480 Volt Wye-Wye Transformer

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curcie

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Hi guys, I have a project I have hit a snag with. I have an AE 333Nx inverter I am tying to 3 phase 4 wire wye grounded gear. The inverter manufacturer is telling me I have a 3 phase grounded wye output from the inverter. There is no neutral coming off the inverter though. I am being asked by the customer to put in an isolation transformer. So then I need a wye-wye 480-480 transformer? At this point I am still trying to understand why I need the transformer and what kind. Any help would be much appreciated. If anyone needs more information just ask. Thanks.
 
curcie said:
Hi guys, I have a project I have hit a snag with. I have an AE 333Nx inverter I am tying to 3 phase 4 wire wye grounded gear. The inverter manufacturer is telling me I have a 3 phase grounded wye output from the inverter. There is no neutral coming off the inverter though. I am being asked by the customer to put in an isolation transformer. So then I need a wye-wye 480-480 transformer? At this point I am still trying to understand why I need the transformer and what kind. Any help would be much appreciated. If anyone needs more information just ask. Thanks.
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Correct me if I am wrong, but a wye to wye 480-480 transformer sounds like it could be replaced with a direct connection. What would it do?

I was told never to use a wye to wye transformer to connect an inverter to a service - something to do with harmonics.
 
ggunn said:
Correct me if I am wrong, but a wye to wye 480-480 transformer sounds like it could be replaced with a direct connection. What would it do?

I was told never to use a wye to wye transformer to connect an inverter to a service - something to do with harmonics.
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Yeah, this is the view I have. But since it was a spec, the engineer of record wants it. Trying to reason with him has failed up to this point. Do you have any info in regards to the harmonics issue?
 
It appears that you only need the transfomer to create a grounded conductor as the output of the inverter does not have a one. A 480 delta to 480/277Y transformer will do the job. There is no need for a wye/wye transformer.
 
don_resqcapt19 said:
It appears that you only need the transfomer to create a grounded conductor as the output of the inverter does not have a one. A 480 delta to 480/277Y transformer will do the job. There is no need for a wye/wye transformer.
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If this is a utility-interactive inverter, shouldn't even need a transformer... the POCO provides that.
 
Delta- inverter side to wye would not work. A delta would cause the inverter to lose the reference to ground. The inverter will more than likely start up but not continue running.
 
curcie said:
Delta- inverter side to wye would not work. A delta would cause the inverter to lose the reference to ground. The inverter will more than likely start up but not continue running.
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Well how would a wye-wye be any different? The inverter output has no neutral to connect to the primary.

That's why I said the POCO transformer is all it should need if a UII.
 
curcie said:
Hi guys, I have a project I have hit a snag with. I have an AE 333Nx inverter I am tying to 3 phase 4 wire wye grounded gear. The inverter manufacturer is telling me I have a 3 phase grounded wye output from the inverter. There is no neutral coming off the inverter though. I am being asked by the customer to put in an isolation transformer. So then I need a wye-wye 480-480 transformer? At this point I am still trying to understand why I need the transformer and what kind. Any help would be much appreciated. If anyone needs more information just ask. Thanks.
Click to expand...

The three phase grounded wye sounds like it may be a system by which the inverter references the voltages it generates to ground rather than to a neutral. If that is the case and the inverter is designed to force balanced currents on the three phase wires, a neutral will not be necessary and the ground will not be carrying any current.
If exposed to an ungrounded delta source, the inverter will not have the expected voltage references for its interactions with the phase lines.
The categorization as a wye means to me that the GTI is trying to match the individual phase voltages rather than the phase-to-phase voltages and is probably delivering a controlled current into each phase wire rather than phase-to-phase and letting it sum.
As long as the utility service is center neutral grounded, it should work without a transformer. But putting in a wye-wye or wye-delta transformer and bonding the inverter-side neutral to ground while not connecting it to the non-existent neutral terminal on the inverter should work. Any fault currents will end up going to the ECG, which is OK. Do you also connect the GTI side neutral to the POCO side neutral? Depends on bonding and separately derived or not.

