Ground/Neutral Overcurrent Protection Settings 2

[Islander1]

Hello All,

I'm trying to provide ground/neutral overcurrent setting for a land based microgrid system (The microgrid is connected to utility via Delta/Wye-grounded transformer and I have a SEL 751 relay for my protection), but I don't have much experience in ground fault protection. Therefore I would like to hear the thoughts of senior engineer in the protection field regarding my protection case.

While reading about ground fault protection, I kind of got confused with neutral overcurrent protection:
a. Neutral Overcurrent [50N]
b. Neutral Time-Overcurrent [51N]
c. Ground Overcurrent [50G]
d. Ground Time-Overcurrent [51G]

My questions are:
1.) when do we use neutral and ground overcurrent protection?
2.) do I need to use all 4 of them for my microgrid system?
3.) what are the things that I should be careful about while setting the pickup and delay settings?

Any help would be appreciated!

Thanks,

 

[RRaghunath]

'Neutral' refers to the earth fault protection that takes residual current input from the phase CTs, whereas, 'Ground' refers to that with input from CBCT or from the CT located in neutral-to-earth connection of the equipment (transformer, generator etc.).
50/51N cannot be set sensitive and typical pickup threshold setting is 15% of phase CT or more.
50/51G can be set sensitive with setting going as low as 5%.

 

[Islander1]

Thank you for your reply RRaghunath,

In my system we have only phase CTs, so I should be using only 50/51N in that case. am I right?
Do you have any suggestion regarding the 51N delay curve? Currently I set it to U4, (extremely inverse) with time delay of 5.0 seconds.

Thanks,

 

[davidbeach]

What RRaghunath says is true per C37.2. But if you're using any recent SEL relay (the U4 suggests that), the relay functions with a G use a current that is the vector sum of the A, B, and C currents while the relay functions with an N use the current measured at the IN terminals. If you are using an SEL relay and you only have phase CTs, and you have not wired the residual connection back into the IN terminals, use of 50N and 51N will not do anything, but 50G/51G will.

 

[Islander1]

Hi David,

Yes, you are I'm using recent SEL 751 relay. I'm actually using typical phase and neutral current connection for a feeder application, please see the attached picture (it is the Figure 2.19 in SEL751A instruction manual). I've wired the residual connection back into IN terminals.
Do you have any suggestion regarding the pickup and delay setting for this current SEL relay?

Thanks,

 

[davidbeach]

In that case you can use either N or G as you choose, both will give the same result.

It's impossible to begin to say what reasonable settings would be without far more information than is available there. Faster than what's upstream of it and slower than what's downstream. Easy to say, not always so easy to implement.