Cahier technique n 186 - Schneider Electric · PDF fileCahier Technique Schneider n° 186 /...

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Cahier technique n°186

Intelligent LV switchboards

A. Jammes

■ Merlin Gerin ■ Square D ■ Telemecanique

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Antoine Jammes

Received his engineering degree from ENSEM (Ecole NationaleSupérieure d'Electricité et de Mécanique de Nancy) in 1979. JoinedSchneider (Merlin Gerin) in 1980 and participated in the developmentof protection software in the Dependability Systems and ElectronicsDepartment (SES). In 1991, he moved to the Low Voltage PowerDistribution SBU where he has played a major role in the developmentof intelligent LV switchboards.

n° 186Intelligent LV switchboards

E/CT 186 first issued, june 1997

Cahier Technique Schneider n° 186 / p.2

Glossary

Application:Set of functions executed using hardware andsoftware.

Architecture:Type of organisation for hardware and/orsoftware components in a system. Also themanner in which functions and processing aredistributed.

Bus:Link used to exchange digital informationbetween the various connected elements.

Communications network:Synonymous with «communications bus».

Connected device:Electronic device connected to the bus via acommunications interface. Sometimes alsoreferred to as a «station».

CSMA (Carrier Sense Multiple Access):Method providing random access to thecommunications network.

CTM (Centralised Technical Management):System grouping all the applications in a giveninstallation, for example process control, powermanagement or building management.

Decentralised processing:System in which a part of information processingis carried out as close as possible to the load.

Dependability:Concept encompassing reliability (of control andmonitoring systems), availability (of devices,machines), maintainability (of productionresources) and the safety of life and property.

Distributed processing:Particular type of decentralised-processingsystem in which processing is carried out inseveral processing units, each having a certaindegree of autonomy and capable ofcommunicating with the other units.

Gateway:A product enabling data exchange between twodifferent communications networks, without localprocessing.

Intelligent:Designates a system with its own processingpower and a certain level of operating autonomywith respect to the computer system to which it isconnected.

LV:Low voltage.

Master / slave:A master regularly polls its slaves and givesthem orders.

MCC (Motor Control Centre):LV switchboard grouping the control andmonitoring devices for several motors, valves,etc.

Power management:Form of technical management specificallyintended for electrical distribution systems. Theintelligent LV switchboard is a major element in apower-management system.

Protocol:Sequence of rules that must be followed toestablish and maintain data exchange betweendevices connected to a bus.

Real time :Designates a control and monitoring system withresponse times compatible with the requirementsof the given process.

Stations:Information processing devices connected to thebus.

Switchboard central unit:Unit which centralises all the informationavailable in the switchboard and the directenvironment, processes the information andcommunicates with a supervision system, thusmaking the LV switchboard intelligent.


Intelligent LV switchboards

In all buildings, regardless of the activity carried out inside, the distributionof electrical power must today satisfy ever-increasing needs fordependability and efficiency.Energy must be available not only to ensure the comfort and safety ofusers, but also to avoid the costs incurred by power failures.Electrical installations must therefore be monitored and be capable ofreacting automatically to optimise power distribution. Informationprocessing makes this possible.Already used in medium-voltage industrial and public-distributionapplications, digital control and monitoring is now becoming a reality forlow-voltage installations as well.

Starting with an analysis of needs, this «Cahier Technique» takes a closelook at how LV power distribution can be managed. Particular emphasis isplaced on decentralising and distributing intelligence in and around the LVswitchboard. Several examples of such installations are also provided.

Contents

1 Control and monitoring needs 1.1 Introduction p. 41.2 Needs p. 4

1.3 Functions p. 7

2 Current solutions 2.1 Currently used solutions p. 9

2.2 Advantages and disavantages of these solutions p. 10

3 Intelligent switchboards 3.1 Définitions - decentralised architecture and distributedintelligence p. 12

3.2 Decentralisation of functions in an electrical installation p. 13

3.3 Advantages of decentralised architecture and distributedprocessing p. 17

3.4 Conclusion on decentralised processing in a LV switchboard p. 18

3.5 A switchboard bus suited to electrical applications p. 20

4 Implementation examples 4.1 Computer centre p. 23

4.2 Hospital p. 24

5 Conclusion and prospects for the future p. 27

Bibliography p. 28


1 Control and monitoring needs

1.1 Introduction

For whatever type of application, whether inoffice buildings, banks, hospitals, supermarkets,airports, tunnels or industrial sites, the need tomonitor and control electrical installations isincreasingly prevalent, to ensure the following:c safety,c availability of power,c optimisation of energy consumption and costs(depending on the energy supplier's tariffschedules),c reduction in operating and maintenance costs;c ease of operation,c maintainability and upgradeability of theelectrical installation.

Power management can today be implementedby a Digital Control System (DCS) designed tomeet all the above needs.

Power management may be combined with themanagement of other facilities:c building management (access control, air-conditioning and heating, anti-intrusion systems,lighting …),

c digital control and monitoring of industrialprocesses.

Due to the wide range of needs and significanttechnological progress over the last few years, anumber of solutions are today available whendesigning systems to monitor and controlelectrical installations. It is now possible to arriveat a judicious balance between needs and thecorresponding solutions through the use ofdigital communications buses and theintegration of microprocessors in electricalequipment.

«Cahier Technique» n° 156 explains how todesign the power section of an electricalswitchboard so that it satisfies needs concerningdependability.

The goal of this document is to discuss theoptimised design of power-management systemsin LV electrical installations.

The first step is to review the needs expressedby users and operators.

1.2 Needs

The needs of users and operators of electricalinstallations are different, depending on whetherthe building is intended for commercial, industrialor infrastructural purposes. A hierarchy of needsmay be established (see fig. 1 ).For example, in a small office building, the costof energy and ease of use of systems by non-specialists are the foremost criteria. On the other

Comfort

Costs

Availability

Safety

Source management

Tariff schedule management

Automatic source changeover

Protection of persons

Examples:

hand, in a hospital or a factory implementing anindustrial process, the most important need iscontinuity of service.

Safety of life and propertyAn electrical installation must distribute electricalpower while ensuring the safety of life and property.A power-management system does not replace theprimary protective functions (reflex-type devices).

fig. 1 : hierarchy of needs in LV electrical.


Insulationresistance

Time

InterventionPeriodic drop in insulation,steadily worsening.

First fault alarm threshold

Given its capacity to communicate as well as storeand process data, it simply assists the operator byindicating the settings of protective devices, thetype of fault that caused a device to trip and thestatus of the installation prior to the incident, etc.

Power management can, however, include overallprotective functions. For example, on IT systems,insulation-monitoring may be implemented towarn the operator of a first fault. It is then possibleto identify and clear the fault without any break inthe continuity of service (see fig. 2 ).

Technological advances have made it possible foroperators to reduce the duration of a fault in aninstallation, thus reducing the probability of asecond fault occurring. Operators can check atany time the insulation measurements at differentpoints in the installation and even the evolution ofthe insulation measurements over time. Preventivemaintenance therefore becomes a real possibility.The insulation monitoring function is autonomousand may be considered a decentralised function inthe framework of a power-management system.

