Protection of Neutral Grounding Resistors

[McDaniel8402]

Hello Folks. New to the forum.

I'd like to get some community input on the topic of neutral grounding resistor protection settings when setting ground fault protection settings on a low resistance grounded system. When I was first introduced to system relaying and protection, the engineers that were training me drilled into my head to always set ground fault settings as low as practical. In real numbers, this meant protecting a NGR that was rated for 400A @ 10sec with a 51G setting of between 50 and 100 amps, set as low/fast(in time) as possible while still coordinating with downstream devices. I always questioned why we couldn't just stay beneath the resistor damage curve.

As time has gone on, i've adopted the philosophy of setting 51G settings as low and fast as practical. However, i've never found any literature that would really support the idea that "anything" below the resistor damage curve is acceptable, or not acceptable.

Per IEEE, 10% of resistor rating is typically considered to be the allowable continuous current rating of an NRG. I'm assuming that the low & fast mentality comes from aiming at the 10% mark, but that's only a guess.

In recent times, i've had folks tell me that setting a 51G at 200 amps for 2 to 3 seconds on a "400A @ 10sec NRG" is perfectly fine, and logically, they should be correct.

Any of you folks have much to say on this? Any good case studies that would suggest a best practice? Did I accept gibberish from my old trainers?

 

[waross]

Not all faults are bolted faults. Your philosophy may serve well for a low resistance fault but not detect a higher resistance fault. In the case of an arcing fault, much damage may be done by the arc if the 51G setting is too high.
As well as fault resistance, the supply conductor impedance to a distant fault may be a factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

If fast and sensitive coordinates and allows for sufficient measurement error for all conditions, I'm not sure what advantage would be gained by increasing time or current.

 

[McDaniel8402]

Thank you for the responses!

I agree completely with issue of not setting the 51G setting too high in order to prevent equipment damage during an arcing ground fault. Clearing the fault quickly and at low magnitude is best.

My question is really in regards to the neutral grounding resistor itself, and not the other equipment in the system. Although it is obvious that clearing fast and low is less detrimental to the equipment, is there anything technically wrong with choosing a setting (any setting) so long as it falls beneath the damage curve of the neutral grounding resistor?

When discussing this with other folks, I would always er on the side of fast and sensitive, but I don't have anything to support my opinion in terms of case studies, white papers, standards, etc. Strictly technically speaking, i'm not sure that there is anything "wrong" with choosing a pickup of 200 amps (or greater, all the way up to the resistor rating), as long as the time dial setting puts the relay curve below the resistor damage curve.

I hope i'm making sense here. I'm not arguing in favor or against anything, just looking for some supportive documentation for the "fast and sensitive" philosophy.

 

[krisys]

The earth fault relay connected to the transformer neutral CT is the last in the line of protection. Hence for a given system, this relay shall operate last. Your primary focus shall be on achieving the correct ground fault relay coordination with the downstream devices.

While doing so, care shall also be taken to ensure that the relay connected to the transformer neutral CT operates sufficiently below the damage curve of NGR.

From my experience, the NGR short time rating was never a concern while ensuring the ground fault relay coordination. However, you may perform a verification check for that at the end, mostly for the formality sake.