Continuoussupply is deplants. Todifferent scconsultantsschemes emore sourbusbar sysschemes cthe users issupply fromother by desired swThis manuacontinuoussubstation supply for ainevitable. and fool prby addingbetween breaker.In case supply is delay needa few changeoveaffected bu

Issued by : Switchgear Contracts Division, LARSEN & TOUBRO LIMITED, Powai Works, Mumbai 400 072

, uninterrupted powermanded by all process

meet this demand,hemes are adopted by and end users. Thesessentially need two orces of supply to thetem. One of the simpleommonly followed by manual changeover of one source to theswitching OFF-ON

itches/circuit breakers.l changeover demands

attentive manning ofand yet interruption oft least some minutes isIn this scheme, safetyoofness is incorporated mechanical interlock

the switches/circuit

where interruption ofvery critical, and times to be brought down toseconds, automatic

r of supply to thes is implemented. In

January-March 1999

In

Prospect / Retrospect

such cases, the affected incomeris switched off and under healthybus conditions, the load istransferred to the otherbus/incoming source.

Since in this case monitoring ofconditions/parameters is carriedout by relays and interlocks/permissives are connectedelectrically, the changeover takesplace in a few seconds. Electricalcontrol circuit is normallydeveloped considering variousinterlocks and safety features. Sooperations are more reliable andindependent of operator's skills.Chances of error get eliminated.

While conceiving such interlockedschemes, different variations arefollowed by users. Depending onprevious experience, probableconditions are added for morereliable and safe operations. Goodengineering practices followed bysome consultants/end users arenot incorporated by many other

users. To attempt standardisationof such logic and benefit entireuser sector with good engineeringpractices, L&T has incorporated acomprehensive auto-changeoverlogic with the facility of manualparalleling in the AdvancedFeedervision relays of theSupervision Series.

Apart from brining advantages of avast experience in this field, thepackage offers economy in termsof space, lesser component count,lesser engineering and testingtime. The operations are simplifieddue to user friendly features of thesystem.

In this issue of L&T CurrentTrends, we bring you a curtainraiser on this subject. Though logiccovered here is mainly discussinga typical two incomer and onebuscoupler scheme, the same canbe extended to three incomers andtwo buscouplers or two incomerswithout a bus sectionaliser.

telligent Changeover

Normally, at distribution voltage(6.6 kV) and low voltage (415V)levels, switchgear has two or morebussections, for reliableoperations. This arrangementensures availability of supply tothe critical drives/feeders even ifthe incomer on that bus is notfeeding them.

The normal sequence of events insuch cases is as follows: -

In healthy condition, the respectiveincomer is ON and feeds the bussection connected to it.

In case there is sustained undervoltage (about 75%) on the lineside (for about 2 - 2.5 sec.), theincomer trips. This tripping isinitiated by the line VT circuit. Inthis case, incomer tripping is notcarried out by the lockout relay.

When the bus voltage is down to20% and if the other bus section ishealthy, (having a voltage above80%), the buscoupler closes andthe supply on the affected bus isrestored. This sequence is true foreither of the incomers on bothsides of buscoupler. Bus VT circuitcarries out this sequence. Closingpermissive of buscoupler checkshealthiness of the lockout relay ofthe open incomer.

In a two incomer and a buscouplerscheme, each incomer has a lineVT circuit and each bus has a bus

VT circuit to carry out thisoperation.

Under voltage on line side or busside is sensed by the line VTcircuit and it initiates tripping of therespective incomer and autoclosure of the buscoupler.

To prevent false undervoltagealarm/signal being generated bythe blowing of fuse in the VTcircuit, healthiness of the VT fuses(both primary and secondary)needs to be monitored.Conventional relays are connectedonly across the secondary fuses ofthe VT circuit. With the availabilityof fuse monitoring MCCBs (whichcan withstand upto 50kA), theprimary and secondary fuses inthe VT circuits can be replacedwith such MCCBs. (e.g. L&T'sMCCB type DF) and thehealthiness of line or bus VTcircuit can also be monitored forinitiating the auto changeover.

In the above-mentioned sequence,tripping of the incomer (due toundervoltage) is unplanned andrestoration of supply occurs onlyafter a break. This necessitatestripping of larger motors(controlled by circuit breakers) onundervoltage of the bus, so thatback emf of these motors does notdelay residual voltage reaching novolt level on the busbar. For thispurpose, a separate undervoltage

relay with a time delay is providedon the busbar.

After the closing of buscoupler andrevival of healthy voltage, theessential drives in a process plantmust start automatically withoutthe need to do so intentionallyfrom a control room. For thispurpose, a re-acceleration schemeis added in the starter circuit ofsuch drives. To prevent all suchdrives from startingsimultaneously, time grading isdone in groups, so that thechangeover bus dip is controlled.

This is only one part of the events.

However auto changeover is notdesirable in all circumstances.Instances when auto changeoveris stalled or delayed are asfollows:

1) At distribution voltage level, thetime delays for sensingsustained undervoltage areshorter compared to the delayson low voltage level. Thisfacilitates changeover first atdistribution voltage withouttripping incomer of low voltageswitchgear.

2) In the event incomer trips onoperation of a protection relay(say overcurrent or earthfault),the lockout relay on respectiveincomer prevents thebuscoupler from closing andfeeding the faulty bus.

