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Source of Information on International Developments in Transmission & Distribution

Issue 68 • Quarter Two - 2005 • Volume 13 - Number 2

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Swedish Bushings Plant Sees Growth in RIP Designs

Johansson. Continued efforts to be placed in process optimization.

Jonsson. RIP manufacturing technology more demanding than for OIP.

Winding machine for OIP bushing condensers up to 550 kV.

Computer-controlled machine winds condensers for large RIP bushings.

The nearly 100 year history ofbushings production at ABB andespecially at its flagship Ludvikaplant has provided management with an interesting perspective on how the changing requirementsof electrical networks has pushedthe advancement of this technology.Bushings Vice President RutgerJohansson and Senior SpecialistLars Jonsson look back at some of the major steps in the evolution of the bushing since the earliestdays of this plant’s operations.

The very first ABB bushing designswere based on an insulation system involving Bakelite material.This type of insulation system, while effective at lower transmissionvoltages, apparently could notaccommodate the move towardhigher voltages without anunacceptable increase in the level of internal partial dischargeactivity.

Therefore, as the Swedish powernetwork developed and moved

to 350 kV during the 1940s, the old Bakelite technology hadto be abandoned and replacedwith OIP dielectric systems,which at the time were regardedas quite novel. Says Jonsson,“with the OIP design, we foundthat we could now handle eventhe highest voltage class. For example, by the early 1960s,ABB was able to supply the first 750 kV bushings for thenew EHV system of HydroQuébec in Canada.”

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Like the insulator, the bushing isanother of those critical componentswhich comprise all electrical networksand whose failure can have veryserious economic consequences.

Although many in the electric utilityindustry still regard it as nothing more than a hollow piece of porcelainwith a conductor running through it,notwithstanding its simple appearance,the task performed by a bushing isactually quite extraordinary.

This is because, for apparatusoperating at many thousands or even hundreds of thousands of volts,it is almost a Herculean task toadequately isolate the current carrierfrom the equipment’s metallic housing.HV bushings accomplish this task byrelying on sophisticated insulationsystems able for example to keep a 550 kV conductor operating safely only some 20 cm from thegrounded flange.

The basic design of most condenserbushings has not changed in decadesand this is perhaps a tribute to howeffective the current technologyactually is. This design involvesspecial paper wrapped around theconductor as well as metallic foilsplaced at strategic locations withinthis wrap. The precise location and the size of these floating metallicelectrodes is very important since their capacitive effect controls theelectric field along the unit’s lengthand radius.

To achieve its required high dielectricstrength, no air or gas bubbles can be trapped between the successivelayers of paper. Therefore, the cylinder of wound paper and foil is thoroughly impregnated with eitheroil or a special resin material. Hence the two major technologies

for HV condenser bushings: OIP (oil-impregnated paper and RIP (resin-impregnated paper).

In this article, INMR ContributingEditor Dr. Claude de Tourreil tours the Swedish plant of ABB - by far

the world’s largest supplier ofbushings. Although the Ludvika facility still devotes most of itsproduction capacity to the OIP style,developments over the past decade are apparently now fueling salesgrowth of RIP designs.

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68 B U S H I N G S

Swedish Bushings Plant Sees Growth in RIP Designs

145 kV RIP bushings. Ph

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Although the majority of the bushings made in Ludvika

are still based on OIP technology, the number of RIP units

produced annually is apparently increasing steadily.

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145 kV OIP bushings being impregnated with oil. Station for vacuum processing and oil impregnation of OIP bushings.

RIP condenser fitted with mounting flange.

According to Jonsson, thetechnology to manufacture RIPbushings is more demanding thanthat for the OIP type. However, this he notes is offset by the factthat RIP technology offers several

distinct benefits such as lowerweight, with such designs being up to 50 per cent less heavy than equivalent OIP designs. This makes such bushings easier to transport and store.

