Investments in transmission and distribution (T&D) networks is increasing across the globe as utilities take steps to expand their grids as well as refurbish ageing power systems to ensure efficient evacuation of renewables. Further, T&D utilities continue to embark on smart grid initiatives to make the grid more resilient and flexible in the wake of the ongoing energy transition. These factors are driving the demand for critical power equipment such as insulators which are used in lines and substations to prevent flow of electrical current to the ground/earth from supporting points or joints. One of the basic functions of the insulator is to prevent a flashover in case of over-voltage. Insulators are widely used in overhead transmission lines to provide mechanical support and electrical protection, as well as in distribution lines and substations.

Depending on voltage considerations, different kinds of insulators are used in power systems. Electrical insulators are generally made of porcelain or ceramic, glass or polymeric composite. Both glass and porcelain are commonly used insulator materials, and there is not much difference in the cost and performance of both. However, composite insulators are increasingly replacing porcelain insulators due to their properties and cost advantages.

Global Transmission Report presents a brief on the types of insulators used in transmission lines, technological evolution, recent trends, global experience and the way forward.

Types of insulators

The most commonly used overhead insulators can be classified into pin-type, suspension, strain and shackle insulators. While pin-type and shackle are used in distribution lines, suspension and strain are used in transmission lines.

A suspension insulator consists of several porcelain discs connected to each other with metal links in the form of a string. The conductor/line is suspended at the bottom end of the suspension string, which is secured to the cross-arm of the tower. Strain insulators are used at dead ends or sharp corners to avoid tension in transmission lines.

Pin-type insulators are secured with steel or lead bolts onto transmission poles and generally used for straight-running transmission lines. However, beyond voltages of 33 kV, these types of insulators become bulky and uneconomical. Shackle or spool insulators are generally deployed in low voltage distribution networks, and can be used in horizontal or vertical positions. Like strain-type insulators, they are also deployed on sharp curves, end poles and section poles.

Some other insulator types include post insulators, cap and pin insulators, stay insulators and hollow insulators. Post insulators are similar to pin-type insulators but they are more suitable for higher voltage applications. These insulators can be mounted on supporting structures, both horizontally and vertically. Cap and pin insulators are generally deployed on overhead T&D lines to evacuate bulk power over long distances. They are also used for substation busbar high-level strained connections.

Stay insulators, also called egg insulators, are primarily used to prevent stay wires from getting energised from accidentally broken live wires. Hence, they function to provide insulation between stay clamps and transmission poles. Hollow insulators are employed by substation equipment manufacturers to house post-type current transformers, voltage transformers, cable bushings, circuit breaker supports with central operating rods and interrupting chamber assemblies, isolator supports, etc. However, such insulators need to be of high mechanical strength as they are used in applications that are subject to sudden pressures, like circuit breakers or surge arresters.

Technology evolution and design requirements

Initially, insulators were made primarily of glass; however, these failed to withstand high temperatures. With the introduction of higher voltages in the 1890s, power utilities started installing porcelain insulators with special coatings and designs. By the 1950s, conventional porcelain and glass insulators were considered as mature and the first International Electrotechnical Commission (IEC) standards were issued for specifying them. Presently, both insulator types are widely in use, with each having their associated benefits and limitations. Porcelain is the most commonly used material for manufacturing insulators for overhead transmission lines.

First developed in the 1970s, composite insulators began to be considered for specific applications including in areas with extreme pollution, vandalism or high seismic risk. These insulators started making significant inroads in the US in the 1980s and soon afterwards in Germany and France. As with any new product, initial acceptance was slow due to various teething issues as well as prohibitive costs. However, the technology is now considered state-of-the-art for overhead transmission line applications. In fact, composite insulators have captured a significant market share due to their low cost (only about 25 per cent the conventional glass/porcelain insulators) and other properties. Besides the early application of composite insulators in polluted service environments, other areas of application include cross-arms, arrays and even entire structures. Composite insulators are particularly suited for compact line construction, which have minimal right-of-way requirements.

While choosing the type of insulator to be deployed, utilities typically consider the line design, service environment, acquisition costs as well as total life cycle cost (which includes acquisition cost, replacement cost and accumulated maintenance cost over its expected service life). Experts favouring composite insulators claim that even in the case of heavy pollution requiring over six washings per year and a pessimistic scenario wherein the insulators would have to be replaced after 15 years, choosing composite insulators could still be cost effective.

Some of the key design requirements of insulators are that they should be able to withstand the over-voltages due to lightning, switching or other causes under severe conditions in addition to the normal working voltages; posses high mechanical strength to bear the conductor load under worst leading conditions; have high resistance to temperature changes to reduce damage from power flashover; and minimise the leakage of current to earth to keep the corona loss and ratio of interference within a reasonable limit.

