Wye to wye questions

[paulengr]

Utilities like wye-wye for two reasons. First it’s a 3 leg transformer instead of a 5 leg. In fact you can just use 3 single phase transformers. Plus you can use thinner insulation because it only has to be insulated to line-neutral levels. Wye-wye is slightly cheaper all things considered. Second is lower cost to repair...because you can just swap out a single phase transformer instead of the whole thing. But here are the negatives. X0 passes through so a ground fault on the primary passes through to the secondary and vice versa. Second circulating currents are blocked in delta-wye but you get parasitic circulating currents in wye-wye. Third delta-wye phase shifts everything 60 degrees so it splits faults and kind of self balances unbalanced loads to a degree, and cancels all even harmonics where wye-wye doesn’t.

So lots of reasons to love delta-wye.

 

[ggunn]

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):

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

Allow it, yes, but if it's not there the secondary is separately derived and if it's there the secondary is not separately derived.

 

[tortuga]

There are no MV guys over here. This is solar farm and solar panels are outside on acre of land no building. I am reviewer of this for AHJ and so I review if everything is per code or not. You are saying primary utility side is typically delta so 13.2kV is delta not wye from utility? I thought it could be wye or delta. I am not the designer so i am not sure why wye is primary. However, I intend to and will ask the designer this question and let you know.

In the meantime if we can please go back to question and situation post #4 how does one ground properly post #8 attachment per code?

hhsting What did you ever find out and decide on this? I find it interesting.

 

[hhsting]

It took this long to get back to you all from the engineer. Hopefully Electrofelon or anyone else experienced and knoweldgeable with this sort of thing here help me out here please.

So here it is after talking with designer:

1. 2000kva transformer has X0 and H0 ie neutral bonded together internally by factory see attached nameplate. So question is it separately dervied?

2. Utility system by electrical designer used is multi grounded neutral system.

3. Utility system is grounded by the electeic utility coming into the 13.2kV switchboard.

4. Switchboard 13.2kV has 3 phase conductors each with concentric neutral says neutral is going to be grounded thru grounding electrode conductor. Question how can neutral to bond exist at swtichboard?

5. The 2000kva transformer also has neutral grounded and then 600V switchboard has neutral to ground bond. Question can you have at two places neutral to ground bond if transformer separately derived?

So now above questions and post #1 questions post #8 attachments what needs to be corrected?

 

[ggunn]

It took this long to get back to you all from the engineer. Hopefully Electrofelon or anyone else experienced and knoweldgeable with this sort of thing here help me out here please.

So here it is after talking with designer:

1. 2000kva transformer has X0 and H0 ie neutral bonded together internally by factory. So question is it separately derived?

No. The secondary side is already referenced to ground via the X0-H0 bond. Don't bond neutral to ground on the secondary side; that would potentially introduce current into the grounding system.

 

[hhsting]

No takers? No one knows? Attached is 2000kVA transformer nameplate.

The 13.2kV primary switchboard which is service disconnect I have TRXLPE cables 3 for each phase and each cable is sourrounded with concentric neutral. Electric utility system is grounded.

With each cable sourrounded with concentric neutral not sure does NEC 2014 Article 250.186(A) or NEC 2014 Article 250.186(B) apply? System is multi grounded neutral meaning no EGC primary side if their is no neutral to ground bond at switchboard 600V secondary then how can EGC of 600V connect to neutral.?

With above question comes next question where would the neutral to ground bond be if not at 600V switchboard secondary at the 13.2kv service disconnect and how with each cable sourrounded by concentric neutral be neutral to ground bonded and can their be EGC?

 

[tortuga]

System is multi grounded neutral meaning no EGC primary side if their is no neutral to ground bond at switchboard 600V secondary then how can EGC of 600V connect to neutral.?

2014 NEC 250.184(C)(2) would require the MGN system neutral to be grounded at each transformer.

With above question comes next question where would the neutral to ground bond be if not at 600V switchboard secondary at the 13.2kv service disconnect and how with each cable sourrounded by concentric neutral be neutral to ground bonded and can their be EGC?

At the service and the wye-wye transformer. No ECG is needed on this system segment.

 

[hhsting]

2014 NEC 250.184(C)(2) would require the MGN system neutral to be grounded at each transformer.


At the service and the wye-wye transformer. No ECG is needed on this system segment.

What i have is 13.2kV primary of 2000kva xfmr is grounded to concentric neutral cable. The xfmr 600V secondary I have 3 phase and neutral to 600V swbd. The 600v swbd has N to G bond and is grounded. From 600V swbd I have EGC to downstream equipment.