AvailabilityEach field of activity has its own requirementsconcerning continuity of service:c in hospitals, operating rooms and reanimationcentres are designed to provide a high level ofdependability,c in commercial buildings, the widespread use ofcomputer systems has led many people to useuninterruptible power supplies (UPS) installedeither locally for individual machines or morecentrally for the supply of entire installations withhigh-quality power,

c in industry, power failures result in productionlosses. For example, a ten-minute power outagein a Danone factory results in a production lossof 20 000 cups of yoghurt.

The need to ensure the availability of power hasled to a number of technological choices forequipment (withdrawable or disconnectabledevices or switchboard units, switchboard forms,etc.) and to the distinction in electricalinstallations between uninterruptible, high priorityand low priority circuits, with different choices forthe system earthing arrangement.In this context, the job of a LV electricalswitchboard is to manage the sources. To beeffective, action taken when a problem occursmust be automatic and immediate.Managing power failures is one function ofpower-management systems.

Energy costsA constant concern for all companies is the needto reduce the cost of energy. Reductions may beachieved by working on two different factors, thelevel of consumption and the pricing system of theenergy supplier. To that end, in-depth knowledgeis required on daily and seasonal fluctuations, aswell as on power and consumption levels.A measuring system providing digital data foruse on a supervision screen is required tomonitor and analyse the above elements.It is then possible to:v undertake action to improve the situation,v check the effects of the action taken,v determine energy costs per workshop,department, etc.

fig. 2 : insulation monitoring for an outgoing circuit (IT systems).


c Reduce consumptionThere are numerous possibilities, depending onthe type of application:v turn off lighting and reduce heating inunoccupied rooms,v use motors equipped with variable-speeddrives for industrial applications,v use conditioners and/or filters to reduce lossesdue to harmonic and capacitive currents incables and transformers.c Reduce costs related to the terms of thecontract with the energy supplierv use capacitor banks to avoid being billed forreactive power,v smoothing peaks in consumption to reduce thesubscribed power and avoid overrun penalties.An intelligent system is capable of makingoptimum use of the subscribed power byshedding certain loads, smoothing peaks andalternating the supply of power to high-inertialoads.v Select the best available contract and programproduction cycles requiring particularly highquantities of electrical power for periods whenthe cost of power is low. These periods may be apart of the day, the season or the year. Forexample, certain contracts offer attractive pricesif the subscriber accepts to reduce hisconsumption on a certain number of peak daysper year.v Use replacement sources. This solution makesits possible not only to have a backup source ofpower in the event of a failure, but also tosmooth peaks in consumption and to avoidmoments when the power costs are highest.Managing consumption and energy costs isanother function of power-management systems.

Operating easeCertain installations are managed remotely,either from a control and monitoring stationinside the building or from a centre coveringseveral sites (remote supervision).

Centralisation of management functions is ameans to optimise human resources and improvethe working conditions for personnel through theuse of ergonomic computerised systems andautomatic execution of repetitive tasks(programmed operating times for air-conditioningor heating of offices, etc.).

Another consideration is the fact that in officebuildings and on small industrial sites, thepersonnel in charge of the installation isincreasingly a non-specialist. The electricalswitchboard is commonly under the responsibility ofthe building watchman or a receptionist. To ensureeffective operation as well as for safety reasons,the information presented to these persons mustbe in the form of a man/switchboard interface thatis as ergonomic and simple as possible.

Operating ease is achieved by an electricalinstallation that is as autonomous as possible(self managed).

MaintainabilityThe primary mission of the electricalmaintenance department in a company is to keepthe electrical installation up and running.

There are two types of servicing:c corrective action following an operating fault;c periodic preventive action.

Maintenance may be enhanced in two ways:c by stressing preventive rather than correctiveaction to avoid breaks in the continuity of service,c for preventive level, by stressing conditionalmaintenance, i.e. action taking into accountmonitored data, rather than systematicmaintenance. The more maintenance is preventiveand based on monitored information, the higherthe availability of the installation (see fig. 3 ).

Depending on the type of application, the timerequired to begin servicing and the duration maybe very different. They may be very short forindustrial processes if there is on-site

Corrective

Systematicpreventive

Conditionalpreventive

Type of maintenance

Availability

fig. 3 : operational availability as a function of the type of maintenance.


maintenance personnel and a stock of spareparts. On the other hand, they may be muchlonger in office buildings if an outside companymust be called in and the spare parts ordered.

The time required to service an installationalways depends on the information available:c when troubleshooting, precise and rapidlyavailable information on the problem and data oninstallation operating parameters prior to the faultare critical to making the right analysis andpreparing the subsequent work (new parts),c when undertaking preventive maintenance, in-depth information on the installation statusmakes it possible to intelligently select thoseelements most requiring servicing.

To carry out effective maintenance, personnelmust have relevant information concerning theinstallation status.Providing information for maintenance is one ofthe advantages of power-management systems.

Upgradeability

The points in an electrical installation that aremost subject to change are those closest to thefinal loads. In a factory, the electricalswitchboard may be upgraded to keep pace withchanges in production facilities. In officebuildings, changes in how rooms are used,increasing use of microcomputers, installation ofair-conditioning, etc., all result in modifications tothe electrical installation.

Improvements in availability and reductions inthe cost of the power consumed are alsoreasons for modifying electrical installations.

To correctly manage these changes, in-depthknowledge of the installation and operatingparameters is required.Power-management systems contribute to easyand effective installation upgrades.

1.3 Functions

Satisfaction of all the above needs by a power-management system requires that a number ofdevices be installed in the electrical system,generally speaking in order to:c carry out a number of automatic actions ,c provide the operator, either locally or remotely,with the information needed to plan ahead andcarry out the required work on the installation.

These devices provide a number of functions,not all of which are required in a giveninstallation.

Automatic-control functions

c Source management. Loads are supplieddepending on the availability of power on thedifferent incomers (source changeover systems,normal and replacement sources, engine-generator sets, etc.).c Load shedding. Only priority loads are suppliedwith power when demand exceeds the availablelevel of power on the incomers (for example, whenpower is supplied by an engine-generator set).c Time management. To reduce consumption.c Tariff schedule management. Installationoperation is organised to respect the terms of thecontract signed with the power distributor(smoothing of peak power levels, special tariffs,etc.).c Protection of the electrical distribution system.In large industrial installations, systemdisturbances (transient voltage drops) may, dueto the presence of large motors, provoke transientinstability phenomena. This function ensures thenecessary load shedding to avoid collapse of theentire electrical distribution system.c Power-factor correction. This functionmanages the switching of capacitor banks.c Switchboard safety (over-temperature, internalarcing, etc.).

c Insulation monitoring and fault locating for ITsystems.

Information to plan and take actionThe purpose of the functions presented above isto make the main LV switchboard autonomous. Itis then capable of reacting to various situationsto ensure continuity of service and optimaloperating conditions.

