PV Transformer connection

[Mou]

Hi guys,

I've 2MW PV project and the voltage at the interconnection point (site substation) is 13.8KV three wire grounded system.
the utility force us to use Y-Y transformer, My concern about the connection of 4 wires transformer primary to 3 wire system?
Should I solidly grounded the neutral or keep it floating? Is there any reference or guidance for connecting 3 Y-Y transformer to delta bus??

Thanks

 

[NYSolar]

Hi guys,

I've 2MW PV project and the voltage at the interconnection point (site substation) is 13.8KV three wire grounded system.
the utility force us to use Y-Y transformer, My concern about the connection of 4 wires transformer primary to 3 wire system?
Should I solidly grounded the neutral or keep it floating? Is there any reference or guidance for connecting 3 Y-Y transformer to delta bus??

Thanks

what type of inverters are being used?

 

[Mou]

Chint 36KW string inverters 4 wires + ground.

 

[ggunn]

Chint 36KW string inverters 4 wires + ground.

The neutral must be grounded on the inverter side, for sure.

 

[Mou]

We know that for sure but should we bonded the neutral on the high side solidly or through grounding reactor?

 

[ggunn]

We know that for sure but should we bonded the neutral on the high side solidly or through grounding reactor?

In the OP you said the MV side was three wire grounded; what is grounded? Is it corner grounded? I don't know a lot about MV, but is there three phase corner grounded MV?

 

[Mou]

on high side ,They have 115KV-D/13.8KV-Y with grounding reactor hooked to the neutral with three wires network, so its grounded on transformer side.

 

[ggunn]

on high side ,They have 115KV-D/13.8KV-Y with grounding reactor hooked to the neutral with three wires network, so its grounded on transformer side.

Like I said, I am not that familiar with MV, so you are going to have to keep it simple and speak slowly... My question is if there are three wires and it is a grounded system, which wire is grounded? If none of them are, it seems to me that you want a delta to wye transformer. But that's what you were saying to begin with, isn't it?

If you have a wye transformer and leave the center open, is it the same as if it were a delta?

 

[Mou]

Like I said, I am not that familiar with MV, so you are going to have to keep it simple and speak slowly... My question is if there are three wires and it is a grounded system, which wire is grounded? If none of them are, it seems to me that you want a delta to wye transformer. But that's what you were saying to begin with, isn't it?

If you have a wye transformer and leave the center open, is it the same as if it were a delta?

As shown the system is grounded at the upstream transformer.
So the PV transformer will have floating Y ( center open) my concern about the fault current path ?

 

[Smart $]

As shown the system is grounded at the upstream transformer.

You have a high-impedance grounded primary, i.e. the 13.8kV system. Your primary wye point must be "floated". You should confirm this with POCO EE.

So the PV transformer will have floating Y ( center open) my concern about the fault current path ?

POCO should have fault detection on the primary.

 

[Mou]

You have a high-impedance grounded primary, i.e. the 13.8kV system. Your primary wye point must be "floated". You should confirm this with POCO EE.



POCO should have fault detection on the primary.

Thanks ,

what's POCO standard for? Do you have any guidance to such type of connection? I ran the Etap system analysis and every thing seem normal.

 

[Smart $]

...
POCO should have fault detection on the primary.

I believe the local main should have fault protection also. The "service" looks like it would be treated the same as or similar to an ungrounded system.

I'm much the same as ggunn in that MV is not an area where I have extensive knowledge. I'd have to research how an MV high impedance service should be handled...

Thanks ,

what's POCO standard for? Do you have any guidance to such type of connection? I ran the Etap system analysis and every thing seem normal.

You posted while I was typing addendum, but reply would be much the same. You'll have to coordinate with your POCO if not spelled out explicitly in their "Service Manual".

 

[Mou]

I believe the local main should have fault protection also. The "service" looks like it would be treated the same as or similar to an ungrounded system.

I'm much the same as ggunn in that MV is not an area where I have extensive knowledge. I'd have to research how an MV high impedance service should be handled...

Thanks, If you got some thing please share.

 

[Smart $]

Thanks, If you got some thing please share.

Just so you know, I will not be researching the matter after writing this post... but I do hope you post updates of your progress on the matter.

 

[ggunn]

Thanks ,

what's POCO standard for?

It means "the utility" but I don't remember what the letters stand for.

A question for the group: If you have a transformer with a wye primary and a wye secondary, and you connect the phase conductors to H1, H2, and H3 but leave the neutral open, is it the same at the secondary as if it had a delta primary?

 

[GoldDigger]

It means "the utility" but I don't remember what the letters stand for.

A question for the group: If you have a transformer with a wye primary and a wye secondary, and you connect the phase conductors to H1, H2, and H3 but leave the neutral open, is it the same at the secondary as if it had a delta primary?

It is POwer COmpany.

If you have a delta secondary you must float the primary wye point. With a Y-Y transformer you may safely "ground" it (i.e. connect H4 to the neutral wire of the grounded wye and possibly, depending on POCO, bond it to ground.)

 

[ggunn]

It is POwer COmpany.

If you have a delta secondary you must float the primary wye point. With a Y-Y transformer you may safely "ground" it (i.e. connect H4 to the neutral wire of the grounded wye and possibly, depending on POCO, bond it to ground.)

Thanks, but that's not what I was asking. What if you have a wye wye transformer and you must have a grounded neutral on the secondary side; if you leave the neutral open on the primary is it the same as if you were using a delta wye transformer? Example: connecting an SMA 480V Tripower inverter to a 208V service through a wye wye transformer. If you just connect the phase conductors on the 208 side and leave the neutral open is it the same as if it were a delta wye tranny?

 

[GoldDigger]

... If you just connect the phase conductors on the 208 side and leave the neutral open is it the same as if it were a delta wye tranny?

In terms of phase relationship between primary and secondary, emphatically not. In terms of voltage balance, power handling, etc I would say yes.

 

[ggunn]

In terms of phase relationship between primary and secondary, emphatically not. In terms of voltage balance, power handling, etc I would say yes.

I wouldn't think that the phase relationship between primary and secondary would matter to the grid tied inverter on the secondary; would you? It seems to me that the OP could just leave the primary neutral floating, connect the inverter to the phase conductors and neutral on the secondary, and ground the neutral on the secondary.

 

[GoldDigger]

I wouldn't think that the phase relationship between primary and secondary would matter to the grid tied inverter on the secondary; would you? It seems to me that the OP could just leave the primary neutral floating, connect the inverter to the phase conductors and neutral on the secondary, and ground the neutral on the secondary.

He could, but it seems to me to be a POCO decision, not his to make.