IEEE Tanzaction, on Nucteat Science, Vot.NS-25, No.1, FebkuaAy 1978

REDUCING CABLING COST AND COMPLEXITY W^7ITH REMOTE MULTIPLEXING

OR DISTRIBUTED CONTROL IN NUCLEAR POWER STATIONS

by

A.J. Stirling and J.V.R. L'Archeveque*Atomic Energy of Canada LimitedChalk River Nuclear Laboratories

Chalk River, Ontario, Canada

Abstract

Since control and instrumentation cab-ling accounts for more than 1% of the cost ofa nuclear power station, interest in remotemultiplexing and distributed control is run-ning high. This paper draws a distinctionbetween these two techniques and examines thepractical implications in each case. Com-mercially available remote multiplexers arereviewed and categorized according to theirfield of application. A detailed method isdeveloped to analyse cabling costs and toestimate the potential savings for both pre-sent and future designs. Estimates show thatCANDU reactor cabling costs are approximately$140 per field point, by comparison with apublished example of $690 for a HTGR. Thesevariations, which are quite typical in theprocess industries, are explained, and thecurrent costs for hard-wiring and remotemultiplexing are compared.

Introduction

Control and instrumentation cabling ac-counts for close to 1% of the total capitalcost of a CANDU generating station. Theinterest in techniques that can help reducecabling cost and complexity is therefore run-ning high. Several factors contribute to thehigh cost of cabling in nuclear powerstations.

with current system structures, are also be-coming commercially available. Reactor sup-pliers and utilities in the U.S.A.1' 2 andCanada3 have therefore supported majorstudies of the use of remote multiplexing.

Remote Multiplexing & Distributed Control

Industrial plants frequently requiremonitoring and control over a wide geographi-cal area while concentrating some process in-formation at a control centre for efficientoperation, protection from hostile environ-ments, or to share expensive equipment.

The trade-off between locating equipmentin the field or the control room depends onmany practical factors and is influenced bythe technology available for signal transmis-sion and processing. The digital computerhas provided greatly expanded control capa-bility but its high cost and intolerance toplant environments have traditionally re-quired that it be shared between many tasksand located in a control room. Consequently,this has further reinforced the need forcentralized control and dedicated centralizedwiring (see Figure 1).

- The complexity of the nuclear reactor de-mands that many parameters be measured forcontrol and monitoring purposes.

- The centralized approach to control re-quires that all signals be transmitted tothe control room.

- Stringent licensing regulations dictate theuse of high quality cabling materials.

In the case of CANDU reactors, cabling isparticularly expensive because of the manyfuel channels and the long signal trans-mission paths in multi-unit stations.

In principle, the cost and complexity ofcabling could be reduced substantially ifcontrol equipment were decentralized or ifsignals were multiplexed to share communica-tions channels. Distributed processing andcontrol systems are becoming available andare undergoing considerable development andinvestigation because of changing cost/performance trade-offs in control systemdesign. Remote multiplexers, more compatible

*Address: CANATOM, Montreal, P.Q., Canada.

CONTROL CENTRE

O ~ ~ b CONTROLSUB ENTR

F IEL TRANSDUCERS

(a) CENTRALIZED CONTROL:CENTRAL ZED POINT-TO-POINT WIRING

Figure 1 -

CONTROL CENTRE

FIELD TRANSDUCERS

(b) INBUTED CONTROL

CONTROL CENTRE

MULT IPLEXERS &DEMULTIPLEXERSr R

F IELD TRAN4SDUCERS

(c) CENTRALIZED CCNTROL;REMOTE MULT IPLEXING

Examples of CentralizedControl, DistributedControl and RemoteMultiplexing

In industries where the distance betweenthe information source and destination islarge, centralized hardwiring has never beenpracticable. Two alternatives have beenused: distributed control and remote multi-plexing.

