BUS BAR Presantation

7/27/2019 BUS BAR Presantation

1/35

BUS BAR

PROTECTION


2/35


3/35

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.


4/35


5/35


6/35

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


7/35

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


8/35

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.


9/35


10/35


11/35

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.


12/35


13/35


14/35

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.


15/35

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.


16/35

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.


17/35

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


18/35

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.


19/35

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


20/35

Differential relays

Relays used for high impedence protection

are high stabililty unbiased and tuned to

nominal frequency.

Two modular types are MFAC14/34


21/35

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.


22/35


23/35


24/35

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


25/35


26/35


27/35


28/35

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


29/35


30/35

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


31/35


32/35

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


33/35


34/35

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


35/35

BUS BAR Presantation

Documents

Transcript of BUS BAR Presantation