The material of the insulator should have properties such as high electric resistance; high relative permittivity in order to augment the dielectric strength; non-porous nature; high mechanical strength; and be free of impurities and cracks, which lowers the permittivity of the insulator material.

In general, one of the major challenges related to the use of insulators is pollution. The surface of insulators in service for several years becomes polluted to some degree. Pollution accumulation and dampening (due to fog/dew) could result in pollution flashover accidents. To counter this issue, some preventive measures can be taken. These include adjusting creepage distance to reinforce insulation by increasing the number of discs in a string; increasing frequency of washing; coating surfaces of porcelain and glass with a thin layer of hydrophobic material; or using silicon rubber composite insulators. Further, in severely polluted zones, a firm maintenance plan should be in place. The insulators should be washed and cleaned regularly, particularly in areas with extreme pollution levels or less rain. In general, the most frequently employed methods are washing with water at a high, average or low pressure; washing with compressed dry air; or washing with spurts of abrasive materials.

The design consideration for insulators have also evolved over the years. Recent advances in the design of circuit breakers and other protective devices have reduced the effects of switching transitions and power frequency in conditions of over-voltage. This has reduced the level of insulation required for extra high voltage (EHV) networks.

The key manufacturers of insulators include GE, Siemens, Toshiba Corporation, Hubbel Incorporated, Bharat Heavy Electricals Limited, Aditya Birla Nuvo Limited, Seves Group, Dalian Yilian Technology Company Limited and Elantas GmbH.

Focus on composite insulators

Composite insulators have gained popularity in the T&D space in certain applications. These insulators comprise a fibre glass reinforced epoxy resin rod-shaped core, which is covered with polymer weather sheds. The rod-shaped core is fixed with hop dip galvanised cast steel end fittings on both sides.

The weather sheds protect the insulator core from the outside environment. The most commonly used polymer shed materials for manufacturing composite insulators include silicone rubber, ethylene propylene rubber, ethylene propylene diene monomer (EPDM) rubber, cycloaliphatic epoxy (CE) resins and polytetrafluoroethylene (Teflon). These basic materials are combined with various fillers like aluminium trihydrate to obtain the desired electrical and mechanical properties. The most popular material used in composite insulators is silicone rubber, due to its long-term dynamic hydrophobicity—which significantly reduces the need for regular washing in polluted areas—as well as its availability at a relatively cheaper cost.

The key benefits of using composite insulators are that they are light in weight (one fifth of the weight of the conventional insulators) and small in size; are flexible, and therefore have less chances of breakage; impose less load on the support structure, leading to an overall low cost of installation; have high creepage offset and resistance to vandalism; and have a higher tensile strength compared to porcelain insulators. Most importantly, these are better suited for polluted areas and require less cleaning due to the hydrophobic nature (water repellency) of the insulator.

Some of the shortcomings of composite insulators include high leakage current and short lifespan. Moisture can enter the core of the insulator if there is any unwanted gap between the core and the weather sheds. Over-crimping in the end fittings may lead to cracks in the core, which would, in turn, lead to mechanical failure of the polymer insulator. These insulators fail to perform in areas close to the coastline where there are high levels of salt in the air and are more prone to bird attacks. Another issue is that polymeric insulators melt and bend in case of a fire, unlike glass or porcelain insulators.

Experience so far

The use of new polymeric materials is becoming increasingly popular in the insulator industry. Previously, glass and porcelain were the main materials used for insulators. New factors like the need to integrate renewables, using existing corridors to the maximum extent possible and deploying compact structures is driving the demand for composite insulators, besides the advantage of low acquisition costs. Over the years, the production of glass insulators has gradually declined and new polymer insulators are finding higher acceptance given their advantages. Globally, various power utilities have been utilising polymer composite-based insulators for certain applications, and the results have been fairly satisfactory under various climatic conditions over the past 20 years.

In developed markets like the US, the use of composite insulators became common in overhead transmission lines in the 1990s and early 2000s. Unlike in China where the major factor driving the switch was the pollution performance, in the US, the key drivers were ease of handling due to the lighter weight, better resistance to vandalism and cheaper cost. However, due to reluctance to perform live line work (due to added precautions that must be taken with composite insulators) and the risk of erosion damage associated with composite insulators, demand for toughened glass is picking up again in the US, particularly in HVDC and UHV transmission projects.

