buck boost transformers for 240V Delta to 208V 3P? 1

[fastline12]

I have a situation where I need to buck voltage of 240-245V down to as close to 200V as possible. 200V is not a common voltage in the US so 208V may have to do. I have a couple single phase dry transformers and was considering configuring in a 3P bucking arrangement. I had to look at the diagrams to ensure there is no way a transformer failure could ever revert back to full 240V going to the equipment.

Does anyone see issue with this or pros/cons of this less expensive setup compared to an isolation set?

I am having to take a refresher on the math but as I recall, the KVA capacity in buck/boost is the voltage ratio * transformer KVA? So a 2KVA transformer operated in bucking at 32V buck would be 240/32 = 7.5 * 2kva 15kva capacity?

Is there any concerns with the delta primary side? All loads are L-L anyway so L-N are not an issue.


I was also curious of overloading capacity of this configuration for transient current draw for motor accelerations on VFDs?

 

[waross]

Do you have a 240 Volt delta supply?
What are you going to run on 200 Volts.
200 Volts is not a common SUPPLY voltage, however 200 Volts is a common rating for motors intended to run on 208 Volts.
Your formula is valid for an auto-transformer boost, and for a buck connection.
Look at the numbers. For a buck connection (not an auto-transformer connection) the 32 Volt winding is rated for 2000 VA / 32 Volts = 62.5 Amps.
240V - 32V = 208V 208 Volts x 62.5 Amps = 13000 Va, or 13 KVA.
That's for single phase. For three phase it gets a little more complicated.

Is there any concerns with the delta primary side? All loads are L-L anyway so L-N are not an issue.

I had to look at the diagrams to ensure there is no way a transformer failure could ever revert back to full 240V going to the equipment.

Look again. An open in the higher voltage winding will put full voltage to the equipment through the lower voltage winding. The lower voltage winding will act as a fairly low impedance reactor. This will reduce full voltage somewhat but the voltage will still be more than you are happy with when the loads are low.

I was also curious of overloading capacity of this configuration for transient current draw for motor accelerations on VFDs?

I have never had a problem with DOL starting, so VFDs should be no problem.
BUT:
I am concerned and frustrated.
Your questions and assumptions suggest that you may be a little over your head.
I have adjusted voltages with auto-transformer connections many times, but I hesitate to make a suggestion based on the lack of information that you have provided.
At this point, in the absence of more information my suggestion would be to go with a full two winding, 240 to 120/208 Volt isolating transformer.
By the way, if you are planning to use three transformers to buck the voltage then you are looking for transformers with a 140 Volt primary and a 24 Volt secondary Voltage.
There may be better ways.
With an auto-transformer connection you would need a primary voltage rating of 120 Volts and a secondary voltage rating of 24 Volts for a voltage of 202 volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fastline12]

The books seem to indicate full ampacity vs highest voltage for the kva calculation so 15kva for the values above? unless I missed step.

200V is a standard Japanese voltage. trying to run a Japanese machine in the states. I believe 208V will be just fine and the electronics have another 1P multi tap transformer so I can dial the voltage back to 200V to make them very happy.

Everything seemed to look good until I start reading about the use of 2 transformers for a closed delta 3P service. They indicate an open delta output, which I honestly don't comprehend. Again my loads do not have a L-N connection, but earth ground obviously needs to carry through to the load.

The load has VFDs, and a couple direct connected small 3P motors. Some info online indicates some phase shift could occur with the transformer arrangement but I need to do something as these electronics will not survive at 245V volts.

there is also some indication of kva capacity changing due to the close delta connection but nothing more is written about it. Looking in the documents though, they indicate a 3P capacity with 2, 2kva transformers connected, of 22.5kva. I simply cannot figure out where they get this figure if they are documenting a 2kva transformer with bucking of 32V, at 15kva.

 

[waross]

In Canada 600 volts is a standard supply voltage rather than 480 Volts.
However there is still a lot of 480 volt equipment around and more coming in new.
I had a customer with both 600 Volts and 480 Volts in his plant.
Unfortunately there was no way to change the voltage at any machine. If it was 480 volts it could not be reconnected to the 600 Volt supply.
As time went on equipment was replaced and equipment was moved. We were frequently faced with a 480V motor on a 600 V feed and 600 Volt motors on 480 Volt feeds.
later working in the oil patch, during shutdowns at plants, rental generators would be brought in. There are not many 600 Volt rental generators around. But the rental companies also have available 480 Volt to 600 Volt three phase auto-transformers.
Now the plant has generator power stepped up to 600 Volts for their temporary lighting and tools. Wait, we need more welders and special exhaust fans. Rentals are available but they are all 480 Volts. Rent another set of auto-transformers and drop the voltage back down to 480 Volts.
There are three possible connections and the buck connection may be the least used. I have never seen the true buck connection used for voltage adjustment.
Note: The auto-transformer step down connection is often called a buck connection but that is not completely accurate.
You have the simplest formula, good for only one of the three connections.
The open delta, auto-transformer connection with two transformers is often used. Three auto-transformer are used for phase shifting and don't change the voltage much. The connection of one of the transfoemres is reversed compared to what you may be using.
By the way I never use that KVA to KVA formula.
I calculate the current that the secondary winding will be asked to carry. I then multiply that value by the voltage of the secondary winding. That is the VA that I need.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fastline12]

I would likely be connecting with a closed delta primary and using 2 transformers. I am trying to arrive the the correct calculations here. Technically, amps are amps, so I would think if the transformer will handle 62A of single phase, that would be 62A using 2 of them on 3P?

Now, I don't want to complicate things but I have discovered through experience that the kva stated on the machine is WAY higher than would ever be experienced. There are 8 motor loads on the device and that would represent a full overload of all of them at the same time. I know that I have one motor load that will pull some power though. It is 7.5kw cont/10kw peak motor load and will likely pull about 40A during acceleration plus some smaller loads. But that accel event will last for 2sec, then pull back to not more than 40A for all loads combined.

 

[waross]

It looks as if you have things well in hand.
I have seen a lot of delta connections but I have not yet seen an closed delta using two transformers.
Congratulations.
You have taught me something new.

Bill
--------------------
"Why not the best?"
Jimmy Carter