The second major type of function in anintelligent switchboard is the capacity tocommunicate information for planning and takingaction.Information includes:c the status of breaking devices (open orclosed),c measurements (U, I, P, cos ϕ),c the settings of protective devices.

These functions require links to:c a power-management system at a higher level,in charge of managing the entire LV or MVinstallation,c a local or remote control and monitoringstation,c where applicable, secondary switchboards,c where applicable, a process-control system.

Before the operator can be informed and takeaction (manually reconfigure the distributionsystem, maintenance, comfort), the electricalswitchboard must first communicate with ahigher-level system that can be consulted by theelectrician and the person in charge of monitoringoperation of facilities in the building or factory.

During normal operation, an intelligent(i.e. communicating) LV switchboard is useful inpiloting and managing the electrical installation,but it is all the more so when «planning andaction» are required in a fault situation.


This is because corrective maintenance ismore effective if each of the persons involved israpidly provided with the relevant information.Below is an example of a circuit breaker,communicating via the intelligentLV switchboard, in the event of a fault:c on the circuit breaker, the information isprovided by an mechanical indicator.c near the circuit breaker, a red light identifiesthe device that has tripped.c on a screen at the head of the switchboard, amessage in clear text is displayed «10:32:23 -outgoer to Lift 2 - section B - position 12b -tripping due to short-circuit».c on the supervisor screen of the electricalmanager, the same message.

c on the main supervisor screen (e.g. in thesecurity room), a message in clear text is displayed«10:32:23 - Lift 2 out of order due to an electricalfault - call the electrical department on line 347».

Note that planning and taking action also relateto preventive maintenance if the following areavailable:c information on the protection and controlswitchgear in the LV switchboards. Thisinformation may be provided by a counter for thenumber of times a device has opened or closed,a maintenance indicator derived from data suchas the sum total of currents interrupted, etc…c information on the electrical installation, forexample, the number of hours the supplied loadsoperate, drops in insulation…


2 Current solutions

2.1 Currently used solutions

The functions presented in chapter 1 are alreadyavailable, either in whole or in part, using anumber of different technical solutions:c in industry, by adding electrical managementto the facilities already implemented forindustrial-process control,c in commercial buildings, by including electrical-distribution management systems in the existingbuilding-management systems.Consider the solutions implemented for a motorcontrol centre (MCC) or a main low-voltageswitchboard (MLVS).

Solution implementing PLCsc PLCs and wired connectionsThe first step toward an intelligent switchboardinvolved the use of industrial PLCs(Programmable Logic Controllers) near theswitchboard.The PLCs serve as interfaces between theswitchboard and the technical managementsystem and are capable of carrying out certainautomatic-control functions.Made up of racks filled with input/output boards,the PLCs are wired to the various sensors andactuators of an electrical switchboard.A specialist is required to program the PLCs andeach application is the result of a specificdevelopment.

This type of solution is subject to the followinglimits and constraints:v great quantities of control wires between theswitchboard and the PLC, with the followingdisadvantages:- very high wiring costs;- a large number of terminal blocks whichincrease the volume and notably the footprint ofthe switchboard;- high risk of latent defects due to the manyconnection points;- risk of malfunction due to the very strongmagnetic field created by a short-circuit on anoutgoer;v significantly reduced capacity for installationupgrading, due to the very specific nature ofthe PLC programming which can rarely bemodified by the in-house electrical department,v a data-processing system poorly suited to thegiven applications in that the main task of a PLCis to continuously poll the status of devices whichin this case often remain in the same positionyear round.

c PLCs and remote input/outputsIn the past few years, PLC manufacturers havetaken advantage of dropping costs in

microelectronics and communications buses anddeveloped remote input/output modules, thusmaking it possible to reduce the quantity andcost of wiring.This solution has been put to very little use in thefield of electrical switchboards because it ispoorly suited to the constraints inherent in thefield, notably the thermal environment, electro-magnetic disturbances, the need to controlswitchgear locally, etc.

Solution implementing automatedswitchboards

In the 1980's, a number of offerings weredeveloped by the major panel builders forapplications in continuous-process industries orin large commercial buildings.

These offerings differ from the solutionspresented above in two aspects:c the development of specialised modules wiredto the switchgear components andcommunicating via a parallel link or a serial buswith a PLC installed at the head of theswitchboard. These modules, designed for useexclusively in switchboards produced by specificpanel builders, are installed on the front panel ofthe switchboard and include built-in local controland status-indication functions.c the development of repetitive functions for theelectrical automatic-control systems. Forexample, source changeover systems with loadshedding and reconnection of outgoers.

These systems are characterised bydecentralised data processing in the switchboardand the fact that the functions can be handled byelectricians. What is more, they contribute to themassive reduction in the quantity of wiring insidethe switchboard.

The limited success of this type of system is dueto the fact that these modules were specific tothe different panel builders.

Communicating components

Microprocessors are now used by manufacturersof electrical equipment to:c improve the performance of their products. Agood example is the widespread use ofelectronic trip units in circuit breakers. The latterare increasingly capable of communicating thedata they process via digital buses.c enhance their offering with new functions, forexample, power and energy measurements at a


given point in an installation, with the capacity tocommunicate the data.In parallel, automatic-control functions such assource changeovers or reactive power regulationcontinue with their own developments.

The increasing use of such products andmodules in electrical switchboards has resultedin a considerable increase in the quantity ofinformation that can be transmitted to a

centralised supervision system. In large-scaleapplications, the engineering firm in charge ofthe entire installation is still obliged toimplement a complex communicationsarchitecture with intermediate levels fulfilling adual mission:c sorting and analysing the available information,c providing communications gateways betweendifferent buses.

2.2 Advantages and disadvantages of these solutions

The three types of solution presented abovewere developed over the last ten years to satisfysome of the needs listed in chapter 1.Their advantages and disadvantages aresummed up in figure 4 .

In conclusion, the following may be observedconcerning the currently implemented solutions:c a tendency, well underway, towarddecentralisation of automatic-control functionsand data processing for electrical switchboards,c a hierarchical structure for data flow,c the need for specific development work and,consequently, for specialists.

In the electrical-switchboard field,decentralisation of data processing has beenmade possible by digital communications buses.This is the case for:c industrial-process control and monitoring.PLCs with hundreds of input/outputs first gaveway to PLCs with remote input/outputs and arenow gradually being replaced by networksof PLCs and micro-PLCs positioned as close aspossible to the controlled sensors or actuators.The near future will see networks comprising«intelligent» sensors and actuators,

c building management. Functions have nowbeen standardised and are carried out byspecific products well suited to needs. Solutionsproviding industrial-process management,building management and electrical-distributionmanagement are now dedicated systems builtaround decentralised architectures thatincreasingly incorporate distributed intelligence(see fig. 5 ).

A notable aspect of low-voltage electricalswitchboards is their great diversity and the widerange of functions that they must provide. On thebasis of existing solutions described in thischapter and that have already been put to use, itis today possible to determine a number ofprinciples that define and specify what isunderstood by the term «intelligent switchboard»and the corresponding control and monitoringsystem.c An «intelligent switchboard» is defined by itscapacity to autonomously carry out the functionsassigned to it and to fit into the control system ofan electrical installation.c To handle the wide variety of installations, thedesign of the switchboard must be based on thefollowing principles:

fig. 4 : advantages and disadvantages of traditional solutions in meeting control and monitoring needs.

