Emergency Generator Cables W/Neutral

[Lightninarrester]

Your Input appreciated, I am working on a 1200KW, 13.8kv, 3P, 4Wire Emergency Generator (Neutral Grounded through resistance). The generator cables from the generator go through a disconnect then to a 15kv breaker then to a step down transformer 1500kva 13.8/480Y-277V, then to another 480 breaker. Q1) Do I carry the neutral from the generator and the ground with the phase cables or just three phase cables and the grounded neutral. Q2) Do I bond the generator neutral to the first 15kV breaker enclosure. Q3) Do I keep carrying the neutral (Ground) all the way to the 480V breaker enclosure. Q4) At the low side of the transformer do I carry in each conduit a neutral.

 

[Nick]

Re: Emergency Generator Cables W/Neutral

The most economical way would be, if space permits, to put the neutral grounding resistor right next to the generator or as close as possible.That way your feeder from the generator to the C/B will not need a neutral. You will only need a fully insulated neutral from the grounding resistor to the generator terminal. If you can't locate it there and you want to put it next to the C/B then you will need to run a fully insulated neutral all the way to the C/B and an equipment ground. Your N-G bonding will take place, through the resistor, at the C/B enclosure. The equipment ground will serve to bond the generator frame and conduit and needs to be connected to the load (equipment ground) side of the neutral grounding resistor.

[ June 23, 2005, 09:03 AM: Message edited by: Nick ]

 

[Lightninarrester]

Re: Emergency Generator Cables W/Neutral

Thanks for the input. A follow up question, all the equipment I mentioned are new accept when we connect to the 480 exising breaker in existing switchgear, how would the ground connection work between the new grounding system and the existing grounding system. Thanks

 

[Nick]

Re: Emergency Generator Cables W/Neutral

You are installing an transformer so you will have a separately derived system. All bonding and grounding at the secondary side of the transformer will be like any other article 250 compliant 480V system. In fact, the primary equipment ground to the transformer is nothing out of the ordinary either. All the non standard bonding and grounding takes place at the resistor.

 

[Lightninarrester]

Re: Emergency Generator Cables W/Neutral

I can put the resistor close to the generator.From the generator to the 15kv breaker to the delta side of the transformer, do you carry a bare ground wire? at the generator when you ground the generator neutral do you connect the neutral to the ground bonded to the generator enclosure?

 

[rcwilson]

Re: Emergency Generator Cables W/Neutral

At the generator, run a fully insulated lead (8 kV minimum) from the generator neutral to the line connection on the grounding transformer/resistor. Do not connect anything else to the neutral. Connect the other end of the transformer/ resistor to ground using bare or insulated cable. (?Ground? = a buried ground grid system tied to building steel, or Ufer system, or ground rods, etc).

Run another bare or insulated (600V) ground wire with your 13.8 kV primary cables and connect it to the ground bus at the 13.8 kV disconnect and switchgear.

Be sure to connect the generator enclosure and frame to ground at two places.

Do not bond the neutral of the 13.8 kV system to ground or equipment anywhere. It should only be connected to the grounding transformer/resistor. Otherwise, you will short out the resistor and have high fault currents.

The resistor or transformer/resistor will limit fault current to a low value (maybe 10-20 amps) so there is a temptation to use a small ground wire. But the ground wires are not for carrying ground fault current. Their primary purpose is to bond all 13.8 kV equipment to keep it at a safe voltage during a fault. It is possible to have two simultaneous ground faults creating a phase-phase-ground fault. Then higher currents will flow between the two fault locations on the ground cables. The current limiting grounding resistor is not in the fault current circuit so the current will be higher.

Treat the 480/277 V side of the transformer as a SDS. Ground that neutral and bond it appropriately. That neutral has nothing to do with the 13.8 kV system neutral except they could share the same ground grid/ grounding electrode system.

 

[Lightninarrester]

Re: Emergency Generator Cables W/Neutral

RCwilson thanks alot for your detailed answer. What size of neutral cable do you recommend to use between the ggenerator and the resistor?
Do we select the size of the grounding conductor from NEC table depending on the size of our 3P cables? Thanks

 

[rcwilson]

Re: Emergency Generator Cables W/Neutral

You can use tabels to size the neutral to resistor conductor but it should be #1 awg or larger. I typically use a #1/0, but my generators are bigger.

Since the cable between the neutral and the resistor is insulated for the line-to-ground voltage you end up with a shielded cable which is not easily available in smaller sizes.

Size also depends on the setting of your overcurrent and ground fault protection. The cable's short circuit withstand characteristic needs to be greater than the tiem it takes teh protective relay and circuti breaker to oepn and clear the maximum fault. You are probably OK with your low fault levels of a typical 1200A, 13.8 kV generator, but teh withstand rating of teh cable should be verified.