Grounding for 13KV System

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timmc

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Here's the scenario:

13,200 Volt 3-phase/4 wire system is supplied from the local utility
company. The service point is on the load side of the cutouts at the top
of a pole, located at the edge of the property.

Customer owned primary is then run down the pole (3-#2
copper, shielded, 133 percent insulation, Type MV-105 (this cable does not
have a (concentric neutral)) to a listed disconnect rated 600 amps and
fused at 125 amps. The disconnect feeds a 2500 kva delta/wye pad mount
transformer which steps the voltage down to 480/277 to feed the building.

Since the system supplied by the utility company is a 4-wire system, is it
required to run a grounding conductor with the MV cable from the top of
the pole to the disconnect and then to the transformer?

I can't find anything in Article 250 THAT REQUIRES a grounded or grounding
conductor to be run with the ungrounded conductors. Part X of Art 250
covers grounding for systems over 1000 volts and it only addresses
grounding requirements if the system IS grounded. 250.24 clearly requires
it for 50 to 1000 volts, but that section doesn't apply to this system.

Or, since the "service point" is at the top of the pole, can the
contractor elect to go with the proposed shielded cable (cable specs
reflect use for an ungrounded system) and supply the premises with an
ungrounding system (all three shields are connected together at both ends)
and derive a grounded conductor/nuetral at the pad mounted delta/wye
transformer?

250.180 states:
"Where high-voltage systems are grounded, they shall comply with all
applicable provisions of the preceding sections of this article and with
250.182 through 250.190, which supplement and modify the preceding
sections."

The difference I see between this section and 250.20 & 250.24, is that
grounding is an option or is permitted for above 1KV, where lower voltage
systems require grounding WHEN SUPPLIED by a grounded system.

Is the fact that the utility system provides a grounded system at the
service point fall under the scope of 250.180, thus declaring it a
grounded system? or does the installer have the option at that point
(which would be the service point) to install a grounded system in
accordance with this section, regardless of the utility supplied system?

Thanks!

Tim
 
I'm no expert on transformers or medium voltage systems but..
450.10 requires that the non-current-carrying metal parts of the transformer installation shall be grounded where required and in the manner specified for electrical equipment and other exposed metal parts in Article 250.

If you don't run a ground with the service conductors, how can you ground the tranformer case, conduit and other metalllic parts?
I may be missing the boat here, but I'm curious.
steve
 
Tim

I believe you must run an equipment ground with the shielded cables. You have a grounded system and the transformer must be grounded. The fact you have a delta connection and don't need a neutral doesn't exempt you from running a ground that will adequately clear a ground fault at the transformer.

250.190 requires it.

Jim T
 
Dave

I'm not sure I understand your post. It is common to have a delta connected transformer bank in a grounded wye system. I believe that is what the OP described. You would have 3 phase wires connected to the transformer windings on the high side. The ground wire would be to ground the transformer case, cable connections, lightning arresters, etc. at the padmounted installation.

Jim T
 
The core of my question relates to where in the code does it require a grounded or grounding conductor to be run with the MV supply conductors?

50 to 1000 volt requirements are very clear in the NEC that when an AC system is supplied by a grounded system, then the grounded conductor must be run to the service disconnect (250.24). I have yet to find anything specifically requiring this for over 1kv. In fact, 250.20(C) makes it optional for grounding systems over 1 kv.

I would agree that the transformer and metal equipment are required to be grounded. In regard to the installation in question, they are. However, they are grounded to the grounding electrode system at the transformer.

I'm still up in the air on this one. Regardless of what type of system (grounded/ungrounded) is supplied by the utility, it appears, by virtue of the above mentioned code sections, the installer has the option to provide either a grounded (could be multi-point or single point) or ungrounded system complying w/part X of Article 250. I don't think the utility supplied system dictates the grounding requirements for systems over 1 kv, like it does for systems 1 kv & less.

Thanks,

Tim
 
I'm on a learning curve and I have another question...If you're talking about not connecting to the utility supplied ground and using a grounding electrode system to ground a secondary leg of the transformer to provide a neutral for the circuit and also use this grounded secondary to ground the transformer case and other metallic objects, any line to case fault in the transformer will place the transformer case and any bonded metallic object at 13KV to earth. At that point, you're depending on the grounding electrode system to open the primary OCP. The last sentence of 250.4(5) states that the earth shall not be considered a effective ground-fault current path.
How do you reconcile this?
As I said, I'm not experienced in medium voltage services and grounding of transformers and I've got a lot to learn about them, so I'm really interested.
Is it ever allowed to use the earth as the only grounding source?
I know that a fairly high grounding impedance will open a OCP at 13,200 Volts.
Is this the logic? What about 250.4(5)?
Just curious, I hope that I'm making sense.
steve
 
timmc said:
The core of my question relates to where in the code does it require a grounded or grounding conductor to be run with the MV supply conductors?

50 to 1000 volt requirements are very clear in the NEC that when an AC system is supplied by a grounded system, then the grounded conductor must be run to the service disconnect (250.24). I have yet to find anything specifically requiring this for over 1kv. In fact, 250.20(C) makes it optional for grounding systems over 1 kv.

I would agree that the transformer and metal equipment are required to be grounded. In regard to the installation in question, they are. However, they are grounded to the grounding electrode system at the transformer.

I'm still up in the air on this one. Regardless of what type of system (grounded/ungrounded) is supplied by the utility, it appears, by virtue of the above mentioned code sections, the installer has the option to provide either a grounded (could be multi-point or single point) or ungrounded system complying w/part X of Article 250. I don't think the utility supplied system dictates the grounding requirements for systems over 1 kv, like it does for systems 1 kv & less.

Thanks,

Tim
Click to expand...

Tim

Your system is a 7620/13200 grounded wye system. It is already grounded. 250.20(C) would apply if it was transformed to a different system, but it isn't. If you want to set transformers and establish a 13200 delta or something else then you can apply 250.20(C), but you are stuck with a grounded system otherwise.

Unless you have a multiple grounded system on your side of the service disconnect, you can't have a ground at the transformer connected to the 13.2 KV side. That will leave the transformer ungrounded, and still a violation of 250.190.

If you don't pull a ground or a neutral with your shielded cables, you will fry the shield with the first ground fault. It will try to be the EGC.

Jim T
 
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