosion phenomena result in:
The most severe corrosion cases occur especially in the tropical areas very near to the sea or marine pollution and areas where the pollution by dust accumulation happen for long periods without rain, plus the high environment humidity.
Insulators fittings corrosion may have the following effects:
Metal parts protections have been developed to avoid or delay corrosion phenomena.
They consist of reinforced galvanized fittings and of a sacrificial zinc sleeve protection.
Fig. 3: Detail of leakage current on insulator
IEC- specifies the minimum requirements for the zinc sleeve; anyway, it can be improved to increase it behaviour.
Fig. 4: Drawing and photo of a pin with zinc sleeve
Also IEC- specifies test method for the zinc sleeve control. Future work of normalization would have to be to include the zinc sleeve requirements and the tests methods on the IEC--1 (for a.c. lines)
The main objective of the overhead line maintenance policy in an electric company is to maintain the number of fault outages at reasonable values. A Data Base with the principal information of the insulation is an efficiency tool for the evaluation of performance. The information that this database should contain is:
Several maintenance indicators are habitual used by the utilities:
Also numerous methods of maintenance procedures are well-known by the utilities:
The evolution of maintenance indicators together with the results of inspections linked with the data base of the line help both the decision making regarding the maintenance or replacement of the insulation and the valuation / comparison of the different types of materials, profiles and manufacturers quality.
Insulators are high-technology products expected to work with high reliability over a long period of time. A great number of successfully balanced design parameter, analyzed in the previous chapters, choice of material as well as mastering of manufacturing processes are required to ensure this high long term reliability.
An insulator comes to the end of its working life either when it fails mechanically, flashover at unacceptably high frequency or gives evidence of deterioration to a condition likely to lower its factor of safety in service. All insulators are affected to some extend by impact, cycling but thermal and mechanical, deterioration from weathering and electrothermal causes, flexure and torsion, ionic motion, corrosion and dement growth.
Determinate the exactly time to replace if it necessary is important to optimise the cost of maintenance. There are a fairly large number of degradation modes. Some modes are easily detectable by visual inspection other, especially for porcelain insulators, need sophisticated methods. Degradation modes causes by easily detectable mechanism like slip of metal fitting, pin corrosion or surface erosion are considered to be reason for replacement of insulators.
CIGRE has established a test procedure to determinate the state of cap and pin and long-rod insulators and to decide on the safe time for their replacement: “Guide for the assessment of old cap and pin and long-rod transmission line insulators made of porcelain or glass: What to check and when to replace”. CIGRE Technical Brochure No. 306, .
This document stablishes a testing sequence with a number of non-destructive tests as visual tests (e.g. degree of corrosion) as well as dimension and thermal and the combined thermal-mechanical tests. This first series of tests is followed by destructive mechanical tests. A probability diagram based on normal distribution is used to analyse the failing load test results. With the probability (risk) for failure on the ordinate and the failing load on the abscissa, the failing load characteristics are represented as straight lines. In that way changes in strength become easily visible.
To help the user, the document includes a number of typical cases of analysis of test result called “Reference Scenarios”. They are useful for the assessment of the present condition of the insulator
The failing load characteristics are represented by:
The SFL (specified failing load) is marked with a solid vertical line.
For the example of ”Reference scenario F1” the reductions in strength shown in this example diagram are not representative of good quality products. Age and TMP-test have only negligible influence on good products.
Parallel to this document, another Technical Brochure published by CIGRE assist to evaluate the technical conditions of aged, old or failed composite insulators: “Guide for the assessment of composite Insulators in the laboratory after their removal from service”. CIGRE Technical Brochure No. 481. Different methods, philosophies and tools are descripted which enable a conclusion regarding the residual life-time of composite insulators of the same age and design family. The document also give indications of work for the case of an investigation a failure or a unit recognized as high risk and evaluation of units for research.
It is based on a recommended sequence of testing on samples taken out different stress zones of the line.
The selection of the insulator type is not a simple job, especially if the insulator will be install in a high polluted area.
