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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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