0018-9499/78/0200-0817$00.75 ® 1978 IEEE 817

In a distributed control system (seeFigure 1) control sub-centres are locatednear the transducers. A control sub-centreperforms all of the functions which do notrequire the facilities of the major centre.Only a small number of information channelsare therefore required between the majorcentre and the sub-centres. Until recently,available technology has limited the capabi-lity of control sub-centres, thereby restric-ting the degree of distribution possible.

Remote multiplexer systems (see Figure 1)can eliminate most of the point-to-point wir-ing without disturbing the centralized con-trol structure. Multiplexers and demulti-plexers scan individual field transducers inturn so that information is carried over mostof the distance on a few time-shared communi-cations channels. Both distributed controland remote multiplexing have the potentialfor reducing nuclear power station cablingcosts.

In addition, distributed control systemsappear attractive for the following reasons 4

- reduced complexity of central computeroperating software;

- simpler management of software developmentthrough partitioning;

- potential for graceful degradation ratherthan catastrophic failure;

- potential for expansion without majordisruption;

- reduced analog signal transmission re-quirements.

Although remote multiplexing may bejustified on economic considerations alone,it may also be selected to meet specific per-formance requirements. Examples of engineer-ing factors which may dictate the use ofremote multiplexing in nuclear plants are

- reduced volumes of cable in confinedspaces,

- availability of existing wiring (forretrofits), and

- reduced number of containment penetrations.

In addition, since most remote multiplexerstransmit data digitally, the followingcharacteristics of digital transmission are

often attributed to remote multiplexing:

- high transmission accuracy in the presenceof noise, and

- compatibility with digital control equip-ment.

Technology Status

Remote multiplexing and distributedcontrol systems exist today as commerciallydeveloped technologies: thirty companiesprovided literature describing products forreducing cabling costs in industrial. plantapplications3 . The equipment available

divided into four categories corresponding tofour separate industries which have contri-buted products to the marketplace.

(a) Industrial Control

The major industrial control manufac-turers have taken a "total systems approach"to minimizing plant wiring with emphasis ondistributed control supplemented by remotemultiplexing. Configured with one or moremultidrop lines linking control centre andsub-centres many thousands of transducers canbe accommodated. The multidrop lines arehigh speed, high volume, bidirectional,digital data busses including coaxial cables,twisted pairs or multiconductor cables.

(b) Digital Computer Manufacturin

Digital computer manufacturers, anxiousto apply their products to process controlapplications, have recognized the need totransfer information from field transducersto the control room computer. On thestrength of experience with remote job entry,common carrier communications techniques wereapplied to multiplexing for data acquisitionand control. Typical field terminals handlea range of analog and digital inputs and out-puts, incorporate a modem, and are treated asremote I/O devices by the computer.

Remote multiplexers supplied by computermanufacturers can be characterized asfollows:

- they handle only computer inputs and out-puts;

- most are conceived as subsystems ofspecif ic computers;

- they generally rely on the host computerfor functions such as error checking.

(c) Specialized Control/Multiplexing

Electronic equipment manufacturers notspecifically associated with the control orcomputer industries produce "wire replacers"for diverse applications. Two distinctclasses of multiplexing systems exist. Someequipment is designed to eliminate humansupervision at remote plants, e.g. hydro-electric power stations 100 km from thecontrol centre. In principle, these could beused within a plant, but the multiplexer isreally intended to provide the link to adistant supervisory station.

To use a specific example, one manufac-turer provides 63 multiplexers, each accept-ing 63 discrete inputs which can transmitdata at 2400 Baud over an unlimited lengthof telephone line. Relatively small numbersof I/O points, and low data rates are typicalof this class of equipment which does notaddress situations where high speed sequence-of-events recording or millisecond responseis required.

Plant monitoring and control which in-volves thousands of analog and digital inputsand outputs and millisecond response can behandled only by the products of a few

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specialized control/multiplexing equipmentsuppliers.

The I/C Engineering Uniplex 600 is usedas an example in this report because it isoffered for use in power stations and isused for cost comparisons later.