PLC-based Automated Communicatingsolutions switchboards components

Weak points Specific to each application, Solution available only Single-function solutionslittle capacity for upgrading to large panel builders

Large quantity of wiring Requires PLC specialists Increase in the numberof modules

Requires PLC specialists Requires electriciansspecialised incommunications

Strong points Reliable equipment, Implementation of the Reliable field-busused on large decentralisation concept technologyindustrial sites

Solutions perfectly Functions may be mastered Industrial productstailored to the by an electrician capable of withstandinginitial needs of each EMC constraints incustomer switchboards

Functions standardisedwith progression ofprojects


v design must be modular in terms of bothhardware and software,v decentralisation of processing must be optimised,v functions and products must be standardised,designed for installation and implementation byan electrician and capable of handling the severeenvironmental conditions prevalent in anelectrical switchboard.

c For users confronted with automation needsand the necessity of obtaining relevantinformation, an intelligent switchboard provides:v an enhanced level of dependability andupgradeability,v standardised solutions, even for large sites,v cost reductions and simplified implementation,v easier operation and maintenance.

Building-managementstation

Electrical-distributionmanagement station

Industrial-processmanagement station

Intrusion

Fireprotection

Personnelpresence

Air-conditioning

Processing units Processing units

MV distributionsystem

LV switchboard

Engine-generator sets

UPSs

PLCs

Companymanagement

Workshop

MCC

Machines

Variable-speed drive, starter

fig. 5 : decentralised architecture for building management, electrical-distribution management and industrial-process management systems.


3 Intelligent switchboards

3.1 Definitions - decentralised architecture and distributed intelligence

c An analogy based on how companies areorganised may be useful in understanding thesetwo terms.In centralised organisations, all decisions aremade by the «boss». Subordinates provide himwith all information and wait for orders.In an effectively decentralised organisation, amajority of decisions are delegated by the bossto the subordinates. Each person, within thelimits of the delegated powers, actsautonomously and reports only the necessaryinformation back to the boss. Only thosefunctions concerning the entire company arecentralised, for example, the payroll.

possible manner, whether for an entire installation(power management), for a low-voltageswitchboard or for a given outgoing circuit.Then the criteria determining the selection of aninternal communications bus forthe LV switchboard, suited to the given needs,will be examined.

Finally, a given function may be distributedbetween a number of subordinates. This form oforganisation implies information exchange and acertain degree of autonomy for the team incharge of the function.

c Figure 6 shows how a function may be:v totally decentralised,v partially decentralised, whereby execution ofthe function is decentralised, but parametersettings remain centralised and common to anumber of functions,v distributed among equipment on the samehierarchical level.

fig. 6 : possibilities ranging from a fully centralised to a decentralised system with distributed intelligence.

Intelligent switchboards, as defined in thepreceding chapter, are based on the concept ofdecentralised architecture with distributedintelligence.After defining these terms, we will go into how thevarious functions of an electrical installation maybe decentralised and distributed in the best

Solution 2.Centralised processing,decentralised acquisition.

F1 F2

F4 F5

F3

Solution 3.Function F3 and F4 decentralised.

F1 F2

F5

F4F3

Solution 4.Distributed processingof functions F2 and F5.

F4 F5F3

F1

F2

Solution 1. Centralised system(acquisition and processing).

F1 F2

F4 F5

F3


3.2 Decentralisation of functions in an electrical installation

Energy contract managementThis type of function requires an overall view ofthe installation.

In many cases (small and mid-sized installations),the LV switchboard is the central element in theinstallation. If this is the case, the contract-management function is handled by the LVswitchboard central unit, with either local or

remote (from a supervision station) parametersettings. On a large site (medium-voltagedistribution system), switchboards receiveoperating orders from a higher-level system.

Time management of outgoers

In centralised systems, this function is traditionallyassigned to the supervision station which can be

c Decentralisation as implemented in a companymay be applied in a similar manner to the controland monitoring system of an electricalinstallation. The Centralised Technical-Management (CTM) concept is now giving wayto the decentralised power-management conceptwith distributed processing.

Note that high-power distribution systems(architecture and protection) follows thesame principles, thus ensuring coherencebetween high- and low-current systems(see fig. 7 ).Below are examples of these concepts appliedto different electrical functions.

Power-supply substation

Operator can monitor, issue orders and set parameters

Switchboard control and monitoring

Autonomous functions for measurements and control and monitoring of outgoers

Configurationconsole

MLVS central unit

LV switchboard

G

VAR

VAR

Supervision station

fig. 7 : small to mid-sized installation with a control and monitoring system (power management), of which the major part is located in the MLVS.


Outgoer 1 Outgoer 2

Q1 Q2

MLVS 1

Non-intelligent outgoers

Outgoer 4Outgoer 3

Q3 Q4

MLVS 2

Intelligent outgoers (equipped with internal clock and memory), capable of handling part of the function

Operating-time parametersset by operator

Execute orders

Transmit parameters

Store parametersIssue and execute orders

Power-management systemCentralised system

Set operating-time parametersStore parametersIssue orders

Transmit orders and acknowledgements

used to set operating-time parameters for outgoersand to issue opening and closing orders for devices.On the other hand, in a decentralised power-management system, these commands areexecuted at the level of the switchboard centralunit or even at each device. A device mustsimply receive the operating set-points and beequipped with an internal clock that is regularlysynchronised by the supervisor.

In figure 8 , the information flow is shown for atraditional centralised solution and adecentralised solution. It is clear that thepermanent information flow is reduced asdecentralisation is increased. On the other hand,new data exchanges, much more limited inscope, are required to periodically synchronisethe various internal clocks and transmit newoperating set-points.

fig. 8 : time management of outgoers, in a centralised system (CTM) and in a decentralised and distributed system (power management).


Source management

This function opens or closes the incomingcircuit breakers in the switchboard, depending onthe data processed either in the switchboard orin the immediate proximity. It is thereforeperfectly logical that the operations required forthis function be carried out in the electricalswitchboard. Note that in relay-based systems,the relay sub-assemblies were installed in theswitchboard and the diagrams were drawn up bythe panel builders. It was only when a newtechnique arrived, one that most panel builderscould not handle, that this processing wasremoted to a centralised PLC.If the incoming diagram is simple, for examplewith a normal and replacement source, thisfunction is totally decentralised and is carried outby an autonomous standard product. If theincoming diagram is more complex or requiresprogrammable shedding of outgoers, thefunction is located at the level of the switchboardcentral unit:c if the replacement source supplies the main LVswitchboard alone, a switchboard central unit willcarry out the function autonomously (see fig. 9 ),c on the other hand, if the replacement sourcesupplies the MV system and/or several MLVSs,

GE

Presence Un

Presence Ur

Generator-start order

main LV switchboard central unit

this function is distributed betweenthe MV switchboard central unit and the centralunits of the various LV switchboards.

