Different types of insulators are available for application on overhead transmission lines, including glass or porcelain string insulators, porcelain long-rods and composite/polymeric insulators. Within each category there are also a range of designs, materials, qualities and prices. At the same time, there are also several alternatives available to improve performance of insulators intended for polluted service areas, from advantageous shed profiles to coating with RTV silicone material.
Link to Yipeng
Given all these options, there are a host of questions when deciding on which insulators to select for any new project, such as: What is the best design for that environment? What is the best material and what criteria must be taken in account when selecting it? Which parameters are most suitable for in service evaluation of condition? What will be the estimated service life? and so on. Unfortunately, there is no simple answer to all these questions. But it is possible to note the different elements to be considered when evaluating and comparing all possible solutions.
This edited contribution to INMR by Javier García, an expert at La Granja Insulators in Spain, offers his views on what type of considerations must be taken into account when selecting an insulator for application on an overhead transmission line.
An insulator acts mainly as a mechanical support. As such, only after all mechanical aspects of any design have been finalized are the required electrical characteristics added. In fact, mechanical characteristics are so important to the function of an insulator that they are the only commonality found in all markings on insulators. Another issue to consider is consequence of mechanical failure, e.g. is it only loss of leakage distance or is it a dropped conductor. This of course depends on design of the insulator. IEC establishes the mechanical residual test methods and acceptance criteria for glass and porcelain string insulators under dielectric breakage.
The user must also determine maximum loading that the line will ever apply to the insulators, including weight of conductor and hardware, ice and wind loading and any other load factors. Suspension insulators are rated in terms of their Specified Mechanical Load (SML). Manufacturers usually recommend that the insulator never be loaded to more than 50% of its SML, which is a guaranteed minimum ultimate strength rating. Each batch of insulators produced is sampled for mechanical strength and all samples must meet or exceed stated SML value based on statistical criteria. The routine test load is the proof load applied to each unit and also the maximum load that the insulator should ever experience in service. IEC -1 and IEC respectively establish mechanical test methods and acceptance criteria for porcelain insulators, glass insulators and composite insulators.
The electrical characteristics of an insulator are imparted to it by the surrounding air. This is defined principally by its arcing distance, namely “the shortest distance in the air external to the insulator between the metallic parts which normally have the operating voltage between them”. Impulse withstand/flashover and dry power frequency characteristics are all based on dry arcing distance. Some might argue that the wet power frequency withstand/flashover characteristics are determined by leakage distance but that argument only holds within a narrow band. Leakage distance plays a role but as a contributing factor. IEC -1 recommends the withstand voltage associated with the highest equipment voltage.
The past edition of the IEC TS series developed new techniques for selecting and dimensioning high voltage insulators and established a process to determine the most efficient insulation. This technical specification recommends three approaches to select suitable insulators based on system requirements and environmental conditions:
• Approach 1: Use past experience
• Approach 2: Measure and test
• Approach 3: Measure and design
The applicability of each approach depends on available data, time and the economics of a project. Some of the parameters required for these approaches include:
1. Determining Reference Unified Specific Creepage Distance (RUSCD)
Fig. 1 shows the relationship between site pollution severity (SPS) class and reference unified specific creepage distance (RUSCD) for insulators. The bars are preferred values representative of a minimum requirement for each class and are given for use with Approach 3 (i.e. measure and design) of IEC/TS -1. If site pollution severities are available, an RUSCD is recommended that corresponds to the position of the SPS measurements within the class, following the curve.
For Type A pollution (i.e. inland, desert or industrial areas), SPS is calculated from ESDD and NSDD values. For Type B pollution (i.e. coastal areas where salt water or conductive fog is deposited onto insulator surfaces), SPS is calculated from SES (site equivalent salinity).
2. Choice of Profile: Glass & Porcelain Insulators
Different types of insulators and even different positions on the same insulator type accumulate pollution at different rates in the same environment. In addition, variations in the nature of pollutants may make some shapes of insulator more effective than others. Table 1 from IEC TS -2 briefly summarizes the principal advantages and disadvantages of the main profiles with respect to pollution performance.
