Reverse fed step down transformer

[MGC]

Due to a time crunch.
I have a case where I may need to use a standard 480V delta to 120/208V Wye in reverse to gain 400v 100A 3 phase.
I am aware of the inrush current and potential nuisance tripping that may occur.
I do have a 200A 120/208V breaker not being used.

 

[kwired]

All you stated were a few facts, do you have a question?

 

[GoldDigger]

That may not be allowed under [2014], unless the transformer is identified for that use too.
You do know that you should not connect the neutral on the wye (source) side?
How do you propose to get the 400V output?

 

[kwired]

That may not be allowed under [2014], unless the transformer is identified for that use too.

Something new that has come up in this type of discussion.

You do know that you should not connect the neutral on the wye (source) side?

Often a topic in this type of discussion.

How do you propose to get the 400V output?

I did notice that and almost commented on it.

We already have three things to consider and we don't really have a question from the OP yet.

 

[MGC]

MGC

MGC

Correct I did not ask a question, I began reading some of the comments on previous threads.
I am new to this site and did not want to break any rules.
Should I use output voltage to calculate connected Amperage?

 

[MGC]

MGC

MGC

Yes I know not to connect a neutral to XO.
I have a call into an engineer to help with achieving 400V, if possible with a standard transformer.
My guess is this will require I special trans.

 

[jim dungar]

How close to 400V do you need?
416Y/240 transformer outputs are being more common, so you may find something from a standard manufacturer. Don't forget to look at 'drive isolation' transformers also.

You might be able to build your own by taking (3) 25kVA 240V:120V single phase units and connecting them into a reverse fed 75kVA wye-wye giving you 208Y/120V to 416Y/240V.

 

[Besoeker]

That may not be allowed under [2014], unless the transformer is identified for that use too.
You do know that you should not connect the neutral on the wye (source) side?

I've seen that here before but don't see why you shouldn't.
Surely a floating neutral on a wye winding would allow the phases to go unbalanced?

 

[don_resqcapt19]

That may not be allowed under [2014], unless the transformer is identified for that use too. ...

The code stops short of requiring that the transformer be identified for use when reverse fed. It only requires that the instructions say you can reverse feed the transformer.

450.11(B) Source Marking. A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the instal lation is in accordance with the manufacturer?s instructions.

 

[MGC]

MGC

MGC

Thanks guys,
I'll follow up on this today with my supplier and engineer.

 

[Jraef]

But the part about getting to 400V is not as simple of a detail as you might be thinking. Then I also question the need. People jump into doing this quite often when they get a piece of machinery in from overseas because they read only the voltage on the nameplate. But 99.9% of the time if something is rated for 400V, it is also rated for 50Hz, not 60Hz, and that can create new problems you might not realize yet. So why not describe the reason for the need, including how the 400V power is used, I.e. motors, drives, heaters, controls etc.

 

[GoldDigger]

The code stops short of requiring that the transformer be identified for use when reverse fed. It only requires that the instructions say you can reverse feed the transformer.

IMO if the msnufacturer's instructions say that you can reverse feed it, it is identified for that purpose. It need not be listed.

 

[GoldDigger]

I've seen that here before but don't see why you shouldn't.
Surely a floating neutral on a wye winding would allow the phases to go unbalanced?

A good question.
With a balanced delta load there will be no current in the wye point. And with an unbalanced line to line load the load current will be supplied in parallel by the obvious winding and the series combination of the other two windings. The sum of those two primary winding currents will balance the current at the wye point to zero.
Now look only at the unloaded transformer. If the voltages on the wye side are not perfectly balanced you will get a circulating current in the closed delta side sufficient to bring the system into balance. This will result in a very high neutral current in the wye side, possibly greater than the transformer capacity would suggest at full load.
If the wye point is left unconnected the applied primary voltages will simply be brought into balance as the wye point floats relative to ground.
With a wye wye transformer there is no balance condition imposed on the output and the vector triangle on the output will automatically be closed, even if unbalanced.
You can see this more easily if you look at what happens with unloaded wye delta and unloaded wye wye when the applied voltages on the primary are 120, 120, and 0. (Line to neutral)

 