Reactive power regulation

Power-factor correction using capacitor banks isan independent automatic-control function builtinto a product called a reactive power regulator.This type of regulator must operateautonomously in over 90% of all installations.A communicating reactive power regulator canbe built into a power-management system toprovide the following additional useful functions:c setting of parameters from a supervisionstation,c action on alarms processed by the switchboardcentral unit,c action on maintenance information in theframework of overall switchboard maintenance,c coordination of the reactive power regulationfunction with other switchboard functions. Forexample, during operation on an engine-generator set, the capacitors must bedisconnected. This can be carried out byopening a circuit breaker upstream from thecapacitor banks or by transmitting a shutdownorder to the regulator if it is connected by bus to

fig. 9 : example of source management. With this solution, the switchboard central unit manages the outgoers. Priority outgoers areprogressively reconnected during operation on an engine-generator set. Definition of outgoers as priority or non-priority is adjustable. Note that itis not necessary to separate the busbars into two parts, thus eliminating the coupling device. Finally, this solution makes it possible to handlemultiple-incomer diagrams with great ease.


the switchboard central unit managing ormonitoring source changeovers.

Threshold-initiated load shedding

In certain situations (voltage drop due to aproblem on the distribution system, failure of asource, demand exceeding the available powerfrom a source supplying the switchboard, etc.), itmay be necessary to rapidly shed a group ofnon-priority outgoers, for example to avoidtransient-stability problems.Figure 10 shows how shedding of non-priorityoutgoers is processed in a decentralisedmanner, following an overload on themain LV switchboard.This example shows that the amount ofinformation exchanged is very small. The centralunit receives a signal, issues an order via the busand the concerned circuit breakers carry out theorder.

Switchboardcentral unit

Q2Q1 Q3

M

Parameter setting of threshold and priority (P)non-priority (NP) status for each outgoer

Store threshold parameterDecide: if measured power > threshold, send load-shedding order

Measure power

Store P/NP parameterDecide: if parameter = NP, open outgoer on receiving load-shedding orderControl the circuit breakers

Management of an incomer or an outgoer

Management of an incomer (or an outgoer)may include some or all of the followingfunctions:c control and monitoring (control of the deviceand monitoring of its status),c measurements (currents, power levels, energydrawn, etc. ),c local or remote operator interface,c communication with the switchboard centralunit.By distributing these functions among differentmodules (see fig. 11 ), it is possible to solvecertain problems:c not all the outgoers require all the functionslisted above,c the operator interface can be remoted,c the interface must be adaptable to thevarious users (language, level of competence,etc.).

fig. 10 : example of decentralised processing of a load-shedding order.


3.3 Advantages of decentralised architecture and distributed processing

Mastering complexityA complex problem can often be broken down intoa set of simple basic problems. Similarly,controlling an electrical installation may prove verycomplicated given the size of the installation andthe number of functions that must be processed.By decentralising a majority of the functions, mostof the processing can be carried out by smallerunits. The processing is then easier to handle andcan be standardised. The concept of type-testedassemblies, already used for the power part ofelectrical switchboards, can now be expanded toinclude power-management functions. The loadon the higher-level processing unit is significantlylightened and it can devote its processing powerto the tasks for which it is specifically intended.

Technical and economical constraintsAs already mentioned in the preceding chapter,the considerable increase in the quantity ofinformation to be transmitted has led to thedevelopment of hierarchical architectures. Justas in large installations where there are levels inthe power-distribution structure (main andsecondary LV switchboards, final distributionenclosures, etc.), the creation of levels for theprocessing of information is the best solution:c constraints (response times, environment,throughputs, etc.) are not the same inside aswitchboard and throughout an entireinstallation,c all the information that is useful for a functionat a given level is not necessarily relevant on a

Module M1 controls the device and reads its status conditions.Module M2 measures the currents and voltages and prepares the power and energy information.Module M3 displays the status and measurement information for the outgoer on the front panel of the switchboard. It can also control the outgoer.Module M4, identical to module M3, is a display remoted to a point outside the electrical room. The information may bedisplayed in a manner different than on M3

Control &Monitoring

M1

Measurement

M2

Localdisplay

M3

Outgoer 3 closedI1 = 125 A E = 327 kVAh

M4

Switchboardcentral unit

fig. 11 : example of distributed processing for outgoer management.


3.4 Conclusion on decentralised processing in a LV switchboard

higher level, for example, all the informationavailable locally on each device is notnecessarily of great use to the operator:v some information is intended strictly formaintenance purposes,v other information must be summarised to avoidsubmerging the operator (see fig. 12 ),v the cost of programming is reduced by usingstandard «codes» for a vast majority of thefunctions.

Continuity of service

In a centralised system, a breakdown results inan interruption of service for the entireinstallation. In a decentralised system, however,the same breakdown can be limited to the single

subsystem where it occurred, thus enabling therest of the installation to continue operating,though perhaps in a downgraded operatingmode. For example, if maintenance is requiredon the switchboard central unit, local functionsinside the switchboard remain operational, giventhe decentralised nature of the installation.

Maintainability

A decentralised system implements a largenumber of processing units, however, theirfailure rate is not cumulative.The limited number of connection points reducesthe number of breakdowns.The self-test system on the digital products andthe communications buses can detect nearly100% of possible breakdowns.

Flexible implementation

c Setting up a new site often takes place overrelatively long periods. It is not uncommon thatfor budgetary reasons, the remote supervisionstation is installed one or two years aftercommissioning of the switchboards. The lattercan, nonetheless, operate autonomously oversuch long periods if decentralised processing iscarried out locally.c When existing installations must be renovated,upgrading can be spread out over several years.Decentralisation makes the replacement of aswitchboard simpler. The new switchboard canbe factory tested and a single serial link is all thatis required to connect the new switchboard to thecontrol system.

The examples presented in chapter 3 show thatthe functions managed by an intelligentLV switchboard can be distributed to varyingdegrees among different processing units.c Certain functions are handled by theswitchboard central unit when:v the processing is complex and cannot becarried out by a standard autonomous module.For example, source management when thereare multiple incomers,v the functions call on processing that iscommon to other functions. For example, asource changeover can be caused by the failureof the normal source or by an order issued by acontract-management function,v the functions must be coordinated with otherequipment. For example, when management ofreplacement sources brings MV equipment intoplay.c Certain autonomous functions can be carriedout by dedicated products that have beenoptimised for the given function. This is the casefor reactive power regulators and sourcechangeover units.

When these autonomous products areincorporated in an intelligent switchboard, they canbe connected via a bus to the switchboard centralunit which provides additional functions such as:v setting of parameters for the products by amore user-friendly device that is common to allthe functions carried out in the switchboard,v minimum management during downgradedoperating modes,v incorporation of the products in predictive- andcorrective-maintenance functions.

