Using 51N on a 3 wire system

[cvirgil]

We plan on using a 51N relay on a 13.8kV system, 3 phase, 3 wire to provide for ground fault protection. A co-wowrker syas we can't use a 51N device since we don't have a neutral.

Any comment or input would be appreciated.

Thanks.

 

[jghrist]

Is the system grounded or ungrounded? If it is a grounded system, you can detect ground faults with a residually connected 51N relay. You can have a neutral on the CT secondary even if you don't carry a neutral on the primary.

If ground faults are low, and you have cable feeders, you may want to get additional sensitivity by using a CT around all three cable cores.

 

[cvirgil]

the system is grounded (according to the utility company they use a uni-ground. never really heard this term used before).

 

[davidbeach]

There seems to more than one definition of "51N" by the manufacturer's and users of relays, but in general a 51N would be a function of calculated residual current while a 51G would be a function of measured residual current. The measured residual current can be either the hardwire summation of the phase CTs or it can be the result of a single CT around all conductors.

If you have a three wire, solidly grounded, system the 51N is probably sensitive enough detect your ground faults. The 51G can be more sensitive since it can use a CT with a lower ratio. If you have 400:5 phase CTs, the 51N is based on that ratio, but if you use a 50:5 core balance CT, you have 8 times the sensitivity.

Which one is right? It depends. Either will work and it is up to you to evaluate the pluses and minuses of each to determine the best course. Many relays allow you to do both.

 

[rbulsara]

You need to find out what "uni-ground" is. There was some discusison on that term a while ago here. No need to assume anything here.

 

[dpc]

A uni-ground system should be solidly grounded, so the 51N residual CT connection should work fine. You just need to get some idea of the maximum ground fault current as well as the minimum fault current you want to be able to detect.

There is no standard definition of the difference between 51N and 51G. These terms are somewhat interchangeable. In the US, the "N" is conventionally used when dealing with a residual CT connection and "G" is used when the relay is connected to a CT in a grounded neutral conductor. However, Schweitzer relay documentation is just the reverse of this, as I recall.

 

[sgtan1]

I am still confused.

Is there any difference in the the two, When one make the same type of connections. Assuming a 4th coil is available for the ground fault detection?
Both being a summation of the 3 phase's individual CT?
Or is it just a difference in measurement method?

51N being protection from a calculation from 3 OC coils?

 

[rbulsara]

For vector sum system (3 or 4 CT), if you have a load neutral (4 wire load) you will need the 4th coil (for neutral) and if you do not have a load neutral you do not need the 4th coil. Even if it is there there the current thru it will be zero and wont make any difference to the vector sum.