Uniplex 600 is a dual multiplexed multi-drop configuration (see Figure 2) whichhandles 64 (or 255) Uniplex Field Multi-plexers (UFM). Each UFM is able to receiveor transmit 188 channels of data up to 1.6 kmover a 462 kbs'1 twinaxial line. The UFMshandle analog inputs and outputs with 12-bitresolution, and also monitor contact closuresand provide digital control signals. Since aredundant transmission line is routed along aseparate path, the probability of failurethrough cable severance is very low. TheCentral Control Unit and its redundantpartner,

- direct computer bound signals to the com-puter input,

- direct signals destined for analog controlboards to be demultiplexed by control roommultiplexers, and

- provide data and system diagnostic checking.

[CENTRALLATN

Figure 2 - Multidrop ConfigurationTypical of Multiplexersfor In-Plant Applications

The Uniplex Field Multiplexers are designedfor location at inaccessible areas of a plant,to operate in adverse environmental conditions,(e.g. steam) and to meet appropriate seismicqualifications.

In summary, the only stand-alone remotemultiplexing products, suitable for widespreaduse in a nuclear power station, are thosewhose capabilities are comparable with theUniplex 600 system.

(d) Communications

The communications industry, which haslong experience in multiplexing voice, videoand data signals, is the source of severalproducts with potential for plant controlapplications. For example, an "In-plantCommunications Information Utility" whichuses CATV technology for voice, video anddata transmission and also provides terminalunits for analog and digital inputs and out-puts is installed at CRNL for development

purposes 4. However, in general, the communi-cations industry products are not designedfor plant control.

Cabling Approaches

The primary economic incentive for re-mote multiplexing is the reduction of capitalcosts for cabling. The magnitude of cablingcosts in a number of industries6-12 iS de-monstrated in Figure 3.

IoI

I0 IIIl

C AN DU

10 100 I000 10.000NUMBER OF FIELD POINTS

COST COMPARISON FOR I00-200 m CABLE RUNS

Figure 3 - Typical Wiring Costs

N.B. Data taken from references 6-12 andnormalized to 1976 dollars assuming8% annual inflation, 1970-76.

For cable runs of 100-200 m,wAiring costsrange from approximately $50 to $1,000 perfield point, depending on the wiring re-quirements in each industry. While thesecost data do include both termination andlength-dependent components, a simpleanalysis shows that cabling costs perpair*metre range from $0.37 to $8.15.

(a) CANDU

The approach to cabling in Canadiannuclear power stations is characterized byhardwiring all field points to a centralcontrol area through a Central DistributionFrame (see Figure 4). The cabling at theBruce A Generating Station in Ontario fol-lows a mature engineering procedure, in-corporates economies based on previousexperience and is representative of otherCANDU power stations. Since the most recentcost estimates available are for Bruce k, itis taken as a reference design in thisreport.

At Bruce there is provision for 29,400field points (80% of which are wired) in eachunit. Wires from the turbine halls andreactors run an average of 206 m to thecentral service area*. Eighty-one percent

*There are 12,100 km (7500 miles) of controland instrumentation cabling at the BruceGenerating Station in Ontario, Canada.

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I I Imux mux NUX

of the field signal lines penetrate the con-crete nuclear containment shield; the remain-der are located in the conventional areas ofthe plant. The signals transmitted includelow level dc from sensors (22%), 45V dc forinstrumentation loop signals (6%), 48V dc forrelay logic (65%),and 120V ac for smallmotors and solenoids (7%). Less than 12% ofthe field wires are connected to the twodigital control computers while the remainderare connected to control panels or equipmentracks in the central service area. Vital in-puts are connected independently to bothcomputers to improve system reliability.

it applies to a nuclear power station design,and it is instructive because it shows howwiring costs can vary widely depending on theapplication and approach. The engineeringdata are derived from an architectural/engineering analysis made for a Gulf-GeneralAtomict high temperature gas reactor with9200 field points, on average 99 m from thecontrol room.

