Bonded neutral 277/480V to neutral 120/208V

dthimot

Member
Location
Haiti
Occupation
Electrical engineer
Hello everyone,
It’s normal to bonded the neutral of an inverter tripower (output star L1,L2,L3,N)277/480V with the secondary transformer neutral tripower (L1,L2,L3,N) 120/208V? While the voltage is different
Can I do that ? I’m a young engineer
Please let me know.
 
Hello everyone,
It’s normal to bonded the neutral of an inverter tripower (output star L1,L2,L3,N)277/480V with the secondary transformer neutral tripower (L1,L2,L3,N) 120/208V? While the voltage is different
Can I do that ? I’m a young engineer
Please let me know.
I'm a little confused what you have and are asking. Sounds like you have a tripower inverter connected to a transformer. What is the winding configuration of the transformer? Normally you would use a 208 Delta to 480/277 wye and you would treat the 480 side like a typical SDS and ground and bond the neutral. Sometimes a wye wye transformer is proposed for this which adds some confusion, and I have never heard a valid reason for using it.
 

retirede

Senior Member
Location
Illinois
I'm a little confused what you have and are asking. Sounds like you have a tripower inverter connected to a transformer. What is the winding configuration of the transformer? Normally you would use a 208 Delta to 480/277 wye and you would treat the 480 side like a typical SDS and ground and bond the neutral. Sometimes a wye wye transformer is proposed for this which adds some confusion, and I have never heard a valid reason for using it.
I think he’s asking if the neutral (wye point) of the inverter should be connected to the wye point of the transformer secondary.

I would presume that the transformer primary is indeed delta.
 

dthimot

Member
Location
Haiti
Occupation
Electrical engineer
I think he’s asking if the neutral (wye point) of the inverter should be connected to the wye point of the transformer secondary.

I would presume that the transformer primary is indeed delta.
Exactly
Is it possible?
 

dthimot

Member
Location
Haiti
Occupation
Electrical engineer
It’s Tesla power pack Tripower 480v wye
With a transformer tripower primary 480V delta
Secondary wye 120/208V
I’ve connected the neutral of the inverter 480V to the secondary transformer neutral 120/208
 

dthimot

Member
Location
Haiti
Occupation
Electrical engineer
I just want to know if it’s normal to connect the two neutral together (bonded) ?
Remark: inverter neutral is a star 277/480V and secondary transformer neutral is a star 120/208V
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
I;m not sure I follow exactly but if you have a 480/277 supply, that neutral will be bonded at the service.
(It will not be present at a 480 delta transformer primary). The 208/1220 output neutral will also be bonded so the two neutrals (277 &* 120) will be "connected" thru their respective bonding.
Does that answer your question ?
 
I;m not sure I follow exactly but if you have a 480/277 supply, that neutral will be bonded at the service.
(It will not be present at a 480 delta transformer primary). The 208/1220 output neutral will also be bonded so the two neutrals (277 &* 120) will be "connected" thru their respective bonding.
Does that answer your question ?
I was thinking the service is 208, and the 277/480 is an SDS, but not sure. Also I'm not clear if this is a wye wye transformer.
 

dthimot

Member
Location
Haiti
Occupation
Electrical engineer
I;m not sure I follow exactly but if you have a 480/277 supply, that neutral will be bonded at the service.
(It will not be present at a 480 delta transformer primary). The 208/1220 output neutral will also be bonded so the two neutrals (277 &* 120) will be "connected" thru their respective bonding.
Does that answer your question ?
I had linked them (the two neutrals together)
Thank you for answering!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
I was thinking the service is 208, and the 277/480 is an SDS, but not sure. Also I'm not clear if this is a wye wye transformer.
agreed. I was not clear on the exact install details either.. just took a stab at answering as the "two neutral" question is often asked.
 

dthimot

Member
Location
Haiti
Occupation
Electrical engineer
I was thinking the service is 208, and the 277/480 is an SDS, but not sure. Also I'm not clear if this is a wye wye transformer.
The transformer is Delta => wye
The service is 120/208V
277/480v initial voltage
Thank for your helps
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
This raises a long standing question I've had when there are two wye voltage systems present: if you want an SDS, then the primary and secondary neutrals are not directly connected, but they end up indirectly connected, as both systems are bonded to a common GES.

