Delta system neutral point

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mivey

mivey

Senior Member
This concerns the NEC definition of neutral point. For the single phase system, I have heard no conflict. For the three phase system, there seems to be some confusion about a neutral point on a delta system.

I contend that there is no neutral point on a delta system but that there is a neutral point on the single phase PORTION of the delta system. Consider the following as an explanation:

deltawyeneutralpoint.jpg
 
quogueelectric

quogueelectric

Senior Member
Location
new york
It is not a sub system it is a part of the whole 4 wire delta system it is by definition a center tapped winding on one side and it is not single phase. There are 3 separate windings and one of them is center tapped simple. 120* out of phase with each other. Dont forget it takes 2 phases off the primary delta to drive this winding. It is not single phase.
 
mivey

mivey

Senior Member
neutral vs ground reference

neutral vs ground reference

I'm think I'm about convinced that the neutral on the wye and delta serves as a neutral for the single-phase loads and as a reference to ground for the three-phase loads.

This being said, the neutral on the wye system is truely a neutral point to all three phases. It is also the neutral for the single phase loads.

The neutral point for a delta system can be derived with a zig-zag transformer. This derived neutral point is NOT the mid-leg tap point reference that is the ground reference and is also used as a neutral for the single phase loads.

With that, a corner grounded delta has no neutral (unless derived using the zig-zag xfmr) but only has a ground reference.

What say ye?
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
mivey said:
What say ye?
Click to expand...

I say for the purposes of the NEC they can call define a neutral in anyway they want.

I do know they have spent at least two code cycles trying to define neutral.

Here is some info you may find interesting.


5-36 Log #1554 NEC-P05 Final Action: Accept in Principle

(100.Neutral Conductor and Neutral Point)

TCC Action: The Technical Correlating Committee directs that the action on this proposal be sent to the Technical Correlating Committee Task
Group on the definition of ?Neutral Conductor? for review and comment.

Submitter: Technical Correlating Committee on National Electrical Code?,

Recommendation: Add the following definitions to Article 100:

Neutral conductor. A circuit conductor connected to the neutral point of a
system.

Neutral point. The common point of a wye-connection in a polyphase system
or midpoint of a single-phase, 3-wire system or midpoint of a single-phase
portion of a 3-phase delta system or midpoint of a 3-wire, direct-current
system.

Substantiation: This proposal was developed by the TCC Task Group on the definition of ?Neutral Conductor.? Task Group members were: Jeffrey Boksiner (Chair) (CMP 5, TCC ), Paul Dobrowsky (CMP 5), Walter Skuggevig (CMP 5), Doug White (CMP 5), Michael Toman (CMP 2, TCC), Bob Wilkinson (CMP 2), Jim Daly (CMP 6, CMP 7, TCC), Bill Laidler (CMP 6), and Oran Post
(CMP 6).

The definition of ``neutral conductor?? and the associated definition for
?neutral point? is needed in the NEC so that the appropriate conductor can be
identified whenever this term is used in a requirement such as in 250.26 and
250.36. The proposed definition is derived from the IEC definition of ?neutral
conductor? and IEEE Std C57.12.80-2002 definition of ?neutral point.? The
proposed definition was adapted into the NEC language and was expanded to
cover the various cases relevant to the NEC.

The attached figures illustrate the meaning of the proposed definition. Note
that according to the proposed definition ?neutral conductor? exists even where it does not function as a neutral conductor (that is, where the conductor is not shared by two or more circuits in the system) as long as it is connected to the neutral point of the system.

Additional information is available in the Task Group report.

Note: Supporting material is available for review at NFPA Headquarters.

Panel Meeting Action: Accept in Principle

Add the following (two) definitions to Article 100 as follows:

Neutral Conductor. The conductor connected to the neutral point of a system
that is intended to carry current under normal conditions.

Neutral point. The common point on a wye-connection in a polyphase system
or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase
portion of a 3-phase delta system, or a midpoint of a 3-wire, direct current
system.

FPN: At the neutral point of the system, the vectorial sum of the nominal
voltages from all other phases within the system that utilize the neutral, with
respect to the neutral point, is zero potential.

