Entrepreneurs in the Industryhave always desired that thelatest technology be madeavailable to them - for betterquality, cost effectiveness,enhanced productivity,….

Technological advances haveintroduced a new concept inIndian industry - the concept ofIntelligent Electronic Devices(IEDs) and communication -based control and dataacquisition. It is beingimplemented in upcomingprojects and also as pilotprojects in the expansion ofestablished industries.

As long as microprocessor -based devices were used onlyfor protection, external factorslike power quality, surges due toswitching devices had very littleimpact on them. Introduction of

PLC based controls (even hardwired) necessitated protectionagainst surges in the circuits.Cable capacitance also causedmalfunction. Cost effectivenumerical controls, variablespeed drives and simple devicesusing semi-conductors andchokes polluted the electricalsupply system. Thisnecessitated connection ofharmonic filters in the systemfor reliable performance of otherdevices.

The associated data acquisitionnetworks demand propershielding of cables, their properrouting and precautionsregarding electromagneticinterference.

IEDs incorporating protection,control, metering, indication andannunciation in a single

intelligent device are availabletoday.

Application of such IEDs forcontrol and protection is alreadycoveted in the previous issues ofL&T Current Trends, but theoverall system needs tailoredengineering solutions for easy,safe and reliable operations.

In this issue of L&T CurrentTrends, we bring you a brief onthe engineering considerationsdesirable for system involvingIEDs.

Wish you aHappy New Year

For The Year 2000

Engineering the System Environment

Prospect / Retrospect

Issued by: Electrical Business Group - Tailormade Products, LARSEN & TOUBRO LIMITED, Powai Works, Mumbai 400 072

October-December 1999

In the earlier articles, we haveseen how the concept of"Integrated Protection andControl" is advantageous forcontrol and protection of feedersand motors. We also consideredhow such Intelligent ElectronicDevices (IEDs) are used forspecific applications asAuto-changeovers and dataacquisition with man machineinterface.

In this article, let us understandhow such IEDs can be used in anindustrial environment and alsotry to analyse variousenvironmental effects on thesystem and find out theengineering solutions for them.

For this purpose, we willconsider the engineeringaspects involved in :-♦ Physically locating the IEDs

in switchboards (both LV andHV);

♦ Electrical connections forprotection & control inputs;

♦ Communication connectivitywithin the switchboard withexternal devices.

Location of the IED inSwitchboardsNormally, IEDs are to be located inthe respective feeders in theswitchboard. We can generallycategorise the IEDs as active(having user interface throughvisual display, interactive buttons,manual interrogation, serial link)and passive (having userinterface only through serial link.)An active device is expected to beaccessible without opening thefeeder door whereas a passive

device can be located insidethe feeder compartment (on abase plate), behind the door.However, for such a passivedevice, access to the port for datadownloading and parametersetting should be available withouta need to switch off the feeder.Hence, the passive device cannotbe mounted inside a compartmentwhich also houses the feederpower switch.

Another aspect that needs to betaken into account is electro-magnetic Interface (EMI) for suchIEDs. It is true that thecommunication network needsspecial precaution against EMI inswitchboards. Even the locationof IEDs needs special attention ineach feeder since it has towithstand the highelectromagnetic fields generatedby the arcing in circuit breakers,power switches and/or contactorswhile opening them.

It is apprehended that location ofsuch IEDs in MV/HV switchboardsneeds more attention. Thisapprehension is incorrect. SinceMV/ HV switchboards have aseparate compartment withmetallic barriers for relays andmeters, it offers magnetic shieldingagainst the field created byopening of the circuit breaker.Hence in MV/HV switchboards,more attention is required to bepaid to communicationconnections.

In LV Switchboards, thecompartments/modules arecompact and the IEDs are locatedin close vicinity of the power and

control contactors or powerswitches or circuit breakers. Arcingdue to opening of these devicesproduces its own field, whichneeds to be withstood by the IEDin that compartment. Evenimproper earthing can causepermanent damage to theprocessor due to high transients.Hence, proper electromagneticshielding, earthing connection to aclean earth and properlyengineered feeder layout offer aneffective solution in this regard.

In case of drawout feeders, if theIED is mounted on the drawoutportion, while replacing the modulewith any other similar module, theIED will lose its data/settings. Forsuch cases, IEDs can be locatedon the fixed portion of the feeder.

Electrical connections forprotections and controlsa) CT/VT Inputs : All IEDs offer a very low

burden for CT and VT inputs.This burden is less than 1VA.Depending on the functions, theauxiliary supply burden varies.This burden is of the order of10-15 VA.

Low burden offered by IEDsfor CT inputs needs specialattention, specially for motorprotection. In case of largemotors, the starting currentsurge is very high. Thisresults in CT saturation. Itcauses unbalance whenresidual connection is givento earth fault element ofIEDs. This results in trippingof the motor if unbalance

Integrated Protection & Control SystemsFor Process Plants - Part – VIII

- R. S. Mahajan, Assistant General Manager

FEATURE

or earth fault setting issensitive. To prevent thisnuisance tripping, a stabilisingresistor is provided in the CTcircuit, as shown in Fig.1. Thisstabilising resistor limits thecurrent and prevents spurioustripping during starting.

However, in case of an earthfault, the CT producesadequate voltage on secondaryand operates the relay. For safeoperation of the circuit, it isdesirable to restrict the peakvoltage experienced by theresistor and associated circuitto a maximum of 3kV. In casethis value is likely to exceed3kV, voltage dependant non-linear resistors (Metrosil) thatare quite expensive arerequired to be used. When theCT burden is as low as 2-2.5VA, the voltage acrossstabilising resistor can berestricted to 3kV.

