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DEVELOPMENTS TOWARDS H.V. SUBSTATIONS WITHOUT DISCONNECTOR SWITCHES AND WITHMODERN CONTROL SYSTEMS

H. Timmerman J. F. Groeman

EDON T ranspor t, The Netherlands KEMA , The NetherlandsAbstractIn open-air high-voltage su bstations, disconn ectorswitches are applied to ensure p ersonnel safety duringmaintenance of circuit breakers. However, disconnectorswitch maintenance has becom e significant in compari-son to circuit break er maintenance. Therefo re, a studyhas been performed to investigate the consequences ofleaving out the circuit breakers from a substation; thiswould result in significant cost reductions. The resultsof the analysis shows that, if certain precautions aretaken, personnel safety of a substation without discon-nectors is comparable to that of a substation with dis-connectors.Next, a new medium-size 110 kV substation had to berealised. It was decided to omit the line disconnectors,i.e. the disconnector switches between circuit breakersand the line connection.A modem SM S (substation management system) hasbeen used with distributed interfacing on primaryequipment level. Optimal serial communication be-tween SM S and protection equipment leads to savingson cabling and measurement equipmentThe savings on secondary equipment are about 25 %and on primary equipment about 10 %. In future theexpected savings on a substation without disconnectorswitches are up to 20% of the total life-cyclexost

INTRODUCTION

This paper focuses on efforts to achieve significant costreduction s in 110 kV and 150 kV substation s in theNetherlands. Highlights are placed on:0

ereduction and elimination of disconnector switches.application of substation automation with its associ-ated advantages.

A discussion on eliminating disconnector switches wasstarted by seve ral individual Dutch utilities in the 1980s.Next, KEMA, in close co-operation with the Dutchutilities, has carried out a study, funded by the associa-tion of Dutch utilities (EnergieNed), giving an indica-tion of the feasibility of the idea. One further step wasto implemen t the idea and to build a substation withoutdisconnector switches. As a first step, EDON decided tobuild a new 110 kV substation without line disconnec-

tors. The experiences during basic design and engineer-ing of a new su bstation are reported in this paper.An important tool to reduce the overall costs of substa-tions is the optimal use of SM S (substation managementsystem). A good example is the distribution of the I/Ointerfacing on the primary equipment level. SM S alsogives the opportunity to reduce the number of primaryequipment and to reduce the maintenance costs by con-tinuously monitoring the equipment. The co-ordinatedcontrol system SAS2000 has been developed by EDONin co-operation with a Dutch manufacturer and beenimplemented in a new substation.

A STUDY INTO HIGH-VOLTAGE SUBSTATIONSWITHOUT DISCONNECTORS

The application of disconnector switches stems fromtimes when circuit breakers required much more pre-ventive and corrective maintenance than they do now. Itseems that the innovation and cost reduction rates ofcircuit breakers are much higher than for disconnectorswitches. The vast improvements in circuit breaker reli-ability have led to a situation in which maintenance ofthe disconnector switch itself has become significant incomparison to circuit breaker maintenance. Could it bebetter to leave out the disconnector switches?Leaving out disconnector switches would reduce sub-station life-cycle cost but safety during maintenancecould be affected.In a feasibility study, safety, economics, reliability anddesign aspects have been studied by means of compar-ing a substations without disconnector switches with atraditional one. The following, typical Dutch open-airhigh-v oltage substatio n has been used as a study case:0 an new open air substation, voltage level 15 0 kV.

tw o power transformer bays and four line bays fortwo double-circuit lines.0 double busbar configuration:- traditional substation: according to figure 1 (in

the Netherlands, most 110 and 150 kV substa-tions are of this type).- substation without disconnector switches: ac-cording to figure 2.

