Re: [RE-wrenches] Transformer Nomenclature/Protection/Grounding

[Michael Howell (m...@pveducation.com)]

Darryl,
My day job is in electrical distribution and the primary is always the
utility side of the transformer. We have custom transformers made all the
time for solar and every manufacturer we work with builds the primary on
the utility.

Corey,
If your utility wants wye on their point of connection side this will turn
into a custom transformer. Your local electrical distributor will mostly
likely give you specs from the factory that will also clarify this. In your
case you need a 208Y120 primary and 480Y277 secondary. I run into this
wye-wye all the time with some utilities.

One of the main reasons for having the primary on the utility side is the
transformer has an internal resistance designed to not trip the breaker
feeding it when you first energize the transformer. I've run into cases
where this has been switched in the field and when you turn the breaker on
for the transformer it will open from the large inrush current.

Definitely a confusing topic and once you get the transformer quote back
for the wye-wye it should be clearly indicated. Federal Pacific it
typically very competitive on these units.
Hope this helps,
Mike



On Wed, Apr 11, 2018 at 1:57 AM, Darryl Thayer 
wrote:

> hi I have copied your text. My comments are in red
>
> - Is there any consensus on what circuit is considered the "primary" and
> which is the "secondary" in this type of scenario?  Yes, the energy
> supply side is the primary, so the solaredge side is the Primary
>- NEC 690.9(D) speaks of "considering first one side of the
> transformer, then the other side of the transformer, as the primary". Is
> this relevant? Yes; you must protect from the grid side (the grid has all
> the real power) then the high-voltage side inverter side is protected by
> the inverter capacity.   i.e. Is it possible for both sides of the
> transformer to be labeled as a primary?  No, Your transformer was made
> with assumed primary and secondary, the energy flows from primary to
> secondary with a slight voltage drop, the turns ratio reflects this slight
> voltage change.  Also, the low voltage side is wound next to the core and
> the higher voltage side is wound away from the core.
>
> - I understand that 450.4(A) requires overcurrent protection on the "input
> conductors" (aka "primary conductors"?), correct? Yes, the grid side must
> be protected in the event of a fault. And the solar side must be protected
> or have conductors sufficient to carry the current as limited by input
> (grid) side OCPD.  The solar side by OCPD or a power limited supply, the
> inverters, but the input from the grid must be protected.  Must we
> provide overcurrent protection on the "other" side in the above scenario? Yes,
> the solar side has to have protection from the grid.
>
>- We were originally planning to land each inverter output on a 50A
> breaker inside a 200A main lug panel - must we also include a main breaker? 
> No,
> not from the inverters, but if you are you are using this as the OCPD on
> the solar side of the transformer yes.  I think the calculation is 125%
> transformer OCPD power or the power of the transformer is the maximum
> current.  For example, 112 kVA / 480 x 125% =292 amps is possible, the
> conductors need to be 292 amp or you protect with OCP. If you protect then
> the conductors need to be 100 kW x125% /480/1.73= 150 amp minimum.
>
> - How do we size the grounding conductors on either side of the
> transformer? For standard distances, the equipment ground for the
> transformer is from 250.122 based on the grid side OCPD.  For 100 kW x 1.25
> /208/1.73 = 350 amps, is #3 cu or equiv.
>  For the solar side it is a separately derived system, The equipment
> ground from 250.122 is #6, However, if you use a corner grounded delta, (if
> SE will allow)  you do not run a separate equipment ground.  Being a
> separately derived system you need an GEC, which is sized from 250.66and it
> will go to a series of ground rods, with a #6
>
>- Is the grounding conductor between MDP and transformer considered an
> EGC (250.122)? Yes,  for the transformer on the grid side
>
>
> On Tue, Apr 10, 2018 at 7:38 PM, Corey Shalanski 
> wrote:
>
>> I'm seeking some guidance on how to treat transformers when included in a
>> PV system.
>>
>> Specific system details:
>> - (3x) SolarEdge 33.3 inverters combined in a 200A panel
>> - 480/208 transformer to be installed between 200A panel and main
>> distribution panel (service is 208 wye)
>>
>> My questions:
>>
>> - Is there any consensus on what circuit is considered the "primary" and
>> which is the "secondary" in this type of scenario?
>>- NEC 690.9(D) speaks of "considering first one side of the
>> transformer, then the other side of the transformer, as the primary". Is
>> this relevant? i.e. Is it possible for both sides of the transformer to be
>> labeled as a primary?
>>
>> - I understand that 450.4(A) requires overcurrent protection on the
>> 

Re: [RE-wrenches] Transformer Nomenclature/Protection/Grounding

[Darryl Thayer]

hi I have copied your text. My comments are in red

- Is there any consensus on what circuit is considered the "primary" and
which is the "secondary" in this type of scenario?  Yes, the energy supply
side is the primary, so the solaredge side is the Primary
   - NEC 690.9(D) speaks of "considering first one side of the transformer,
then the other side of the transformer, as the primary". Is this relevant? Yes;
you must protect from the grid side (the grid has all the real power) then
the high-voltage side inverter side is protected by the inverter
capacity.   i.e.
Is it possible for both sides of the transformer to be labeled as a
primary?  No, Your transformer was made with assumed primary and secondary,
the energy flows from primary to secondary with a slight voltage drop, the
turns ratio reflects this slight voltage change.  Also, the low voltage
side is wound next to the core and the higher voltage side is wound away
from the core.

