Answers for energy.

Retrofitting for medium-voltage switchgearReducing maintenance costs

Reprint from ew, Volume 108 (2009), Issue 22 – 23

Author: Dr.-Ing. Dieter Sämann, Siemens AG, Energy Sector, Erlangen

ew SUBJECT AREASwitchgear

In a medium-voltage system, elec-tric power is generally distributedin the voltage range between 6 kV

and 30 kV. Medium-voltage switch-ing stations house the equip-mentrequired for both switching andprotection, on which safe and reli-able power distribution heavily de-pends. Formerly, air-insulatedswitchgear was built with circuit-breakers on trucks or withdrawableparts in order to facilitate inspec-tion and maintenance work outsidethe switch bay by pulling out thebreaker. Today, gas-insulatedswitchgear is used as well as air-in-sulated switchgear. It is less depen-dent on the environment and,thanks to maintenance-free circuit-breakers, hardly requires any main-tenance. Modern circuit-breakersbased on the vacuum-switchingprinciple can also be integratedwith existing switchgear, resultingin higher availability, reduced oper-ating costs, and a smaller invest-ment.

Switchgear development overthe years

Circuit-breakers are the core ele-ment of medium-voltage switchge-ar. They ensure that electric power ismade available to the consumerand that the short-circuit current isinterrupted quickly and reliably inthe event of a fault. The technology,properties, and behavior of circuit-breakers have evolved continuouslyover the past 100 years, and todayprovide largely maintenance-freeoperation.The following short review will de-scribe the changes that have takenplace in switchgear; see Fig. 1.

Dead-tank oil circuit-breakers didnot yet have an arcing chamber, butinstead generated two partial arcs ina large oil tank. The high arc energygenerated resulted in considerablestressing, and so required consider-able maintenance.

Expansion circuit-breakers usedwater as the arc-extinguishingmedium. This first use of an arcingchamber for interrupting the cur-rent enabled contact wear to be re-duced compared with dead-tank oilcircuit-breakers. These circuit-breakers were manufactured from1930 onward, and there are still afew of them in operation today.

With the development of the min-imum oil circuit-breaker – whichonly needed a small amount of oilcompared with dead-tank oil cir-cuit-breakers and a combined arc-extinguishing medium flow – it waspossible to improve switching char-acteristics and reduce maintenance

Medium-voltage switchgear has a servicelife of 30 to 40 years. As it ages and needsmore frequent servicing, maintenancecosts increase to the point where replace-ment of the entire switchgear becomes un-avoidable. An alternative to exchanging thewhole switchgear is to replace the compo-nents that are subject to wear. This makesit possible to continue using existing intactswitchgear with modern technology. By ex-changing components, both the invest-ment costs and the time spent on planningand approval are lower than with a com-pletely new system. The possibilities for ex-changing components and the advantagesfor the operator are explored in this paper.

Reducing maintenance costs

Retrofitting for medium-voltageswitchgear

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Vacuum Switchgear

No other switching principle can match these outstanding properties: Constant dielectric• Virtually no change in internal pressure even at the end of the service life• No decomposition products • No impact on the environmentConstant contact resistance• No oxidation• Low wearHigh total current• Very low arc power • Special contact material from in-house production • High degree of reliability• Very low number of moving parts • High-quality materials

Dr.-Ing. Dieter Sämann, BusinessDevelopment Management medium-voltage components, Siemens AG, Erlangen

intensity. These breakers are still inoperation in large numbers to thepresent day. Assuming they wereregularly maintained after fiveyears, or after five to ten short-cir-cuit-breaking operations, these de-vices will continue to function welltoday.

The development of the vacuumcircuit-breaker marked a great leapforward in technology. A totally newarc-extinguishing principle meantthat the circuit-breaker could beused universally and hardly re-quired any maintenance. Vacuumcircuit-breakers were launched onthe German market by Siemens in1977 and have now been in servicefor more than 30 years. Over this pe-riod, operators at power supplycompanies and in industry haveseen for themselves how reliableand cost-efficient these modern de-vices are.

There has also been further devel-opment of the vacuum circuit-breaker. Thanks to new productiontechniques, the contact material ofthe vacuum circuit-breakers can bematched more effectively with theirmultiple applications. Detailedknowledge of the switching processin a vacuum acquired over this peri-od and improved production meth-ods have led to a reduction in thesize of the vacuum interrupters. Theresult today is a modern vacuum

circuit-breaker with efficient vacu-um interrupters of such small di-mensions that they fit into any ex-isting medium-voltage switchgear.