The various functions mentioned in thisdocument may be included in the architecturepresented in figure 13 :c The switchboard central unit is in charge of:v processing the general switchboard functionsand the interdependent functions,v coordinating the functions managed by lower-level modules,v integrating in a higher-level control system,v communicating with a terminal intended for theelectrician in charge of implementation andmaintenance operations. During servicing, thisterminal is connected locally in front of the

fig. 12 : example of sorting information to be madeavailable to different users.

Information UserMaintenance Supervisionroom station

Device position X XFaulty outgoer X XEnergy measurement XOutgoer not available (summary) Xv Disconnected/locked out Xv Not supplied XTrip unit setting XLoad shedding in progress X X


switchboard. In no way does it replace thecentralised supervision station from which theoperator manages the installation.c Certain modules are in charge of processingautonomous functions (reactive powerregulation, insulation monitoring, etc.).c Other modules are in charge of managing anincomer or an outgoer.

Modularity makes it possible to:v integrate control and monitoring in the conceptof tested, standardised functional units,v standardise connections between the moduleand the switchgear, thus reducing the risks ofbreakdowns due to faulty connections,v take action on a given outgoer without shuttingdown other elements in the switchboard, in theevent of a breakdown or installation upgrading.

The modules and the switchboard central unitare connected via a digital bus. Note that use ofthis type of bus offers a wide range ofadvantages:c massive reductions in the quantity of controlwires in the switchboard and consequently in thecost of wiring and the space required,c reduced risks of breakdowns due to faultyconnectionsc less design and wiring time for the panelbuilder,c greater installation upgradeability, for example,the addition of outgoers or functions in anexisting installation.

The next chapter describes the types of busesbest suited to power managementapplications.

Switchboardcentral

unit

Central functions

Autonomous functions

Local functions

Installation supervision station

Source management with load shedding / reconnection

Generator managementReactive power regulationUPSsInsulation monitoring

Outgoer control and monitoring and measurements

Secondaryswitchboards

Configuration

fig. 13 : distribution of functions in the architecture of a switchboard.


3.5 A switchboard bus suited to electrical applications

Before selecting a suitable field bus, it is essentialto fully comprehend the constraints weighing onan electrical application, notably the informationflow, response times, the environment, etc.

Characteristics of an electrical applicationc A naturally stable and continuous applicationThe purpose of an electrical installation is todistribute power to each load. The purpose ofan LV switchboard is therefore to permanentlymaintain the operational status of outgoers.Opening of a device may be related to one of thefollowing events:v reaction of a protective device to an electricalfault,v operator intervention to isolate a circuit (forservicing, to turn off the lights on a floor at theend of the day, etc.),v intervention of an automatic-control function toshed non-priority loads, for example, followingthe failure of the normal source.The change in status of a device is therefore anexceptional event. An electrical switchboard iscontinuously in a naturally stable state. Note thatcircuit breakers are by nature bistable devices.

c Certain situations cause an informationavalancheOn the other hand, certain situations may resultin an avalanche of information over very shorttime periods. For example, following the failureof the source supplying the switchboard, mono-stable devices such as contactorssimultaneously open and the automatic sourcechangeover and load-shedding functions issueorders to the circuit breakers.

c Limited real-time constraintsIn an electrical installation, the response time ofthe system to an event depends on the nature ofthe event:v when the operator issues an order from thesupervision station, the system must respondwithin an time delay that is «acceptable» to theoperator, i.e. one or two seconds betweenconfirmation of the order by the operator and thechange in status of the device displayed on thescreen,v for source-changeover automatic-controlfunctions, no specific constraints concerning theresponse time weigh on the application. The goalis simply to reduce to the strict minimum the timethat the loads are not supplied with power.Response times of several hundred millisecondsare perfectly reasonablev if, during operation on an engine-generator set,the rated output of the set is overrun, certainnon-priority loads must be shed. The authorisedoverload time is indicated by the manufacturer ofthe engine-generator set and depends on thelevel of the overload.In complex installations where local powergeneration facilities are coupled with the power

supplied by the utility, in the event of a utilityfailure, certain loads must be shed in a fractionof a second, before the engine-generator setprotective functions can react.

c Data flow capacity sized for the number ofmeasurementsElectrical measurements may result is a constantflow of information on the switchboard bus. Themost common measurements concern voltages,currents, power levels and quantities of energy.Sizing of the bus therefore depends not only onthe quantity of information that must betransmitted, but above all on how often theinformation must be transmitted:v measurement values for currents or powerlevels may be used by the operator to monitorthe distribution system in real time and thevalues may therefore have to be transmittedevery few seconds,v values concerning the quantity of energyconsumed are required only every few minutes,at most, i.e. the frequency of transmission forthese values is very low.

c Implementation constraints in an electricalswitchboardInstallation of a bus inside an electricalswitchboard must take into account the followingconstraints:v the bus must not be sensitive to the majorelectromagnetic disturbances that exist in a low-voltage switchboard,v it must be easy to install during wiring of theswitchboard and be easily modified duringswitchboard upgrades,v the cost of each connection point, which is adecisive element in selecting a bus in that a low-voltage switchboard comprises great numbers ofconnection points.

Master/slave protocols are inadequate

For the solutions discussed in chapter 2, master/slave protocols are commonly used. An exampleis ModBus (for further information, see «CahierTechnique» n° 147).

For a basic automated switchboard, i.e. one thatmanages only orders and acknowledgements, amaster/slave protocol is sufficient to satisfy therequired functions. For example, given aswitchboard with 50 incomers and outgoers anda polling time of 20 milliseconds for each one,approximately one second is required to poll allthe incomers and outgoers. When an eventoccurs (order from the supervisor or interventionof an automatic-control function in theswitchboard central unit), polling of the status ofeach incomer or outgoer can be interrupted tosend the necessary orders.

But when the system functions require thetransmission of measurement values, the


weaknesses of master/slave protocols, i.e. theincreased time required for each polling cycle,rapidly become apparent.

What is more, when a device-status changeoccurs following tripping, the information is madeavailable to the switchboard central unit onlyduring the next polling cycle.

Finally, this type of protocol is inadequate fordistributed processing because the central unitcan act as the master only if all the informationruns through it.

CSMA protocols

Contrary to protocols using the master/slaveaccess method, CSMA (Carrier Sense MultipleAccess) protocols allow the various stationsconnected to the bus to spontaneously transmitdata only when there is a real need.

c CSMA constraintsRandom access to the bus creates threeconstraints that do not exist in master/slavesystems which are, by definition, centralised.Solutions for these difficulties are easy toimplement.

v Risk of collision. Several connected stationsmay transmit data simultaneously. Rules are setup to avoid collision between the differentmessages.Two different solutions exist:- CSMA-CD (Collision Detection). Using thistechnique, the stations detect the interference on

Station 1

Station 2

Line status

Line monitoring by station 1

Line monitoring by station 2

15 V

0 V

OK OKOKOKOKOKOKOK

1 111 10 00

1 0 0 Stop transmitting

_ ErrorOK OK_ _

Station 1 pinches the line and short-circuits it.

Stations 1 and 2 pinches the line.

