Ungrounded 11KV system

[julianov]

Hello everyone,

Does the NEC say anything about Ungrounded 11KV systems? I noticed that section 250.21(B) addresses systems with less than 1000 volts.

 

[Elect117]

See Art 250 Part X (part 10). 250.180 - 250.194

 

[julianov]

See Art 250 Part X (part 10). 250.180 - 250.194

Thanks but that for Grounded systems. How about ungrounded system over 1KV?

 

[Elect117]

Is this a utility system? Or could you provide more detail on the system?

The question is vague. Something will be grounded.

 

[julianov]

Yes sure,
We are using two transformers because the motor is far away, and there isn't enough room to use a large wire size due to the existing conduit constraints. The motors and Panel A operate at 480V, with the first transformer stepping up to 11KV and the second transformer stepping down from 11KV to 480V.

The first transformer is configured as 480V Y / 11KV Δ, and the second transformer is configured as 11KV Δ / 480V Y.

As you can see, the feeder between the transformers is ungrounded. Which NEC articles cover the rules we need to follow in this situation?

 

[Elect117]

Yes sure,
We are using two transformers because the motor is far away, and there isn't enough room to use a large wire size due to the existing conduit constraints. The motors and Panel A operate at 480V, with the first transformer stepping up to 11KV and the second transformer stepping down from 11KV to 480V.

The first transformer is configured as 480V Y / 11KV Δ, and the second transformer is configured as 11KV Δ / 480V Y.

As you can see, the feeder between the transformers is ungrounded. Which NEC articles cover the rules we need to follow in this situation?

View attachment 2572204

480V delta to 11kv wye and ground the wye point and dont run the neutral. then 11kv delta to 480v wye, and ground the wye point and dont run a neutral to the motor.

wye-delta transformer config is not advised. delta primary is better.

what is the distance to make two transformers worth it?

 

[julianov]

Because by NEC 240.21(C)

The six applications covered in 240.21(C)(1) through (6)
permit transformer secondary conductors without an over-current
protective device at the point the secondary conduc-tors
receive their supply under one of the following
conditions:

1. The primary overcurrent protective device, as described
in 240.21(C)(1), can protect single-phase (2-wire) and
3-phase (delta-delta) transformer secondary conduc-tors.

Therefore, we always use a delta configuration on the secondary side to avoid adding an 11 KV circuit breaker at the secondary. Does the NEC mention the need for a ground detector in systems over 1KV that are ungrounded?

what is the distance to make two transformers worth it?

1400 feet

 

[Elect117]

450.3 still requires protection on both sides in other than supervised locations.


1400ft isn't that long.

Voltage Drop Calculator | Southwire

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I got 1/0 and larger will suffice for 55A at 1400ft.

But then again, I am not the one buying everything.


And I am needing one, no. They will have to be grounded in accordance with 250.30 (GECs, EGCs, SSBJs, etc.) and since this is new, you will probably have to make sure you ground it in some fashion. The methods are described in the sections I sent. Probably corner grounded will be the easiest.

 

[augie47]

Did you look at the possibility of 600v or 1000v units considering the distance and load ?
. 11kv sure brings about higher expense

 

[zbang]

there isn't enough room to use a large wire size due to the existing conduit constraints.

Is there enough room for three 15kv cables and can you actually pull the stuff in without damage? I'd be barking up that tree before worrying about ungrounded systems. And I think if it's 11kv corner-grounded, the grounded lead can be 600v wire instead of 15kv (smaller and cheaper).

 

[julianov]

Thank you very much, everyone. I was looking for where in the NEC rules and guidelines for ungrounded systems over 1000 volts are provided. Mostly to keep in mind for the future and for purely academic reasons. Are ungrounded systems over 1000 volts permitted in USA? From my perspective, there are no ungrounded high voltage systems. I do know that, for example, in Japan, they use an ungrounded system in their high voltage distribution system, but not in the USA.
But I'm looking for a section of the NEC that clearly communicates this.

 

[tortuga]

Thank you very much, everyone. I was looking for where in the NEC rules and guidelines for ungrounded systems over 1000 volts are provided. Mostly to keep in mind for the future and for purely academic reasons. Are ungrounded systems over 1000 volts permitted in USA?

Yes, See 90.2 for the scope of the NEC and 250.20(C) for the requirements for AC systems over 1000V.
There have been lots of changes recently in the over 1000V space and I am not up on it. I don't think over 1000V has to be grounded per the code.

