Impedance Grounding

[Nick]

Here is the installation. It?s a central plant for a hospital. There are going to be two 1000KVA 4160V generators with provisions for two more in the future. A line up of paralleling gear ties everything together. All overcurrent/disconnecting means are in the paralleling gear. The plans call for neutral grounding resistors for each generator. These will consist of one Avtron N/G resistor for each genset.
250-180 states that high voltage systems shall comply with all applicable provisions of the preceding sections. What I am unclear on is the actual wiring method for the resistors. Does the conductor have to be fully insulated to 5KV? 250-36(d) allows the conductor to be run in its own raceway. Does this apply to >1000V systems too?
If I am interpreting this right I can run the grounding electrode conductor to ground the gensets into the paralleling gear and make all the grounding connections there. Then run a (600V or 5KV?) insulated conductor in a conduit to the NGR. From there that conductor can continue to the genset in its own conduit to the neutral point of the generator. The 5KV circuit conductors would then run from the paralleling gear to each generator without an equipment grounding conductor. (If one were run it would effectively shunt the resistor) IS this correct?
I tend to want to run two conductors to the N/G resistor in the same conduit. One from the equipment ground bus to the resistor, one back to the gear to run back with the 5KV circuit conductors. Does anyone have some experience with this?

 

[jcormack]

Re: Impedance Grounding

The leads from the genset neutral to the resistor must be fully insulated - - the leads from the resistor to the ground grid may be insulated or bare - I prefer insulated - Do not run the leads from the genset neutral to the resistor and the resistor to ground leads in the same raceway unless both are insulated - otherwise a fault in the insulation of the one lead could result in a bypass of the resistor.

 

[bennie]

Re: Impedance Grounding

Nick: You indicate "central plant", are you saying there is no utility service?

 

[bennie]

Re: Impedance Grounding

Nick: Go to the EC&M magazine site.
Select Feb.1,1999 issue, article title "Solid Grounding for Your Generator".

There is some good information on impedance grounding techniques.

 

[Nick]

Re: Impedance Grounding

Thanks for the replies.
I got a response from Avtron today. They basically said what jcormack said. They added to keep the resistor as close as possible to the ground tie point.

Bennie,
It?s a central mechanical plant and these are emergency generators for the entire campus. The normal power comes in at 12.47KV in some switchgear outside.

I guess 250-36(d) just strikes me as odd. It is completely opposite of what we do everywhere else in electrical systems! Running the grounded conductor of an AC system by itself in a conduit and not bonding it to both ends just seems strange.
Now I will write Avtron back and ask if the neutral (grounded conductor) has to be shielded like the phase conductors.

 

[bennie]

Re: Impedance Grounding

Nick: There is probably no current on the neutral, it will be a MGN. Shielding will not be required.

You can use cable with a concentric neutral.

Be sure and use stress cones on the terminations.

 

[bennie]

Re: Impedance Grounding

Nick: How is the utility power switched? Is it shut off, and the generator power put on line?
I would think the utility neutral is switched due to the impedance grounding.

 

[Nick]

Re: Impedance Grounding

Bennie,
This is a customer owned 12KV normal and 4160V emergency distribution system. Both normal and emergency power is stepped down to 480V outside each building with oil filled pad mounts. Emergency and normal power is switched via 4 pole ATS?s inside the buildings. A very typical installation. SCE is the serving utility and they usually use a MGN system but that would stop at the 12 KV service gear. There are no ?neutrals? utilized in the medium voltage systems with the exception of the conductors from the ?neutral grounding resistors? to the neutral terminal of the generators.

 

[bennie]

Re: Impedance Grounding

Nick: Check your single line...I think you will find the MGN is electrically complete to the premises wiring.

 

[Nick]

Re: Impedance Grounding

Bennie,
Nope. The conductors are shielded 15 KV MV-90 with 133% insulation level. The shield gets bonded to the equipment grounding conductor at each man hole and each termination. Unless my idea of a multi grounded neutral system differs from yours!?

 

[bennie]

Re: Impedance Grounding

Nick: The conductor you call the equipment ground conductor, I call it the multi-ground neutral.