zig zag transformer

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electrofelon

electrofelon

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Cherry Valley NY, Seattle, WA
Sahib said:
If it is one time phase unbalance, the system may shut down to pass the unbalance condition. What if it persists which is usually the case? In such cases, neutral grounding or a zigzag transformer is required.
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I do not understand your logic. If it persists than something is wrong and putting in a zig zag isn't going to fix it. Please note that in the OP there was,a wye wye transformer Plus a zig zag. The question was what the purpose of the zig zag was. I, and I assume the others, understand using a zig zag on a delta source to get a ground reference.
 
Sahib

Sahib

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electrofelon: If the neutrals of inverters are connected together and grounded, circulating current in parallel connected inverters will flow through neutrals also. So to avoid it, inverter neutrals not to be connected together and grounded. So a zigzag transformer is used for grounding. Just IMHO.
 
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electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
Sahib said:
electrofelon:It looks like there is no common neutral for the inverters. Is that true?
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The inverters DO have a netural connection.

Sahib said:
electrofelon: If the neutrals of inverters are connected together and grounded, circulating current in parallel connected inverters will flow through neutrals also. So to avoid it, inverter neutrals not to be connected together and grounded. So a zigzag transformer is used for grounding. Just IMHO.
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That is not how the system is supposed to be set up, nor have I ever heard of such an arrangement. My theory is the zig zag is from a utility requirement, and it wasn't implemented properly.
 
electrofelon

electrofelon

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Cherry Valley NY, Seattle, WA
Sahib said:
. The paper I posted in my last post states there are grounded and ungrounded inverters marketed.
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Transformerless inverters connected to grounded WYE AC systems have become dominant in the US, comprising the vast majority of systems. Although the PV systems that connect to transformerless inverters are sometimes called "ungrounded" This is a poor choice of words because the DC side is referenced to the grounded AC side because there is no transformer to isolate the two sides. There were some inverters that were isolated and ungrounded, I do not know if there still are.


It actually looks like EGC tapped from star connected transformer neutral.
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Yes that is correct. It is a little confusing that they dont show the neutral.
 
Sahib

Sahib

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electrofelon: Your inverter output neutral should be ungrounded. Otherwise there is no need for a zigzag transformer as explained in the paper in post #45.
 
david luchini

david luchini

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Sahib said:
electrofelon: Your inverter output neutral should be ungrounded. Otherwise there is no need for a zigzag transformer as explained in the paper in post #45.
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The inverter neutral should be grounded as it connected to a grounded wye system.

The zig-zag transformer is installed to reduce current flowing in the neutral due to harmonics, as explained in post #23.
 
david luchini

david luchini

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Sahib said:
Correct.

Nowhere it is shown in the picture of the OP that the inverter neutral connects to the system neutral. The fourth wires coming from the inverters are actually EGC's bonded to the star connected transformer neutral.
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I don't believe that the diagram shows EGC's. It is not common in the US to indicate EGC's on a riser diagram.

But even it is showing EGCs, the inverters are still connected to a GROUNDED system. A zig-zag transformer would not be installed to create a ground on a system that is already grounded. It would be installed to reduce current flowing in the neutral due to harmonics.
 
electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
david luchini said:
I don't believe that the diagram shows EGC's. It is not common in the US to indicate EGC's on a riser diagram.

But even it is showing EGCs, the inverters are still connected to a GROUNDED system. A zig-zag transformer would not be installed to create a ground on a system that is already grounded. It would be installed to reduce current flowing in the neutral due to harmonics.
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I certainly agree, although based on the utility requirements here, it seems plausible that there was,a mistake or misunderstanding in the design. I have never heard of harmonic mitigation for pv systems.
 
Sahib

Sahib

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Location
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David & electrofelon: Would you mind giving your comments on the paper in post#45. That may clarify the matter a bit.:)
 
G

ggunn

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Consulting Electrical Engineer - Photovoltaic Systems
I believe that the inverters in question are SMA Tripowers. If so (I'm not slogging through the thread to verify it) all but the 30kW version (it can run either way) require a neutral, and in most if not all cases this neutral is grounded.
 
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