The question was if N to G bond cannot be in 600V swbd since their is X0 and H0 bond in xfmr would be considered not separately derived then where can it be? EGC of 600V swbd has to be bonded to neutral some point if not then EGC would be floating. You are now saying 13.2kv side does not require EGC so then how does EGC in 600V swbd get bonded to neutral?

 

[electrofelon]

I agree with gunny. This is NOT an SDS, and although you can bond equipment via the grounded/neutral conductor with MV MGN systems, you can't with low voltage systems per 150.142(B), thus I don't see how you can bond the XO/HO at the transformer (unless you separate them, but then it is an SDS and you bond and ground the XO, then it's basically the same thing :?:dunce .it's odd. I don't think the code really anticipated that situation.

If I were the decider, I would probably just ground the comb XO/HO (that's what any utility would do), NOT run an EGC to the LV switchboard, and bond and ground the LV switchboard (likely) per 230.50(A)(1) exception 2.

That's what I would do.

 

[hhsting]

I agree with gunny. This is NOT an SDS, and although you can bond equipment via the grounded/neutral conductor with MV MGN systems, you can't with low voltage systems per 150.142(B), thus I don't see how you can bond the XO/HO at the transformer (unless you separate them, but then it is an SDS and you bond and ground the XO, then it's basically the same thing :?:dunce .it's odd. I don't think the code really anticipated that situation.

If I were the decider, I would probably just ground the comb XO/HO (that's what any utility would do), NOT run an EGC to the LV switchboard, and bond and ground the LV switchboard (likely) per 230.50(A)(1) exception 2.

That's what I would do.

Are you saying something like the attached sketch? If not then not sure what you mean.

 

[electrofelon]

I see two options: One is per your sketch which IIRC is also per the original drawing submitted:

Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), MGN provision between HV switchboard and transformer (250.184(C) with transformer XO/HO bonded to neutral and grounded, LV switchboard bonded and grounded per 250.50.30(A)(1) exception 2 (calling transformer an SDS which it technically is not). So yes I am actually saying if i was the decider I would do it this way even though it is technically not code compliant. This is the way I always see it done, utilities would do it, and in this situation I see it as better than the alternative. IMO there the NEC's definition of SDS is irrational and they didnt consider the MV to LV wye wye.

I think the fully correct way is:


Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), single point grounded system run from HV switchboard to transformer (250.184(B)) with EGC hitting transformer, neutral hitting HO/XO but not grounded, separate neutral and ground running between transformer and LV switchboard with separate N-G at switchboard. Probably a few rods at each piece of equipment.

As I said, I dont like the latter for several reasons.

 

[hhsting]

I see two options: One is per your sketch which IIRC is also per the original drawing submitted:

Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), MGN provision between HV switchboard and transformer (250.184(C) with transformer XO/HO bonded to neutral and grounded, LV switchboard bonded and grounded per 250.50.30(A)(1) exception 2 (calling transformer an SDS which it technically is not). So yes I am actually saying if i was the decider I would do it this way even though it is technically not code compliant. This is the way I always see it done, utilities would do it, and in this situation I see it as better than the alternative. IMO there the NEC's definition of SDS is irrational and they didnt consider the MV to LV wye wye.

I think the fully correct way is:


Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), single point grounded system run from HV switchboard to transformer (250.184(B)) with EGC hitting transformer, neutral hitting HO/XO but not grounded, separate neutral and ground running between transformer and LV switchboard with separate N-G at switchboard. Probably a few rods at each piece of equipment.

As I said, I dont like the latter for several reasons.

Funny thing NEC 2014 Article 250.30(A)(1) exeption no, 2 says if building or structure is fed from outdoor transformer.... The transformer is outdoor and so is switchboard however the 600V switchboard and the transformer are on one concrete pad and has one grounding electrode around it ground ring encircling the pad. Would that matter?

 

[tortuga]

I see two options: One is per your sketch which IIRC is also per the original drawing submitted:

Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), MGN provision between HV switchboard and transformer (250.184(C) with transformer XO/HO bonded to neutral and grounded, LV switchboard bonded and grounded per 250.50.30(A)(1) exception 2 (calling transformer an SDS which it technically is not). So yes I am actually saying if i was the decider I would do it this way even though it is technically not code compliant. This is the way I always see it done, utilities would do it, and in this situation I see it as better than the alternative. IMO there the NEC's definition of SDS is irrational and they didnt consider the MV to LV wye wye.