[Besoeker]

A good question.
With a balanced delta load there will be no current in the wye point. And with an unbalanced line to line load the load current will be supplied in parallel by the obvious winding and the series combination of the other two windings. The sum of those two primary winding currents will balance the current at the wye point to zero.
Now look only at the unloaded transformer. If the voltages on the wye side are not perfectly balanced you will get a circulating current in the closed delta side sufficient to bring the system into balance. This will result in a very high neutral current in the wye side, possibly greater than the transformer capacity would suggest at full load.
If the wye point is left unconnected the applied primary voltages will simply be brought into balance as the wye point floats relative to ground.
With a wye wye transformer there is no balance condition imposed on the output and the vector triangle on the output will automatically be closed, even if unbalanced.
You can see this more easily if you look at what happens with unloaded wye delta and unloaded wye wye when the applied voltages on the primary are 120, 120, and 0. (Line to neutral)

Still make no sense to me.
Perhaps I framed my question badly.
A floating neutral on the input wye side would allow the phase voltages to go unbalanced.
So, to make the question simpler, what is the downside to connecting the star point?

 

[GoldDigger]

Still make no sense to me.
Perhaps I framed my question badly.
A floating neutral on the input wye side would allow the phase voltages to go unbalanced.
So, to make the question simpler, what is the downside to connecting the star point?

I also explained the downside of connecting the star point. If the incoming line to ground voltages are unbalanced (unequal), then even a few volts of unbalance can cause enormous currents in the primary neutral.
And going back to your first statement, a floating neutral on the input wye will allow the voltage balance to be forced by the delta secondary, regardless of whether the primary voltages are closely matched or even roughly matched. It will not allow the delta voltages on the output to become unbalanced because those are constrained by the closed delta.

 

[Besoeker]

I also explained the downside of connecting the star point. If the incoming line to ground voltages are unbalanced (unequal),

Surely we should be looking at line to neutral voltages.

 

[kwired]

POCO's build wye primary transformer banks out of single phase transformers with the neutral connected all the time with no problems, but for reasons I don't fully understand you get high circulating currents if you do this with all windings on a common core.

 

[mike_kilroy]

Due to a time crunch.
I have a case where I may need to use a standard 480V delta to 120/208V Wye in reverse to gain 400v 100A 3 phase.
I am aware of the inrush current and potential nuisance tripping that may occur.
I do have a 200A 120/208V breaker not being used.

I'd hate to see you compound your time crunch problem by blowing up the item being powered this way and thus have even longer down time....

So, I ask, do you have a particular transformer in mind to do this? If so, I assume it has taps on it so you CAN get 400v from the 480v nominal side? If so, post the particular xfmr spec and folks will give opinions on its usefulness....

You should be REAL sure too what your input voltage will be: is it indeed 208v or it 240v? Will make a difference too on the output voltage....

Last comment, pay close attention to Jraef's post: If you simply throw 400v 60hz into a device designed for 400v 50hz you have potential to do damage too. His suggestion for you to give a bunch more detail on who, what, where, when, how you are doing is a really good idea. Some of us have been there, done that, got the shirt, and might save your bacon

 

[iceworm]

POCO's build wye primary transformer banks out of single phase transformers with the neutral connected all the time with no problems, but for reasons I don't fully understand you get high circulating currents if you do this with all windings on a common core.

Aren't the POCO xfm banks connected Y-Y? I've never seen one connected Y-D - Well, one that worked anyway.

ice

 

[kwired]

Aren't the POCO xfm banks connected Y-Y? I've never seen one connected Y-D - Well, one that worked anyway.

ice

Pretty much all the banks here are Y primary built out of one bushing transformers the MGN must connect to the neutral of the primary if that is the case

Secondary side can be wye or delta - depends on what service is being supplied, and of course either setting taps or using right transformer to get desired secondary voltage.

Open delta systems are fairly common around here as well, the primary side of those banks are two phase conductors again connected to single bushing transformers and MGN connects as well - so you have an "open wye" so to speak for the primary side.

You don't see two bushing transformers around here, if you do it is probably some older setup, they don't use anything but single bushing transformers for pole mounted transformers.