1. Bus Bar Protection

Discussion on BusBus-bar -bar protection for different Bus bar arrangement of Sub--station. Sub -station. Different types of problem & its remedy. calculation, per unit and % impedance.

Engr. Mor Morshed shed la lam lam m !h !han an E"ecutive Engineer #rid Maintenance Division Dha$aDha$a -orth,#'B -orth,#'B.

At a glance: A Bus arrangements

A Bus components A Bus protection techni2ues A ') Saturation A pplication 'onsiderations(  +i h im edance bus differential rela in  4o impedance bus differential relaying

Bus Bar system system in Electrical subsub-station -station • Single Bus-bar arrangement arrangement • Double Main Bus-bar scheme • Main and Transfer bus-bar scheme • • Ring Main arrangement arrangement scheme

Single BusBus-bar -bar scheme This is the simplest bus bar scheme available available which consists of single set of bus bars connected to the generators, transformers and load feeders !ll the feeders are connected b" circuit breaker and set of isolators This arrangement helps to remove the , , maintenance b" opening the circuit breaker breaker contacts and further opening the isolators isolators

dvantages( arrangement en&o"s less cost of installation • This bus bar arrangement • 'ess maintenance • simple operation

Disadvantages( is un under maintenance total su su • when Bus bar is

l and al all

feeders feeders should be disconnected reliabilit" • 'east fle(ibilit" and reliabilit" •

)ault on the bus bar all the feeders feeders connected to bus bars should be disconnected

the

Double Main Bus bar Scheme *ormall" in double main bus-bar scheme each circuit is connected to to both the buses +n some cases half of the circuits can be connected and operated on each bus, in these cases bus or circuit breaker breaker failure would cause loss to half of the circuits +n double main bus-bar arrangement one or two breakers breakers can be provided for for each circuit Double main bus-bar and double breaker scheme provides high reliabilit" in the case of fault or outage of one of the breaker

dvantages( taken out of circuit for maintenance • !n" circuit can be taken • )le(ibilit" in connecting the feeder circuit to either of

the bus-bars

Disadvantages(  • 'oose circuits connected to bus-bar when fault occurs

on the bus-bar

Main and )ransfer Bus-bar Scheme Main and Transfer bus-bar scheme is similar to single bus-bar arrangement arrangement with additional transfer transfer bus connected Tie circuit breaker is provided to tie both the main and transfer bus During normal operation all the circuits are connected to the main bus hen circuit breaker breaker connected to the circuit , circuit breaker breaker connecting the main and transfer bus is closed The rela" protection for the circuits connected to the transfer bus is taken care b" the tie circuit breaker 

dvantages( • 'ow initial cost • !n" breaker can be taken of circuit for maintenance

Disadvantages( • Reuires one e(tra breaker for bus tie • Switc ing is somew at comp icate w en rea er is

under maintenance

*ne and +alf brea$er brea$er Bus-bar scheme +n One and half breaker breaker scheme, two circuits c ircuits are are connected between the three circuit breakers breakers .ence One and .alf breaker breaker name was coined for this t"pe of arrangement /nder normal operating conditions all the breakers breakers are closed and both the bus-bars are energi0ed !n" 1ircuit fault will trip two circuit breakers and no other circuit will be affected in this arrangement arrangement hen a bus-bar fault occur onl" breakers breakers ad&acent to bus-bars trips and no circuit will loose power Two bus-bars can also be taken out of service ser vice with out affecting affecting the power flow if the power source circuit # alternator alternator circuit% and receiving circuit #transmission line% available available in the same ba"

dvantages( • Most fle(ible operation possible

reliabilit" • .igh reliabilit" • Bus failure will not remove an" circuit from service

Disadvantages( •

g cos 

• Rela"ing is somewhat complicated since the

middle breaker must responsible for both the circuits on either direction and should operate operate

ing bus bar scheme +n this ring main bus bar scheme arrangement, breakers breakers are connected in ring and circuits are connected between the breakers breakers There will be same number of circuits as the number of breakers breakers in the arrangement arrangement During normal operation all the breakers are closed During circuit fault two brea break kers ers conn onnecti ectin n the the cir circuit cuit tri tri s Duri Durin n brea break ker maintenance the ring is broken but all the lines remain in service

dvantages( • 'ow cost • )le(ible operation for breaker maintenance

breaker can be taken taken out of service ser vice without • !n" breaker interrupting load 2ower can be fed from both the direction • 2ower

sa van ages( • )ault occur during maintenance will break the ring • Rela"ing is comple(

Breaker failure during fault will trip tr ip one additional addition al circuit • Breaker

Fandamentals of Bus bar protection

+n earl" da"s da"s onl" conventional conventional over current rela"s were used for bus-bar protection But it is desired that fault in an" feeder or transformer connected to the bus bar should not disturb bus bar s"stem s"stem +n viewing of this time setting of bus bar protection rela"s are made length" So when faults occurs on bus bar itself, it takes much time to isolate the bus from source which ma" came much damage in the bus s"stem

Basic rotection rotection Scheme

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+n recent da"s, the second 0one distance protection rela"s on incoming feeder, with operating time of 34 to 35 seconds have been applied for bus-bar prote p rotection ction

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But this scheme has also a main disadvantage This scheme of protection can not discriminate the fault" section of the bus-bar

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*ow da"s, 2ower s"stem s"stem deals dea ls with huge amount of o f power power .ence an" interruption in total bus s"stem causes big loss to the compan" So it becomes essential to isolate onl" fault" section of bus-bar during bus 

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!nother drawback of second 0one distance protection scheme is that, sometime the clearing time is not short enough to ensure the s"stem stabilit"

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)o overcome the above mentioned difficulties, differe differential ntial bus-bar b us-bar protection scheme ith an operating time less than /.0 sec., is is commonly applied to many bus b us system.

Differential BusBus-bar -bar rotection • 'urrent Differential rotection • 1oltage Differential rotection rotection

'urrent Differential rotection • The scheme of bus-bar protection, involv involves, es, 6irchoff7 6irchoff7ss current current

law, which states that, total current entering an electrical node is e(actl" eual to total current leaving the node •

.ence, total current entering into into a bus section is eual to total current current leaving the bus section

• The principle of differential differential bus-bar protection is ver" simple

.ere, secondar"8s of 1Ts are connected parallel That means, S9 terminals of all 1Ts connected together and forms a bus wire Similarl" S : terminals of all 1Ts connected together to form another bus wire •

! tripping rela" is connected across these two bus wires

So, it is clear that under normal condition there is no current flows through the bus-bar rippiing rela" la" Thi This rela" la" is gener enera all" ll" referred rred as Rela ela" bus-bar prot protect ection ion tripp ; or =unbiased>9 differential.

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*ver current percent 5=restrained> or =biased>9

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4inear couplers

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+igh-impedance bus differential differential schemes

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4o-impedance bus differential differential schemes