In the wiring analysis, interconnectionsare point-to-point, and 1 or 6 pair 12 AWGcables are used in control circuits while 1or 6 individually shielded pairs are used forinstrumentation (see Figure 5). Wires whichenter or leave the containment are terminatedon both sides of the wall at penetrationassemblies.

Tl1ttSOUCER EBMEMBTEo F S TS11DO PANELF PENETRATION FRAtE (COF)

TRAltSOUCEIS TBMBL °e R CABLES

CONtTAltINENT AREA rClTX < O L L E S ETC

WIRING S CEEPACIY FDAEURPBUENT3 OR PAIN DotTs __

lb ATt6 CABLES

TEtNtATION 30t. CONFPUTER

r'Ir -I 1 5|7 5PAtt 0H OtERUN r.a

TRStttSOUCE!5 0 4 4W r ~~~CABLES 2DSXt) 1WOUTStO:E XRUCLFA.1.AtCONTAINNtE MT AREA r -'Zr_,WlIRING SCHEME FOR BRUCE A

TRANSDUCER TERtt I,NT OTERttItATION (IUNCTION BOX)

TRANSDUCERS r rttlTHlt NUCLAR -- -COttT 11NTENT r

TRAttSD UCER TE Rott w.t^TTERMtINATIONS JlItCT ON BOX)

TRANSDUCERSOUTSIOE tlCL' T _

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TERMINBTISE EMETBETITE T E TION SPREW DIMNG....tsLIT t5 ^I E CONTROTLLERSASNSE LIS CN PAIBR

r -j HOttERUN CABLStE

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PAIR TABLES HOTtERUN r

(AtV 99tt) Si

WIRING SCHEME USING CONVENTIONAL HARDWIRING PRACTICE FOR A

Figure 4GULF-GENERAL ATOMIC HTGR

The use of a Central Distribution Frame(CDF) is a dominant feature of the field wir-ing in Canadian nuclear reactors. Althoughthe CDF approach requires more wires and moreterminations than point-to-point wiring, uni-formity in materials and procedures yieldsconsiderable cost savings in design andgreater flexibility and convenience at in-stallation.

Field wiring at Bruce A radiates fromthe CDF in cables of 25 or 50, 16 AWG twistedpairs. These trunk cables have a singleground sheath, and are laid in 30 and 61 cmcable pans with up to 16 other cables. Wir-ing between the CDF and the control room or

equipment room is identical to field trunkcabling. In the field runs, the cables whichmust penetrate the containment are fedthrough gland-seal* fittings, while theinterstices between pairs are impregnatedwith epoxy to form an internal seal.

Strategically placed junction boxes,each with 100 terminals, are provided toterminate trunk cables and to marshal thewires from individual transducers.

(b) Other

Two alternative cabling approaches haverecently been discussed, compared andpublished5' 12. That study is relevant since

*The cable is not cut at the point of pene-tration. The outer jacket is stripped and anannular seal is compressed around thebundle of wires.

Figure 5

In the remote multiplexing analysis,Uniplex Field Multiplexers (UFMs) replace thejunction boxes and are connected in a multi-drop configuration (see Figure 6)

TRINSOUCER WIRN

TTEII %AT ION TERMINAT I ON PENETRTION OBEiULT ILEXER

TRANSOUCEFS - DA|TBTTA INBICATORSNiS TAP CONTROLLERS ETt

CONTROu@4 ~~~~~~~~~~~~~~~COMtPUTE R

TRAtSWcER FIELO IlRlttGTERMtlNA lOl TERttlZtATION OiTAr- -1 r- _-- BU

.....UTSIDEZtUCLER - @f

EttOTE MtULTIPLEXER

WI RING SCHEME USING REMSOTE MULTI PLEXING FOR A GULF-GENERAL ATOMIC HTGR

Figure 6

tThe reactor for which the study was madewas never built.

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-H)

Comparative Cost Analysis

A number of assumptions and some normali-zation of data were made to permit meaningfulcomparisons between cabling costs for:

(a) future CANDU reactors,

(b) the hypothetical Gulf-GA reactor, and

(c) remote multiplexing.