Trends in Distribution Switchgear, 10-12 November 1998,Conference PublicationNo . 4590 IEE 7998

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'igure 1: Single-line diagram of the (double-busbarsub-station with discon nector switches as used for the study

Tr 1 Tr 2

igure 2: Single-line diagram of the double-busbar sub-station without disconnector switch.es as used for thestudy

Safety comparison - regarding safety quantitativelyMost important of a substation without disconnectorswitches is the safety issue. New legal requirements inthe Netherlands concerning safety give more freedom toensure safety than previously. Safety should, however,not only include satisfaction of legal requirements. Autility refraining from safety provisions (such as dis-connector switches) should also be convinced that therisks of a disconnectorless substation are acceptable. Inthe study, the latter argument has been treated by requir-ing that the disconnectorless substation should not beless safe than a traditional one with disconnectorswitches. In order to quantify this requirement, a q uanti-tative comparison has been made of the probability ofan accident, using event and fault trees.In order to determine the accident rate, the possiblecombinations of failures and human errors leading to anaccident have been determined. Possible technical fail-ures are, e.g. flashovers over open circuit breaker con-

tacts. Possible human errors are forgetting basic safetymeasures, starting to work in the wrong bay etc. Fromthese basic ev ents, and their associated probabilities, thechance of an accident have been estimated.Application of fault and event trees is relatively tradi-tional failure probability assessment method. It is suit-able in simple situations in which all failure modes andcombinations thereof can be foreseen. The method hasbeen used for assessing the safety of e.g. nuclear plants.For more complex situations, the number of probablefailure mechanisms increases drastically. In that case,the manual fault and event tree calculations become toolaborious and, not unimportant, prone to mistakes. Insuch cases, it is recommended that a monte carlo analy-sis be carried out, to account for all possible combina-tions of failures and mistakes.The results of the analysis showed that, if certain pre-cautions are taken, personnel safety of a substationwithout disconnectors is comparable to that of a substa-tion with disconnectors, provided following conditionsbe fulfilled:circuit breakers and earthing switches can bemanually locked against accidental operation duringmaintenance.earthing switches are remotely controlled.personnel may not be in the open-air substation

during switching.the circuit breaker type should have an excellentreliability record.the circuit breaker should have a withstand voltageacross open contacts in accordance with values usedfor disconnector sw itches or better.the circuit breakers should have a clear position in-dication.at some locations, removable parts are necessary toensure sustained safety.

Economic comparison

In order to determine the economical advantage of asubstation without disconnector switches, not only theinitial investment cost, but also the maintenance costshould be taken into account, capitalised to the momentof the investment. The investment cost include high-voltage equipment, control and protection, civil works,land area and engineering cost.The result of the cost estimation has yielded a potentialsaving of 15% by elimination of the disconnectorswitches, even though the number of circuit breakers ishigher in the scheme without disconnector switches (12instead of 7). The advantage is caused by:elimination of the disconnector sw itches themselves.

reduction of supports.

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reduction of control equipment.reduction in land area.0 reduction of engineering and maintenance costs.

Reliability comparison

Not applying disconnector switches has an influence onthe expected and unexpected non-availability of a sub-station.For this study, an analytical comparison has been madeof the unavailability of a substation with or without dis-connector switches. Several assumptions have beenmade, amongst which:0 the line bays belong to two double-circuit lines,which are fully redundant, i.e. in case of de-energisation of one circuit of a double-circuit line,the remaining circuit will not get overloaded.0 the two power transformers are fully redundant, i.e.

in case of de-energisation of one transformer, theother transformer will not become overloaded.Under these assumptions, the unexpected unavailabilityof the transmission and distribution function appears tobe dominated by the situation when one bay is de-energised (e.g. for maintenance) and a failure of anotherbay occurs at the same time. In the case where no dis-connector switches are applied, the reduced mainte-nance period gives rise to a reduction of the periodwhen the system is most vulnerable to equipment fail-ures. The calculations show a slight decrease (5-10%) ofthe unavailability of transmission and distribution func-tions of the substation.

A MODERN CONTROL SYSTEM CONCEPT

As a second cost reduction item, the optimal use of aSM S (substation management system ) is discussed.The co-ordinated control system consists of centralisedredundant multi-processor units (user interface, substa-tion processor and bay processing) and bay wise processinterface modules. The interface modules are connectedto the central control units with a number of independ-ent local area networks (LAN's) as redundant glass fibrerings. The concept is shown in Figure 3.

Dabbuted P m c e nterfaceModule.II Figure 3: Schematic principle of SM SThe communication between the distributed interfacemodules and the centralised control units is extremelyreliable and with a high performance.The interface modules of the co-ordinated control sys-tem, as well as the protection equipment, are bay ori-ented. They are mounted into cubicles in the centralcontrol building or in bay kiosks. The cabling of allprimary equipment of a specific bay is connected to amarshalling cabinet in the switch yard (figure 4) . Fromthere on connections are made with multi-core cables tothe co-ordinated control system and the protectionequipment in the central control building. The length ofthe cabling in the switch yard can add up to many kilo-metres.