Numerous documentations (IEC Standards, CIGRE Technical Brochure…) are available to help on the selection of the most appropriate insulator, for monitoring the behavior in operation and for the determination of the finite insulator life.
Different solutions are available to improve the insulator performance on high corrosion areas.
Several factors must be taken in account on the insulator type selection:
[1] IEC -471: International Electrotechnical Vocabulary. Part 471: Insulators
[2] IEC -1: Insulation co-ordination – Part 1: Definitions, principles and rules
[3] IEC : Artificial pollution tests on high-voltage ceramic glass insulators to be used on a.c. systems.
[4] IEC : Residual strength of string insulator units of glass or ceramic material for overhead lines after mechanical damage of the dielectric.
[5] IEC--1: Insulators for overhead lines with a nominal voltage above 1 000V: Ceramic or glass insulators units for a.c. systems – Definitions, test methods and acceptance criteria
[6] IEC/TS -1: Selection and dimensioning of high-voltage insulators intended for use in polluted condition – Part 1: Definitions, information and general principles
[7] IEC/TS -2: Selection and dimensioning of high-voltage insulators indented for use in polluted condition – Part 2: Ceramic and glass insulators for a.c. systems
[8] IEC/TS -3: Selection and dimensioning of high-voltage insulators indented for use in polluted condition – Part 3: Polymer insulators for a.c. systems
[9] IEC : Insulators for overhead lines – Composite suspensions and tension insulators for a.c. systems with a nominal voltage greater than 1 000 V – Definitions, test methods and acceptance criteria
[10] IEC-: Insulators for overhead lines with a nominal voltage above 1 000V: Ceramic or glass insulators units for d.c. systems – Definitions, test methods and acceptance criteria
[11] I. Gutman, Wallace Vosloo, “Development of Time-and Cost-Effective Pollution Test Methods Applicable for Difference Station Insulation Option”. IEEE Vol. 21, No. 6. TDEI submitted .
[12] M. Marzinotto, J-M. George, S. Prat, C. Lumb, F. Virlogeux, I. Gutman, and J. Lundengard, “Field Experience and Laboratory Investigation of glass Insulators Having a Factory-Applied Silicone Coating”, TDEI submitted .
[13] I. Gutman, J. Shamsujjoha, C. Lumb, J-M. George, and S. Roude: “Investigation of Rapid flashover solid layer pollution testing as an alternative to current standard method”, IEEE ISEI-, paper 17 pp.73-77
[14] CIGRE Task Force 33.04.07, “Natural and artificial ageing and pollution testing of polymeric insulators” CIGRE Technical Brochure No. 142,
[15] CIGRE WG B2.03, “Guide for the assessment of old cap and pin and long-rod transmission line insulators made of porcelain or glass: What to check and when to replace”. CIGRE Technical Brochure No. 306,
[16] CIGRE WG B2.03, “Guide for the establishment of naturally polluted insulator testing stations” CIGRE Technical Brochure No. 333,
[17] CIGRE WG B2.21, “Guide for the assessment of composite Insulators in the laboratory after their removal from service”. CIGRE Technical Brochure No. 481,
[18] CIGRE WG D1.44, “Pollution Test of Naturally and Artificially contaminated insulators” CIGRE Technical Brochure No. 691,
[19] Javier GARCIA /Philippe Platteau. Glass insulators in polluted environment: design, test, experiences and benchmarking with other materials. Cigre regional meeting outdoor insulation. Tunisia,
[20] R. García Fernández, M.A. Perez Louzao, I. Serrano “REE’s insulator global maintenance policy”, CIGRE ; B2-206.
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CookieDuraciónDescripciónA material that does not let electricity and heat travel through it is known as an insulator. The electrical insulator is a protector or protective device that finds a use for connecting many electrical components. It plays a notable role in the making of various electrical and electronic circuits and overhead power systems.
