Replacement of Inverter Skid transformer with different vector configuration 1

[OOO3291]

This is related to the Operating Solar plant. Our contractor has replaced an Inverter skid unit transformer of ungrounded Yd1 configuration with the similar KVA rating of ungrounded Dy11 Transformer. It worked for few months and failed recently. We came to know about this vector configuration change recently and working on RCA. What are the consequences of this vector configuration change and affects on the Inverters as well as substation protection? All the remaining electrical parameters of both the transformers are same. Please see the attached nameplate details of both units. The RCA might be something else also, we are trying to analyze whether we can do this replacement again with Dy11 as we don't have any Yd1 Transformers available immediately?

Original configuration: Yd1 Ungrounded
Replacement configuration: Dy11 Ungrounded

 

[Kiribanda]

The inverters used in PV systems are compatible with wye floating, delta floating, and wye grounded transformer winding configurations.
Therefore, your original transformer connection YNd1 (low side 630V delta) was working fine. At that time the PE terminal of the inverter
must have been bonded to nearby system ground. Because you have 3 wires only.
But when it was changed to Dyn11 (Low side is now 630V ungrounded wye), did your contractor connect the wye point of the new transformer
to the PE terminal of the inverter? If yes, then the INV generates lot of harmonics and even neg seq. currents too which will flow into the
new transformer. Due to that, it may have got failed. Going forward, could you please confirm the above?

 

[OOO3291]

The inverters used in PV systems are compatible with wye floating, delta floating, and wye grounded transformer winding configurations.
Therefore, your original transformer connection YNd1 (low side 630V delta) was working fine. At that time the PE terminal of the inverter
must have been bonded to nearby system ground. Because you have 3 wires only.
But when it was changed to Dyn11 (Low side is now 630V ungrounded wye), did your contractor connect the wye point of the new transformer
to the PE terminal of the inverter? If yes, then the INV generates lot of harmonics and even neg seq. currents too which will flow into the
new transformer. Due to that, it may have got failed. Going forward, could you please confirm the above?

Thank you for your comments. We are still checking on the PE terminal connection of the inverter in the replacement transformer. If the Inverter PE terminal connection is not connected to the wye point, do you think we can use Dy11 as a replacement ?

 

[RRaghunath]

This is related to the Operating Solar plant. Our contractor has replaced an Inverter skid unit transformer of ungrounded Yd1 configuration with the similar KVA rating of ungrounded Dy11 Transformer. It worked for few months and failed recently. We came to know about this vector configuration change recently and working on RCA. What are the consequences of this vector configuration change and affects on the Inverters as well as substation protection? All the remaining electrical parameters of both the transformers are same. Please see the attached nameplate details of both units. The RCA might be something else also, we are trying to analyze whether we can do this replacement again with Dy11 as we don't have any Yd1 Transformers available immediately?

Original configuration: Yd1 Ungrounded
Replacement configuration: Dy11 Ungrounded

When you say 'failed', what does preliminary investigation suggest the cause of failure to be - Is it overload of windings or short circuit fault or voltage surge!

 

[OOO3291]

When you say 'failed', what does preliminary investigation suggest the cause of failure to be - Is it overload of windings or short circuit fault or voltage surge!

We are still identifying the cause of failure, but we can see the C phase bushing failed and leakage in the Transformer.

 

[Kiribanda]

If the Inverter PE terminal connection is not connected to the wye point, do you think we can use Dy11 as a replacement ?
Yes. As I mentioned before, PE terminal has to be bonded to the nearest plant grounding system At the same time, please make sure that your N point of the Dyn1 replacement transformer is NOT bonded to any ground point.