The most important normalizations were tocalculate costs of CANDU wiring based on theuse of the more expensive flame retardantcable and underground concrete cabling ductsbetween units. It has also been assumed thatcable runs will be 15% longer in futurelarger stations. All basic data came inImperial Units, so apparent minor discrepan-cies are due to round-off errors in conver-sion to SI units. Costs are in 1976 dollars.

The comparison between the costs ofcabling in the CANDU and the hypotheticalGulf-GA reactors is shown in Table 1, whichis a summary of the detailed cost comparison3.Total cabling costs include materials andlabor for cable, cable pulling, cable pans,terminations, penetrations and design andproject management. The total cost of trunkcabling for a single unit of a multi-unitCANDU will exceed 4 million dollars or $140per field point.

By comparison, the cost per point forhardwired trunk cabling in the Gulf-GAexample is $687. The large differencebetween the CANDU and Gulf-GA costs is evenmore striking when costs per pair*metre arecompared. The CANDU cost of $0.63 perpair*metre is a factor of 11 smaller thanthe Gulf-GA cost of $6.94 per pair*metre.However, cabling cost differences of thismagnitude have been widely reported (seeFigure 3). In this comparison, the lowercost for cabling in the Canadian reactor isexplained by,

(a) extensive use of 50 pair cable (ratherthan single and 6 pair cable),

(b) no terminations at the penetrationassembly,

(c) less labor involved in terminatingwire, and

(d) slightly lower labor rate.

The costs for remote multiplexersS inthe Gulf-GA example can be assumed to berepresentative for equipment which meets thespecifications for nuclear power stations.On an individual signal basis remote multi-plexing costs $260 per point, intermediatebetween the cost of $142 per point for CANDUReactors and the cost of $687 per point forthe Gulf-GA example. The remote multiplexersupplier claims that installation costs areonly 9% of the system purchase price but doesnot break down the remainder between materials,labor and amortized development changes. Bycomparison, labor represents 68% of the wir-ing costs for CANDU reactors (See Figure 7).The remaining components are materials whose

costs are unlikely to be reduced by technolo-gical developments in the foreseeable future.

CANDU GULF-GA

ITEM UNIT UNITCOST TOTAL COST TOTAL($) ($k) ($) ($k)

Cable (M) 0.19 1248 0.97 887Cable Pulling (L) 0.06 385 0.69 1537Pans & Tunnels 0.13 872 0.26 218(M+L)

Terminations (M) 0.17 39 0.75 76Terminations (L) 2.7 1239 10.5 1851Central Distribu- 23tion Frame (M&L)

Penetrations (M&L) 5 64.2 620Design, Doc. & 381 1136Project Mgt.

TOTAL 4192 6325

Cost per Signal $142 $687Cost per pair*metre $0.63 $6.94

NOTES: M = Materials L = Labor

Quantities: CANDU Reactor has 29,400 points,at an average of 226 m from the control room.Uses 6.6 x 106 pair.metres of cable. Gulf-GAestimate assumes 9200 points, at an averageof 99 m from the control room. Uses9.15 x 105 pair*metres of cable. The numbersof terminations included in materials esti-mate is not equal to number included inlabor estimate (due to CDF), but is compar-able between CANDU and Gulf-GA estimates.

Unit Costs: Cable related costs are quotedin $/signal. Design, Doc. & ProjectManagement costs are taken as 10% for CANDUreactors, 16.7% for Gulf-GA estimate.

Table 1 - Cabling Cost Comparison(for details see Ref. 3)

SHADED AREASINDICATE MATERIALS(32% OF TOTAL)

)LENGTH DEPENDENTCOMPONENTS 52%

COMPONENTS OF TRUNK CABLING COSTS FORCURRENT CANOU REACTORS

Figure 7

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Hardwiring costs include length dependentcomponents (e.g. wire) and fixed components(e.g. terminations). The cost of wiring for aCANDU reactor, plotted as a function of cablelength, is shown in Figure 8.