The overhead line conductors on the poles are supported by an Insulator, which prevents current flow toward the ground. The transmission lines must function properly. Different materials, such as rubber, wood, plastic, mica, and others, can be used to create an insulator. insulators are available from a variety of Suppliers and Companies, as well as manufacturers and distributors, and there are many Insulators for Sale on Linquip.
There is a complete list of insulator services on the Linquip website that meets all of your needs. Linquip can connect you with a number of insulator service providers and experts who can help you. Linquip offers a team of Insulator Professionals and subject matter experts available to help you test your equipment.
Here, we are describing one of the most significant overhead system’s components and its different types. Read this new blog in Linquip to find out more about types of insulator.
What is an insulator?
Insulators are materials that inhibit the flow of electrical current. They are implemented in household items and electrical circuits as protection and provide the required insulation between the line conductor and earth. Insulators possess high resistivity and low conductivity. In addition to protecting the loss of current, insulators make an electrical current more efficient by concentrating the flow.
Insulator properties
Insulators have some specific properties that make them different from other electrical devices. These are some features of insulators:
High resistivity
Good mechanical strength for the conductor load
The high relative permittivity of insulator material
Good dielectric strength
Waterproof or non-porous
Types of insulator materials
Insulators consist of different types of insulator materials like plastic, rubber, mica, wood, glass, etc. In the electrical system, specific insulating materials are used like porcelain, glass, steatite, polymer, ceramic, PVC.
The importance of insulators
Help to protect from heat, noise, and electricity
Support the overhead conductor
Insulate the live parts of equipment or conductor from the earth
Help to save switchgear, transformer, and other systems in a substation
Types of insulator
4 Main Types of Insulator are:
Pin insulator
Post insulator
Suspension insulator
Shackle insulator
Insulators are classified into different types based on their voltage rating. There are several types of insulators, but the most commonly used are:
Let’s dive in each one-by-one.
An insulator that isolates a wire from physical support, such as a pin on a utility pole or tower, is known as pin type insulator. It is the first developed insulators and being used for overhead lines. As the name suggests, the pin insulator is secured to the cross-arm on the pole. A pin insulator consists of nonconducting material such as porcelain, glass, plastic, polymer, or wood. Pin type insulators find a use for transmission and distribution of electric power at voltages up to 33 kV. Beyond operating voltage of 33 kV, the pin type insulators become too bulky and hence uneconomical.
Pin insulator specifications
It has high mechanical strength.
It requires less maintenance as compared to other insulator types.
Pin insulator pros
Good creepage distance
Simple in construction and cheap in cost
Easy maintenance
Can be used both vertically & horizontally
Applicable to a high voltage transmission line.
The mechanical strength of this insulator is high.
Pin insulator cons
It applies only to transmission lines.
It must be used by the spindle.
The insulator pin can damage the thread of an insulator.
Post insulators are similar to pin-type insulators, but they are more suitable for higher voltage applications. Post insulators are commonly used in the substation or generating substation. They are made in ceramic and also composite material for special applications.
Post Insulator specifications
In a substation, it is suitable from a higher to a lower voltage.
It helps to protect switchgear, transformers, and other connecting devices.
Strong mechanical strength
Can be mounted both horizontally and vertically
A suspension type insulator protects an over headed transmission line like a conductor. It operates at above 33KV and overcomes the limitation of pin type insulator. Mostly, suspension insulator is made of glass or porcelain discs connected in series by metal links in the form of a string.
Suspension insulator specifications
The voltage operating capacity of the insulator is from 11 kV to 765 kV.
Mostly used in the overhead transmission system or line.
It provides more flexibility to the overhead lines.
The multiple discs find usage based on voltage level (low to high).
Generally placed with the steel tower.
Suspension insulator pros
If anyone of the disc insulators in a suspension string is damaged, it can be replaced easily.
Mechanical stresses on the suspension insulator are less since the line hanged on a flexible suspension string.
By using different numbers of discs, a suspension string can be made suitable for any voltage level.
Suspension insulator cons
Suspension insulator string costlier than pin and post type insulator.