FEATURE

Integrated Protection & Control SystemsFor Process Plants – Part V

- R. S. Mahajan, Assistant General Manager

3. Intentional opening/tripping ofthe incomer is distinguished bya memory contact (lost motiondevice contact in conventionalschemes) and it also preventsclosing of buscoupler.

4. In the event that both incomerson the two bus sectionsexperience simultaneousundervoltage on their line side,none trips and the systemhangs on till the time any oneor both supplies restore.

The above auto changeoversequence covers situation ofundervoltage on the line side andinterruption in the bus supply.There can be a requirement formaintenance to take outage onany one of thetransformers/incomer circuitbreakers. In this event, it is desiredto feed the respective bus from theother healthy bus - obviouslywithout interruption of the supply.This situation is "Planned outageof the desired circuit breaker' andit necessitates paralleling both theincomers - momentarily - and thentripping the desired circuit breaker.As a prerequisite, obviouslysynchronization of the suppliesneeds to be checked beforeparalleling the sources. Normally,using line side supply, checksynchronization function is addedon to the bus-coupler.

In most of the cases, "close"permissive is given to the circuitbreakers only from the controlroom, when they are in serviceposition. In case the control logicis desired to be tested, the circuitbreakers need to be drawn in testposition (out of service position)and then the operation can betested, without affecting othercircuit breakers, which are inservice position and feeding the

bus. However, for momentaryparalleling and planned outage,the close command may be givenfrom the control room or from theswitchgear.

When wired with conventionalelectromechanical devices, allthese logical requirementsdemand extensive wiring betweenthe Incomers and Buscouplercircuits, in addition to a largenumber of auxiliary contactors andtimers. Electrical antipumpingfeature is desired in both incomersand buscouplers.

Protection

Normally on distribution voltage(6.6/11kV) and low voltage (415V),Incomers are provided with thefollowing protections:

51 - Inverse overcurrent (2 elements)

51N - Earth fault

50 - Highset overcurrent

64S - Standby earthfault

64R - Restricted earthfault

There is a lockout relay (86) for allthese protective functions.

To monitor voltage on line and busside, undervoltage (27u) and no-volt relays (27n) with timers (2) areprovided in the line VT and Bus VTmodules. Trip circuit healthiness(95) is monitored using a separaterelay.Buscoupler have normally 51, 51Nprotections with lockout relay (86)and trip circuit supervision (95).

Metering

For Incomers, meteringrequirement includes current,

voltage, power factor and energy.For Buscoupler, normally it is onlycurrent that is measured anddisplayed. For remote metering,transducers are used.

Annunciation and Indication:Apart from feeder status, lampsand annunciators are used forvarious alarms and faults. For thispurpose, auxiliary circuit needssome auxiliary relays, specially toannounce transformer faults andalarms.

Single line diagram and panellayoutA typical single line diagram withall these requirements is shown inFig. 1. For simplicity, only2:Incomers and 1:Buscoupler areshown.

A typical switchboard layout for3200A ACBs, using conventional

electromechanical relays isshown in Fig. 2. It is clear thatdummy panels are to beadded to accommodate meters /relays in the switchboard.Additional relays / timers are to be

Considered for " Autochangeoveroccurred " and "AutochangeoverFailed" annunciations wheneverdesired.

Using Integrated ProtectionControllers:L&T has introduced a concept ofIntegrated Protection Controller,combining protection and controlwith metering and indication inSupervision series relays. Theseintegrated protection controllershave been developed for totalflexibility in protection and controlof feeders and motors. Keeping

specific application in mind, asophisticated and powerfulcontroller in the form of AdvancedFeedervision is developedincorporating the complexAutochangeover logic of Incomersand Buscouplers mentionedabove. Its software is developedsuch that in addition to current andvoltage inputs, digital inputs/statusinputs are taken by the relay ashard wired signals and the entirelogical operation is carried out bysoftware similar to a PLC. It notonly eliminates large number offunction specific devices andauxiliaries but also offers thebenefit of a foolproof logic andstandardisation to the user. L&T'sexperience, merged withinternationally acclaimed goodengineering practices followed byleading consultants in the world isincorporated in this software.

The single line diagram giving acomprehensive list of functionscarried out by the respectiveAdvanced Feedervision relays isshown in Fig. 3. Switchboardlayout with Advanced FeedervisionRelays incorporated is shown inFig. 4. Comparing Fig. 2 with

Fig.4, it is obvious that IntegratedProtection Controllers simplify theswitchboard layout and wiring. Inaddition, they also reduceengineering, commissioning,testing and trouble shooting time.

Capability to record fault historyfacilitates fault analysis.

Conclusion

In today's context, cost of spaceand interruption in production arevery important issues. AnIntegrated Protection Controller,offering a comprehensive packageand occupying less space, offers aeconomical solution in terms of :-- reduced time for system

design, commissioning andtrouble shooting

- lesser number of failures andlower downtime

- saving in space.

In addition, features ofmicroprocessor based relays viz.Self diagnostic feature, datalogging, fault history add furthervalue to the package.

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, Mumbai400 001 and Printed at Uma Offset, 359, A to Z Industrial Estate, Lower Parel, Mumbai 400 013. Associate Editor: Luis S. R. Vas