Using of 67N and 51N

[freetown]

Hi all,

When we have to use residual earth protection 51N and 67N and what's the difference?
Is it true that 67N is only used on the incomers and 51N on the outgoing feeders?

 

[System Protection]

51 is Inverse Time Overcurrent, so the more current there is the faster it will trip. There are curves associated with the inverse time overcurrent element that will give you the relay operate time with different current levels and time dials. With a modern relay you get to pick the pickup, time dial and curve shape.

67 is a directional overcurrent element/relay. Generally, with either an instantaneous operation or definite time delay.

 

[wcaseyharman]

At the utility I work at 51N is typical for distribution ground protection, and 67N is used in transmission. 51N works just fine if you only have one sources, but becomes problematic once you have multiple sources like a distributed generator, where fault current varies significantly depending on the direction of the fault.

 

[RRaghunath]

Hi all,

When we have to use residual earth protection 51N and 67N and what's the difference?
Is it true that 67N is only used on the incomers and 51N on the outgoing feeders?

67N set to look towards source (could be generator or transformer) is relevant if there is another source in the same system that can feed the fault in the generator / transformer.
In case of simple distribution station such as a transformer connected to a busbar with number of outgoing feeders from the busbar, 67N has no relevance.
51N is used in all outgoing feeders as well as in the incomer. The 51N in the incomer acts as back up protection for outgoing feeder faults, while also being primary protection for faults in the busbar.

 

[freetown]

67N set to look towards source (could be generator or transformer) is relevant if there is another source in the same system that can feed the fault in the generator / transformer.
In case of simple distribution station such as a transformer connected to a busbar with number of outgoing feeders from the busbar, 67N has no relevance.
51N is used in all outgoing feeders as well as in the incomer. The 51N in the incomer acts as back up protection for outgoing feeder faults, while also being primary protection for faults in the busbar.

The configuration we have is 2 incomers 1 & 2 (powered from the power plant) so two busbars with the ability to couple the two with a bus coupler breaker
Some outgoing feeders are connected on each incomer busbar. We had 51N protection on each incomer but the power plant asked to disable it and enable only 67N on each incomer, outgoing feeders still have 51N protection.
It has a thing related to protect their generator. Can you explain please how 67N in this case will work ?

 

[RRaghunath]

The configuration we have is 2 incomers 1 & 2 (powered from the power plant) so two busbars with the ability to couple the two with a bus coupler breaker
Some outgoing feeders are connected on each incomer busbar. We had 51N protection on each incomer but the power plant asked to disable it and enable only 67N on each incomer, outgoing feeders still have 51N protection.
It has a thing related to protect their generator. Can you explain please how 67N in this case will work ?

You seem to have no other incomers to the busbar other than those from the power plant. So, 51N in the incomers will be required as a back up protection for faults in the outgoing feeders. The other option in case this is to be disabled is the 51N at the upstream of the power plant incomers, if one exists there.
With regard to 67N, this will be required if the buscoupler is kept closed. 67N set to look towards upstream of the incomer will trip the incomer breaker in case there is a fault in the incomer feeder or somewhere upstream. This way, the switchboard can continue to remain live supplying the downstream loads when there is a fault in the upstream of one of the incomers. Further, 67N can be set to act faster as it doesn't have to coordinate with downstream feeders and hence the fault can be cleared faster.
If 67N is not set and the buscoupler is in closed position with both incomers ON, both incomers will trip simultaneously (in case of upstream fault) with 51N operating.

 

[NickParker]

This is a bit confusing to me as well, because in some relays, such as the REF 615, even the 51N protection has a directional mode that can be configured.

 

[System Protection]

I imagine most modern relays have that functionality.

Modern SEL relays can have the 51 made directional through the Torque Control setting.






Definitely need to pay close attention to directionality with your 51 elements.