Station 1 pinches the line.Station 2 leaves the line at rest. It detects the short-circuit on the line, analyses a collision and stops transmitting.

Stations 1 and 2 leave the line at rest.

Station 1 leaves the line at rest.

Station 1 did not detect a collision and continues transmitting its message.

fig. 14 : collision analysis in the BatiBus system.

the bus due to the two simultaneous messagesand stop transmitting. Each station will thenattempt to retransmit its message as some latertime. This is the solution used by Ethernet- CSMA-CA (Collision Analysis). Using thistechnique, the station transmitting the messagewith the lowest priority level stops, thus allowingthe higher-priority level message continue.Management of priorities is based the coding ofthe frames transmitted. This is the solution usedby BatiBus (see fig. 14 ).

v Non-deterministic response times. Dependingon the information load on the bus, thetransmission time for a frame is not constant. It istherefore not possible to guarantee a maximumtransmission time using a protocol implementingthis type of access to the bus. However, acertain number of devices and design rulesmake it possible to ensure maximumtransmission times that are nearly 100% certain.For example, in the BatiBus system, commandsare priority messages. This is a means to avoidthe response-time constraint.

v Detection of faulty stations.In a system using the master/slave method ofaccess to the bus, the breakdown of a slave isdetected by the master during the next pollingcycle. But for protocols in which messages aretransmitted only when necessary, a faultymodule is not detected. Each application musttherefore develop and implement the necessarymonitoring devices required to periodically checkthe status of each module.


c Advantages of a CSMA bus for an electricalinstallationIn the preceding paragraphs, it was shown howthe constraints specific to CSMA buses may beavoided.The main advantages that may be gained fromthis type of bus are listed below:

v Optimised data exchange: a CSMA protocoloptimises data exchange because the bus is notclogged with continuous polling operations.Consequently, for a stable application, which isgenerally the case for an electrical installation,with the same transmission speeds as a master/slave protocol, the quantity of useful informationtransmitted is significantly increased andresponse times can even be shorter.

v Reduced costs: the greater the transmissionspeed on the bus, the greater the systemconstraints concerning protection againstelectromagnetic disturbances and, consequently,the greater the costs. A CSMA protocol makes itpossible to select slower transmission speedsand thus reduce transmission-related costs.

v Decentralised processing: a protocol offeringthis type of access to the bus makes foroptimised processing of decentralised and/ordistributed operations. The example in figure 15(decentralisation) shows how data exchange canbe simplified (opening orders to non-prioritycircuits) with respect to a centralised master/slave system.Note that for distributed processing, it is themeasurement module which directly transmitsthe opening order to the non-priority loads.Consequently, even if the switchboard centralunit has failed, load shedding remains possible.

Using FIP for MCC applicationsCertain industrial applications impose verysevere demands in terms of continuity of serviceand performance levels. For example, aguaranteed response time (deterministic) may berequired for an order issued by an automatic-control function managing the industrial process.This is the case for certain MCC (Motor ControlCentre) switchboards. Contrary to a mainLV switchboard, opening and closing orders fordevices are by no means exceptional.In this case, the performance levels offered bybuses implementing master/slave protocols arenot sufficient, unless very high transmissionspeeds are used, with the corresponding highcosts. Buses implementing random-accessprotocols are not up to the job either.

It was for this type of application that the FIP buswas designed by industrial companies andmanufacturers. It is not within the scope of thisdocument to present the FIP bus in detail,however, it should be noted that it combines theadvantages of both master/slave and random-access protocols:c access to the bus is controlled by a busmanager located in the switchboard central unit(LV switchboards),

Time

0 ms

10 ms

20 ms

30 ms

40 ms

Status

Status

Status

Status

Status

Commands

Commands

Commands

c Status information is sent as periodic variables every 10 milliseconds.

c Commands are sent as periodic messages every 20 milliseconds.

c The remaining available time (shown in grey) can be used to transmit parameters, measurements, diagnostics information, etc., as non-periodic messages.

fig. 15 : time diagram for an MCC implementing FIP.

c data may be periodically transmitted over thebus (orders and status information, for example),c stations may request permission from the busmanager to transmit information, as needed, forexample, in the event of a significant change inthe value of a measurement, etc,c the data issued by one station may be used byone or several other stations, for example, fordistributed processing,c finally, the protocol has a number of built-insystems that make it possible to guarantee avery high level of transmission dependability.

The FIP bus thus combines the advantages of:c master/slave protocols (deterministic andguaranteed response time),c random-access protocols (transmission ofuseful information or following an event).The FIP bus offers a high level of performanceand meets very severe dependabilityconstraints.


4 Implementation examples

4.1 Computer centre

NeedsIn a computer centre operating 24 hours per day,the primary concern of the electrical manager isto ensure continuous availability of power andfast response when maintenance is required.In addition to these basic needs, the customermay want to reduce his energy bill by:c improving the power factor;c taking advantage of special tariffs with theutility, for example by agreeing to sharply reduceconsumption on peak-demand days uponreception of a special signal.

Surveillance room

VAR

MLVS

G

1 000 kVA20 kV /230 - 400 V

550 kVA

UPSs

MV cubicle

A A A A

A A

D

B

C

C

F

A

VAR

E

fig. 16 : solution implemented for a computer centre.

Through these two modifications, the return oninvestment for the installation drops to less thanthree years due to the sharp decrease inelectricity bills.

Implemented solutionc Electrical installationThe electrical installation is supplied by a 20 kVmedium-voltage loop. The MV loop supplies a1 000 kVA transformer which in turn supplies amain LV switchboard (see fig. 16 ).The main LV switchboard is made up of


withdrawable units. It supplies 23 outgoers,including two reserve outgoers. The high-poweroutgoers are equipped with motor mechanismsfor remote-control purposes. The computers areprotected by two UPSs set up in a redundantconfiguration. A 550 kVA engine-generator setcan replace the power supply for all theelectrical equipment in the computer centre.

c Required functions

v Source changeover. In the event of a voltageloss downstream of the MV/LV transformer (or ifa load reduction signal is received from theutility), the switchboard is automatically suppliedby the replacement source. The cabinetmanaging the engine-generator set receives theshutdown and start orders from the main LVswitchboard and autonomously manages theengine-generator set.When the load is transferred to the engine-generator set, the high-power outgoers are shedto reduce the load step change during switching,and are then reconnected one after the otheraccording to an adjustable individual time delay.When utility power returns (or the special utilitysignal is discontinued), the switchboardautomatically transfers the load back to thenormal source and requests shutdown of theengine-generator set.

v Contract management. For special tariffcontracts, the power utility sends a loadreduction signal to the customer 30 minutesbefore actually shifting to the special mode. Thesignal, transmitted via the field bus, is decodedby a specific relay. On reception of the signal,the switchboard transfers the load to theengine-generator set, exactly as if a powerfailure had occurred. When the signal isdiscontinued, the load is automaticallytransferred back.

v Reactive power regulation. A 100 kVARcapacitor bank used to compensate for reactive

energy consumption is managed by a reactivepower regulator.

