Low Resistance Grounded 35.5kV System

[JBUDA54]

I am trying to size the NGR's conductor from a wye delta isolation transformer. I believe the NGR is rated 75A continuous and can allow ~900A for 10 sec and has an 8 ohm impedance.

When I model the system using SKM the damage curve of the conductor requires a 500kCMIL cable to withstand the fault current that the system will see. I have a co-worker that plugged in the ~900A SC rating of the NGR at 10 sec into the Thermnal damage curve equation and came up with a much smaller cable size. see Link

Copper conductors

t = 0.0297 log10[(T2+234)/(T1+234)] (A/I)2 (1)

My question is... Was the thermal calculation done improperly by limiting the SC current to ~900A for 10 sec that resistor can withstand? Does the Resistor open circuit if its rating is exceeded and limit the fauilt current of to ~900A over 10 seconds or does the fault continue to flow and require the SKM result of a 500kCMIL that the TCC curve is illustrating?

My guess is that my coworker using the NGR limitation as the Cable damage curve threshold is an incorrect approach.

I can't conclude this as I don't know what happens to the NGR once is rating is exceeded.

Thanks in advance.

 

[waross]

This is a 35500 volt system, right.
Line to neutral is 35500 V / √3 = 20596 Volts.
20595 Volts / 8 Ohms = 2562 Amps.

I can't conclude this as I don't know what happens to the NGR once is rating is exceeded.

It gets hotter. If it is a resistor bank it will generate about 52 MW of heat.
At the 10 second rating (900 Amps) it will generate about 6.5 MW of heat.
At about 8 times the 10 second rated current you can expect the NGR to get very hot, very fast. Expect it to melt soon, possibly in less than 10 seconds.
Then the impedance becomes the resistance across the arc across where the NGR used to be. Eventually the arc may burn clear or some protection may clear the arc. When the arc is extinguished the NGR will most likely become an open circuit.
These values may be a little less due to MVA limitations of the source system.
You may want to consider a little higher impedance NGR at that voltage.

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

 

[Parchie]

"Low-resistance grounding" usually limits the ground fault around 200-400A. Choosing a 900 amperes level means your resistor will be 35,000/(1.732*900)= 23 ohms, not 8 ohms. What @ waross said, you will have a hard time finding equipment to carry 2kA+.

 

[JBUDA54]

Sorry Gents. My ratings were going from memory when I wrote this the value of the NGR is 20 Ohms.and the 10 sec rating is closer to 1000A.

I believe that if my coworker were to plug the SLG fault values to the cable thermal damage equation they would get similar results as what SKM is producing.

Thanks for your comments.