Line Differential

It's a big substation with a 119Km line in it!

Don't post duplicate threads!

You can consider both types of line protection relaying, but a qualified protection engineer could tell you which is preferable. There are likely other pieces of information that will be required, such as is there generation at one end or the other (or along the line), is there a requirement for fast tripping (5 cycles or less), any requirements from your electric utility, or other considerations.

Remember too that differential protection will require some sort of communication path along the 10km path of the line, so that the differential relays at the ends can communicate with each other. Differential won't work if the relays can't compare what goes in to what comes out. Distance relays work just from their end by looking at line impedance.

Hire a qualified professional to determine this for you.

And I'm telling you that you could use either, depending on the situation.

If you want the fastest, most sensitive protection, the line differential will be your better choice. However, you will need to put relays at both ends of the line (so you need access to the station at the other end, or the cooperation of its owner), and have a communication path between them for the differential scheme to work.

If there are no requirements for extremely fast, sensitive protection, use line distance relays. They are the simplest and least costly. They are used for probably 90% of line protection schemes.

You can use either. You didn't even specify if the lines is radial or not, which in my point could even point to using overcurrent current relays.. hahaha..

But always consider the fault resistance reach in settings if using such relays.