3. Profile Suitability: Glass & Porcelain Insulators
Tables 2 & 3 in IEC TS -2 give simple merit values for porcelain and glass insulator profiles. Table 2 gives profile suitability, relative to standard profile assuming the same creepage distance per unit or string. Table 3 assumes the same insulation length. Both review the principal advantages and disadvantages of the main profile types with respect to pollution performance.
Moreover, IEC TS -2 also gives profile parameters to take into account, e.g.:
• Alternating sheds and shed overhang;
• Spacing versus shed overhang;
• Minimum distance between sheds;
• Creepage distance versus clearances;
• Shed angle;
• Creepage factor.
4. Polymeric Insulator Profiles & Parameters
Chapters 8 & 9 of IEC TS -3 give recommendations for polymeric/composite insulators profiles and parameters to take into account, including:
• Alternating sheds and shed overhang;
• Spacing versus shed overhang;
• Minimum distance between sheds;
• Creepage distance versus clearances;
• Shed angle;
• Creepage factor.
5. Pollution Test Standards
Pollution tests on glass and porcelain insulators in a laboratory can be carried out with two main objectives:
• To obtain information about the pollution performance of insulators (i.e. comparing different insulator types/profiles);
• To verify performance in a configuration as close as possible to that in-service.
IEC prescribes the procedures for artificial pollution tests applicable to porcelain and glass insulators for overhead lines. Two categories of pollution test methods are recommended for these standard tests:
• Salt fog method in which the insulators are subjected to a defined ambient pollution;
• Solid layer method in which a fairly uniform layer of a defined solid pollution is deposited onto the insulator surface.
These standardized laboratory pollution test methods are not applicable for composite (polymeric) or RTV coated insulators, although a proposed test method for artificially polluted composite insulators is covered in CIGRE TB 555: “Artificial Pollution Test for Polymer Insulators”. In the case of naturally polluted insulators removed from service, a recent CIGRE TB 691 (WG D1.44), “Pollution Test of Naturally and Artificially Contaminated Insulators” summarized recent experience with the so-called rapid flashover test methods:
• Rapid flashover Test (RFO, based on IEC solid layer test);
• Quick flashover (QF, based on IEC salt fog test).
Both tests can be applied for glass and porcelain as well as for composite insulators for AC and DC applications. The objective of these tests is based on the need for a reliable diagnostic of naturally polluted insulators so as to evaluate residual dielectric strength. Also considered is the trend to make testing more cost-effective and time-efficient, even for artificially polluted insulators.
Any reduction in performance can be due to pollution in the case of ceramic insulators or due to a combination of pollution and ageing in the case of polymeric insulators. In both cases, however, residual pollution strength should be quantified in terms of flashover voltage and not withstand voltage. This is because withstand voltage does not provide the user with information about the probability of flashover or the standard deviation in flashover voltage.
6. Insulator Test Stations
Sometimes, the combination of all the varying environmental parameters that influence insulator behaviour over its lifetime are difficult to simulate and accelerate. The validity of laboratory testing is thus often questioned since the procedures adopted for these tests may not take into account significant factors that would be encountered in service; or they may overemphasize others.
Given this, evaluation of insulator performance at naturally polluted outdoor test stations is becoming more important. Although involving longer test durations and still requiring care in correct interpretation of test data, results tend to be accepted with more confidence. An outdoor test station is also a valuable tool for new insulation technologies for which there is still no technical or normative specification for testing or characterization.
CIGRE Technical Brochure No. 333, “Guide for the establishment of naturally polluted insulator testing stations” serves as a general guide for establishing natural test stations that will facilitate comparison of various insulator designs, exploration of particular aspects of insulator performance and/or selection of the most appropriate insulation for a particular application. While such testing relates specifically to insulators intended for use under AC conditions, certain aspects are applicable to DC as well. Typical goals for such testing could be one or more of the following:
• To compare performance of insulators of different design;
• To compare performance of insulators from different manufacturers;
• To dimension insulators for a particular environment or application;
• To examine behaviour of insulators of different dielectric materials;
• To compare performance of insulators in different orientations;
• To explore effects of specific parameters such as profile geometries or insulators diameters;
• To identify possible weaknesses or failure mechanisms of an insulator design;
• To estimate life expectancy of various insulators;
• To serve as a qualification test for potential suppliers;
• To establish effectiveness and service life of special insulator treatments such as washing, greasing, silicone rubber coating, shed extenders, etc.;
• To assess performance of other outdoor equipment insulation such as transformer bushings, surge arresters, cable terminations, etc.
