Transformer with PV system

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If I was using SMA tri power 24000 that put out 277/480 volt, then wired to production meter. Then go to transformer , that goes to a disconnect witch line side taps a 120/208v service. My question is that if my numbers all worked out could I get a 120/208v boost transformer to 277/480v and use the PV ocpd to comply with OCP for primary side of transformer disco?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If I was using SMA tri power 24000 that put out 277/480 volt, then wired to production meter. Then go to transformer , that goes to a disconnect witch line side taps a 120/208v service. My question is that if my numbers all worked out could I get a 120/208v boost transformer to 277/480v and use the PV ocpd to comply with OCP for primary side of transformer disco?
Yes, you can use a 480V to 208V step down transformer to interconnect a 480V inverter to a 208V service, but I don't understand the rest of your question. Are you asking if a 208V to 480V step up transformer is usable as a step down? That would be a question for the transformer manufacturer but in at least some cases the answer is no, you cannot. I think that the makers of many of these "smaller" 480V inverters are recommending the use of autotransformers to interconnect at 208V. Ask SMA tech support for a recommendation.
 
I think he is asking if the service disconnect for the "line side tap" can also meet the transformer protection requirements in 430.3(B). I dont see any problem if all the numbers worked out of course. However I think due to 240.21(C) he will need an OCPD between the transformer and inverter. I dont see any way to feed the inverter directly from the transformer secondary conductors.
 

Carultch

Senior Member
Location
Massachusetts
I think he is asking if the service disconnect for the "line side tap" can also meet the transformer protection requirements in 430.3(B). I dont see any problem if all the numbers worked out of course. However I think due to 240.21(C) he will need an OCPD between the transformer and inverter. I dont see any way to feed the inverter directly from the transformer secondary conductors.

If it is a system where all fault current is guaranteed to line up "winding-to-winding" from primary to secondary, a primary OCPD can protect the secondary by proxy. Examples are single phase and delta-delta.

If you have a WYE system on either side, you have to have OCPD on both sides. Because fault current from two phases can land on just one, when it is passed across the transformer.



This kind of transformer application is particularly interesting. Which side do you define as the primary, and why? For purposes of 240.21(C), which side do you define as secondary and why?
 
If it is a system where all fault current is guaranteed to line up "winding-to-winding" from primary to secondary, a primary OCPD can protect the secondary by proxy. Examples are single phase and delta-delta.

If you have a WYE system on either side, you have to have OCPD on both sides. Because fault current from two phases can land on just one, when it is passed across the transformer.

FWIW I was aware of that. I have used the sunny tripower and they require a neutral hence it would need a Y secondary.



This kind of transformer application is particularly interesting. Which side do you define as the primary, and why? For purposes of 240.21(C), which side do you define as secondary and why?

Yes an interesting question. This came up a while back and I dont remember much of the discussion other than looking into it and realizing that the NEC doesnt address how to determine which side is the primary or secondary AFAIK. Particularly important with the new requirements for using a transformer "backwards"
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
.... However I think due to 240.21(C) he will need an OCPD between the transformer and inverter. I dont see any way to feed the inverter directly from the transformer secondary conductors.

See 705.65(B), Exception. Maybe I need to read it a couple more times to make sure I'm really reading it right, but I think it says you don't need OCPD there if you're numbers work out.
 

iwire

Moderator
Staff member
Location
Massachusetts
This came up a while back and I dont remember much of the discussion other than looking into it and realizing that the NEC doesnt address how to determine which side is the primary or secondary AFAIK. Particularly important with the new requirements for using a transformer "backwards"

Is it an issue much now?

I see many / most transformers being labeled with 'high and 'low' voltage in place of primary and secondary.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Is it an issue much now?

I see many / most transformers being labeled with 'high and 'low' voltage in place of primary and secondary.

It is a question for the transformer manufacturer. I had to do this (interconnect a bank of 480VAC inverters with a 208VAC service), and the transformer company told me that their 208VAC to and from 480VAC of the same kVA rating had different product codes and were not interchangeable.
 

iwire

Moderator
Staff member
Location
Massachusetts
It is a question for the transformer manufacturer. I had to do this (interconnect a bank of 480VAC inverters with a 208VAC service), and the transformer company told me that their 208VAC to and from 480VAC of the same kVA rating had different product codes and were not interchangeable.

In that case they must be labeled primary and secondary.

However I am finding more and more transformers marked high and low vs primary and secondary.

If they are labeled high and low the NEC would allow either side to be the primary.
 

Smart $

Esteemed Member
Location
Ohio
See 705.65(B), Exception. Maybe I need to read it a couple more times to make sure I'm really reading it right, but I think it says you don't need OCPD there if you're numbers work out.
Correct!!!

(B) Power Transformers. Overcurrent protection for a transformer with a source(s) on each side shall be provided in accordance with 450.3 by considering first one side of the transformer, then the other side of the transformer, as the primary.

Exception: A power transformer with a current rating on the side connected toward the utility-interactive inverter output that is not less than the rated continuous output current of the inverter shall be permitted without overcurrent protection from that source.

However, the question remains about what to do when the numbers don't work out. Code says to consider first one side, then the other as primary... but it doesn't say what to do when you do consider it in that manner. All transformers require primary protection. It is secondary protection that is in doubt. :blink:
 

Carultch

Senior Member
Location
Massachusetts
Correct!!!

However, the question remains about what to do when the numbers don't work out. Code says to consider first one side, then the other as primary... but it doesn't say what to do when you do consider it in that manner. All transformers require primary protection. It is secondary protection that is in doubt. :blink:

Interesting.

I did an example where on the PV inverter side of the transformer, I had an AC combiner with 300A worth of inverters, and 350 kcmil wiring. The people who bought the panelboard, upsized the main to 350A, and I allowed the 350 kcmil wiring to remain in place, justifying it with the next size up rule that doesn't apply to secondary conductors. But it DOES apply to primary conductors.

And on the placards, I was calling the PV system side the primary.

It says that code rules require both sides to be considered as primary. It doesn't say to simultaneously consider both sides as secondary.
 

Smart $

Esteemed Member
Location
Ohio
Interesting.

I did an example where on the PV inverter side of the transformer, I had an AC combiner with 300A worth of inverters, and 350 kcmil wiring. The people who bought the panelboard, upsized the main to 350A, and I allowed the 350 kcmil wiring to remain in place, justifying it with the next size up rule that doesn't apply to secondary conductors. But it DOES apply to primary conductors.

And on the placards, I was calling the PV system side the primary.

It says that code rules require both sides to be considered as primary. It doesn't say to simultaneously consider both sides as secondary.
I can read ya know...:D

The question is what do you extract from considering one then the other as the primary???

Do I look at it as if using primary-only protection or primary and secondary protection, and what do I do when I do???

It would be clearer (to me anyway) if they just said protect both sides as a primary using primary-only protection method for not more than 1000V.
 
Correct!!!



However, the question remains about what to do when the numbers don't work out. Code says to consider first one side, then the other as primary... but it doesn't say what to do when you do consider it in that manner. All transformers require primary protection. It is secondary protection that is in doubt. :blink:

Ok but that sounds like transformer protection. What about transformer secondary conductors?
 

Smart $

Esteemed Member
Location
Ohio
Ok but that sounds like transformer protection. What about transformer secondary conductors?
Well from the way the section words it, there are no secondary conductors. Each side is considered as a primary and protected accordingly... whatever that is supposed to mean. :p
 
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