Wye to wye questions

[tortuga]

​​​​​​I disagree. We cannot tell if that is an ungrounded utility system. In fact that code reference is for grounded systems, but where Poco does not provide a grounded conductor. OP needs to find out f I'm POCO what kind of system this is

Thats a good point. The drawing does not show a neutral coming from the utility side, however we don't know thats its ungrounded without more info.

 

[ggunn]

snapshot attached

OK, the centers of the two wyes in the transformer are both connected to ground, but they are not bonded within the transformer if the drawing is accurate. The fact that on the secondary side neutral is bonded to ground supports this; if the neutrals were bonded in the transformer the secondary would not be separately derived and there would be no ground to neutral bond on the secondary.

 

[hhsting]

OK, the centers of the two wyes in the transformer are both connected to ground, but they are not bonded within the transformer if the drawing is accurate. The fact that on the secondary side neutral is bonded to ground supports this; if the neutrals were bonded in the transformer the secondary would not be separately derived and there would be no ground to neutral bond on the secondary.

Centers from both wyes connect to same ground. Would not that make it one?
Also how can you ground at the transformer and then at the 600vac swbd again? I thought 250.30(A)(1)(a) or 250.30(A)(1)(b) but not both.

 

[ActionDave]

Centers from both wyes connect to same ground. Would not that make it one?
Also how can you ground at the transformer and then at the 600vac swbd again? I thought 250.30(A)(1)(a) or 250.30(A)(1)(b) but not both.

Grounded - Physically connected to the earth. 99% of every transformer in North America is connected to the earth. That does not make every transformer in North America "one".

Bonded - Mechanically connecting the neutral point in a transformer to the same conductor that is connected to the earth. That is what a Main Bonding Jumper and System Bonding Jumper is all about.

The secondary side of your photo shows a bonded neutral and the primary side is not bonded.

 

[Hv&Lv]

Take a pIcture of the transformer dateplate.
take a picture of the bushings with the labeling.
That should stop all the conjecture.

 

[tortuga]

I suppose no matter what the utility calls it the code requires a ground buss at the main:
490.47 Switchgear Used as Service Equipment shall include a ground buss.
Then the code requires:
250.24(A)(1)Neutral Ground bond at service equipment.
I don't see where in the code they would be allowed to have a neutral - ground bond elsewhere.

 

[hhsting]

I have asked the desginer to forward manufacturer datasheet of the wye to wye transformer. In meantime if H0 and X0 of the transformer are bonded internally would the transformer be separately derived?

I can recall if generator ATS neutral is not switch then it would not be separately dervied. However should the generator ATS switches neutral then generator is separately derived.

I am wondering would same analogy apply to separately derived or not generator above with tranformer neutral in wye wye transformer? If neutral is bonded or not H0 and Xo would depend transformer is separately derived or not?

 

[tortuga]

I have asked the desginer to forward manufacturer datasheet of the wye to wye transformer. In meantime if H0 and X0 of the transformer are bonded internally would the transformer be separately derived?

I can recall if generator ATS neutral is not switch then it would not be separately dervied. However should the generator ATS switches neutral then generator is separately derived.

I am wondering would same analogy apply to separately derived or not generator above with tranformer neutral in wye wye transformer? If neutral is bonded or not H0 and Xo would depend transformer is separately derived or not?

I seems like it meets the definition:

Separately Derived System: An electrical source, other than
a service, having no direct connection(s) to circuit conductors
of any other electrical source other than those established by
grounding and bonding connections.

 

[hhsting]

I seems like it meets the definition:

So tell me would generator whose ATS neutral is not switched i.e ATS is 3 pole would be considered separately derived? Would you have system bonding jumper for ATS 3 pole generator? If no then how come wye to wye transformer whose neutral secondary X0 and neutral primary H0 are bonded together internally be separately derived and require to have system bonding jumper?

 

[jaggedben]

So tell me would generator whose ATS neutral is not switched i.e ATS is 3 pole would be considered separately derived? Would you have system bonding jumper for ATS 3 pole generator? If no then how come wye to wye transformer whose neutral secondary X0 and neutral primary H0 are bonded together internally be separately derived and require to have system bonding jumper?

It actually depends if that internal bond wire is white or green/bare. At least it seems so to me by the definition.

 

[tortuga]

So tell me would generator whose ATS neutral is not switched i.e ATS is 3 pole would be considered separately derived? Would you have system bonding jumper for ATS 3 pole generator? If no then how come wye to wye transformer whose neutral secondary X0 and neutral primary H0 are bonded together internally be separately derived and require to have system bonding jumper?

I think a generator is a different case than a transformer. If you put a ammeter on that H0 to Xo bond no current would flow in normal conditions.

 

[ActionDave]

If the neutral from the generator and the neutral from the power company are joined together at the transfer switch then you have a direct electrical connection and you have a non separately derived system.

If the XO and HO are bonded together then you have a direct electrical connection and you have a non separately derived system. Nobody besides the power company does this that I know of.

 

[tortuga]

If the neutral from the generator and the neutral from the power company are joined together at the transfer switch then you have a direct electrical connection and you have a non separately derived system.