It requires more height of supporting structure than that for pin or post insulator to maintain the same ground clearance of the current conductor.
The amplitude of the free swing of conductors is vaster in the suspension insulator system, so it should provide more spacing between conductors.
Shackle type insulator
An insulator that finds use in distribution networks that works with low voltage is known as a shackle insulator. This insulator, that is also known as a spool insulator, is used on sharp curves, end poles, and section poles. Shackle insulator occurs in small size on the overhead distribution system. At present, the usage of this insulator has decreased because of the underground cable used for distribution purposes. It includes a conductor within the groove and can be fixed by using a soft binding wire.
Shackle insulator specifications
It is used in a distribution system by arranging in between tower & conductors to support & insulate.
Can be used in overhead lines with low & medium voltage
It is used with a bolt by placing on the pole otherwise telegraph to avoid outflow current from conductors
Can be used in both horizontal & vertical position.
Shackle insulator pros
Extremely dependable for conductors
Designed to meet up the demands of electricity
The best solution for maintaining protection in different electrical appliances
Bears a high amount of current and temperature
Shackle insulator cons
These apply to low voltage distribution networks only.
Here we provide a table of voltage capacity and use of each types of insulator .
Insulator
Voltage capacity
Use
Pin insulator
< 33 kV
Distribution system
Post insulator
11 kV to 765 kV
Substation system
Suspension insulator
> 11 kV (High)
Transmission system
Shackle insulator
< 33 kV
Distribution system
So, there you have a detailed description of different types of insulator and their specifications. If you have any points to discuss in this article in Linquip, let us know by leaving a reply in the comment section. Is there any question we can help you through? Feel free to sign up on our website to get the most professional advice from our experts.
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Insulators: Types, Properties, Material, Causes of Failure and Applications
The insulators or line insulators are used in overhead transmission and distribution lines. The overhead conductors are supported on the poles or towers. In order to avoid the flow of current through supporting poles, the insulators are used to support the line conductors. Hence, the line insulators are used to provide insulation to the power conductor from the ground.
In a high-voltage transmission line, the conductors are bare and it uses are as an insulator between two conductors. Also, it is not covered with any insulation coating. Generally, the insulators are connected on the cross-arm of the supporting structure to provide necessary clearance between conductors and the metal body. And an insulator clamp is used to pass the conductor. To prevent the flow of any leakage current from line to ground, these insulators provide necessary insulation.
Properties of Insulator
The following properties are desirable in insulators that are used in overhead lines;
It has high electrical resistance to avoid flowing any leakage current.
The insulators are able to withstand conductor load and wind. Hence, it should have high mechanical strength.
It should have high relative permittivity to increase the dielectric strength.
Puncture to flashover ratio must be high.
The material used for an insulator is non-porous, free from impurities and cracks. Impurities may reduce permittivity.
The insulator material must not be affected by a change in temperature.
Insulator Materials
The following different types of insulating materials are used to make a typical insulator.
Porcelain Insulator: (based on ceramic material i.e. china clay, quarts, aluminum silicate, plastic kaolin, feldspar)
Glass Insulator: based on toughened glass having high dielectric strength up to 140kV/cm.
Steatite Insulator: bases on magnesium silicate (different % of magnesium oxide + silica).
Polymer Insulator: based on fiberglass and epoxy polymer.
Porcelain Insulator
Porcelain is the most commonly used material to make insulators for an overhead transmission line. Porcelain does not allow to pass electric charge and it works efficiently when it is combined with other non-conductive materials. It is aluminum silicate mixed with quartz, feldspar, and plastic kaolin. This results in hard and glazed material that is ideal for an insulator.
Glass Insulator
Annealed glass material is used for insulators and it is useful in transmission and distribution systems. The glass is cheaper than porcelain. The dielectric strength of glass is 140 kV/cm and it is more than porcelain. It has high resistivity and a low coefficient of thermal expansion.
