Communicating the right way

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Issued by : EBG - Electrical Systems & Equipment, LARSEN & TOUBRO LIMITED, Powai Works, Mumbai 400 072

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Prospect / Retrospect

With more and more automation making inroadsin various types of industries, it's becomingimportant to make the system extremely reliable.This issue is being discussed in various forumsand we have also addressed the same in L&TCurrent Trends from time to time.

One more aspect that needs to be taken care ofin an integrated system is the language ofcommunication or the Protocol.

This is the issue which needs a lot of detailing ofthe entire integrated system. It also need toaddress various parameters like the type ofcommunication to be handled (whether dataintensive or not), speed of communicationexpected, types of interruptions expected, .... Theselection of communication protocol will basicallydepend on the type of application if is required tohandle.

The issues that need to be kept in mind whiledeciding on a communication protocol are, itsadaptability with existing system and flexibility withrespect to future upgradation of the entire system.

The protocol may differ for different applicationsa single integrated system, but all the devices,though on different protocols, must talk to eachother for any upward or downward integration.

It must also not restrict the user to a single systemsupplier and pose no limitations whenever theplant automation system goes for an upwardintegration with higher level automation system orundergoes expansion with newer technology.

The support available from the system supplieralso forms a critical parameter.

This issue of L&T Current Trends discusses indetails issues related to protocol selection.

Communicating the right way

Selection of communication protocol- Mr. Gagan Kapoor

Electrical Systems & Equipment

Present Scenario

Today’s increased competitiveenvironment is driving the industrytowards higher level of automationusing integrated electrical systemfor their plants. Advent of newcommunication systems is forcingindustries to spend a good amountof time in strategising theirautomation requirements. Beforelooking at the specific automationneeds of the industry, we canclassify the industry into threecategories:

� Power generation / distributioni.e. utility

� Process Industry e.g.continuous running plants

� Manufacturing / MaterialHandling industry

The Utility industry prefers acommunication system that canhandle a huge amount of data ata reasonable communicationrate. Normally, the requirementdoes not call for prioritisation ofactivities, transmission ofcommand or interruptions throughcommunication bus. The basicneed is just for data logging.Hence, they opt for supervisorycontrol systems, energymanagement systems etc.

In a continuous process industry,the system is dominated byinterlocks, logics, controls andinformation collection in differentformats with different priority.What plays an important role hereis the time taken to executecommands and the reliability ofthe entire communication system.Hence, they opt for priority based

� Facility for having all eventsdate and time stamped

� Remote data acquisition� Extraction of disturbance

recording� Data logging

Generally used protocols for aSCADA are :� IEC� DNP� Ethernet based protocols� Profibus FMS

Or� Protocols that are of protective

nature, but shall haveproperties listed above.

2. DCS:

Distributed Control Systems(DCS) are more of processoriented automation systems thatgovern the process, continuouslymonitoring the commands,interlocks,.... It takes very fewcritical signals from electricalequipment e.g. motor, transformer,breakers etc. Most of its data is inthe form of hardwired I/O ortransducer inputs from theprocess equipment. e.g. valves,pumps which may or may nothave Intelligent Electronic Devices(IEDs). Its functionality requiressequential interlock logics, controlsof process equipment responsiblefor a particular process. UnlikeSCADA, DCS depends on presentand past state of data / informationtoo. Since DCS often runssequentially rather than eventbased, the communication systemadapted shall be of sequentialtype. Hence, communicationprotocol selection shall be basedon the following :

system in a continuously polledenvironment e.g. DCS, ProcessHMIs or ECS for electricalequipment.

Manufacturing industry needscustom-built automation systemwith extensive interlocks. Hence,they detail their requirement basedon which a system can beadopted.

The need of automation systemalso varies with the voltages of thepower system. e.g. HV substationneeds a continuous monitoringsystem with a reasonable rate ofcommunication and very fewinterruptions. Whereas, when itcomes to LV system, the speedof communication, parametermonitoring and prioritization playsan important role.

Integrated Systems in Industry

1. SCADA System:

Supervisory Control & DataAcquisition (SCADA) carries outall the functions like supervision,control and data acquisition.SCADA is basically an event drivensystem. SCADA is a masterstation which monitors change ofstate i.e. data collection on anyevent / alarm or change of valuebeyond a permissible band andrepresentation of the same.