If the XO and HO are bonded together then you have a direct electrical connection and you have a non separately derived system. Nobody besides the power company does this that I know of.

So your thinking a wye - wye can never be considered separately derived unless HO or XO are floated?
These solar sites are essentially the definition of a utility, they serve no other purpose other then to generate electricity and feed it into the grid, they are regulated as such. The CT meter meters 'output'. The wye - wye probably is due to the inverter needing to sync with other equipment at a precise phase angle. But this is conjecture.

 

[ActionDave]

So your thinking a wye - wye can never be considered separately derived unless HO or XO are floated?

Correct.

These solar sites are essentially the definition of a utility, they serve no other purpose other then to generate electricity and feed it into the grid, they are regulated as such. The CT meter meters 'output'. The wye - wye probably is due to the inverter needing to sync with other equipment at a precise phase angle. But this is conjecture.

Interesting. I've never been involved in such a project.

 

[electrofelon]

So your thinking a wye - wye can never be considered separately derived unless HO or XO are floated?

I do not agree with that. First let's note that grounded conductors between different systems are always bonded together, just sometimes not "directly" (i.e. may be through grounding and bonding connections). A grounded conductor can't float.

Take a wye wye transformer. If the HO and XO are common, as in the same terminal or buss then that is a direct connection, it's a non SDS.

Let's look at other possible configurations where the HO and XO have SEPARATE terminals.

1. HO landed but floating, XO grounded. This would clearly be an SDS. If we are talking a single point grounded MV system, this may be how it is setup and there would be a bonding jumper going from the tranny case back to the MV neutral somewhere.

2. HO and XO landed AND grounded. I think this is still and SDS as the neutrals are not "directly" connected. It's really no different than how the neutrals of a typical <600v Delta wye transformer are indirectly connected. This is how utilities usually do it, and how it would be for a NEC MGN MV system. Note I don't believe you can do this with low voltage as you are making a neutral to ground connection with the primary neutral which is generally prohibited past the service disconnect.

So to summarize, IMO, any sort of accessible strap connecting the bushings/terminals of the O's makes it an direct connection.

 

[tortuga]

I find this 'NEC MGN MV system' wye - wye thing really interesting from a theory perspective.

Take a wye wye transformer. If the HO and XO are common, as in the same terminal or buss then that is a direct connection, it's a non SDS.

OK

2. HO and XO landed AND grounded. I think this is still and SDS as the neutrals are not "directly" connected.

If they are both bonded to the case can you elaborate on how electrically this is different than our HO and XO come bonded at the factory?

If our wye - wye H0 and X0 are connected directly or indirectly (via the case) In what case would any current flow from the X0 secondary side to the H0 primary side?
Say there is a
MV looses its neutral shield on all three cables somewhere somehow?
MV ground fault?
Low voltage (600V) ground fault?
Unbalanced loads?
lighting strike?

I would speculate that it does not matter where they are connected. If they are measurable current would never flow between the two systems.

Where's @garr?

 

[electrofelon]

I find this 'NEC MGN MV system' wye - wye thing really interesting from a theory perspective.
If they are both bonded to the case can you elaborate on how electrically this is different than our HO and XO come bonded at the factory?

Electrically its no different of course. I am not sure what the reason is for the grounded conductor clause in the definition of SDS. I would have to cogitate on it and sketch some stuff out, but my first thought it is to prevent/lessen neutral currunt flowing on grounding and bonding conductors.

If our wye - wye H0 and X0 are connected directly or indirectly (via the case) In what case would any current flow from the X0 secondary side to the H0 primary side?
Say there is a
MV looses its neutral shield on all three cables somewhere somehow?
MV ground fault?
Low voltage (600V) ground fault?
Unbalanced loads?
lighting strike?

I would speculate that it does not matter where they are connected. If they are measurable current would never flow between the two systems.

Where's @garr?

I think it gets back to that same question about that definition of SDS. I am too tired to think about it right now, but I agree thanks to Kirchhoff, everything should be returning to its source so I dont see neutral current flowing between the wye points. I recall some past threads about the utility practice of interconnecting the primary and secondary neutrals and people not liking it. It does seem that potentially invites a MV path back to the source through your living room. There is a thing called a ronk blocker to address just that. Hopefully someone else will jump in and help us out......

 

[Hv&Lv]

Here...
all for $.50..

http://files.engineering.com/downloa...rmer_Paper.pdf

primary floating neutral problem page one.

 

[electrofelon]

Here...
all for $.50..

http://files.engineering.com/downloa...rmer_Paper.pdf

primary floating neutral problem page one.

A classic. I saw it in the theater when I was little

 

[tortuga]

A classic. I saw it in the theater when I was little

Yeah reading that and my "American electricians handbook" on it, it seems extremely likely any wye - wye over 1000V is a 'Multigrounded Neutral System' and 2014 NEC 250.184(C):

(1) The neutral conductor of a solidly grounded neutral system
shall be permitted to be grounded at more than one point.
Grounding shall be permitted at one or more of the fol-
lowing locations:
a. Transformers supplying conductors to a building or
other structure

So the NEC would allow the H0-X0 bond in either case.