We are developing a protection requirement to a power line of 69kV. This line is about 7 kilometer. Because of costs we use 50/51 protection and 67N to backup protection. Is this scheme efficient??? What are the disadvantages of it?
If the line is truly radial, then there are no real disadvantages of using 50/51 relays. For the 67N, you might want to look at magnetic coupling between the lines if they are parallel.
Hello.
I would like add several points.
Standard solution in this case is 50/51, but don't forget:
On the second side( w/o generation) we must install
directional overcurrent 67 with time delay about 0.2-0.5sec.
If we use 67N as back-up protection, we also must install on other side 67N with reverse direction ( time setting lower then 67N on generation side).