v Remote surveillance. In the event of anincident in the electrical installation, thewatchperson is immediately informed via asupervision console which transmits any alarmsissued by the main LV switchboard.

c The control and monitoring system

v Each incomer and outgoer in the main LVswitchboard is managed by a module (marked Ain the diagram), which:- acquires the position of the device (open,closed, tripped, withdrawn, etc.);- displays this status locally;- for the remote-controlled incomers andoutgoers, orders opening, closing or resetting.These orders may be given locally or sent via theswitchboard bus;- dialogues with the switchboard central unit viathe digital communications bus.

v The switchboard control unit (marked B in thediagram) located inside the main LV switchboardis in charge of:- managing the control and monitoring functionsfor the incomers and outgoers via the modulesmarked A in the diagram;- directly acquiring two elements of information,namely the presence of utility voltage or engine-generator set voltage (via voltage relays markedC in the diagram), and passage to the specialutility mode (special utility relay marked E in thediagram);- transmitting a start order to the cabinet (markedD in the diagram) managing the engine-generator set;- processing the source changeovers caused bya failure in utility power or reception of thespecial utility signal;- generating and transmitting any alarms to thesupervision console (marked F in the diagram)which then displays them in the appropriatemanner.

4.2 Hospital

NeedsIn a hospital, the continuity of electrical powerservice is critical. The example below deals witha mid-sized hospital.To provide optimum management of theelectrical distribution system and in compliancewith the expressed wishes of the operator:c Outgoers are divided into three categories,«backed-up» (by a generator set), «priority»(protected by a UPS) and «no-break» (protectedby a UPS and a generator set). Each incomerand outgoer is monitored and may be remotelycontrolled from the supervisor;c The entire installation is remotely supervised.

Implemented solutionc Electrical installation

v The electrical installation is supplied by a20 kV medium-voltage loop. The MV loopsupplies three 1 000 kVA transformers which inturn supply an LV distribution switchboard.v Two 400 kVA engine-generator sets can stepin to provide back-up power to certain electricalequipment in the hospital.

v Two UPSs supply the no-break and priorityoutgoers.

v The outgoers are grouped in three LV switch-boards. The diagram in figure 17 makes clear thesupply system for each outgoer in LV switchboard 1.

c Organisation of power management

v A supervision station (supervisor) may be usedby the operator to monitor the installation, issueorders and set parameters.


v The switchboard central unit in the LVdistribution switchboard:- provides control and monitoring of the circuitbreakers on incomers and outgoers;- checks operation of the reactive power relays(reactive-energy compensation) and stopscompensation if utility voltage is absent(operation on the engine-generator sets);- dialogues with the UPS processing units;- supplies the «transformers ON» information.v The switchboard central unit in the engine-generator set LV switchboard:- provides control and monitoring of the circuitbreakers;

- dialogues with the control cabinets of theengine-generator sets for monitoring andtransmission of ON and OFF orders;- dialogues with the LV switchboards (1, 2, 3 andno-break) which issue a starting order for theengine-generator sets and receive a set-pointindicating the maximum power availabledepending on the operating engine-generatorsets and the LV switchboards supplied withpower (all are not necessarily supplied duringmaintenance operations).

v The central units in the LV switchboards 1, 2and 3:- control and monitor the circuit breakers;

fig. 17 : solution implemented for a hospital.

Priority outgoers(mains / UPSs / mains)

No-break outgoers(mains / UPSs / generators)

Backed-up outgoers(mains / generators)

MV cubicle

20 kV line

MLVS 2 MLVS 3

MLVS 2 MLVS 320 kV / 230 - 400 V transformer

3 x 1 000 kVA

2 x 400 kVA

GE GE

MLVS 2 MLVS 3

MLVS 1

MLVS 2 MLVS 3

No-break MLVS

Gen. MLVS

Distribution MLVS

UPSs withautomatic

bypass


- provide the automatic source changeoverfunction for the backed-up outgoers on the basis ofthe information supplied by the voltage relays, i.e.:. load shedding of the major outgoers;. transmission of a start request for the engine-generator sets;. closing of the engine-generator set incomingcircuit breaker;- provide load regulation. Depending on thepriority rating of the backed-up outgoers, theyare shed and reconnected according to thepower supplied by one or both engine-generatorsets or by the transformers (1 and/or 2 and/or 3);- dialogue with the insulation monitor for the«no-break» outgoers.

v The central unit in the no-break LV switchboard:- controls and monitors the circuit breakers;- controls source changeovers, after requestingstarting of an engine-generator set if the UPSsignals a problem.

In this example, not all the available power-management functions are implemented (theynever are).c Time management is not implementedbecause a hospital operates 24 hours per day.c Contract management (smoothing of peaks,special utility modes) was not applicable, onlyreactive-energy compensation was implemented.c The power-management system set up isentirely dedicated to ensuring maximumavailability of electrical power.c Each switchboard received local andautonomous processing capacity to carry out itsassigned functions.c Very little event information circulates on thebus (automatic source changeover, engine-generator sets, load regulation) and nomeasurement values except for metering valuesin the MV switchboard.Status checks are run periodically.


5 Conclusion and prospects for the future

Intelligent switchboards, a critical element inelectrical distribution systems, provide solutionsmeeting the needs of managers and operators ofelectrical installations, notably:c energy savings;c dependability;c remote control of the installation (with possibleextensions to building management forcommercial applications and processmanagement in industry);c installation maintainability and upgradeability;c gradual evolution of the installation over timetoward greater intelligence.

The construction of switchboards with integratedmanagement functions, but with decentralised anddistributed intelligence, is today made much easierdue to the existence of standardised modules,equipment and software that will remain availableover long periods. In this sense, control andmonitoring can now be implemented usingconcepts similar to those of type-tested assembliesand shows sharp differences with respect toautomatic-control functions for industrial processes.

Integration of intelligence in switchboards hasmade it possible to:c simplify switchboard and electrical-installationarchitecture, during the initial design process andlater during upgrades (distributed electricaldistribution, elimination of half busbararrangements, discrimination, knowledge ofswitchboard reserves, management of operatingconditions at switchboard maximum limits(temperature, overloads, etc.)):c manage the switchboard over time (black-boxfunction, up-to-date diagram file, etc.);c combine communications functions (lowcurrents) with power functions (high currents).

In the near future, communication andprocessing will continue even furtherdownstream to the individual devices, sensorsand actuators. This will make the distribution ofintelligence easier and thus further reducecentralisation. Considerable advances may beexpected in the fields of design, wiring,installation, operation, dependability andupgradeability.


Bibliography

Schneider Cahier Technique publicationsc Introduction to dependability designCahier Technique n° 144P. BONNEFOIc Initiation aux réseaux de communicationnumériques.Cahier Technique n° 147E. KOENIGc High availability electrical power distributionCahier Technique n° 148A. LONGCHAMPT - G. GATINEc Dependability and LV SwitchboardsCahier Technique n° 156O. BOUJU

Schneider Direction Scientifique et Technique,Service Communication TechniqueF-38050 Grenoble cedex 9Fax. (33) 04 76 57 98 60

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