Line Protection: Ground Instantaneous Element Setting Policy Based on Remote End Open. 1

[Distribution73]

Hi,
I would appreciate very much to have your opinion on the following:
I am trying to set a directional ground instantaneous element (34.5kV Line Prot. Application). The utility policy on this regard is that it should be set as "1.25 x 3Io for the maximum line end fault, with the remote end open".
I am curious about the reason behind the requirement to have the remote end open. I can see, when doing fault simulations, that the fault infeed from the end I am trying to set increases when the remote end is open. Therefore the setting I calculate is more conservative. However I cannot see any reason why I cant use the fault values obtained when the remote end is closed (more sensitive setting).
What do you think is the background for such requirement?
Thank you very much in advance.

 

[protoslash]

what is the purpose of this ground instantaneous element?

if you set the pick up to 1.23 x 3Io of fault current at line end, does this mean you don't want the protection to pick this fault up?

 

[jwatty]

This eliminates the need for a more elaborate rule. I've seen other utilities use 125% x maximum ground current under worst case N-1; if you're following this rule you need to be cognizant of auto-reclose conditions as well. This often will look like an N-2 condition but if you have relatively few lines into a substation this will end up very close to just opening the remote end of the line in question.

 

[protoslash]

what im getting at is:
by setting the pick up at 125% of max ground fault current at end of line. what are you selectively trying to trip and what are you selectively not trip by using this instantaneous ground overcurrent element?

 

[jwatty]

protoslash - replying to your first comment. That is precisely what the intent is. The "remote end" will be wherever there is another protective device that is expected to operate past that point, so you do not want the local instantaneous overcurrent to operate when the remote end device should. Since the real world isn't perfect, you must pull the instantaneous "reach" back some amount. Typical margins are in the 120-150% range for overcurrents. The smaller you make this margin the more vigilant you must be in making sure system changes don't require tweaking settings.

 

[protoslash]

ok i get it. Is this like a ground zone 1 setting?

well every pick up setting is a trade off between selectivity vs security.

by choosing a higher pick up (utility policy) you want to be conservative so that the protection does not overreach. It is much more important to not reach beyond the remote end than to cover the whole zone in between. It is ok to not be as sensitive because i assume there is overlapping zone 2 and zone 3 as back up.

 

[Distribution73]

jwatty: your indications make perfect sense. Thank you very much.
protoslash: you are right in your last comment. The reason for the 125% is to avoid overreaching due to any error in calcs, relays, cts, etc.
Thank you both very much for your feedback.