Properties of present-day vacu-um switchgear

Vacuum circuit-breakers are nowavailable for the entire medium-voltage range, and comprise the fol-lowing spectrum of electrical data: • Operating voltages from 7.2 kV to 40.5 kV• Operating currents from800 A to 6.300 A• Short-circuit currents from 12.5 kA to 72 kA

Vacuum circuit-breakers fromSiemens are employed for allswitching tasks at medium voltagelevels. A special contact material forthe vacuum interrupters, producedin a special in-house productionprocess, ensures that a very widerange of different currents can beswitched reliably. The low choppingcurrent of 3 to 4 A prevents non-per-missible overvoltages when switch-ing small inductive currents. Therapid dielectric recovery of thebreaker gap guarantees that all ca-pacitive currents occurring in net-works, as well as high short-circuitcurrents up to 72 kA, are interruptedreliably. This means that vacuumcircuit-breakers can be used to re-

place all old circuit-breakers of theconventional type. The new SIONseries is particularly suitable for re-placing old circuit-breakers, since ituses the highly compact vacuum in-terrupters and has minimal dimen-sions due to its special design.

What does retrofitting mean?

At the time when it was installed,switchgear would have conformedto the current state-of-the-art andits design matched the technical

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Fig. 2: SION vacuum circuit-breaker

Fig. 1: Dead-tank oil circuit-breaker, expansion circuit-breaker, and minimum oil circuit-breaker

specifications of the networks.However, over the course of years ordecades, conditions change and to-tally different factors can result inthe need to renew the entireswitchgear or parts of it: • Availability o Transmission performance of thenetwork has increasedo Supply reliability needs to be in-creasedo Individual devices are worn out• Operating costso Costs for maintenance have in-creased o Supply of spare parts becomes in-creasingly difficult • Personnel protection o Retrofitting for protection of per-sonnel• Legal situationo Regulations or standards havechanged The measure that is required de-pends significantly on the reasonfor the modification or renewal, butalso on financial resources. If costsdo not have top priority and thereare no other constraints, the deci-sion will go in favor of installingcompletely new switchgear. How-ever, there can be many reasons infavor of replacing parts of theequipment:• The premises do not allow the en-tire switchgear to be exchanged• The switchgear is in good condi-tion

• The switching device requires in-tensive maintenance• Spare parts can only be procuredat high cost or are no longer avail-able• There are no personnel left whoknow how to maintain the oldequipment

There are also non-technical ar-guments in favor of retrofitting:• Construction measures• Interruption of the power supply• Costs for a provisional solution• Official certification procedures• Investment and budget issues

In most cases of retrofitting, theold circuit-breakers are replacedwith new ones. Depending on theswitchgear type, either the circuit-breaker only is replaced or the com-plete withdrawable part orswitchgear truck. Because of itssmall dimensions, the SION vacu-um circuit-breaker has proved to bean excellent solution for retro-fitting. Siemens offers switchgeartrucks or withdrawable parts for allkinds of different switchgear.

A possible solution is describedhere using the example of Siemens8BD1 switchgear. These switchgearwere equipped with minimum oilcircuit-breakers, which may be over40 years old now. The production oflow-oil-content circuit-breakersand the switchgear were discontin-ued in 1998, and it is becomingmore and more difficult and expen-sive to obtain spare parts for theseproducts today. The switchgear assuch is in good condition, but thecosts for maintenance are no longeracceptable. The electrical specifica-tions of the network have notchanged since the time of initial in-stallation. This is a simple and typi-cal case for replacement of the min-imum oil circuit-breaker with amodern vacuum circuit-breaker.

Before the replacement is carriedout, the system is inspected and thedimensions measured. The condi-tion of the switchgear is assessedand existing documents are re-viewed in order to take into accountmodifications that have been car-ried out over the course of time. Acheck is made to see whether thecircuit diagrams correspond to thecurrent state so that the new solu-tion can replace the old one directly.

The object is to directly replicatethe existing switchgear truck. Thismeans that the new truck will havethe same dimensions, the samecontact arm system, the same inter-

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Fig. 4: SION circuit-breaker as retrofit solution on a switchgear truck

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Switchgear truck

Fig. 3: Switchgear truck for switch-gear 8BD with SION circuit-breaker

locking, the same secondary equip-ment and wiring, and the same low-voltage connectors. The switchgeartruck is completed at the factory sothat replacement is a simple “plugand play” operation at the cus-tomer’s location. Naturally, theswitchgear truck with SION is testedin an 8BD switchgear, the circuit-breaker in accordance with IEC62271-100, and the completeswitchgear truck in the bay accord-ing to the main criteria of IEC62271-200. The circuit-breaker isthus upgraded compared with theold one and complies with the re-quirements of the latest standards.The entire switchgear is upgradedin the same way: even though itcannot be brought completely up tothe current state-of-the-art, it willbe operational as long as some ofthe built-in components still com-ply with old standards.