0I

400-IJ REMOTE MULTIPLEXING

200

BREAK EVEN POINTt /FOR REMOTE MULTIPLEXING

vr ~~~~~~~~~~~~~~~~~~~~~~I I I 1 IIj 0 500 1000

AVERAGE CABLE LENGTH (m)

WIRING COST AS A FUNCTION OF AVERAGE CABLE LENGTH

Figure 8

Remote multiplexing costs are almost indepen-dent of length and are shown as constants inFigure 8. For a typical new CANDU reactorthe break-even point is reached for trunkcables approximately 500 m long, more thantwice the average length of Bruce A trunkcables.

Summary and Conclusions

The comparisons show that:

(a) Due to the large plant-to-plant varia-tions in cabling procedures and require-ments, remote multiplexing and distri-buted control do not provide universalsolutions for reducing cabling costs.

The cost studies for U.S. and Canadiannuclear povwer stations show that cablingin the Canadian reactors is a factor of11 less expensive per pairmetre. Whilethe reasons have been discussed, themagnitude of the discrepancy explainsthe higher degree of interest in remotemultiplexing for nuclear applications inthe U.S.A.

(b) When a large number of points is moni-tored, wiring costs per field point can

be reduced through the economies as-

sociated with multiconductor cable. Insuch cases, remote multiplexing becomesless attractive financially.

(c) Point-to-point wiring is labor intensiveand is becoming more costly. Remotemultiplexing can benefit from the de-creasing cost of integrated electronicsand can take advantage of improvedfabrication techniques. Projectionshows that remote multiplexing may be-come competitive for CANDU powerstations before presently committedreactors are commissioned.

Before remote multiplexing can gain wide-spread acceptance in nuclear power stations,field experience in plants where they are al-ready cost effective will be vital to estab-lish the reliability of the equipment andbuild up confidence in the technique. At theChalk River Nuclear Laboratories remote multi-plexers are being introduced in a developmentaldata acquisition and experiment control systemto study the characteristics of functionallydistributed architectures.

References

1. Electrical Power Research Institute(EPRI), "Study of Remote Multiplexing forNuclear Power Plant Applications",EPRI NP-254, Project 513-1, Volumes I -

III, 1976-1977.

2. M. Kayton, J. Kininger and A.B. Long,"Functional Requirements, Availabilityand Cost Analyses for Remote Multi-plexing in Power Plants", theseProceedings.

3. A.J. Stirling and J.V.R. L'Archeveque,"Potential of Remote Multiplexing Systemsin Reducing Cabling Cost and Complexityin Nuclear Power Stations", AECL-5700,March 1977.

4. G. Yan and J.V.R. L'Archeveque, "OnDistributed Control and InstrumentationSystems for Future Nuclear Power Plants",IEEE Transactions on Nuclear Science,Vol. NS-23, No. 1, February 1976.

5. I/C Engineering Corporation, Los Angeles,California, "Uniplex Data System Model600, Features and Economics", June 1975.

6. N.E. Archibald and C.A. Wiatrowski,"Remote Multiplexing", Burr-Brown Re-search Corp. Application Note 80,January 1976.

7. HI. Simon, "Multiplex Systems Save Multi-bucks in Refinery and Chemical Plants",ISA International Conference paper70-564, Philadelphia, Pa., October 1970.

8. American Multiplex Systems Inc.,"Systems Design Manual", AMX 674, 1974.

9. M.A. Keyes, "Distributed Digital Control",Control Engineering, Vol. 20, No. 9, page77, September 1973.

10. J. Fling, "Multiplexing, Once Mistrusted,Now in Demand", Control Engineering, Vol.23, No. 4, page 87, April 1976.

11. S. Godfrey, AECL Private Communication,November 1975 to June 1976.

12. E.TA. Weideman, "Slash Costs with Multi-plex Systems", Power, Vol. 119, No. 12,page 34, December 1975.

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