Solar wye/wye transformer

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
But if the nuetral conductor from the service is not terminated on the XO lug then it shouldnt matter if the XO terminal is bonded at thr transformer, should it?

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
There is a very fine point here that people need to understand. There are inverters that use the EGC in place of a neutral if the neutral is not run to the inverter (SMA for instance) but they require a grounded WYE service. The EGC connected to the neutral at the service entrance allows the inverter to still monitor the phase to neutral voltage by monitoring the phase to EGC voltage at the inverter.
Then there are inverters that don't need any neutral reference and can be connected to a delta service. Do not confuse the two types of inverters even though both say they don't need a neutral. If an inverter that uses the EGC for phase to neutral voltage monitoring is connected to a delta it will probably run but will have intermittent shutdowns as the phantom phase to neutral voltage bounces around.
Yes, of course. When I have to put in a transformer to connect a 480/277V inverter to a 208/120V service, I use a delta-wye (delta to the service) and I bond N to G at the transformer.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Got it Jon, thank you! Yes I explained it in detail to Hammond.

I think the only question left is if the SMA Core 1 inverters need the neutral for voltage stability? It sounds like according to pvn00b's post #72 they do not and can use the EGC for that.
SMA Core 1 inverters are shipped with N and G bonded with a strap that must be removed if you run a neutral conductor to the inverter.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Code:
If the X0 of the transformer _is_ bonded to the case, then what you are doing is using the EGC as the neutral, which is a separate no-no from multiple neutral bonds, and arguably worse.

Jon, can you explain how the EGC would be carrying nuetral current and to where if I do not connect the nuetral from the service to XO and I am unable to break the XO bond to the case of the transformer?

I think I can break the XO bond to the case so maybe its all for nothing, but still not following how the EGC would carry nuetral current in the situation I described above.

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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Jon, can you explain how the EGC would be carrying nuetral current and to where if I do not connect the nuetral from the service to XO and I am unable to break the XO bond to the case of the transformer?
Because of the neutral-EGC bond at the service, if the transformer "wants" to draw any neutral current to/from X0, and X0 is bonded to the case, it will have a current path. X0 - case - EGC - neutral bond at service.

Cheers, Wayne
 
There is a very fine point here that people need to understand. There are inverters that use the EGC in place of a neutral if the neutral is not run to the inverter (SMA for instance) but they require a grounded WYE service. The EGC connected to the neutral at the service entrance allows the inverter to still monitor the phase to neutral voltage by monitoring the phase to EGC voltage at the inverter.
Then there are inverters that don't need any neutral reference and can be connected to a delta service. Do not confuse the two types of inverters even though both say they don't need a neutral. If an inverter that uses the EGC for phase to neutral voltage monitoring is connected to a delta it will probably run but will have intermittent shutdowns as the phantom phase to neutral voltage bounces around.
YES, but the question remains for the case of the inverter that needs a neutral connection but can use the EGC as a substitute: Why is there even an option? Why not just say "no neutral connection required" ? Saying the installer can put a jumper between N-G freaks everyone out because they don't want to "use the EGC as a neutral" so they end up running the neutral conductor. I have done dozens of megawatts of utility scale string inverter based ground mounts, and they all had a full size neutral run to the combiner panel boards and to the inverters, a complete waste, thousands and thousands of dollars. As I said, I have been out of the utility scale stuff for 4-5 years, maybe it's changing.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Code:
 Because of the neutral-EGC bond at the service, if the transformer "wants" to draw any neutral current to/from X0, and X0 is bonded to the case, it will have a current path. X0 - case - EGC - neutral bond at service.

I see, I see. Thanks Wayne

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electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Yeah that bonding strap once removed does break the XO bond to the case. When I was measuring continuity in an earlier post the nuetral from the service was terminated on the XO lug. Im all good now.

