difference between directional and non directional over current relay

[simatic7]

hi
I am a new engineer and want to know what is the precise difference between the AC directional over current relay (67) and ac non directional over current relay (51). In which case we have to use 67 and when to use 51. whether they can be used interchangebly or can not be used in that way. How does it affect in protection coordination.
regards
simatic

 

[slavag]

Hi simatic7.
Please start from:

http://www.geindustrial.com/pm/notes/artsci/artsci.pdf

page 57.
Simple answer: directional is usually used in a closed ring connection, paralleling circuits and more and more.

Good Luck.
Slava

 

[davidbeach]

The protection references in faq238-1287, Blackburn in particular, would be a good source of information. If you're going to be doing relaying you ought to have a copy of Blackburn on your shelf.