Can I interconnection on the high voltage side of the existing transformer?

[Frank6172]

Hi all,

The site got 208V, 1200A utility service and there's an existing 208/120V to 480V step-up transformer to feed some 480V machine. Also there's a 480V, 400A AC disconnect on high voltage side of the transformer. We want to avoid 208V inverters as much as possible in this project so can we do the interconnection on the 480V disconnect of the transformer?

The transformer nameplate is below and our intended inverter is 480V, 120kW.

 

[winnie]

The questions you will need to answer:

1) What does the utility say about the interconnection?

2) What sort of grounding can your inverters tolerate?

It appears that the transformer is a delta primary wye secondary transformer being run in reverse (the wye _secondary_ is on the utility supply side). This means that the 480V side is either ungrounded or corner grounded, something that is a problem for some inverters.

The utility may have requirements for 'effective grounding' which may force a specific transformer and primary connection.

 

[Frank6172]

The questions you will need to answer:

1) What does the utility say about the interconnection?

2) What sort of grounding can your inverters tolerate?

It appears that the transformer is a delta primary wye secondary transformer being run in reverse (the wye _secondary_ is on the utility supply side). This means that the 480V side is either ungrounded or corner grounded, something that is a problem for some inverters.

The utility may have requirements for 'effective grounding' which may force a specific transformer and primary connection.

We are using the SolarEdge 480V transformer which says supporting Delta: IT. Does it mean it's compatible with the high voltage side winding? Thanks,

 

[winnie]

'Delta: IT' means the inverter is compatible with an _ungrounded_ delta, however your 480V system might be 'corner grounded'.

 

[zbang]

You're also going to waste some power in the transformer; not a lot, but more than zero.

 

[Frank6172]

'Delta: IT' means the inverter is compatible with an _ungrounded_ delta, however your 480V system might be 'corner grounded'.

Thanks! Do you know how to verify if the transformer is ungrounded or corner grounded?

 

[Frank6172]

You're also going to waste some power in the transformer; not a lot, but more than zero.

Yes we understood that. If the the 480V side machine is pulling enough power we don't anticipate a lot of backfeed current will be pushed to the utility side.

 

[winnie]

Thanks! Do you know how to verify if the transformer is ungrounded or corner grounded?

That's all in the design of the wiring connected to the transformer. Is there an intentional connection to ground on at any of the transformer terminals or the disconnect after the transformer.

You said that the transformer is feeding some existing machines. Do you know anything about the 480V distribution system currently in use?

 

[ggunn]

Thanks! Do you know how to verify if the transformer is ungrounded or corner grounded?

Two of the lines will be 277V to ground and one will be 0V to ground if it is corner grounded. If it is ungrounded all the lines will have undefined voltage to ground.

 

[winnie]

Two of the lines will be 277V to ground and one will be 0V to ground if it is corner grounded. If it is ungrounded all the lines will have undefined voltage to ground.

I think that would be 480V to ground.

If the system is corner grounded, then you will measure 480V to ground on 2 lines, and 0 on the 3rd line. However this will only tell you that there is a ground, not if it is intentional or a fault.

If the system is ungrounded, then the voltage to ground is poorly defined. Often it will be roughly 277V to ground on all phases. If you measure with a very low impedance meter, you might even measure 0V to ground on all phases. One clue that the system is ungrounded will be some sort of ground fault detection system.

 

[ggunn]

I think that would be 480V to ground.

Oops! You are correct, sir!