Mostly the glass used as an insulator is transparent. Therefore, the impurities or air bubbles can be easily detected, and also, it is not heated up from sunlight like porcelain. But the disadvantage of a glass insulator is that the moisture can condense the glass surface and provide a path to the leakage current.
Note: If you want to know more, we have a detailed post on the “differences between Glass Insulators & Porcelain Insulators“.
Polymer insulator
There are two types of polymer material used in the system; glass fiber reinforced epoxy resin and silicon rubber. Silicon rubber is also known as EPDC (Ethylene Propylene Diene Monomer) and it is made from weather sheds. Polymer insulators are also known as composite insulators. The polymer insulator is lightweight compared to glass and porcelain insulators. But it has more moisture effects.
Suitable Materials used for Insulators
The suitable material to achieve the above properties are; toughened glass, glazed porcelain, and composite polymer. The toughened glass is obtained by rapidly cooling the glass insulator after shaping and allowing the interior to cool slowly. In this way, the glass insulator has the outer surface in compression and the inner surface is in tension.
If you are looking for more details, kindly visit Aerodynamic Glass Insulator.
Toughened glass can withstand greater tension compared to annealed glass. The cost of toughened glass is less compared to porcelain. In most cases, the toughened glass is transparent. Therefore, the flaws in a material can be detected easily by visual. But the moisture affects the surface of the glass and it causes leakage in the surface of an insulator. Another disadvantage of glass is that the mass of a high-voltage line may irregulate the shape and it may result in internal strain. Due to this, the toughened glass is rarely used in a system and it is only suitable for lines below 25 kV.
The porcelain is made up of clay, quartz or alumina, and feldspar. It is necessary to glaze the porcelain to keep the surface relatively free from dirt and moisture. The dielectric strength of porcelain is about 4-10 kV/mm. The dielectric strength of glass is more than that of porcelain. Any impurities available in porcelain may reduce the dielectric strength of the insulator. Hence, it is essential to confirm the porcelain is air free and impervious to the entry of gases and liquid.
Some utilities have started to use insulators made from polymer composite materials. This type of insulator is composed of fiber-reinforced plastic and the outer surface is silicon rubber or EPDM (Ethylene Propylene Diene Monomer) rubber to avoid weather hazards. The composite insulators are cheap in cost and light in weight. This type of insulator is also suitable in a polluted environment.
Related Posts:
Types of Insulators
There are various types of insulators are used in the transmission and distribution network which are listed below;
Pin Type Insulator
Disk Type Insulator
Suspension Type Insulator
Post Insulator
Strain Insulator
Shackle Insulator (Spool)
Stay or Egg Insulator
Special Types of Insulators
Click image to enlarge
Pin Type Insulator
A pin type insulator or pin insulator is small, simple in construction, and cheap in cost. In most cases, this type of insulator is used in the distribution system up to 33 kV lines (they are generally used for 11kV but not more than 50kV). The pin insulator is mounted on a pin on the cross-arm of the supporting structure.
There is a groove on the top of the insulator to place the conductor. A soft metal (generally lead) is used to avoid direct contact between the porcelain and metal pin. One piece of an insulator is used for lower voltages and an adequate length of leakage path is obtained by providing the insulator with two or three rain sheds or petticoats. The rain sheds or petticoats are designed to have sufficient electrical resistance even if the outer surface is wet due to rain.
The thickness of the insulator is required high in case of higher voltage. And it is practically difficult to manufacture pin insulators with high thickness. Hence, two or three insulators are joined and used for higher voltage systems. Generally, pin type insulators are used up to 33 kV system because for higher voltage it becomes heavy and more costly. So, for higher voltage levels, the suspension type insulator is preferred compared pin type insulator for economic reasons.
Disk Type Insulator
An insulator in the shape of a disk or a plate is known as a disk type insulator. These types of insulators are widely used in low and medium as well as high voltage transmission and distribution due to the required electro-mechanical strength in the polluted environment at comparable low cost.
