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Defnite Time Overcurrent Relays
Definite Time Over current Relays:
Defnite Time Overcurrent Relays
1) (Extract from https://electricalnotes.wordpress.com/2013/01/01/types-of-over-crrent-relay/)
!n this type" two conditions mst #e satisfied for operation (trippin$)" crrent mst
exceed the settin$ vale and the falt mst #e continos at least a time e%al to time
settin$ of the relay. &odern relays may contain more than one sta$e of protection
each sta$e incldes each own crrent and time settin$.
'or peration of efinite *ime ver +rrent ,elay operatin$ time is constant
!ts operation is independent of the ma$nitde of crrent a#ove the pic-p vale.
!t has pic-p and time dial settin$s" desired time delay can #e set with the help of an
intentional time delay mechanism.
Easy to coordinate.
+onstant trippin$ time independent of in feed variation and falt location.
Drawback of Relay:
*he continity in the spply cannot #e maintained at the load end in the event of falt.
*ime la$ is provided which is not desira#le in on short circits.
!t is difficlt to co-ordinate and re%ires chan$es with the addition of load.
!t is not sita#le for lon$ distance transmission lines where rapid falt clearance is
necessary for sta#ility.
,elay have difficlties in distin$ishin$ #etween 'alt crrents at one point or
another when falt impedances #etween these points are small" ths poordiscrimination.
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Application:efinite time over crrent relay is sed as:
ac p protection of distance relay of transmission line with time delay.
ac p protection to differential relay of power transformer with time delay.
&ain protection to ot$oin$ feeders and #s coplers with adsta#le time delay
settin$.
2) Extract rom:
http://power-systems-protection!lo"spotcou#/2$%%/$%/&efnite-time-over-
current-relayhtml
In this type, Two conditions must be satisfied for operation (tripping), Current must exceed the
setting value and the fault must be continuous at least for a time equal to time setting of the relay.
odern relays may contain more than one stage of protection each stage includes each own
current and time setting. !igure ("") shows a multi stage over current relay.
The connection diagram of definite
time over current relays with
internal and external timers are as
shown in figure ("#), and figure
("$) respectively. %efinite time
over current relay is the most
applied type of over current. It is
used as &
". 'acup protection of
distance relay of
transmission line with time delay equal to fourth stage of distance
#. relay which is #. second in ##* + lines, and ". second in + lines.
http://power-systems-protection.blogspot.co.uk/2011/01/definite-time-over-current-relay.htmlhttp://power-systems-protection.blogspot.co.uk/2011/01/definite-time-over-current-relay.htmlhttp://power-systems-protection.blogspot.co.uk/2011/01/definite-time-over-current-relay.htmlhttp://power-systems-protection.blogspot.co.uk/2011/01/definite-time-over-current-relay.html -
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$. 'acup protection to differential relay of power transformer with time delay equal to #.*
second in ##*- + transformers, and "." second in the -"" + transformers.
. ain protection to out going feeders and bus couplers with ad/ustable time delay setting.
0ee also
http&--www.slideshare.net-sspalayi-basics1of1overcurrent1protection
http://www.slideshare.net/sspalayi/basics-of-overcurrent-protectionhttp://www.slideshare.net/sspalayi/basics-of-overcurrent-protection -
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