BBC Distance relay

[frelin]

Hi, I'm in need for some help, it's regarding an old distance relay of electromechanical type, BBC LI41b, see attached image. The protection relay will be exchanged with a newer one, not decided yet, because of age and no spare parts available.
I have managed to get hold of a data paper of a BBC LI41a, but i cannot properly grasp the settings, to transform them to a newer protection relay.

The starting element seems to be of overcurrent type and a setting of 5A.

Zone1: 120%, zone2:67%, zone3: 0% and zoneA: 67%.

The replica impedance setting is, cot fi=0,6 and tan fi=1 with a reach of 0,2 ohm/ph.
Is there a way to get this vector in a+bi form?
Isn't 0,2 ohm/ph with a angle of (cot fi = 0,6 => fi = 59 degrees) very low?

No k0-factor.

The transmission line is 980m AXKJ 3x1x300mm2 (underground) + 1150m FeAl 3x234mm2 (overhead).

Or is there a better way to protect this short line?, directional overcurrent relay?

Sincerly Fredrik

 

[davidbeach]

I certainly wouldn't try to translate old relay settings into a new relay; I'd just apply today's criteria to the new relay. If the line is 0.2Ω and the minimum Zone 1 setting is 0.05Ω you should be fine, particularly if the VT is a wound transformer rather than a CVT.

 

[frelin]

Thanks for your reply David, that I can agree on but still you have to base the new settings on something. Regarding the replica impedance vector, I have: 2,6 + 4,3j, which is the impedance of the line if my calculations are correct (below).

cot φ = 0,6 → φ = 59˚
cos 59 = r/0,2 → r = 2,6
sin 59 = x/0,2 → x = 4,3j

 

[davidbeach]

Right, you have to base the new settings on something; to me that would be raw data. I'd look up (or recalculate) the line data. I'd never assume that an old setting was correct. That 0.6 may have been a compromise setting; maybe you'd be better off with something else with a new relay, maybe not. But you'll never know if you just copy the old settings.