- I understand that 450.4(A) requires overcurrent protection on the "input
conductors" (aka "primary conductors"?), correct? Yes, the grid side must
be protected in the event of a fault. And the solar side must be protected
or have conductors sufficient to carry the current as limited by input
(grid) side OCPD.  The solar side by OCPD or a power limited supply, the
inverters, but the input from the grid must be protected.  Must we provide
overcurrent protection on the "other" side in the above scenario? Yes, the
solar side has to have protection from the grid.

   - We were originally planning to land each inverter output on a 50A
breaker inside a 200A main lug panel - must we also include a main breaker? No,
not from the inverters, but if you are you are using this as the OCPD on
the solar side of the transformer yes.  I think the calculation is 125%
transformer OCPD power or the power of the transformer is the maximum
current.  For example, 112 kVA / 480 x 125% =292 amps is possible, the
conductors need to be 292 amp or you protect with OCP. If you protect then
the conductors need to be 100 kW x125% /480/1.73= 150 amp minimum.

- How do we size the grounding conductors on either side of the
transformer? For standard distances, the equipment ground for the
transformer is from 250.122 based on the grid side OCPD.  For 100 kW x 1.25
/208/1.73 = 350 amps, is #3 cu or equiv.
 For the solar side it is a separately derived system, The equipment ground
from 250.122 is #6, However, if you use a corner grounded delta, (if SE
will allow)  you do not run a separate equipment ground.  Being a
separately derived system you need an GEC, which is sized from 250.66and it
will go to a series of ground rods, with a #6

   - Is the grounding conductor between MDP and transformer considered an
EGC (250.122)? Yes,  for the transformer on the grid side


On Tue, Apr 10, 2018 at 7:38 PM, Corey Shalanski 
wrote:

> I'm seeking some guidance on how to treat transformers when included in a
> PV system.
>
> Specific system details:
> - (3x) SolarEdge 33.3 inverters combined in a 200A panel
> - 480/208 transformer to be installed between 200A panel and main
> distribution panel (service is 208 wye)
>
> My questions:
>
> - Is there any consensus on what circuit is considered the "primary" and
> which is the "secondary" in this type of scenario?
>- NEC 690.9(D) speaks of "considering first one side of the
> transformer, then the other side of the transformer, as the primary". Is
> this relevant? i.e. Is it possible for both sides of the transformer to be
> labeled as a primary?
>
> - I understand that 450.4(A) requires overcurrent protection on the "input
> conductors" (aka "primary conductors"?), correct? Must we provide
> overcurrent protection on the "other" side in the above scenario?
>- We were originally planning to land each inverter output on a 50A
> breaker inside a 200A main lug panel - must we also include a main breaker?
>
> - How do we size the grounding conductors on either side of the
> transformer?
>- Is the grounding conductor between MDP and transformer considered an
> EGC (250.122)?
>- Is the inverter-side of the transformer considered a separately
> derived system, in which case the grounding conductor would be considered a
> GEC (250.102(C)(1))?
>
> Thanks for any advice!
>
> --
> Corey Shalanski
> Joule Energy
> New Orleans, LA
>
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[RE-wrenches] Transformer Nomenclature/Protection/Grounding

[Corey Shalanski]

I'm seeking some guidance on how to treat transformers when included in a
PV system.

Specific system details:
- (3x) SolarEdge 33.3 inverters combined in a 200A panel
- 480/208 transformer to be installed between 200A panel and main
distribution panel (service is 208 wye)

My questions:

- Is there any consensus on what circuit is considered the "primary" and
which is the "secondary" in this type of scenario?
   - NEC 690.9(D) speaks of "considering first one side of the transformer,
then the other side of the transformer, as the primary". Is this relevant?
i.e. Is it possible for both sides of the transformer to be labeled as a
primary?

- I understand that 450.4(A) requires overcurrent protection on the "input
conductors" (aka "primary conductors"?), correct? Must we provide
overcurrent protection on the "other" side in the above scenario?
   - We were originally planning to land each inverter output on a 50A
breaker inside a 200A main lug panel - must we also include a main breaker?

- How do we size the grounding conductors on either side of the transformer?
   - Is the grounding conductor between MDP and transformer considered an
EGC (250.122)?
   - Is the inverter-side of the transformer considered a separately
derived system, in which case the grounding conductor would be considered a
GEC (250.102(C)(1))?

Thanks for any advice!

--
Corey Shalanski
Joule Energy
New Orleans, LA
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