The severity of pollution and prevailing climate of an outdoor test station should ideally be representative of conditions found on the system. As is the case for laboratory tests, over-acceleration of ambient stresses can yield misleading results. Contamination severity assessment by means of ESDD and NSDD measurements and/or directional dust deposit gauges should be undertaken to ensure that the appropriate site has been selected.
Insulator test stations have a range of sizes and levels of sophistication and can be categorized as:
• Research stations;
• Simplified, on-line stations;
• In service test structures;
• Mobile insulator test stations.
Leakage current activity (including number of flashovers experienced), climatic effects and pollution severity are all usually monitored at these sites. In addition, performance of test samples should be judged based on regular inspection of insulators, including close-up visual examination of surfaces, assessment of the hydrophobicity of the dielectric material and evidence of electrical activity.
Insulator Fitting Corrosion Mechanism
Insulator corrosion generally occurs whenever an insulator is polluted and there is presence of humidity. Leakage currents start when the surface is covered by a deposit of wet pollution, with amplitude a function of degree of pollution (i.e. 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 pins. Impact of leakage current is most harmful when frequency and duration of wetting periods are high, such as in tropical climates, and also when pollution finds a hygroscopic surface. Hence the special importance of monitoring for inert contaminants that absorb or retain humidity.
Such corrosion is more important for DC than for AC voltages given the same site due to unidirectional current and electrostatic phenomena that contribute to pollution deposition. For insulators, dominant electrolytic effects only add to atmospheric initiated corrosion, particularly those due to formation of oxidizing agents caused by presence of arcs near fittings. These can be initiated and maintained during periods of humidification and drying that precede and follow critical conditions or whenever the insulator is more humid. Protective field dispatch accessories can be beneficial to limit such humidification and drying periods, which accelerate insulator fitting corrosion in those units that are most electrically stressed. Corrosion can result in:
1. attack on galvanization;
2. attack on internal steel structure with formation of a conductive rust deposit that can flow onto the dielectric
The most severe cases of corrosion can be found in tropical areas with heavy marine pollution and in areas where pollution by dust accumulation occurs over long periods without rain in combination with high environmental humidity.
Corrosion of insulator fittings can have the following effects:
1. Impact on mechanical resistance
This applies particularly to the pin of the insulator when the section of the corroded part becomes reduced, such as reduction in pin diameter;
2. Impact on electrical resistance due to formation of rust deposit on surface
This deposit can also cause damage to the insulation due to concentrated electric field around this new electrode.
3. Breakage of dielectric due to expansion of corroded pin
Remedies to improve resistance to corrosion on insulators typically involve special metal protection developed to avoid or delay this phenomenon.
These remedies consist of reinforced galvanized fittings and use of sacrificial zinc sleeve protection.
• Reinforced galvanized fittings
Ch. 26 of IEC--1 standardizes minimum average coating mass for the metal fitting of insulators: 600 g/m2 (85 µm) but this value can increase to 140 µm for insulators installed in high corrosion areas in order to prolong service life.
• Zinc sleeve is galvanically positive and has a large potential difference from iron
This works as a sacrificial electrode at the cement boundary where current flows. The zinc sleeve is free from accumulation of corrosive products.
IEC- specifies minimum requirements for a zinc sleeve but this can also be improved to increase corrosion performance.
Also, IEC- specifies a test method for control of the zinc sleeve. Future work in standards and norms would have to include zinc sleeve requirements and tests methods within IEC--1 (for AC lines).
Among the principal objectives of any overhead line maintenance policy is to maintain the number of fault outages at acceptable levels. In this regard, a database containing key information on line insulation is an efficient tool to track and evaluate performance. The information this database should contain includes:
• Type/sub-type of strings;
• Type of insulation: glass, ceramic, composite, coated glass, etc.
• Sub-type of insulator: standard profile, pollution profile, etc.
• Number of insulators per string;
• Manufacturer of the insulation;
• Insulator traceability data (production order, date, etc.)
