67N Vs 51N

[NickParker]

If both 67N (directional ground fault) and 51N (non-directional ground fault) are available in a relay, which one would you prefer to use?
My colleague suggests that 67N is a better choice because it incorporates zero-sequence voltage, while 51N does not. As a result, 67N is less likely to cause nuisance trips and operates only for actual faults. What are your thoughts on this? Also, how do you typically test 67N (directional earth fault) during commissioning? Is it necessary to earth one phase of the system for testing, especially in a high-impedance grounded system?

 

[wcaseyharman]

Depends if you need the directional capability or not. My company uses 67N on networked transmission to prevent reverse fault tripping, 51N on distribution where reverse faults aren’t an issue.
Our techs simulate a ground fault with a Doble test set, often with the relay out of service but the breaker in service, so no changes are made to the actual system. It’s not a problem to simulate zero sequence voltages and currents.

 

[crshears]

wcaseyharman: 51N on distribution where reverse faults aren’t an issue.

Being a picky bastard, I find this wording just a bit unclear, depending on how one reads it . . .

51N may well suffice on some feeders, but the presence of distributed generation on a given feeder and its ability to supply a portion or all or even in excess of the load on the feeder may well dictate the use of 67N.

 

[wcaseyharman]

Our system protection group does not use 67N even when distributed generation is present. They don’t typically set an instantaneous element in the distribution breaker. Faults in the bus will be cleared very quickly by a bus differential or bus overcurrent relay, and faults on other feeders will trip faster due to the combined fault current from the system and the DG.
We had a misoperation where a fault on a nearby feeder tripped a 50N on a breaker that solely served a 10MW hydro. I advocated for new relays with directional settings. They did install new relays but didn’t set an instantaneous element, just used time overcurrent phase and ground.