Thanks for the help guys. Dont think I will do a wye wye again:/

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Saying the installer can put a jumper between N-G freaks everyone out because they don't want to "use the EGC as a neutral" so they end up running the neutral conductor.
FWIW, the SMA Core 1 inverters ship with the jumper in place; the installers have to remove it to run a neutral.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
YES, but the question remains for the case of the inverter that needs a neutral connection but can use the EGC as a substitute: Why is there even an option? Why not just say "no neutral connection required" ? Saying the installer can put a jumper between N-G freaks everyone out because they don't want to "use the EGC as a neutral" so they end up running the neutral conductor. I have done dozens of megawatts of utility scale string inverter based ground mounts, and they all had a full size neutral run to the combiner panel boards and to the inverters, a complete waste, thousands and thousands of dollars. As I said, I have been out of the utility scale stuff for 4-5 years, maybe it's changing.
Perhaps a shift in terminology would help. That is, manuals could distinguish between requiring a 'neutral conductor' to the inverter in the one case, and a 'bonding conductor to the neutral point of the system' in the other. Probably pie in the sky optimism on my part, but there you go.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Funny enough I ran into an EE tonight who installed some of the original SMA Tripowers on a 480V ungrounded system at a factory. He said they got the okay from SMA but that some of the inverters started blowing up. They ended up having to install a transformer to establish the ground. Sounded like a 480V Delta 3 wire to 480V Wye 4 wire transformer.



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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Funny enough I ran into an EE tonight who installed some of the original SMA Tripowers on a 480V ungrounded system at a factory. He said they got the okay from SMA but that some of the inverters started blowing up. They ended up having to install a transformer to establish the ground. Sounded like a 480V Delta 3 wire to 480V Wye 4 wire transformer.



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Seems understandable. There is a big difference between an ungrounded source and a grounded source without a neutral (e.g. corner grounded) or with the neutral not extended.
 

pv_n00b

Senior Member
Location
CA, USA
I think the only question left is if the SMA Core 1 inverters need the neutral for voltage stability? It sounds like according to pvn00b's post #72 they do not and can use the EGC for that.
The SMA inverter can use the EGC as a proxy for the neutral if a neutral is not run to the inverter, but a functioning neutral is still needed where the neutral is bonded to the EGC at the source of the separately derived system. So the SMA inverter needs a grounded WYE source.
 

pv_n00b

Senior Member
Location
CA, USA
YES, but the question remains for the case of the inverter that needs a neutral connection but can use the EGC as a substitute: Why is there even an option? Why not just say "no neutral connection required" ? Saying the installer can put a jumper between N-G freaks everyone out because they don't want to "use the EGC as a neutral" so they end up running the neutral conductor. I have done dozens of megawatts of utility scale string inverter based ground mounts, and they all had a full size neutral run to the combiner panel boards and to the inverters, a complete waste, thousands and thousands of dollars. As I said, I have been out of the utility scale stuff for 4-5 years, maybe it's changing.
I think the point is moot for large MMW utility scale inverters that still need full size neutrals. They do not have the reduced size neutral option that is needed to even consider using the EGC as a proxy. That only applies to smaller inverters that only use the neutral for voltage sensing.
 

Carultch

Senior Member
Location
Massachusetts
The SMA inverter can use the EGC as a proxy for the neutral if a neutral is not run to the inverter, but a functioning neutral is still needed where the neutral is bonded to the EGC at the source of the separately derived system. So the SMA inverter needs a grounded WYE source.

What is the reason for NOT using the EGC as a proxy for the neutral, with any inverter that allows you to do so, and never has a reason to put a full amp of current on any zero-voltage wire? This seems like an obvious solution to economize the installation. The manual shows that it is possible to do this, but doesn't provide any guidance on the pros and cons.

I think the point is moot for large MMW utility scale inverters that still need full size neutrals. They do not have the reduced size neutral option that is needed to even consider using the EGC as a proxy. That only applies to smaller inverters that only use the neutral for voltage sensing.
Do you have an example of any utility scale inverters that are currently made today, and require full size neutrals? And is it just lack of a statement otherwise that causes them to require it, or does it actually have an operational reason for a significant amount of amps on the neutral?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What is the reason for NOT using the EGC as a proxy for the neutral, with any inverter that allows you to do so, and never has a reason to put a full amp of current on any zero-voltage wire? This seems like an obvious solution to economize the installation. The manual shows that it is possible to do this, but doesn't provide any guidance on the pros and cons.


Do you have an example of any utility scale inverters that are currently made today, and require full size neutrals? And is it just lack of a statement otherwise that causes them to require it, or does it actually have an operational reason for a significant amount of amps on the neutral?
I don't do utility scale, but of the commercial inverters I use, only SolarEdge does not show the neutral as optional. I do not know the reason, only that it shows a neutral connection on their data sheets and does not say that it is optional..
 
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