12,470V high impedance

[ryan_618]

I have a job that I am inspecting that has a 12,470 volt, ungrounded service. From the utility transformer, they come into the building and immediatly to a service disconnect. After the disconenct is a transfer switch that is attached to a 12,470/7,200 volt, 2.5Mw generator. The generator is impedance grounded. The grounding impedance is 36 Ohms, limitting the fault current to 200 A.

My question: What is the point of having an impedance grounded system with the impedance set so high that it allows 200 amps of fault current??? The 12,470 equipment (after the transfer switch, of course) is all transformers. 12,470 to 4,160 and 12,470 to 480. The circuit breakers for the primary to the 480 transformers will be less than 200 amps.

Any thoughts?

 

[jtester]

If you assume that the current on the ground is supposed to be zero, 200 amps would be a cinch to detect, and would significantly limit the damage compared to high current l-g faults.

Jim T

 

[ron]

The 200A is acceptable on the gen windings for a period of time.

 

[ryan_618]

I guess what I am trying to say, is that most people install a high impedance system so the system stays online during a ground fault. Why would you design a high impedance system that will iniate OCPD's on the first fault?

 

[Nick]

It sounds like the grounding impedance is there more to protect the generator from high fault current damage that for system continuity. Typical levels a 200-400A on these systems.

 

[ron]

I agree with Nick. I often design the standby generator (especially if it is an 'N' system) as high impedance, to allow it to run, and save the windings.

 

[zyhml]

is there any code about generator impedence grounded? what if low impedence was designed what would happen? how to do bonding in high impedance grounded generator and low impedance grounding system case? is it practical?