• Standards;
• Year of installation;
• Manufacturer/applicator of silicone material;
• Estimated end of life;
• Degradation environment: Normal, hard or very hard.
Several maintenance indicators are normally used by utilities:
• Number of faults;
• Insulator breakage rate;
• Washing frequency.
Also a range of maintenance methods and procedures are known:
• Aerial inspection;
• Ground patrol inspection;
• HD recording;
• Infrared inspection.
Trends in maintenance indicators together with findings from inspections can then link with the database to help decision-making with respect to maintenance or replacement of insulation. There is also the opportunity for evaluation and comparison of different types of materials, insulator profiles and manufacturer qualities.
Insulators are expected to perform with high reliability over long periods of time. A large number of design parameters (discussed above), choice of material as well as mastering manufacturing processes are all required in order to maintain such reliability over the long-term.
An insulator comes to the end of its working life when it fails mechanically, flashes over with unacceptably high frequency or gives evidence of deterioration to a condition likely to lower its safety factor in service. All insulators are affected to some extent by impact, cycling (both thermal and mechanical), weathering, conductor motion, corrosion and cement growth. Determining when is the right time to replace insulators is key to optimizing maintenance costs and there are a large number of possible degradation modes. Some are easily detectable by visual inspection while others, such as porcelain and composite insulators, may require more sophisticated methods. Degradation modes can also be due to easily detectable mechanisms such as slip of metal fittings, pin corrosion or surface erosion – all considered to be valid reasons for insulator replacement.
CIGRE has established a test procedure to determine the state of cap & pin as well as long-rod insulators and to decide on time for 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, established a testing sequence with a number of non-destructive tests including visual tests (e.g. degree of corrosion) as well as dimensional, thermal and combined thermo-mechanical tests. This first series of tests is followed by destructive mechanical testing. A probability diagram based on a normal distribution is used to analyze failing load test results. With probability (risk) of failure on the ordinate and failing load on the abscissa, failing load characteristics are represented as straight lines. That way, changes in strength are easily seen. To help users, the document includes a number of typical cases of analysis of test result called “Reference Scenarios” that are useful to assess the condition of the insulator.
Failing load characteristics are represented by:
• dashed line for an insulator sample tested when new;
• solid line for insulators as received from a line;
• dashed/dotted line for insulators that have been submitted to thermo-mechanical testing (TMP test).
The SFL (specified failing load) is marked with a solid vertical line. For the example of ”Reference scenario F1”, the reductions in strength in this diagram are not representative of high quality products. Ageing and TMP tests should have only negligible impact on products of high quality.
Parallel to this document, another Technical Brochure published by CIGRE assists evaluation of the technical condition 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 described which enable some conclusion regarding the residual lifetime of composite insulators of the same age and design family. The document also gives indications for research and evaluation in the case of investigating a failure or a unit considered at high risk of failing. This is based on a recommended sequence of testing on samples removed from different stress zones on the line.
Selection of insulator type is not a simple task, especially if the insulator will be installed in a highly polluted area. Numerous documents (e.g. IEC standards, CIGRE Technical Brochures, etc.) are available to help select the most appropriate insulator, to monitor its behaviour in service and to determine when it is nearing end-of-life.
Different solutions are available to improve insulator performance in high corrosion areas.
Several factors must be taken into account when it comes to optimizing selection of insulator type:
• More effective designs/materials;
• Maintenance costs: Inspection cost, cleaning cost, replacement cost, etc.;
• Breakage rate in service, to be guaranteed by the supplier;
• Severity of consequences in case of failure (mechanical breakage or electrical failure;
• Expected end-of-life.
References
A 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.
Download Types of Insulator Used in Overhead Lines PDF
By using Linquip RFQ Service, you can expect to receive quotations from various suppliers across multiple industries and regions.
Click Here to Request a Quotation From Suppliers and Service Providers
Read More on Linquip
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
For more Aerodynamic Glass Insulatorinformation, please contact us. We will provide professional answers.
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.
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.
Related Posts:
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
Related Posts:
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.
Related Posts:
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
Related Posts:
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.
Related Posts:
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.
Related Posts:
URL Copied
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.