I think the fully correct way is:


Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), single point grounded system run from HV switchboard to transformer (250.184(B)) with EGC hitting transformer, neutral hitting HO/XO but not grounded, separate neutral and ground running between transformer and LV switchboard with separate N-G at switchboard. Probably a few rods at each piece of equipment.

As I said, I dont like the latter for several reasons.

​​​​​​​Hmmm seems like we should stick with the 'fully correct way'.
What would be to possible safety problems with either of the above?

Suppose we got a open neutral on the utility side because of a storm or something but we still had all 3 hots? If we had continued our MGN system seems like more voltage likely present of exposed metal of equipment than if we did it 'fully correct way'.
If neutral suffers direct lighting strike the wye-wye is handled by GEC in HV switchboard.

 

[hhsting]

Hmmm seems like we should stick with the 'fully correct way'.
What would be to possible safety problems with either of the above?

Suppose we got a open neutral on the utility side because of a storm or something but we still had all 3 hots? If we had continued our MGN system seems like more voltage likely present of exposed metal of equipment than if we did it 'fully correct way'.
If neutral suffers direct lighting strike the wye-wye is handled by GEC in HV switchboard.

​​​​​​​Can you switch if utility is grounded has MGN then to switch to single point grounded system?

​​​​​​​NEC 2014 Article 250.184 says single point grounded system or MGN can be used. With MGN though neutral is grounded via GEC to ground ring each equipment should not that handle the exposed voltage on metal? Also choosing which one would that not be design issue? Engineer decided on MGN. I am not the designer though.

 

[tortuga]

1

 

[hhsting]

To summarize we are discussing these 3 options that dont involve parallel paths call them A, B and C:

No. None of the three. Two options either MGN or single point (fully correct). However, Engineer says its MGN. The MGN version option is in Post #70 attachment but not exactly shown by the engineer as he has bonding jumper between ground bars 13.2kv swbd, xfmr and 600V swbd. I suspect they are supply side bonding jumpers per NEC 2014 Article 250.30 when seeing from solar side and when seeing from utility side but 250.30(A)(1) exception no.2 not needed also MGN their should not be bonding jumper. Thats a question will be asking engineer

I responded previously thinking you are electrofelon

 

[hhsting]

I see two options: One is per your sketch which IIRC is also per the original drawing submitted:

Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), MGN provision between HV switchboard and transformer (250.184(C) with transformer XO/HO bonded to neutral and grounded, LV switchboard bonded and grounded per 250.50.30(A)(1) exception 2 (calling transformer an SDS which it technically is not). So yes I am actually saying if i was the decider I would do it this way even though it is technically not code compliant. This is the way I always see it done, utilities would do it, and in this situation I see it as better than the alternative. IMO there the NEC's definition of SDS is irrational and they didnt consider the MV to LV wye wye.

I think the fully correct way is:


Concentric grounded neutral brought to and bonded and grounded in HV switchboard (250.186(A)), single point grounded system run from HV switchboard to transformer (250.184(B)) with EGC hitting transformer, neutral hitting HO/XO but not grounded, separate neutral and ground running between transformer and LV switchboard with separate N-G at switchboard. Probably a few rods at each piece of equipment.

As I said, I dont like the latter for several reasons.

With MGN system can you bond neutral to ground downstream of service disco like in 600V Swbd? 250.184 does not indicate you cannot do it for MGN system. Single point can be supplied from MGN no?

 

[electrofelon]

With MGN system can you bond neutral to ground downstream of service disco like in 600V Swbd? 250.184 does not indicate you cannot do it for MGN system. Single point can be supplied from MGN no?

The MGN provision is for over 1kv. That is why it gets sticky with a non SDS transformer. You can't bond neutral to ground past the service disconnect or first disconnect of an SDS.

 

[Sahib]

It is a balanced three phase system and so no current in the neutral during normal operation. Any current in neutral due to any unbalance fault may be rectified rather quickly as it is a technically supervised installation. Bonding of transformer neutrals on both sides to ground (back to respective sources) is required for proper protection of Y-Y transformer (operation of protective device) in the OP case.

 

[ggunn]

It is a balanced three phase system and so no current in the neutral during normal operation. Any current in neutral due to any unbalance fault may be rectified rather quickly as it is a technically supervised installation. Bonding of transformer neutrals on both sides to ground (back to respective sources) is required for proper protection of Y-Y transformer (operation of protective device) in the OP case.

Irrespective of that, the secondary is not separately derived if the neutrals are bonded in the transformer. Whether or not it is anticipated that there will be neutral current on the secondary is irrelevant.