IFigure 4: Typical hardware configurationIn the past years, we have achieved considerably sav-ings in the construction and refurbishment of substation,both in primary and secondary equipment. The activitieswe employed are described below.

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Integration of auxiliary relays and measurementconverters

The digital output channels of the SMS are equippedwith on-board relays, which can switch the current (upto 5 A) that flows through the switch's coils.The analogue input channels of SMS can be wired di-rectly to the output of voltage and current transformers(1 10 Vac and 5 or 1 Aac, respectively). This means areduction of engineering effort, as well as a consider-able saving on material costs. The return on investmentfor the development cost was 2 to 3 substations.

Leave out VT's in feeder bays

Many of EDON's substations have 6 feeder bays and abusbar system. The feeders are protected by a distancerelay, which requires the current and voltage for thatbay. Instead of the line voltage, we measure the busbarvoltage, which is wired to all protection relays. In caseof a double rail system, we use a simple software func-tion and kipp relays to select the right voltage fo r a par-ticular protection.The savings are considerable.

Serial commun ication with protection equip ment

Most vendors of digital protection equipment supportthe IEC 870-5-103 protocol standard. Using this stan-dard, we exchange information with the control system.This not only lead to cost reduction, but also to addi-tional functionality:Serial exchange of alarm indications.Only trip signals, required by other equipment, arehard-wired. All other signals are exchanged via theserial link.

0 Serial exchange of measurement data like voltages,currents, power, reactive power and frequen cy.Mass storage of fault recordings (disturbance data).

0 The protection relay has limited storage c apabilities,whereas the control unit is capable to store all re-cordings that may occur during the life-cycle of thesubstation.

0 Central access to fault recordings.0 The recordings of all protection equipment (if con-

nected via serial link) is stored centrally. You justneed to have access to the control system.

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Software transformer regulator

In conventional substations, a separate transformerregulator is applied. In substations, equipped with SMS,this function is implemented in software, eliminatingthe cost for the regulator.

Leave out b reaker fa i lure protect ion

Modern circuit breakers have extremely low failurerates; the main remaining failure m echanism is a fault inthe control circuit.This leads to the decision not to implement the breakerfailure protection in substations, equipped with moderncircuit breakers, if we could eliminate the dominantproblem that can still lead to failing breakers. This canbe achieved by continuously monitoring the closing coilof the breaker by the SMS.Distributed YO interfacing on equ ipment level

The distributed 1/0 concept on equipment level isshown in figure 5.

Snitch-yard

14-Control Building

Figure 5 : Process interfacing in the switchyard

Most primary equipment (circuit breakers, disconnec-tors and transformer) has his own interface module(outdoor mini IM), mounted in a water resistant cabinetclose to or mounted on the primary equipment. Themini interface module has only a few digital inputs andoutputs to control the primary equipment.The protection equipment is installed in the controlbuilding and is serial connected to the interface modulein the control building.

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The analogue signals of the measurement transformersare directly connected to the interface modules in thecontrol building, as well as the protections.The outdoor mini interface modules and the central lo-cated interface modules are connected to the centralcontrol units in the control room via a fibre optical localarea network.In comparison with the present concept the main differ-ences are:0 "bay control" cubicles do not exist any more.0 extra outdoor mini interface modules.0 saving on cubicles in the control building.0 saving on cabling in the switch yard.0 saving on marshalling cabinets in the switch yard.0 a fibre optical local area network in the switchyard.0 saving of personnel costs, especially the mounting

of cabinets and cabling and testing.0 saving on cable ducts in the switchyard.In comparison with the present concept the total sav-ings are about 25 % of the secondary equipment. Themost important savings are on engineering, constructionand testing costs (84 % of the total savings). The sav-ings on equipment amount to 5%. In comparison withthe I/O concept on bay level 4 to 6 field IM's are neededinstead of 1 IM (depends on the number of the primaryapparatus in a bay field).