Disc type insulators are used in high voltage transmission and distribution systems. It is also used in industrial and commercial applications. The disc insulators are cost-effective and can be used in medium and low-polluted environments. The shape of this insulator looks like a disc. Hence, it is known as a disc insulator.
Suspension and strain insulators are two types of disc insulators. It provides good mechanical strength and high resistivity. Dialectic strength is also good in disc insulators. Rubber, plastic, glass, mica, or wood can be used to manufacture the disc insulator. The material used to make a disc insulator depends on the voltage rating of the system and environmental conditions.
There are additional three types of disk insulators based on degree of pollution and creepage distance (The shortest distance of outer surface of insulation material between two conducting parts).
Normal Type
Semi-fog Type
Anti-fog Type
The following table shows the creepage distance and degree of pollution for insulators.
S. No
Degree of Pollution
Recommended Creepage Distance
1
Clean areas
16mm/kV
2
Moderately Polluted Area
20mm/kV
3
Industrial Area
22mm/kV
4
Heavily polluted Area
25mm/kV
Disk insulator (both glass and porcelain insulators) has more additional types such as suspension type and strain insulators which are furtherly discussed below.
Suspension Type Insulator
The suspension type insulator is also known as the disc type insulator. The cost of the pin type of insulator increases rapidly with an increase in line voltage. Hence, the suspension type insulator is used over 33 kV line voltage.
This type of insulator uses porcelain discs. The porcelain discs are mounted one above the other and make a string. To increase the creepage distance, each disc consists of a single shed of porcelain grooved under surface. The upper surface of a disc is inclined at an angle to ensure free draining of water. Each disc has a metal cap and a metal pin. The metal cap is provided on the top and the metal pin is provided underneath. So, the cap is fitted with the pin of another unit and it helps to connect a number of discs together.
The conductor is suspended below the point of support with insulation string. Hence, this type of insulator is known as a suspension type insulator. The string of an insulator is flexible and it provides adequate clearance between conductors and between conductor and metal body when the string swings in wind.
There are three types of suspension type of insulators as follows:
Hewlett Type Insulator: It is the old school design and contains only one porcelain disk.
Cemented Cap Type Insulator: It is also contained on one porcelain disk and there is a slot in the bottom (to improve the flashover ability) where a conductor can be connected to it. There is a galvanized cast iron cemented cap on the upper portion of the insulator, that’s why it is known as a cemented cap type insulator.
Core & Link Type Insulator: It is the combination of above mentioned two types of insulators e.g. Hewlett + cemented Cap.
The suspension type insulator has many advantages compared to the pin type insulator. Some of these advantages are;
The insulation string is flexible. Hence, it reduces mechanical stress.
Each disc is designed for low voltage. But a number of discs are connected together to make a string according to a required voltage level. If one disc is failed, only that disc is needed to replace instead of an entire string.
If it is proposed to increase the line voltage, the line insulation is increased by the increased number of discs.
The number of discs in a string depends on the line voltage and the atmospheric conditions. But, in most cases, a number of discs for different volage ratings are shown in the table below.
Voltage kV
33
66
110
132
220
440
500
Number of Discs (Suspension)
3-4
5-6
7-8
9-10
13-16
22-23
25-26
Voltage Distribution Over Suspension Insulator String
In a suspension insulator, the number of discs is connected in series with a metal cap and pin. Therefore, each disc acts as a capacitor C. Also, there is a mutual capacitance between the metal connector to earth and it is represented by kC. The illustration of four suspension insulators is shown in the figure below.
Fig-3
Consider that V1 is the voltage difference between two metal conductors of the first insulator.