Siemens offers retrofit solutionsfor a large number of differentswitchgear. This is illustrated by afew examples here.

Since SION circuit-breakers havesmaller dimensions than conven-tional switchgear, they can be in-stalled on any switchgear truck.They are adapted to the existingswitchgear via the contact arms.

What needs to be taken intoaccount for retrofitting?

Retrofitting means the partial re-newal of existing switchgear. Itmakes a big difference whether amajor modification is intended ornot, since this will involve a kind of“status protection.” A major modifi-cation by definition is accompaniedby a change of function, e.g.:• Change of the rated voltage bymore than 20 percent• Increased power output• Change in the supply• Change in measures to protectagainst electric shock

In a case such as this, the entireswitchgear has to be adapted to thecurrent electro-technical safety reg-ulations, which may require addi-tional measures as well as addi-tional tests. In this instance, thecost-effectiveness of retrofittingcompared with a new systemshould be examined.

If there is no change of function,for example, exchange or repair ofthe switchgear, the regulations ap-plicable at the time the switchgearwas installed remain valid. It is an

advantage if the new circuit-breakeris type-tested in the particularswitchgear, since then only a fewtests will be required on site.The “plug and play” solution causesonly a brief interruption of opera-tion.

What are the benefits for the -operator?

Opting for an entirely newswitchgear certainly has many ad-vantages. However, the fact shouldnot be overlooked that in such acase it may be necessary to performwork in an existing building on thefloor openings and on the cables.Old cables in particular must becarefully adapted to the new cablebushings and then subjected to atest. Installation of a newswitchgear in a new building can in-volve lengthy approval procedures,and so create costs that are not in-curred through a retrofit.

By replacing the most importantequipment in a switchgear, namelythe circuit-breaker, the operator in-creases the reliability and availabili-ty of its switchgear quite substan-tially without incurring costs formodifications to the building. Themaintenance-free breakers avail-able today hardly incur any costs formaintenance; see Fig. 6. Devicesstill in operation that have a liquid

arc-extinguishing medium, partic-ularly if they are more than 30 yearsold, require intensive maintenance.The contact systems have to be in-spected at short intervals, and mayneed to be exchanged. The costs forreplacing these worn parts increaseconsiderably once the manufactur-ers have discontinued productionof the devices, since the parts nowhave to be manufactured on a one-time basis. This development isshown in the maintenance cost pro-gression in Fig. 6. A decision in favorof a new maintenance-free breakeris immediately reflected in reducedmaintenance costs.

Apart from the reduced mainte-nance costs, a retrofit also increasesthe reliability of the operator’sswitchgear. The new circuit-break-ers are tested according to the lateststandards, which have significantlystricter requirements in terms ofverifying switching capacity thanconventional breakers. Moreover, acircuit-breaker approaching theend of their service life are morelikely to develop a fault than a newbreaker. And with modern vacuumswitching technology, the conse-quences of a fault are far less seriousthan with conventional low-oil-content and expansion circuit-breakers, since explosions do notoccur with vacuum circuit-break-ers.

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Fig. 5: SION circuit-breakers as replacement for an air-magnetic circuit-breaker

SummaryModern vacuum circuit-breakershave proved ideal for use in routineoperation, a fact that is confirmedby the more than 600,000 vacuumcircuit-breakers from Siemens thatare in service worldwide today.SION vacuum circuit-breakers arean outstanding solution for retro-fitting on medium-voltageswitchgear. The investment costsare paid off quickly, first becauseservicing costs are very low due tothe maintenance-free circuit-breakers, and secondly becauseavailability is increased because of

the high degree of reliability. Withtype-tested retrofit solutions, re-placement of the switchgear truckonly requires the power supply to beinterrupted briefly because no workor only a very limited amount needsto be performed on the existingswitchgear.

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dieter.saemann@siemens.com

www.siemens.com

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Maintenance costs

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Fig. 6: Maintenance costs of old and modern circuit-breakers

www.siemens.com/energy

This article appeared in:ewVolume 108 (2009), Issue 22 – 23Copyright © 2009 by EW Medien und Kongresse GmbH

This reprint is published by:Siemens AGEnergy SectorFreyeslebenstrasse 191058 Erlangen, Germany

Siemens AGEnergy SectorPower Distribution DivisionMedium VoltageMozartstrasse 31c91052 Erlangen, Germany

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Subject to change without prior notice.The information in this document contains generaldescriptions of the technical options available, whichmay not apply in all cases. The required technicaloptions should therefore be specified in the contract.