“Given the advantages, the difference between the cost

of a porcelain insulator and that of a composite insulator

with silicone sheds should not really be the decisive factor

in the selection of one bushing type over the other.”

Resin impregnation vessels forRIP bushings up to 550 kV.

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new dry RIP technologyhad grown substantially,triggered by increasingrequirements foravailability and eventssuch as major firesoccurring at HVDCconverter stations. RIP bushings, beingmade with a basically dry insulation systemand with silicone rubber insulators, havetherefore become the

preferred choicefor these types of applications.

Although themajority of thebushings made in Ludvika are still based on OIP technology, the number of RIP unitsproduced annuallyis apparentlyincreasingsteadily. This, says Jonsson,is due to some significantadvantages RIP technology

Hedlund. Performance increasingly more decisive a factor in purchasing than cost.

After the merger between Asea andBrown Boveri in the mid 1980s, the Ludvika plant and Swiss-basedMicafil joined forces in continueddevelopment of dry bushingtechnology. Ludvika also became a major producer of the RIP bushing designs which had firstbeen developed and which today are still mainly manufactured at the Micafil facility.

By the 1990s, notes Jonsson, the demand for the comparatively

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While the technology to manufacture RIP bushings

is more demanding than that for the OIP type,

this is offset by the fact that RIP technology offers

several important advantages to users.

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has over the more established OIP design - especially from the point of view of the end user.

Winding machine for RIP bushing condenser up to 170 kV.

RIP bushings prepared for vacuumprocessing and injection molding.

Machine winds condensers up to 12 m in lengthfor use in UHV OIP bushings.

Petrici. Recent large investments made in Ludvika plant.

The wrapped condenser paper is then dried to ensure it iscompletely free of moisture beforebeing impregnated with oil or resin. Impregnation, notes Petrici, is quite a delicate operation and this is much more the case for resin than for oil.

For higher voltage RIP bushings up to a maximum of 550 kV, resinimpregnation is performed in vesselsof varying size. It is critical that noair bubbles remain in the bulk of the insulation after curing becausethese could become the site ofinternal partial discharges whichwould eventually lead to productionrejects.

After impregnation, the mountingflange and the insulator housing arefitted onto the resin-impregnatedcondenser. In the case of condensersystems for RIP bushings up to 170 kV, the external siliconehousing is generally molded directly on to the entire unit.

For OIP bushings, which ABB offersup to 800 kV as part of its standardproduct range, impregnation with oilis carried out in a special computer-controlled installation after thecondenser has been placed insidethe insulator and flange assembly.

Each bushing is first put undervacuum during which time a leaktest is performed. Subsequently, oil is introduced into the bushing to completely impregnate theinternal condenser. Together, thesetwo operations take approximately24 hours.

Both RIP and OIP bushings thenpass through a series of routine testsbefore being cleared for shipment.Capacitance and dielectric losses are measured up to the powerfrequency withstand voltage which is maintained for a duration of one minute.

The magnitude of any dielectriclosses is directly related to themoisture content of the oil-impregnated paper dielectric system(see Figure 1). Detection of partialdischarges is carried out during thisentire test. Some of this testing

requires fairly sophisticated set-upsbecause part or all of the bushinghas to be submerged in oil andbecause there are very tightacceptance criteria on partialdischarges.

Preparation for shipment is then the last step of the manufacturingprocess. Shipping bushings in thelower voltage range does not usuallyrequire any special precautions in the way of packaging. However, large bushings, especially of the OIP type, require sturdy crateswhich can sometimes measure more than 10 meters in length.

Petrici and Jonsson indicate that, in order to remain a market leader,ABB has invested much in theLudvika production facility in orderto bring it to a level which theyregard as state-of-the-art. At thesame time, they have tried tooptimize all processes for both OIPand RIP designs. Indeed, Johanssonsees that future efforts will likelycontinue to be placed on processoptimization rather than ondeveloping radically new designs.