Now,
I1 = V1 ω C
i1 = V1 ω kC
According to KCL;
I2 = I1 + i1
I2 = V1 ω C + V1 ω kC
I2 = V1 ω C (1 + k)
Also, for the second insulator;
I2 = V2 ω C
Hence,
V1 ω C (1 + k) = V2 ω C
V2 = V1 (1 + k)
i2 = k ω C (V1 + V2)
i2 = k (2 + k) V1 ω C
For the third insulator;
I3 = I2 + i2
I3 = k (1 + 3k + k2) V1 ω C
I3 = V3 ω C
V3 = k (1 + 3k + k2) V1
i3 = k ω C (V1 + V2 + V3) = k (3 + 4k + + k2) V1 ω C
For the fourth insulator;
I4 = I3 + i3
I4 = k (1 + 6k + 5k2 + k3) V1 ω C
I4 = V4 ω C
V4 = k (1 + 6k + 5k2 + k3) V1
The value of ‘k’ lies between 0.1 to 0.2.
String Efficiency:
String efficiency of ‘n’ units is defined as;
The string efficiency decreases with an increase in the number of disc units.
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Post Insulator
A post insulator is similar to a pin type insulator. But post insulator has a metal base and metal cap that helps to mount more than one insulator in series. This type of insulator is used in supporting bus bars and disconnecting switches in the sub-station.
The post insulator is used in extra high-voltage sub-stations. Polycon post insulators are used in 400 kV and above systems. This insulator is a puncher-proof, solid core insulator suitable for outdoor applications.
This insulator is mostly used in substations and similar applications. Hence, this insulator is also known as a station post insulator. And in transmission line applications, this insulator is known as a line insulator. There are five types of line post insulators;
Vertical clamp top line posts
Tie top-line posts
Horizontal (stud mount) clamp top line posts
Brace post insulators
Horizontal (gain base) line posts
Strain Insulator
The strain insulators are specially designed to have high mechanical strength. And these insulators are able to withstand the tension of the conductor at the line ends. The strain insulators are used to support overhead transmissions and radio antennas. It is also used to separate two wires electrically.
The strain insulator is made up of glass, porcelain, or fiberglass. For a lower voltage system, a single insulator is used. But for a higher rating, the number of insulators are connected in series. This type of insulator is used in long transmission lines. And also, it is suitable to provide mechanical strength when the line crosses the lake, river, or pond.
The following table shows the required number of strain string (discs) and related voltage levels.
Voltage kV
33
66
110
132
220
440
500
Number of Discs (Strain String)
4-5
6-7
8-9
10-11
14-17
23-24
26-27
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Shackle Insulator
A shackle insulator is used in a low-voltage distribution network. This type of insulator can be used either in vertical or horizontal positions. The shackle insulator is also known as a spool insulator. In a recent power system (transmission and distribution) scenario, the distribution networks use an underground system. Hence, usage of shackle insulators is decreased.
The shackle insulator is available in different sizes and round shapes. It has a hole in the middle for boulting. A wide galvanized plate is placed on both sides. The surface of an insulator is covered to resist water flow. It is manufactured from porcelain and aluminum silicate. The conductors are fixed within the groove using a soft binding wire. The shackle insulators are designed to provide insulation between the tower and conductor in the distribution system.
Stay or Egg Type Insulator
Egg or stay insulators are used to insulate the stay or guy wire from the low voltage lines. Stays are installed minimum 13 meters above the ground level and insulated from the ground via porcelain insulators. The special design of the stay insulator won’t allow the guy wire on the ground even if it breaks.
In stay paths, this type of insulator is used. In case of wire cuts or falls on the ground, the stay insulators help to avoid current flow from conductor to ground. Therefore, this type of insulator is placed between the tower and the line conductor. This insulator is mainly used on guy wire to balance tensile.
Generally, the stay insulators have been installed in the middle of the stay path. And in most cases, it is made up of porcelain material. The stay insulators are used for low voltage overhead transmission systems.
There are two vertical slots in the stay insulator which are used to fix the guy wire and line wire in each of them. Generally, these insulators are used in stays, that’s why it is known as stay insulators.
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Special Types of Insulators
Areas where the air conditions are not favorable or normal, for example where there is smoke, chemicals, salts or humidity in the environment etc. then special types of insulators are used instead of normal insulators e.g.:
Pin Type Anti-dirt insulators: It has more creepage distance (490mm) than the normal insulator which is generally 325 mm. The rain water can easily go inside the shades of these insulators which helps to clean the dirt.