Remote MMI via TCP/IP

A major break-through was the implementation of theTCP/IP protocol stack in the control unit. We can nowplace our MMI anywhere we choose, as long as there isa TCPIIP connection with the control unit.Apart from bringing the process mimics to the remotePC, we can also download files (e.g. alarm lists, trenddata, fault recordings) from the control system. We ex-pect that this will reduce the number of visits to the(unmanned) substations.

THE WEIWERD SUBSTATION

In April 1998 EDON put into use a new open air sub-station, voltage level 110 kV in Weiwerd. It has a dou-ble busbar, single-breaker configuration with four over-head line bays for two double circuit lines, one over-head line bay for a coupling with the 220 kV networkand one cable bay for the coupling with a 520 MVAgenerator (figure 6) .

1 11 1

IFigure 6: Single line diagram of the W eiwerd substation

Several of the previously described issues are applied inthis substation:0 leave out the line disconnectors, i.e. the disconn ectorswitches, which are usually applied between the cir-

cuit breakers and the line connection. In the previ-ously mentioned study, it was disconnected to leaveout all disconnector switches. This step was nottaken as the legal requirements at the moment ofdecision were not yet clear.0 leave out the VT's in the four line bays (in the cou-

pling bay with the 22 0 kV network and cable bay tothe generator a VT is still applied to enable syn-chronisation and metering)0 the IM's (interface modules) of the SMS are build inthe driving-cabinets of the breakers and disconnec-tors0 Only the voltage and frequency of the both busbar

systems are directly measured by SMS. All the othermeasurements are derived o f the protection systems.

0 remote diagnosisAlthough there is not yet operational experiences withthis substation (April 1998) the expected savings havealready been reached during engineering and construc-tion, amounting to 25 % of the cost of the secondaryequipment and to 10 YO f the cost of primary equip-ment.

FUTURE DEVELOPMENTS

High-voltage installation

As has been shown, a substation without disconnectorswitches promises significant cost reductions. Apartfrom implementing this idea, future developments mayinclude:0 a review of the reliability of circuit breakers.

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a review of removing disconnector switches in a GISsubstation.

0 a review of removing disconnector switches fromHV substations.

Leaving out disconnector switches from a substation isnot the only way of cost reduction. Several competingdevelopments include:0 skipping most of the circuit breakers and using dis-

connector switches instead (together with a controlstrategy to ensure a switching sequence in which thedisconnector switch switches at zero current).0 circuit breakers integrated with disconnectorswitches. For air-insulated metal clad medium volt-age switchgear, this option is not uncommon, forhigher voltages, assemblies have been developed tointegrate the breaking and disconnecting function in

one apparatus. This yields reductions in installationcost.

Which of these developments will finally prove mostattractive will depend upon the situation. In meshednetworks, the applicability of the first solution will belimited due to selectivity requirements. The second op-tion looks promising, although the extra cost of the dis-connecting mechanism more or less seem to counterbal-ance the cost reduction by omitting two disconnectorswitches.

1 CT & VT IIigure 7: Singular ring shaped busbar without disconn-ectors

0 SMS monitors on line the state of th e HV breakers.In that case it is possible to take action if mainte-nance is necessary.SMS derives the line voltage from the busbar volt-age, dependant on the switching configuration.0 protection concept need s still more attention.legislation on safety and working procedures should

not form barriers.

CONCLUSION

Removing disconnector switches from a substationpromises great cost reductions. The safety of such asubstation without disconnectors seems to be compara-ble to a traditional substation, under conditions that areoutlined in this paper. A substation without any discon-nector sw itches has not been realised yet. As a first step,a substation without line disconnector switches has beenrealised.The optimal use of SM S also leads to cost reduction:0 integration of secondary functions into SMS andleave out secondary equipment.0 distributing of the interface modules on the primary

equipment level.0 serial communication with protection equipment and

leave out VTs in feeder bays.0 by on line monitoring the primary equipment pre-

ventive maintenance can be replaced by conditionbased maintenance.remote control gives a lot of information to the pro-tection engineer and the maintenance engineer with-out visiting the unmanned substation.

A singular ring shaped busbar without disconnectors isshown in figure 7. Each bay consists of a maintenancefree circuit breaker, removable links on either side ofthe circuit breaker and a line earthing. Consequences orpossibilities can be summarised as follows:savings on primary and secondary components andtherefore on maintenance.