5kV cable 1

[james001]

I'm working on the 5kV standby generation at the water treatment plant. I spec'd out that each 2000kW gen must have 500MCM cable connected to the 4160V bus. My question is, does the cable needs 1/3 concnetric neutral, if not, how are we going to ground this, or does it need the ground at all?

Thank

jimmy

 

[dpc]

You don't need a concentric neutral - just run a separate ground conductor along with the phase conductors. It can be low voltage (600V) insulation. You do need a ground wire.

Concentric neutral cables are used by utilities for UG distribution systems to avoid the need for a second conductor. They are not often used for industrial/commerical power systems.

 

[busbar]

The “one-third neutral” cable is a typical rating for underground residential distribution in electric utilities. For industrial applications, it is more common to use tape-shielded cable, or for 4160V, unshielded cable.

In all cases, do not rely solely on the cables’ concentric neutral or tape shield for equipment grounding/bonding of the generator-set enclosure . System grounding {of the genset stator winding} is a completely different issue.

 

[alehman]

If you are refering to grounding of the generator neutrals, that depends on a lot of factors. Are the generators to be connected to a system with a solidly grounded neutral? If so you may want to connect the generator neutrals to the system neutral, without separately grounding the generator neutrals.

You will need to do a fault current calculation and look at both 3phase and phase-to-neutral fault levels. Depending on the generator's zero sequence impedance, you may need to install current limiting resistors or reactors in the neutral circuits. Consult carefully with your generator manufacturer.

I agree with the above comments regarding concentric neutral cable. You also need to ensure that your overcurrent protection is adequate to prevent damage to the grounding conductor whether it is cable shield and/or a separate wire.

 

[jbartos]

Suggestion: Very often 4160V uses the high-resistance system grounding. The concentric neutral could be used to connect the neutral to the distribution grounding transformer and to ground to implement the system grounding. The high-resistance grounding calculation is normally performed to determine a size of the distribution grounding transformer and its secondary resistor.

 

[james001]

thank you all for the comments...

 

[peebee]

I agree with all above posts, with one exception: dpc's statement that "You do need a ground wire."

I don't believe NEC would require any ground wire be run with the phase conductors [please verify we're all talking US codes here]. I believe this statement is true regardless of system grounding (solid/resistance/reactance/ungrounded). I could be wrong and would certainly invite comment on this.

 

[jbartos]

Suggestion to the previous posting: PVC or non-metallic conduits, in general, need to have a ground conductor to be run inside conduit with phase conductors.