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7/27/2019 BUS BAR Presantation
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BUS BAR
PROTECTION
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Requirement of bus bar protection
scheme Complete reliability
Must be completely stable under all through fault conditions.
Must be capable of complete discrimination between sections
of bus bars.
Must operate with high speed to minimize damage andmaintain system stabillity.
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Types of protection system
A number of protection systems have
been devised popular among them are
Frame earth protection
High impedance differential protection
Low impedance differential protection
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Frame earth protection
Only earth fault system
Involves measuring fault current from switchgear
frame to earth
Switchgear insulated by standing on concrete plinth Only one earth point allowed switchgear
C.T mounted on single earth conductor use to
energize instantaneous relays
All cable glands must be insulated
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Differential protection
Uses Merz-price circulating current
principle . All current entering and leaving
bus bar are compared. One set of CT,s for
each circuit associated with a particularzone are all connected to a relay
A single element relay gives earth fault
protection only. A three element relaygives phase and earth fault protection.
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High impedance differential
protection This is a versatile and reliable protection system
applied to many different Busbar configurations.if CT requirements are met , schemeperformance may be predicated by calculation
without heavy current conjunctive tests. Uses equal ratio CT,s .Scheme assumes that
with heaviest through fault , one CT saturates ,the other not.To ensure stability , voltage settingof relay circuit made higher than voltagedeveloped accros relay circuit. To achieve thisan extra relay circuit resistance is required this isnon as stabilizing resistance.
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aEffective settings
Since in each zone of protection there are sevaeral CT,sin the paralell with the relay and each other, thecombined CT magnitising currents will increase theprimary operating current.
POC=CT ratio (Ir+INLR+NIm) Where
Ir = relay setting current
IM =CT magnetising current
N = number of paralleled CT,s INLR = non linear resistor current at relay setting
voltage.
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Primary operating current
The value of primary operating current
should be arround 30%of minimum fault
current avalible . This ensures sufficient
relay current during internal faultconditions for high speed operation.
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Through fault stability
Busbar protection stability limit is based on
maximum through faulrt current.
Generally this value is derived from the
rating of the associtiated switchgear
irrespective of existing faul level,since it
can be expected that system will develop
up to limit of rating.
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Check feature
Usually provided by duplication of primary
protection using second set of CTs on all
circuits other than bus section and coupler
units.Check system forms one zoneonly,covering whole of the busbar system
and not discriminating between faults on
various sections
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Use of non linear resistor
Under in zone faults conditions it is possible for voltagesabove the relay withstand of 3kv peak to beproduced.metrosil non linear resistor may be necessaryto limit the peak voltage below this level.
Approximately peak volts=2 Metrosil characteristics :- v=cib
where:-v and I are peak values
c=constant depending on metrosil
construction b=constant in range of .2-.25
The values of c and b are chosen to limit metrosil voltageto less than 3kv peak at maximum fault current.
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Metrosil limitation of ray voltage
To ensure primary operating current not
adversly affected metrosil constant c
must be sufficently high to restrict metrosil
current at relay setting voltage VS.
Typical currents :-
30 ma for use with one
ampere CTs100 ma for use with 5ampere CTs
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Differential relays
Relays used for high impedence protection
are high stabililty unbiased and tuned to
nominal frequency.
Two modular types are MFAC14/34
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CT wiring supervision
Open circuit connections between CTs and relay
circuits result in unbalanced currents whioch
may operate the protection.
Supervision is applied by voltage relay acrossdifferential relay circuit.supervision relays is time
delayed gives alarm and also short out bus
wires to protect differential relay circuit.typiucal
effective setting is 25 primary amperes or 10%of lowest circuit rating which ever is greater.
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Current transformers and wiring
CT must be low reactance tyoe(class x)and haveidentical terms ratio. They should be of similardesign or if not of rreasonabely matchedmagnetic characteristic.it is common character
to use CTs having one ampere secendories Load burdens between various sets of CTs must
be kept low . Usually bus wires are run in closedring between breaker control panels.
Typical routes:-CTs to marshelling kiosk, MK toisolator yo auxallaries, loop between MK
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Isolator auxillary switches
On double bus installations , the two bars
are treated as separate zones. The bus
isolator switches must carry auxillary
contacts to transfer CTs and also trippingsircuits between zones.
Auxillary switches should be early make
Late break
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Tripping circuits
One trip[ping rel;ay device is required for
each feeder breaker and two far each bus
section or bus coupler breakers.both
ma8in and check relays must beenergized before triiping relays trip all
breakers assocaited with zones
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Busbar protection and breaker fail
Where breaker fail protection is applied to
a system, back tripping of associated
breaker is required in the event of breaker
failure. Often,breaker fail protection isarranged in conjunctio with busbar
protection tripping circuits to initiate triiping
of breakers opn busbar zone associatedwith field breaker
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Low inmpedence busbar protection
Low impedence protection has been used
for many years.in paricular on installations
where CT output not suuficient for high
impedence application.
Low impedence protection also operates
on circulating cuurrents. Through fault
stability often assured by low valuestabilizing resistance plus throug load bias
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