Waiting eagerly for the fur to fly.....
 
GoldDigger said:
.... But putting in a wye-wye or wye-delta transformer and bonding the inverter-side neutral to ground while not connecting it to the non-existent neutral terminal on the inverter should work. ...
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If you bond the inverter-side transformer neutral to ground, any unbalance of current on the windings will cause neutral current to flow on ground back to inverter. That's a no-no.

I was trying to access literature online, but none of the links to literature worked on the mfr's website when I tried.

Do you also connect the GTI side neutral to the POCO side neutral? Depends on bonding and separately derived or not.

Waiting eagerly for the fur to fly.....
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GTI = Grid To Inverter...???

BTW, same as above. The neutral current just takes a different route.
 
Smart $ said:
If you bond the inverter-side transformer neutral to ground, any unbalance of current on the windings will cause neutral current to flow on ground back to inverter. That's a no-no.

I was trying to access literature online, but none of the links to literature worked on the mfr's website when I tried.


GTI = Grid To Inverter...???

BTW, same as above. The neutral current just takes a different route.
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Sorry. Standard abbreviation in Solar PV context. GTI is Grid Tied Inverter, the equipment, not the Grid To Inverter, the connection.

If the inverter is designed to maintain a vector balance on its phase outputs, it will not, under normal circumstances, allow any current to flow in the neutral.
Whether it does that or not is a good question, and if the manufacturer cannot answer that question the equipment is either really delta, but with equal limits on phase to ground voltage on all phases rather than being truly isolated or the manufacturer does not know what is really going on either. The inverter case needs to be bonded to earth, and the inverter-side wye cannot be allowed to float. Connecting it to the grid-side neutral would be just fine, IMHO, or connecting it to ground, as long as the only current ever flowing in it would be fault current as the GTI was in the process of failing or shutting down in reaction to a fault detection.
 
GoldDigger said:
Sorry. Standard abbreviation in Solar PV context. GTI is Grid Tied Inverter, the equipment, not the Grid To Inverter, the connection.

If the inverter is designed to maintain a vector balance on its phase outputs, it will not, under normal circumstances, allow any current to flow in the neutral.
Whether it does that or not is a good question, and if the manufacturer cannot answer that question the equipment is either really delta, but with equal limits on phase to ground voltage on all phases rather than being truly isolated or the manufacturer does not know what is really going on either. ... Connecting it to the grid-side neutral would be just fine, IMHO, or connecting it to ground, as long as the only current ever flowing in it would be fault current as the GTI was in the process of failing or shutting down in reaction to a fault detection.
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A bit iffy, eh! On the last part, I'd agree if the inverter has ground-fault detection with automatic shutdown.

The inverter case needs to be bonded to earth, and the inverter-side wye cannot be allowed to float.
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Why not?

This all leads back to why use a 1:1 voltage transformer? The system already has a grounded wye secondary in the POCO transformers. The only difference is which side of the transformer the loads are on.
 
Smart $ said:
This all leads back to why use a 1:1 voltage transformer? The system already has a grounded wye secondary in the POCO transformers. The only difference is which side of the transformer the loads are on.
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That was my question. If the inverter needs a four wire three phase 480V interconnect, and the service is four wire three phase 480V, why do you need a transformer?
 
Answer back from manufacturer

Answer back from manufacturer

Response:
Regarding your question, you need to ensure the transformer is adequately sized and will have a 480VAC grounded wye winding facing the inverter. All you need is a 1:1 isolation transformer to couple the inverter output to the 480Y/277 panelboard. I?d recommend a delta secondary and grounded WYE primary if at all possible, otherwise if you have a Yg Yg transformer ground fault current from anywhere on the panelboard will pass through the neutrals of the inverter transformer. Much simpler if it?s a Dyg winding config.
 
GoldDigger said:
OP still has to decide just what to do with the inverter-side X0 terminal. Before you document what you did, you need to decide what to do.
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Well if the transfomer is required by spec', then the transformer should also be spec'd. Additionally, see post #13.
 
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