 

[julianov]

Thanks.
Article 250.20(C) addresses ungrounded systems above 1000 volts, permitting them provided they do not supply mobile or portable equipment. However, it does not require the installation of ground fault detectors, which is noteworthy because it could delay the detection of initial ground faults.

On the other hand, according to 250.21(B), ungrounded AC systems allowed under 250.21(A)(1) through (A)(4), operating between 120 volts and 1000 volts, must have ground fault detectors installed. However, there is no such requirement specified for systems operating above 1000 volts.

 

[GoldDigger]

Keep in mind through this discussion that the POCO-owned transmission facilities are governed by the NESC, not the NEC. The NEC only comes into play when the circuit(s) meet the consumer (or anything outside the system of the POCOs involved.)

 

[tortuga]

However, it does not require the installation of ground fault detectors, which is noteworthy because it could delay the detection of initial ground faults.

There is 110.10 which is a catch all that generally requires you can clear a fault without excessively damaging the equipment. Systems over 600V typically have to be designed and stamped by a PE around here, I don't think the regular electrical inspectors even inspect them. One of the few over 1000V I know of the customer pays the utility to have their linemen maintain it.

 

[LarryFine]

We are using two transformers because the motor is far away, and there isn't enough room to use a large wire size due to the existing conduit constraints.

Let's start there: What size is the conduit, and while I'm asking, what type, any interruptions, etc?

We need to sort and define the constants before juggling the variables.

 

[kwired]

Because by NEC 240.21(C)



Therefore, we always use a delta configuration on the secondary side to avoid adding an 11 KV circuit breaker at the secondary. Does the NEC mention the need for a ground detector in systems over 1KV that are ungrounded?



1400 feet

What size motor is being supplied?

For irrigation wells around here we run into similar distance on most installations and run up to 125 HP motors all the time with up to around 2000 feet of 480 volt supply conductors. Transformers and other needed items to go that route cost more than increased conductor size, never seen one using copper conductor though that maybe would make a difference. Copper usually only used for the short run from a splice box to pump panel and then to motor. I've even made parallel runs across the field a time or two when large conductors were needed for voltage drop on longer than usual runs.

 

[tortuga]

What size motor is being supplied?

For irrigation wells around here we run into similar distance on most installations and run up to 125 HP motors all the time with up to around 2000 feet of 480 volt supply conductors.

You've mentioned that before is that becasue your POCO does not supply 600V service or do your irrigation suppliers not offer 600V pump panels?

 

[kwired]

You've mentioned that before is that becasue your POCO does not supply 600V service or do your irrigation suppliers not offer 600V pump panels?

Center pivot irrigation machines are all 480 volt, never seen one that was any other voltage. Well pump motors are only stocked in 480 volt. For whatever reason that is what they started out with long ago and it likely not going to change now with so many out there that are 480 volts. Under 75 HP you do find them as 230/460 dual volts but 75 and up is almost always single voltage 460, may have leads for part winding start or wye/delta start though nobody uses that anymore. POCO's usually only require reduced voltage starting on motors over 100 HP but most those anymore are done with soft starters.

Pump panels are 600 volt rated and have 600 volt fuse holders, the ones we typically use would only need to replace the contactor coil with a 600 volt coil but would otherwise be the same thing. Maybe would be a change in which panel is selected at times because of lower current at 600 volts but otherwise same ones with different contactor coils would still be what is stocked.

I ran into a 200 HP well before, I did not install it, but it had primary lines ran out and transformer was out in field near the well. That one was reduced voltage transformer type starting method and was also 480 volts.

 

[electrofelon]

Thanks.
Article 250.20(C) addresses ungrounded systems above 1000 volts, permitting them provided they do not supply mobile or portable equipment. However, it does not require the installation of ground fault detectors, which is noteworthy because it could delay the detection of initial ground faults.

On the other hand, according to 250.21(B), ungrounded AC systems allowed under 250.21(A)(1) through (A)(4), operating between 120 volts and 1000 volts, must have ground fault detectors installed. However, there is no such requirement specified for systems operating above 1000 volts.

Since no one has come out and said it, No you do not need to ground MV systems, and you do not even need ground detectors.

I am very skeptical it's worth messing with transformers at 1400 feet. Have you built in the poor voltage regulation/voltage drop of having two transformers into your evaluation? Most people simply leave that out and of course that creates a very unfair comparison.