Delta system neutral point

[LarryFine]

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...

You're not as far off as you think. Don't forget the open Delta. Dr. Frankenstein needed 3-phase for his experiments, and the Transylvania Power Co. found a way to convert his existing 1-ph service by stringing only one new wire.

 

[mivey]

If you took any winding by itself you could call it single phase

Exactly right.

I agree as well. No problem here. [edit to add: as long as the connections were handled correctly]

[edit: added quogueelectric's quote for clarity]

 

[mivey]

I agree that the high leg needs a neutral if you run single-phase 208 volts loads. I also agree that the neutral of the delta leg is the same as a single-phase 120/240.

I am saying the neutral in this configuration only applies to the single phase sub-system, not the three phase system. I'll add a disclaimer that it would not be a valid neutral for a single phase system pulled off of one of the other legs (if you were going to that, put in a wye) because the existing ground point would have an equal potential to the two terminals.

Consider how a neutral point is defined by the ANSI/IEEE Standard 100:
"the neutral point of a system is that point which has the same potential as the point of junction of a group of equal non-reactive resistances if connected at their free ends to the appropriate main terminals or lines of the system."

In other words, The neutral point for the 3-phase delta system is going to be different for the three phase loads than it is for the single phase loads. A three phase delta system would have to have a derived neutral...not sure what you would do with it though.

Also, for a corner grounded delta, The grounded phase could be the neutral point for single phase loads that use the grounded terminal but that grounded point would not be the neutral for the three phase system.

 

[iwire]

In my opinion the CMP was not trying to make an absolute definition of neutral for use by all, just a definition that would work for the purposes of enforcing the NEC.

Look at the definition of 'Separately Derived System', that has some issues as well. :smile:

Or consider that the NEC allows some conductors that do carry current to be considered non-current carrying. 310.15(B)(4)

 

[mivey]

In my opinion the CMP was not trying to make an absolute definition of neutral for use by all, just a definition that would work for the purposes of enforcing the NEC.

But if we could get them onto this forum, we could badger them until they came around to our way of thinking. Of course, the code would then have to have a multiple choice section.

 

[mivey]

Practical NEC vs engineering

Practical NEC vs engineering

In my opinion the CMP was not trying to make an absolute definition of neutral for use by all, just a definition that would work for the purposes of enforcing the NEC.

But, from what you sent me, it would appear they do consider IEEE standards, etc. I guess to keep from inventing a practical world that is at odds with the engineering world.

Leaning away from the practical world just a little bit, a neutral and ground are different. I guess that's why I'm starting to think a system's neutral point and ground point are different. Maybe that is trying to sew two different ideas together, I'm not clear yet.

It is known that zero sequence current flow only produces zero sequence voltage drops. Zero sequence is no big mystery. We normally deal with what we call balanced, three phase systems, even though they don't really exist. A system becomes unbalanced due to unsymmetrical faults, unbalanced loads, open conductors, etc. We just take the unbalanced system and split it into 3 balanced systems. One has a positive sequence rotation, one has a negative sequence rotation, and the last has no "sequence" because instead of the legs being 120 degrees out of phase, all of the legs are in phase or face the same direction. This last system is our zero sequence.

This means the neutral and ground are NOT the same. While the voltage from neutral to ground is zero when no zero sequence voltage is present, When zero sequence current flows, there will be a voltage from neutral to ground.

[edit: spelling]

 

[mivey]

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).

I agree. I didn't catch that the first time around.