Another area which Jonsson says hasreceived much attention in recentyears is technical support. With theoptimization (read downsizing) ofengineering staff at many electricutilities, which represent the endmarket for HV bushings, he statesthat technical support offered bymanufacturers has become moreimportant than ever before.

Johansson indicates that ABB ownsbushing manufacturing units locatedall around the world, includingRussia, Brazil, South Africa, Indiaand China. The main productioncenters, however are in Sweden,Switzerland and the United Statesand this is where engineeringspecialists are employed to furtherdevelop and improve the technology.

As for the impact of growingcompetition, especially coming fromlow cost suppliers based in Asia,Johansson feels that this does notrepresent a serious threat. The laborcost component in the manufactureof HV bushings, he claims, is justnot a large part of the total cost ofthe product and what really counts,he says, are modern facilities andwell-trained personnel. ⌧

RIP bushings prepared for shipment.

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With the downsizing of engineering staff at electric utilities,

technical support offered by bushing manufacturers

has become more important than ever before.

In addition, RIP bushings have low dielectric losses and, like OIPdesigns, are partial discharge free.Moreover, they can be installed at any angle and can be energizedimmediately after installation.Finally, having no oil, they requireless maintenance.

Apart from the increase in marketinterest in RIP bushing technologydue to these advantages, there has been another design elementchange which is now increasinglyimpacting the bushings industry.This involves the substitution oftraditional porcelain housings withhollow core composite insulators.

Bushings, especially of the RIP type,when equipped with compositeinsulators having silicone sheds are increasingly regarded as anexcellent choice when safety andpollution performance are keydecision criteria.

For example, in case of explosion or vandalism, there is no risk of

fire and the silicone shedsensure superior performanceunder polluted serviceconditions. Yet anotherconsideration promotingincreased market interest,say Jonsson and Johansson,is the superior performanceof these bushings underseismic conditions.

“Given these manyadvantages,” remarks ProjectManager, Roger Hedlund,“the difference between thecost of a porcelain insulator and that of a compositeinsulator with silicone sheds should not really be the decisive factor in the selection of one typeover the other.”

At the Ludvika bushingsplant, both OIP and RIP bushingsare manufactured in the same large hall but on separate production lines.

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According to Production Manager,Fredrik Petrici, the characteristics of all incoming materials are routinelyverified to ensure that they meet very tight specifications. This isapparently especially important since the insulation systems beingproduced are subjected to very highelectrical stresses yet have to deliverproblem-free service for severaldecades. Cleanliness is also stressed,so much so that certain vitalproduction processes are performedonly in enclosed areas.

Specialized computer-controlledwinding machines of different sizeswrap the grade of paper, which varies by specific type of condenser,as well as the metallic foils onto acentral tube or solid mandrel rod. A comparatively small unit is used for RIP bushings up to 170 kV, whilea very large winding machine canproduce 12 meter long condensersfor OIP bushings having this length.

OIP oil-to-air transformer bushings near test bay. Two 420 units standing vertically in front of four 550 kV units.

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Figure 1: Dissipation Factor VersusTemperature & Moisture Content

Is your electrical network reliable?

Read more about bushings underwww.abb.com/electricalcomponents

©2005 ABB

Dry/RIP Bushings from ABBElectrical networks are aging and nearing their capacity while environmentaldisasters are becoming more frequent and severe. Therefore, one must rely ona supplier with the experience and knowledge to deliver innovative solutionsmeeting today’s demanding requirements while lowering life cycle cost.ABB Bushings have been designed and built considering this. We have over100 years of experience delivering bushings for transformer, switchgear andother electrical apparatus application. Our dry resin impregnated paper (RIP)bushings provide reliable oil-free service for the most demanding applications.

When combined with a silicone rubber insulator instead of porcelain, one has a self-cleaning, non-brittle,low mass and high mechanical strength design providing increased network reliability during severeweather conditions, contamination and earthquakes.

Dry or oil-filled, ABB has the right bushing solution for your application. This includes custom solutions forreplacement of ABB or other brand bushings at existing installations.