As far as I'm aware, you also have the option to directly connect the primary and secondary neutrals. Then it is not an SDS, and there's no need for a GEC on the secondary side of the transformer, to my knowledge.

So is that in fact allowed? (If not, what is the NEC reference?) And if so, what are the pros and cons of each approach? The SDS seems to be the default choice, but I'm not clear why.

Cheers, Wayne
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
This raises a long standing question I've had when there are two wye voltage systems present: if you want an SDS, then the primary and secondary neutrals are not directly connected, but they end up indirectly connected, as both systems are bonded to a common GES.

As far as I'm aware, you also have the option to directly connect the primary and secondary neutrals. Then it is not an SDS, and there's no need for a GEC on the secondary side of the transformer, to my knowledge.
Rarely do LV primary windings, connected in wye, have the H0/neutral point connected to a system conductor or ground. This point usually floats just like it does in a motor.
MV systems, particularly utilities, regularly connect the primary H0 to the secondary X0 at the transformer, often internally at the factory.
 

jap

Senior Member
The transformer is Delta => wye
The service is 120/208V
277/480v initial voltage
Thank for your helps
This is still confusing to me possibly due to the way it's worded.

Above indicates the service is 120/208v (Which usually indicates the input voltage to the transformer)

Yet, above also indicates the "initial voltage" is 277/480v with the transformer being a Delta primary with a Wye Secondary.

Regardless,

If the transformer is a Delta primary with a 3ph Wye Secondary, Just because the "Service" Voltage or "Initial" Voltage is a "Wye Configuration with a Neutral", that does not mean a "Neutral" or grounded conductor is brought to the transformer's "line side" or "Delta" input,
nor should it be.

The neutral would only be brought to the primary if the transformer is a Wye input /Wye output type configuration.

Unless I'm wrong, (and I am often), it seems you have a standard 480v Delta Primary to 120/208v Wye secondary transformer.

In that case the primary feed to the transformer will only contain 3 phase conductors along with an equipment grounding conductor of some sort to bond the transformer's casing for fault clearing purposes.

The transformer produces it's own neutral on the secondary side or XO , which is required to be bonded.

Hence the interconnection of the grounded conductors on the primary and secondary side of the transformer, (Even though a Neutral is produced in the transformer) because they generally share a common grounding or bonding system.

JAP>
 
This raises a long standing question I've had when there are two wye voltage systems present: if you want an SDS, then the primary and secondary neutrals are not directly connected, but they end up indirectly connected, as both systems are bonded to a common GES.

As far as I'm aware, you also have the option to directly connect the primary and secondary neutrals. Then it is not an SDS, and there's no need for a GEC on the secondary side of the transformer, to my knowledge.

So is that in fact allowed? (If not, what is the NEC reference?) And if so, what are the pros and cons of each approach? The SDS seems to be the default choice, but I'm not clear why.

Cheers, Wayne
The dreaded wye wye has come up before as I'm sure you are aware. I do not believe there is any code issue with bonding the two wye points to make it a non SDS, as long as a neutral is not reconnected to ground. In that case one would not have to run a GEC, but would have to run the primary neutral so it seems like a wash.

I believe that historically the Y-Y was avoided for some time because it has several issues. I believe with proper engineering it is not much of an issue anymore and of course they are now nearly universally used by utilities running MGN systems. There are some different considerations for LV vs MV systems and I am assuming the Delta wye is still preferable for LV, perhaps others can elaborate on why?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
My questions didn't really have anything to do with wye-wye transformers. It could be a delta-wye transformer, supplied by a wye system, with the primary system neutral brought to the transformer and directly connected with the secondary system neutral (both connected to the neutral point of the secondary side of the transformer). Which is a connection that occurs anyway, in an SDS it's just indirect via the GES; in this non-SDS it would be a direct connection, with no secondary GEC.

Or to make it even more funky, if the primary side system is a corner grounded delta, then you could do the same thing with the primary grounded conductor. Same reasoning.

Cheers, Wayne
 
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