Panel Statement: The revised wording removes the term ?circuit? as was pointed out in the TCC ballot, there is no definition for a ?circuit conductor? and the ?neutral conductor? could be in a branch circuit, feeder or otherwise. The revised text also establishes a differentiation between the ?neutral conductor? and the ?equipment grounding conductor? which are in fact both ultimately connected to the neutral point of a system. The differentiation is that under some normal conditions, the ?neutral conductor? is expected to be current carrying while under normal conditions the equipment-grounding conductor is never a current carrying conductor.

Number Eligible to Vote: 15

Ballot Results: Affirmative: 15
Click to expand...
 
Rick Christopherson

Rick Christopherson

Senior Member
iwire's NEC Ref said:
FPN: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.
Click to expand...
I believe that this may be the statement that leads to the confusion. The FPN is not saying that the neutral point needs to be in the vectorial center of the whole system, but in the vectorial center of those phases that use it, and disregarding the vectorial sum of those phases that do not use it.
 
quogueelectric

quogueelectric

Senior Member
Location
new york
I dont understand why they define the centertapped widing as single phase portion. If you took any winding by itself you could call it single phase. Yet it is a part of 3 windings 120* apart fed by 3 windings 120* apart the definition is confusing to me. They should have called it a 3 phase 4 wire delta yet they called it a 3phase with a single phase portion crazy.
 
mivey

mivey

Senior Member
iwire said:
Here is some info you may find interesting.
Click to expand...

I have seen other references to panel discussions, etc. Where can us regular folks get hold of this type info?
 
mivey

mivey

Senior Member
Rick Christopherson said:
I believe that this may be the statement that leads to the confusion. The FPN is not saying that the neutral point needs to be in the vectorial center of the whole system, but in the vectorial center of those phases that use it, and disregarding the vectorial sum of those phases that do not use it.
Click to expand...

I see where you are coming from.

Just thinking of the term "neutral": if it applies to all three phases, doesn't that imply some kind of "neutrality" to all three phases?
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
quogueelectric said:
Dont forget it takes 2 phases off the primary delta to drive this winding. It is not single phase.
Click to expand...

It is single phase.

It does not take two phases, it takes a single voltage from two points in the primary system. The fact that you choose to use the term "phase" to describe a single conductor does not change the physics of the system. Try changing your term to "line" or "hot conductor" and then describe the system.

This is why discussions can be long and drawn out. The electrical industry in the US often uses a single term to describe mutliple items where the correct definition can only be taken from the context of its use. While you can talk about a phase current (and have most people follow the conversation) you really need to describe a voltage with two points (i.e. V12, Vab, V1n, Vng).
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
mivey said:
I have seen other references to panel discussions, etc. Where can us regular folks get hold of this type info?
Click to expand...

You can download some NEC documents at the NFPAs web site.

I think right now the reports are available in pdf for the 2002, 2005 and 2008 NEC.
 
D

dbuckley

Senior Member
Location
Canterbury, New Zealand
Hail Jim. The USA electrical industry (NEC included) also use terms that have meanings everywhere else in the world to describe stuff slightly differently...

In respect of this question; the neutral in this frankenstein crossbreed of a system is not a "natural" neutral point (eg the centre of a wye transformer), it is a neutral point that suits the circuit arrangement, it is correctish for the "single phase" portion of the supply, but meaningless if looked at as a three wire delta, which is how three phase loads see this thing, except when a fault to ground (a/k/a neutral) occurs.

So in general terms, Mivey's analysis looks good to me, even if I don't understand the math :)

I call this a frankenstein system as its really two "normal" systems overlaid; it's the classic three seperate transformer delta/delta system, except one of the transformers happens to be a centre tapped device that fell in there by accident...

Only in America...
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
mivey said:
What say ye?
Click to expand...
I say that the neutral of a center-tapped secondary of such a Delta, open or closed, is exactly, and I mean exactly, the same as that of a single-phase 120/240 system's neutral.

Adding another transformer or two to make a Delta (and this is how many early 3-phase systems were developed: converting an existing 1-ph service) doesn't change that.

After all, a high leg Delta needs to have such a neutral reference in order to have a high leg.
 
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