Vandalismduring the 1970s on railway insulators in the United Kingdom led to researchinto plastic insulators to replace porcelain. Pultruded epoxy glass fibreglassrods had already been developed for the high voltage switchgear industry andthis same rod was used in development of 25 kV catenary insulators. At thetime, these rods were available only up to 25 mm diameter whereas structuralporcelain insulators typically had 70 mm core diameters and post insulators upto 80 mm diameters. It was therefore proposed to make polymeric-housedinsulators using cylindrical porcelain cores of the same diameter and with thesame end fittings as existing porcelain insulators. This would ensure thatreplacements were identical in mechanical performance to existing porcelaininsulators. With this, the hybrid insulator came into existence, i.e. acombination of the mechanical strength of porcelain and the hydrophobicity andimpact resistance of a polymeric housing.

The core ofa hybrid insulator is the internal insulating part and consists of porcelain orglass insulator designed to ensure mechanical characteristics. The housing isthe external insulating part, made of a polymeric material providing thenecessary creepage distance and protecting the core from the environment. Theend-fitting device is the integral component or formed part of an insulatorintended to connect it to a supporting structure, conductor, an item ofequipment or another insulator. Yet in spite of these basic commoncharacteristics, as defined within IEC 62896, the materials and constructiondetails offered by different manufacturers vary considerably. For example, thecore can be of porcelain or glass and there are even products that use acomposite insulator fiberglass core but with a porcelain head – a design whichincorporates the weakness of original composite insulator designs.

Hybridinsulators these days are being used across a range of applications, fromoverhead lines to posts to hollow core equipment insulators. When it comes todesign testing, IEC 62217 is applied for the polymeric housing as well as forthe interfaces between core and housing. For the core itself, IEC 60168, 60383and 62155 test standards are applied for the respective ceramic material. Sometests have been grouped as design tests, i.e. performed only once on insulatorssatisfying the same design conditions. The common clauses defined in IEC 62217are applied for all design tests for hybrid insulators.

Changes toelectrical and mechanical properties of the components (i.e. core material,housing, interfaces, etc.) and of the complete hybrid insulator itself overtime has also been considered in design tests to ensure satisfactory lifetimeunder normal stress conditions. Polymeric housing materials that offer thehydrophobicity transfer mechanism (HTM) are preferred for hybrid insulators,particularly where these are applied as a countermeasure in severely pollutedservice areas. For the time being, while no ageing or pollution tests have yetbeen developed to verify this property, CIGRE Technical Brochure No. 442 isavailable for evaluating retention of hydrophobicity and the HTM of polymerichousing materials.

Advantagesof Hybrid Insulators

The core ismade by a simple extrusion process and this yields a low weight mechanical partwith consistent mechanical properties as in any porcelain with sheds. Thepolymeric housing offers protection to this core as well as greater resistanceto impact damage than the sheds in a conventional porcelain insulator. Thehybrid insulator is typically half to one-quarter the weight of a conventionalporcelain. This reduces transport costs, risk of damage during transport andinstallation and easier installation. Moreover, the polymeric housing normallyallows for thinner sheds and greater creepage distances with resulting improvedlong-term pollution flashover performance. In addition, the hydrophobicity ofthe polymeric housing offers excellent performance and self-cleaning inpolluted environments. Finally, the polymeric housing protects the core fromshort circuit arc damage that can fracture conventional ceramic insulators.

Typically,most dry band arcing and related damage on composite insulators occurs nearestto end fittings. By leaving an exposed band of glazed porcelain at this pointalong a hybrid insulator, partial discharges occur over the glazed porcelainand not on the more vulnerable polymeric housing. Also, the core material isnot susceptible to moisture ingress problems and, if the housing is damaged,the porcelain core remains unaffected by moisture.

Hybridinsulators can easily be made as replacement parts for conventional porcelainposts used at substations, switchgear and overhead lines. Moreover,manufacturing processes are easier to control compared to crimping end fittingsonto epoxy fibreglass core rods, as done for composite insulators. Hybrid postinsulators can even include the same lipped and grooved ends to accept OHLconductors as used on conventional line posts.

ProblemsShared with Polymeric Insulators

Selectingthe appropriate housing material is important to guarantee good fieldperformance in a hybrid insulator. In this regard, long-term stability inresistance to UV weathering, retention of hydrophobicity and resistance totracking and erosion must all be considered. Another key issue is forming anelectrically stable interfacial seal between the housing and the glazed ceramiccore. This is normally done either with a suitable material, e.g. an RTVsilicone material, or moulding the housing in place using appropriate couplingagents, as done with resin glass cores. Manufacturing experience suggests thatthe best option is using a high temperature vulcanizing (HTV) silicone mouldeddirectly onto the porcelain surface under high pressure. The silicone housingis then fully bonded onto the solid porcelain core, with ideal management ofthe critical triple point, where fitting meets silicone housing meets porcelaincore. Due to the high pressure involved in this operation, the silicone housingadheres directly onto the fitting or to the porcelain without need foradditional sealing.

ApplicationExperience in Brazil

Brazil hasa densely populated coastline of approximately 8000 km and this polluted marineenvironment has caused serious insulation problems for many distributionutilities. Hybrid insulators have been applied in these harsh coastal areaswith favourable results that include less need for maintenance and washing,fewer service interruptions and reduced operating costs. Hybrid insulators havebeen successfully tested in other countries as well across a variety ofpolluted conditions. In the desert areas of Chile, for example, thousands ofpin type porcelain insulators with large protected creepage distance werereplaced with hybrid insulators that have performed well.

R&DWith Neoenergia & Lactec

Field testsof hybrid insulators were conducted in Brazil involving Neoenergia Group, a utilitylocated in the Northeast, in co-operation with Lactec, an R&D Institute. Apilot network running 15 m and composed of four spaced poles was set-up in thearea near Pituba Substation, located about 500 m from the seacoast in BahiaState. Cables, insulators and crossarms were installed on these poles and twolines were assembled in parallel, using these poles: a single-phase lineoperating at 34.5 kV and another single-phase line operating at 13.8 kV.Porcelain insulators and hybrid insulators of the same pollution class, LevelIV according to IEC 60815, were installed in this pilot grid for the purpose ofmonitoring leakage current. it shows an overview of the pilot network beforeinstallation of insulators while it shows the insulators installed on leadwires to measure leakage current. The metal pin of each insulator was connectedto the grounding wire and 1kOhm resistors were connected to the groundingconnections. Lactec developed a special instrument to measure leakage currentwith data obtained through an Agilent data acquisition unit (Model 34970A) andstored in a microcomputer. A 3G modem connected to the microcomputer to allowremote access by internet.

Summary

Hybridinsulators are among the most important recent innovations for LV and MV overheadlines and substations. Despite the seeming simplicity of a solution thatcombines a porcelain core with a polymeric housing, successful design in thiscase requires quality materials. The housing should be a long-life hydrophobiccomponent that, according to best practice and field experience, should utilizeHTV silicone designed with specific fillers to protect the silicone fromerosion. These fillers, typically alumina trihydrate (ATH), must beincorporated into the polymer in specific minimum quantities to be effective.The core should be a continuous and single piece porcelain made ofhigh-strength alumina oxide (C130 mass according to IEC 60672). This ensuresavoiding risk of ageing of housing from erosion while also taking fulladvantage of the high mechanical strength, unique stability and long-termperformance of this material. Hybrid insulators combine the known advantages ofa porcelain core with the excellent performance of a silicone housing and offeran ideal solution for distribution systems operating in highly contaminatedconditions.