step up transformers

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mshields

mshields

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Boston, MA
I've got a building with only 208V and a piece of equipment that needs 480V.

Am going to provide a 45kVA step up transformer designed for step up so I don't run into the inrush problems associated with reverse feeding a standard unit.

The equipment only requires 3 phase, 3 wire. I believe the way to go is to nevertheless provide a delta wye and simply not bring the neutral out to the load.

Do you agree that this is the appropriate approach.

Related for my general knowledge, if I did provide a wye delta, would it still be considered a separately derived system. Is the ground connected to one of the corners. Something I've always had a hard time understanding.

Thanks,
 
mshields said:
I've got a building with only 208V and a piece of equipment that needs 480V.

Am going to provide a 45kVA step up transformer designed for step up so I don't run into the inrush problems associated with reverse feeding a standard unit.

The equipment only requires 3 phase, 3 wire. I believe the way to go is to nevertheless provide a delta wye and simply not bring the neutral out to the load.

Do you agree that this is the appropriate approach.

Related for my general knowledge, if I did provide a wye delta, would it still be considered a separately derived system. Is the ground connected to one of the corners. Something I've always had a hard time understanding.

Thanks,
Click to expand...

Yep, take neutral point to ground, and just don't run neutral.

A wye - delta, would be an ungrounded system.
 
Some equipment does not operate well on corner grounded or ungrounded systems. I see little if any advantage in you having a delta secondary. Taking NEC requirements into account, I would recommend a 208 to480Y/277 transformer. Do not connect the neutral on your 208 side and ground the 277 per 250.30.
 
mshields said:
Related for my general knowledge, if I did provide a wye delta, would it still be considered a separately derived system. Is the ground connected to one of the corners. Something I've always had a hard time understanding.

Thanks,
Click to expand...

The key feature that makes something a 'separately derived system' is that it is a _new_ source of EMF electrically isolated from other sources of EMF. Electricity is always looking for a closed circuit path back to the source, with an SDS this path must go back to the SDS itself.

The secondary of a normal transformer is electrically isolated from the primary. (Or at least it is until you provide your intentional grounding connection.) Current flowing from any of the terminals on the secondary is trying to get to one of the other terminals on the secondary, and no where else. (I am ignoring capacitance here, which provides a very small path for current to flow without apparently returning to the source...)

So you can pick any _one_ of the secondary terminals and 'ground' it to make a grounded system. This grounding connection will 'define' that terminal to be 0V and all of the other terminals will be forced (by the transformer) to be at some other voltage relative to ground. With a 'wye' secondary code requires the neutral to be grounded, but in theory _any_ of the terminals could be grounded.

If the secondary is a delta, then you simply don't have the three phase neutral available to ground, so you have to pick one of the other terminals. If you have a 480V delta secondary and ground one of the terminals, than that terminal is forced to be 0V to ground...and the other two terminals are now at 480V each other and to ground. If you had instead a wye secondary and grounded the neutral, then each terminal would be still be at 480V relative to the others, but would now only be at 277V to ground.

Jon
 
I'm a box of Rocks!

The hardest thing for me to remember about transformers is the
phrase - it is considered a separately derived system. SDS.

So in any case if it a step up or step down or even a isolation
transformer, it's still a separately derived system? Right?
 
cadpoint said:
I'm a box of Rocks!

The hardest thing for me to remember about transformers is the
phrase - it is considered a separately derived system. SDS.

So in any case if it a step up or step down or even a isolation
transformer, it's still a separately derived system? Right?
Click to expand...
Yup, and if you use a combination of step up and step down transformers to deal with a long haul VD issue, then you have two separately derived systems, the feeder and the ultimate output.
 
cadpoint said:
So in any case if it a step up or step down or even a isolation
transformer, it's still a separately derived system? Right?
Click to expand...

Step up or step down, makes no difference.

What _does_ make a difference is 'autotransformer' or not.

An autotransformer is one in which the inputs and outputs _share_ the same coil, or where the coils are connected together to form a common circuit. In this case you do _not_ have an SDS.

Buck-boost transformers are not SDSs; step up and step down transformers are.

-Jon
 
Thanks Big W - I read and re-read your post and others, thanks for the
clear and concise wording! I appreciate your time as well as others
that take the time, Thank You! :thumbsup:
 
GoldDigger said:
Yup, and if you use a combination of step up and step down transformers to deal with a long haul VD issue, then you have two separately derived systems, the feeder and the ultimate output.
Click to expand...

And a phase shift that will not allow you to interconnect to another source without an equally adjusted phase shift.
 
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