For CTS with higher burden,this voltage goes sharply up.Hence, it is ideal to use CTs of2 or 2.5 VA burden and VTs of25 VA burden in IED circuits.This ensures space reductionand higher safety of operations.

b) Power Supply :IEDs have very sensitive

circuits and it is desirable thatthe power supply should beclean and the earth connectionshould be proper.Conventionally, forelectromechanical relays,switchboard earth is consideredto be adequate for relayearthing. However, IEDs needproper earth connection. Aseparate earth-pit with cleanearth connection is essentialspecially for the control supply.

Circulating currents due topotential difference betweensystem earth and clean earthmust be avoided, since both aregetting connected to the IED atdifferent points. This potentialdifference can damagesensitive circuitry in the IEDs.

This phenomenon can beexplained with the circuitdiagram shown in Fig.2. Here asingle phase supply tapped forusing a hand held drillingmachine can cause damage tothe IEDs, through earth circuit,due to mixing up instrumentearth and system earth indifferent tap-offs.

If the control supply is DC andis not earthed or mid-pointearthed, care should be takento ensure that external earthfaults in the control circuit donot lead to excessivecurrents in the IED controlcircuits/earth connections. Insuch cases, resistors in theIED earth circuit will helprestrict the circulating current.

Quality of power supply is alsoa critical factor for reliableperformance of IEDs. In thepresent industrial scenario,except for incandescent lamps,virtually all devices generateharmonics. Electrical switchingdevices like contactors, relayshave electromagnetic coils.These electromagnetic coils,while getting activated, generatelarge spikes in the supply, someeven exceeding 10kV. Whenconstruction activity is on at site,welding operations deterioratequality of power supply further.

All these demand specificprecautions to be taken.

Especially in Indianenvironment, where grid powerquality is very poor, performanceof IEDs gets badly impaired iftransients pass through earthconnections.

Torroidal cores connected in theauxiliary power supply circuitoffer good protection againstsuch damaging elements. Theyalso improve theelectromagnetic compatibility.Care must be taken whilerouting the wires in theswitchboard, specially betweentorroidal cores and the IEDs, so

that the wires do not pick-upspikes from adjacent circuits.

Provision of diodes and surgesuppressors for DC and ACcircuits respectively restrict thehigh surges caused by coils incontrol circuit to the point ofinitiation.

a) Control :

IEDs integrating protection andcontrol need special engineeringsince a sensitive Digital Input(dig-in) circuit is added intothem. The supply to dig-ins androuting of cables to each dig-inrequires the same attention asthat demanded by a PLC.

Many PLCs/IEDs need specificlow DC voltages for dig-ins, butin a switchboard, it may becumbersome to have specificDC voltage for dig-ins alone. Insuch cases, external devicescan be connected which will

ensure a surge/transient freesupply to the dig-in circuits,operating at lower DC voltages.

Certain dig-ins are connected to field devices, which are placed

at distant locations. They can bepush buttons located near themotor or limit switches. If thecable distance is long, it will

result in high cable capacitancewhich may give "always high"

signal to the respective input.This can cause malfunction ofthe IED.

Measures to nullify this effect ofcable capacitance on the controlcircuit, for those specific inputs,offer effective solution in thisregard. However, many a time,this cable length data is notavailable/overlooked at the timeof engineering and surprises areexperienced duringcommissioning.

Communication ConnectivityIEDs of present generation offer avery cost effective solution usingshielded twisted pair (STP) wireinstead of expensive fibre opticnetwork. Limitation of twisted pairwire network is the distance, whichcan be maximum 1.5kms for RS422/485 bus. In case of longerdistances, repeaters can be used.STP connections are easy to makeand maintain. STP cables arecheaper and readily available inIndia as against imported fibre opticcables.However, while selecting STP cablesfor communication connectivity, it isessential to get a proper cable withmatched impedance. This eliminatescable capacitance-related issues.

Daisy chain arrangement ofcommunication bus provides cost-effectiveness but can cause a majorproblem of loss of connectivity whena single device goes out of circuit.Multi-drop connections, thoughexpensive, offer a much morereliable solution. Multi droparrangement demands specificattention at each tap-off connectionin the switchboard. Simple screwpinching termination on thecommunication bus may causeloose connections and loss of data.

Communication connections withinthe switchboard need properengineering, so that EMI effects oflarge current in busbars and cables,arcs generated by opening of circuitbreakers and contactors do notaffect communication signals.

For external connections, shieldedtwisted pair cables can be used.They can be run in the cable tray,which is common for control orpower cables. A PVC conduit can beconsidered for additional mechanicalprotection. In very polluted (noisy)environment, MS conduit canprovide better noise shielding aswell as effective mechanicalprotection

ConclusionThese are some of the aspects thatneed attention while engineering,manufacturing and commissioningthe intelligent systems involvingIEDs. Needless to say, one of theimportant aspects, not listed above,is proper ventilation of the sub-stations. It can reduce effect ofhumidity and temperature on theIEDs, thereby enhancing life of thetotal system.

For Further details on this subject, please contact :Switchgear Contracts Division, Larsen & Toubro Limited, Saki-Vihar Road, P.O. Box 8901, Powai, Mumbai 400 072

Fax : 022-8581024 e-mail : nk-swtc@pgriii.ltindia.com

Edited, printed and published by S. R. Talwar for Larsen & Toubro Limited from L&T House, Narottam Morarji Marg, Ballard Estate, Mumbai 400 001 and Printed at Uma Offset, 359, A to Z Industrial Estate, Lower Parel, Mumbai 400 013. Associate Editor: Luis S. R. Vas