Hence, in a SCADA system, thecommunication protocol selectedshall be the one which supports :

� Exhaustive data availability withchange of state, sequence ofevents and alarm generation

FEATURE

� Continuous polling to suit theDCS software functionality

� Facility of interrupts� Facility of control / commands

on priority� Very fast scan of few critical

parameters collected fromIEDs

� Sequence of events records

Majority of DCS worldwide followany one of the following protocols:� Modbus RTU� DNP

3. ECS

Electrical Control System is asystem similar to DCS but itfunctionally differs from DCS. ECScontrols the electrical parametersof a process like load, power etc.rather than the process equipment

e.g. various sensors, interlocksetc. It also encompassesimportant functions like loadmanagement and load shedding.Other requirements remain thesame as DCS.

4. HMI

Human Machine Interface (HMI)is basically an engineeringworkstation monitoring theprocess, its parameters,interlocks, issuing commands,logging data etc.

Since this system is monitoring alarge amount of engineering datafrom each and every IED, itbasically needs prioritisation. Thismeans it necessarily has to pollsome high priority data at a fasterspeed like status, currents etc.

and rest of the bulk at a moderatespeed. Since it also issuescommands, the commandsalways need to get top priority.

Thus, for HMI, prioritising the datatraffic is of utmost importance.

Selection of protocol

As discussed earlier, protocolselection is an important functionwhile deciding a IntegratedSystem. With a wide choiceavailable in protocols, the userneeds to identify the requirementsand the application before goingin for this decision. The higherlevel automation system also playsan important role while decidingupon a particular communicationprotocol. Some of the factors thatcan be kept in mind are:

Plant System Architecture

� Current presence andacceptance of a protocol in theindustry

� Protocol support availablefrom the higher endautomation system supplieri.e. DCS, SCADA, ECS etc.

� Suitability of integrating theexisting devices in a plant withthe newer automation systembeing adopted. The mereselection of protocol shouldnot necessitate changing ofexisting devices due to non-compatibility of communicationprotocols.

� New protocols must take intoaccount the interchangeabilityof existing protocols.

� A time tested, proven protocol,for which the performanceparameters are alreadychecked. e.g. scan time,system response time etc.

� Life time of a communicationprotocol i.e. whether a protocoland its associatedaccessories are going tobecome obsolete in nearfuture. This will lead to loss ofsupport and will become abottleneck for furtherupgradation/integration.

� A new protocol whenintroduced in the field must besupported by a wider base ofsuppliers. If it is popularisedby a select set of suppliers,then there must exist acommitment regarding the lifeof the protocol, supportavailability and possibilityregarding further andbackward integration.

� There should be a goodnumber of higher level of

For further details on this subject, please contact:EBG - Electrical Systems & Equipment, Larsen & Toubro Limited, Saki-Vihar Road, P.O. Box 8901, Powai, Mumbai 400 072

Fax: 022-5505 1024 * E-mail: kotnisnd@Intebg.com

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automation system suppliersthat can support thecommunication protocol. Thisis absolutely necessary forupward integration. This willalso give the user a choice ofhigher level automation toadapt for. Else the wholesystem may get stuck withonly one supplier.

� The protocol should be trulyinter-operable. i.e. Thereshould not exist multipleversions of the same standardprotocol which any vendor canmodify to suit their system.

� The communication protocolshould neither beunnecessarily expensive norshould it make the systemexpensive due to its ownlimitations.

� The communication protocolmust be capable of supportingboth distributed andcentralised type ofarchitecture.

� The integrated system shouldhave future expandabilitywithout any economic andphysical constraint. Thesystem should offer openended connectivity both atupper as well as lower level,thus leaving the choice ofselection of equipment opento the user.

� Integrated system adoptedwith the protocol selectedshould take care of thereliability of communicationsystem.

� Security of the systemcommunication should beverified. e.g. redundancy.

� Complete documentation and

technical support should beavailable for the protocolselected.

Conclusion

A communication standard/protocol should be strictly selectedbased on application. Itsadaptability among major higherlevel system suppliers, its provenperformance for a specifiedfunction and the possibility of itsexistence for a considerablelifetime of the plant are also someof the essentials to be considered.

Type and design of a protocolneeds to be standardised at theapplication level making itindependent of devices. i.e.application in the communicationnetwork should decide the protocoland not the device / supplier.

The various integrated devicesused in the plant may havedifferent communication protocolbased on the predominant field ofapplication. The plant level systemintegration should take care ofcommunication of differentIntelligent devices with each other.

While integrating, a variety of suchdevices, independent of theirmakes, specifics etc. can beconnected together throughsuitable converters or gateways toachieve the desired functionality.

Being the brain of an automationsystem, selection of thecommunication protocol must begiven due consideration includingits thorough testing under practicalcircumstances.