Inclined Disk Insulators: Looks like disk type suspension insulator but not in complete horizontal plane but bend-over at a specific angle.
Deep Shade Insulators: It has deep and large shades as compared to prevent the puncturing problems of Inclined Disk Insulators in the rainy seasons.
Solid Core Insulators: It has a solid core without a slot which is used to fit the pin inside it. The pin is connected to the lower portion of the insulator using cement.
Aerodynamic Insulators: It has small shades as compared to the deep shade insulators hence, it prevents the dirt and resist even in the strong wind. They are used in sandy areas and desert landscapes.
Except all of the above mentioned types, there are some additional types of insulators used in substations as follows:.
Substation Insulators:
Post Insulators for Support
Sectionalizing Insulators
Bracket Insulators
Tensile Load Insulators for Driving / Pulling Live parts
Operating Rod Insulators
Stray Arm Insulators
Shaft Insulators
Roofline Insulators
Connecting Rod Insulators
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Selection of
Insulators
The first thing before proper selection of the insulator is to know the exact value of the insulation level of the lines in terms of the terminal equipment. In addition, the normal frequency flashover voltage and lighting voltages are considered while selecting an insulator.
Another factor is the effect of the line insulation efficiency on supporting structures (wood, steel etc.) this way, the construction, design and usable age of the cross arms and poles & towers should be considered for choosing the suitable insulators.
The overvoltage due to switching surges and arcing ground because of isolated neutral systems can affect the over insulation, In addition, the proper information about the mechanical stress should be taken into account while selecting an appropriate insulator for the designed system.
As a final touch, the manufacturers catalogs based credible data helps a lot to select the suitable insulator for reliable operation of the systems.
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Causes of Failure in Insulators
There are some common causes of failure in the insulator listed below.
Cracking of Insulator:
Due to unequal expansion, there is a chance of a crack in the porcelain insulator. Also, by the seasonal effect, there is a chance to crack in an insulator. There are many improvements needed to avoid such cracking. Sometimes, the cushions are used between shells and steel pins to allow some expansion.
Defective material:
Generally, the insulator is manufactured from good materials. But there is a chance of defective material. And it causes puncture that allows to flow of the current. These defective insulators are not used in electrical systems.
Porosity of material:
Generally, the insulators are manufactured from porcelain. If the porcelain is manufactured at a lower temperature, it will make it porose. Hence, it will absorb moisture from the air and it may allow flowing leakage current. Therefore, this is unfit to use in electrical networks.
Improper glazing on the surface:
The outer surface of the insulator is glazed properly to free the draining of water. But, if the glazing is not properly done, there is a chance to stick moisture on it and depositing dust on the surface of an insulator. It causes a conducting path and it results in a reduction in a flash over distance. Hence, there is more chance of flashover in an insulator.
Flashover
If the flashover occurs between the two metallic ends of the insulator and generates high temperature and heat which leads to breaking the insulator.
Short circuit
If the distance between the conductors is short, there is a chance of arcing due to the bird strikes on line conductors. This cause of short circuit may also lead to failure of the insulator.
Mechanical stress:
The mechanical stress on the insulator is continuously applied by the conductor. If the insulator is manufactured from defective material, it may break down from a weak point and causes the failure of an insulator.
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Insulators: Materials that do not allow electricity to pass through them are called insulators. Example: Rubber, glass, plastic, etc
Types of insulators:
There are mainly five types of insulators, such as:
Suspension insulators - An insulator that acts as a conductor to protect an overhead transmission cable is called a suspension insulator.
Strain insulators - String insulators are utilized whenever a suspension string is needed to support a high tensile pressure on some kind of conductor.
Pin insulators - The first ceiling-mounted insulators to be created, and they've been widely employed in power networks up to
33kV
is called pin insulators.
Stay insulators - A type of low voltage insulator used to balance and secure the dead-end pole using stay wire or guy grip is called stay insulators.
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