New insulator designs and structures are being experimented with in the US. For instance, double-V composite insulated cross arm (CICA) has been developed which can be used as both an insulator and a structure. It aims to address the financial and technical challenges associated with upgrading the existing grid. The double-V assembly replaces the previous horizontal two-dimensional single-V technology thereby helping to optimise the geometry of transmission towers and making the overall structure more compact. It also provides greater resilience against flashovers caused due to pollution, elastic resistance against shock loads, as well as has a high strength-to-weight ratio.

Meanwhile, in China, the use of silicone rubber composite insulators has dominated the power industry for over three decades.  These insulators were initially put into operation in the Shitai Line and Baocheng Line electrified railway tunnels in 1983 and have today become the mainstay of ultra-high voltage (UHV) AC and DC systems of the rapidly expanding Chinese power grid. These composite insulators considerably helped to overcome the trippings caused by large-scale pollution flashovers of porcelain insulators in lates 1980s-1990s in China. In the later decades, as China developed an extensive network of DC and UHV AC transmission lines, composite insulators played a key role owing to their superior mechanical properties and electrical field besides pollution resistance.

In some markets, composite insulators have not yet made inroads. For instance, in Canada, the use of glass and porcelain insulators, rather than composite insulators, has dominated high voltage lines.

While transmission system operators (TSOs) in Europe have been using composite insulators in overhead line applications for quite some time, glass insulators, which have long been popular in the region, account for a significant share. However, the increase in demand for uninterrupted transmission without faults has resulted in highly stressed insulators. This has imposed a greater need for uniform high quality of glass insulators being installed on their networks since the failure of any disc in a string can have an impact on its performance.

European TSOs are also replacing old insulators with new composite ones as part of grid modernisation initiatives. A case in point is Romania which is undertaking the replacement of insulators in high voltage overhead lines with silicone composite insulators for distribution grid modernisation and smartification under the Carpathian Modernisation of Energy Network (CARMEN) Project. The project is being promoted by Romanian distribution system operator (DSO), DELGAZ GRID, with support of the Romanian TSO, Transelectrica and the Hungarian TSO, MAVIR. The CARMEN project received the final approval for the inclusion on the fifth list of projects of common interest (PCI) of the European Union (EU).

Fingrid, Finland’s TSO, has deployed composite insulators at substations and on transmission lines for over two decades years without any serious concerns. However, in recent years biological growths have begun to be observed on some of these insulators. These growths have been cleaned and an R&D project has been initiated to establish periodic cleaning guidelines for overcoming this issue to prevent from flashover risks and study its impact on ageing T&D infrastructure.

Similarly, Belgium’s TSO Elia started using composite insulators on its overhead networks in early 2010 and in 2017it built 21 km of new compact structures that employ pivoting and non-pivoting assemblies of composite insulators as part of the EUR240 million Stevin project. The project involved the construction of 47 km of 380 kV double-circuit line between the Steven substation in Zeebrugge and the Horta substation near Zomergem (including 37 km of overhead lines and 10 km of underground cables). Elia has deployed a different insulator design for each of the two circuits. One uses high temperature vulcanised (HTV) or solid silicone rubber with silicone extruded FRP (fibre reinforced polymer) rods of 130 mm diameter while the other uses liquid silicone rubber (LSR) with rods of 110 mm diameter. In this project, the dimensions of insulated cross-arms were specifically designed so as not to experience corona. In fact, Elia now uses composite insulators for all new lines at 150 kV and above and on refurbishment projects, mainly due to its low acquisition costs.

Way forward

The use of superior materials, which are super hydrophobic and environmentally friendly (due to nanotechnology), could result in a new generation of composite insulators designed for specific climatic/pollution conditions. While silicon rubber is a preferred material in most composite insulators, further research is required in silicone formulations to rectify aspects such as surface silicification, deep surface cracking and mould growth. The availability of advanced software could help in customising insulator designs depending on the mechanical and electrical needs. Improved prediction methods along with offline and online monitoring tools will allow system operators to ascertain the overall condition of the insulation in their systems. This will help reduce maintenance costs. Expanding existing test criteria to ensure quality of insulators is an area that needs to be looked into by IEC, CIGRE and other organisations responsible for formulating test standards. Better insulator diagnostics will permit more live line work without shutting down of lines.

While transmission operators globally are deploying all three types of insulators available in the market, the share of composite insulators has grown significantly in recent years. Given the emerging scenario where renewable energy sources are expected to play a greater role in the global energy mix, power systems must become robust and smarter to be able to handle such large amounts of variable power. This is driving the demand for new solutions, which necessitate continued research on various insulator materials, test procedures, diagnostics and designs.