the distance relay calculate tthe negative distance to fault location

Dear Sirs,
Could you explain why the distance relays such as 7SA 511, SEL 321 which protect for the 110KV radial transmission lines calculated the negative distance to the fault location?
Thanh you very much for your explainations.

 

[busbar]

I wouldn’t waste a heartbeat waiting to contact SEL's technical support. Most will agree it is second to none.

If you want to do your homework first, most everything they have authored is available on line.

 

[electricpete]

I think the link is

http://www.selinc.com

Here's what little I know on the subject. The distance relay actually measures the impedance from the point of measurement to the fault. If the impedance consists primarily of line inductance and you know the inductance per unit length of line, it seems pretty straightforward to compute the distance as Inductance/(inductance per length). Fault resistance may complicate that a little. Sorry if I'm telling you what you already know.

 

[peterb]

Did you have actual cases where the relays gave a negative distance readout for a forward fault?
I have had fairly extensive experience with the SEL relays, having installed some 70+ in a retrofit exercise, and have had nothing but positive feedback in terms of the distance to fault feature. Accuracy has certainly been well within 5% of line length and has considerably reduced line patrol effort - the linesmen love it.
BTW, Electricpete, the relays use the line reactance to determine distance to fault, which takes the fault resistance out of the picture.