Basics of Busbar and Local Breaker Back up

Advertisements

REQUIREMENT ON BREAKER- FAILURE RELAYING

• High security against undesired tripping of adjacent breakers

• Current detectors (RXIB)

– Short resetting time(<12 ms)
– Short operating time(~4 ms)
– High reset ratio (<88%)
– Insensitive for the dc component in the secondary current
– Insensitive for CT saturation

• Time measuring unit

– Accurate time-measuring >>> ± 0.5% of highest scale value
– Negligible overtravel >>> Resetting and recovery time 20ms

LOGIC DIAGRAM OF BFR

LOGIC DIAGRAM OF BFR

FAULT DETECTOR

C.B. Aux contact should not be used to indicate if C.B. is carrying current or not unless there is no other way to do the job

• The reasons are

– Operating linkage is broken or out of adjustment
– CB mechanism is open but CB has failed to interrupt current
– CB open but fault current continues
– A current detector gives more positive indication

CLEARANCE OF ASYMMETRICAL FAULT CURRENT


In the current detector RXIB 2 the influence of the dc component is suppressed by the air gap in the input current transformer of the relay

LOCAL BACK UP

 Local back – up protection can be divided into two categories

1. Relay back – up
2. Breaker back – up

——————————————–

 RELAY BACK – UP

 Duplicate primary protection,
e.g. one non-switched and
one switched distance protn.
To increase the security, the
circuit-breaker has two trip
coils, one connected to the
non-switched relay, the other
connected to the switched
relay.
——————————————– 

BREAKER BACK – UP

Because of the high cost of high voltage circuit-breakers, it is not feasible to duplicate them.
In case of a breaker failure, the other circuit-breakers connected to the same bus as the faulted breaker must therefore be tripped.For uncleared fault shown CB’s to be tripped are 1, 3, 4 & 6

Local Breaker Back-up Protection

A Protection which is designed to clear a system fault by initiating tripping other circuit-breaker(s) in the case of failure to trip of the appropriate circuit-breaker.

STABILITY

• In modern networks the critical fault clearing time may be less than 200 ms.Hence, if the fault is not cleared due to failure of the primary protective relays or their associated circuit breaker, a fast-acting back-up protective relay must clear the fault
• There are two basic forms

        Remote back – up

        Local back – up
REMOTE BACK UP

   The measured impedance Z will vary with the switching conditions.To maintain the selectivity in the system a relatively long time delay is necessary for the second step of the distance relay. Variation of the infeeds at the remote busbars may virtually prevent the application of remote back-up protection.

Advantage: Provides back-up protection for both the relays and breakers at the remote station

SINGLE BUSBAR SYSTEM WITH SECTIONALIZING BREAKER

………………………………..
CT DISCONNECTION UNIT
………………………………..
 CT DISCONNECTION UNIT
      Where there is only one CT in the bus coupler bay, there will be a blind Zone or dead zone between the breaker and the CT, and for a fault in This zone the selectivity of the busbar protection is lost resulting in Mal-operation and unwanted operation of wrong bus bar protection and Non-operation of the required busbar protection. If this is allowed to Persist, it will result in tripping of remote end feeders in Zone-II. Further, when both the buses are coupled through bus coupler and if The bus coupler breaker fails to trip for a fault on any of the buses, the Other bus continue to feed the fault till all the feeders connected to that Bus trip at remote end in Zone-II or by LBB of bus coupler breaker. To take care of this type of faults, the bus bar protection is provided with A feature called bus coupler CT disconnection , which shorts the bus Coupler CT after a preset time after the operation any of the bus bar Protections irrespective of the status of bus coupler breaker.

TYPICAL BUS ARRANGEMENTS

• SINGLE BUSBAR SYSTEM
• SINGLE BUSBAR SYSTEM WITH SECTIONALIZING BREAKER
• SINGLE BUSBAR SYSTEM WITH SECTIONALIZING ISOLATOR
• DOUBLE BUSBAR WITH TRANSFER BUS
• 1&1/2 BREAKER SYSTEM

SINGLE BUSBAR SYSTEM


SINGLE BUSBAR SYSTEM WITH SECTIONALIZING ISOLATOR

SINGLE ZONE RECONNECTION UNIT

 SINGLE ZONE RECONNECTION UNIT
    When both the buses are coupled through isolators I.e. when both the bus isolators of any feeder is closed during bus change over, both the Bus bars will act as single bus bar and bus coupler gets bypassed. In The event of fault on any one of the buses during this condition, feeders Connected to both the buses will be tripped. In the case of bus bar Protection type RADSS, this logic is achieved through a separate bus Interconnection unit or Single zone reconnection unit.

Merits of Percentage Biased Bus Differential Protection Over High Impedance Protection

• In case of High impedance differential protection, it is necessary that magnetizing Impedance and turns ratios of all the CTs exactly match. Otherwise, for an external Fault differential voltage gets developed across the relay and the magnitude of this Might be sufficient to cause relay operation for a heavy external fault. There have Been instances where even minor turns ratio have resulted in mal-operation of high Impedance differential relay for an external faults.

• Any inclusion of auxiliary CT in one or more of the CT circuits means different Magnetizing impedances of such composite CTs and also this might introduce turns Ratio errors however small.

• In Percent biased relay (RADSS), the required voltage to operate, increases a the Restraint voltage increases in proportion to the severity of external fault. In addition Percent bias relay (RADSS) has following advantages

• Basic operating time of RADSS is 1-2m secs, and total time including trip relays Is 5-7m secs.

• In comparison the operating time of High impedance differential relay (including Trip relay) is 20-25m secs.

• RADSS is stable against through fault currents even for infinite fault MVA. The Growing fault MVA levels does not affect the stability of the relay and only CTs With moderate knee-point voltage (500V) need to be connected.

• In case of High impedance relay an increase of fault MVA results in an increase Of the setting, which can only be taken to a maximum of Vk/2 where Vk is CT Knee-point voltage. Thus, if fault levels increase, the CTs may need to be replaced With those of higher knee-point voltage..