When is a neutral required? 3

[paradigm187]

I'm helping design a 22kV overhead line and was recently told that the owner doesn't want a neutral in order to save money. All loads are three phase motors so imbalance shouldn't be a problem, but I'm wondering if there are any rules regarding when you MUST use a neutral. From what I've read in the NESC, there are rules about the use neutrals and grounding, but nothing that states when they must be used. Am I missing something?

 

[cuky2000]

See the enclosed link:

https://www.ecmweb.com/code-basics/characteristics-neutral-conductor

 

[itsmoked]

cuky did you miss the "22kV overhead"?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RRaghunath]

At 22kV level, the neutral is earthed at the transformer itself and is not extended to switchgear and beyond (OHL etc.).
22kV OHL will have three phases and a shield conductor on top.
Shield conductor is optional depending on the OHL route, lightning possibility and whether the area is rocky etc.

Rompicherla Raghunath

 

[waross]

Are the motors 22 kV or is the 22 kV transformed down for the motors?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Fritzy92]

If the NEC is applicable, it seems that you would need to run a conductor to serve as an "Equipment Grounding Conductor" if you don't use a multigrounded neutral system.

 

[paradigm187]

Thank you everyone for your replies. I'm still looking for a definitive answer, preferably one that has origins in the NESC.

Cuky and Fritzy - The NEC is not applicable for this project.

RRaghunath - Are you suggesting that all 22kV lines don't have a neutral? I assumed that most low voltage lines would be used for distribution.

waross - The voltage is stepped down with wye-delta transformer to 480 for the motors.

 

[waross]

That's a big problem.
the 480 Volts is a separately derived system and will need to be grounded.
You may consider using a delta-wye transformer or transformer bank.
That will give you a neutral to provide a symmetrical system ground for the 480 Volt system.
That will also avoid the switching over-voltage transients that may occur on a wye-delta transformer with a floating wye point.
You don't need a neutral on the 22 kV.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Paradigm187, When the loads downstream are all three phase type, which is the case at 22kV level, there is no need to provide neutral in distribution OHL.
The step-down transformer 22/0.480kV is YNd connected as mentioned by you. But, there is no need to wire the neutral to the transformer HV. It is sufficient if the transformer 22kV neutral is earthed at the transformer.

Rompicherla Raghunath

 

[waross]

I agree, RRaghunath,that the 480 Volt does not need to be grounded under some codes, including the code that I am subject to..
However grounding the 480 Volt system avoids over-voltages due to discontinuous ground faults and inadvertent common mode over-voltages due to accidental contact between a 480 Volt phase conductor and higher voltages.
In the absence of a neutral the 480 volt system may be corner grounded or if a 240:480 Volt transformer is used, the center tap may be grounded.
The wye:delta arrangement may have issues with circulating currents and with back-feeding into lost phases.
Changing to a delta wye transformer avoids the wye:delta issues and provides a neutral for system grounding.
Although the 480 Volt system does not need to be grounded grounding the 480 Volt system is highly recommended.
I note that the OP's original question concerned running a neutral conductor with the overhead.
We are in agreement that there is no need for a neutral to be run with the 22kV circuit conductors.
Interestingly, the utility that supplies my residence does not run neutrals for single phase distribution circuits. One wire only.
There is many miles of single wire, single phase distribution in rural areas.
At each service transformer, the neutral side of the transformer is grounded with a driven ground rod. A neutral conductor is carried overhead for one span and grounded a second time with a driven ground rod. End of story, end of neutral conductor.
I was quite taken aback when I moved to this area 10 years ago. The practice is widespread and I have never heard of an issue or accident attributed to the lack of a neutral conductor.
If it stops snowing I may be able to get a picture but no promise.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

Interesting waross. Note to the OP, I believe the NESC would prohibit that practice, section 314.C.4.a in the 2007 version.

 

[paradigm187]

Thank you for your helpful replies! Here's what I conclude: A distribution line with 3 phase loads does not require a neutral to simply operate, however the NESC requires pole grounds every 1/4 mile so either an OHGW or neutral is required to accomplish this.

I live in the U.S. and have seen a few lines with 3 phases and no OHGW or neutral. I suppose these must have been built before the NESC started requiring grounds. Even RUS has drawings in their current bulletins of transmission poles with no OHGW. From what I've heard, these lines are more common in Canada.

 

[bacon4life]

paradigm-Are you referring to NESC 96C note 2 and NESC 97C? "Multi-grounded systems" are a specific type of construction that allows smaller clearance distances between the conductors and other objects. Rule 97D clearly states that ungrounded systems are allowed.

 

[paradigm187]

Bacon, Yes I was referring to 96C. Thanks for correcting me!