Buck/Boost primary voltage question

A

acolella

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Engineering Technician
I have been using a few different buck/boost transformer arrangements lately. One of them is used to buck 600V down to 480V with transformers that have 600V primaries and 120V/240V secondaries. They are 15 kVA transformers in an open delta buck/boost configuration to handle about 125 amps on the 600V side. I have a general understanding of the theory but don't have a great deal of experience with these setups. My question is, could transformers with a 480V/240V primary and 120V/240V secondary be used in a situation similar to this or is the primary required to be rated at 600V?
 
In general, being in the range of ratings is ideal. As well, I have no idea how you would get a typical 120/240 to the %V requirement of 120V drop.
 
I know you can make a 480V to 600V autotransformer (buck-boost). I did it a few decades ago, so I don't remember where I found the diagram, but I think it was from Schneider Electric or in an Ugly's book.

I also recall something about a change in the NEC that prohibited this voltage change, so a connection diagram may be difficult to find.
 
jim dungar said:
I also recall something about a change in the NEC that prohibited this voltage change, so a connection diagram may be difficult to find.
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This is permitted by 210.9 Exception No. 2, but it must be an industrial occupancy. The NEC doesn't define exactly what this is but NFPA 101 does.

210.9 Circuits Derived from Autotransformers.
Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts.

Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons service the installation, autotransformers shall be permitted to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from nominal 600-volt systems, without the connection to a similar grounded conductor.
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6.1.12.1* Definition — Industrial Occupancy.
An occupancy in which products are manufactured or in which processing, assembling, mixing, packaging, finishing, decorating, or repair operations are conducted.
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I made a mistake in my original post. The setup that I was referring to is primarily used to boost 480V to 575V. I have been studying typical buck/boost connections and was looking at the guide in the link that @MTW shared above. I think I'm now understanding how the voltage ratings of the windings affect how the transformer operates.

I think 600-120/240 transformers were selected for the setup I was referring to in order to get a ~95V boost with a 480V input. A 240x480-120/240 transformer would yield a 120 volt boost. If I am understanding this correctly, an input voltage of 480V to a 240x480-120/240 transformer will result in an output voltage of 600V, which is a 25% increase. An input voltage of 480V to a 600-120/240 transformer will result in an output voltage of 576V, which is a 20% increase. 480V being 80% of 600V reduces the boost to 80% of 120V which is 96V. Am I understanding this correctly? Thanks for the replies!
 
acolella said:
Thank you. This is exactly what I was looking for.
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In rough terms, once the primary coils are energized the individual secondary coils can be added or subtracted to the primary coil voltage, depending on the polarity of connection as well as series or parallel connections.

there is a percentage of difference based on how close the input voltage matches the intended primarily winding design voltage, the secondary coils voltage value will be affected by a percentage difference that the input voltage varies from the coil design voltage. take a look at some of the manufacturers selection charts and you will begin to notice these relationships.

Standard 4 winding units 480/240-240/120 can have a wide variety of combinations with varying input voltage.
 
acolella said:
I made a mistake in my original post. The setup that I was referring to is primarily used to boost 480V to 575V. I have been studying typical buck/boost connections and was looking at the guide in the link that @MTW shared above. I think I'm now understanding how the voltage ratings of the windings affect how the transformer operates.

I think 600-120/240 transformers were selected for the setup I was referring to in order to get a ~95V boost with a 480V input. A 240x480-120/240 transformer would yield a 120 volt boost. If I am understanding this correctly, an input voltage of 480V to a 240x480-120/240 transformer will result in an output voltage of 600V, which is a 25% increase. An input voltage of 480V to a 600-120/240 transformer will result in an output voltage of 576V, which is a 20% increase. 480V being 80% of 600V reduces the boost to 80% of 120V which is 96V. Am I understanding this correctly? Thanks for the replies!
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Pretty correct, but confusing as you don't state the connection so we should guess? Anyway, there are a few ways to drive the math but I always refer to the step ratio. 600-120/240 is a step ratio of other 5 or 2.5. Converting to % that is 20% or 40%. The % is also used to size the transformer. The smaller the dV, the smaller the buck/boost can be. Example above using 20% connection, Load requires 100kva, changing voltage by 20%, that is a 20kva buck/boost requirement.
 
fastline said:
Pretty correct, but confusing as you don't state the connection so we should guess? Anyway, there are a few ways to drive the math but I always refer to the step ratio. 600-120/240 is a step ratio of other 5 or 2.5. Converting to % that is 20% or 40%. The % is also used to size the transformer. The smaller the dV, the smaller the buck/boost can be. Example above using 20% connection, Load requires 100kva, changing voltage by 20%, that is a 20kva buck/boost requirement.
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Ah yes, I should have provided more detail on the connection. Two 15 kVA 600-120/240 transformers in an open delta, connected as shown below. LV: 480V, HV: 575V. The capacity should be about 125 kVA if I'm doing this right. 15000 VA/120V=125A. 125A*575V*1.732=124.488 kVA.

1763500261077.png
 
Not sure I follow your math, but there are a few ways to do it.

Here is one. Based on the step ratio, you will use 20% dV, so that 15kva transformer /20% is capable of 75kva of load as single phase. If in 3P, that is 75kva*1.732=129.9kva available to load.

I think you are tossing amps in the mix, where you can keep everything as kva to keep it simple.
 
fastline said:
Not sure I follow your math, but there are a few ways to do it.

Here is one. Based on the step ratio, you will use 20% dV, so that 15kva transformer /20% is capable of 75kva of load as single phase. If in 3P, that is 75kva*1.732=129.9kva available to load.

I think you are tossing amps in the mix, where you can keep everything as kva to keep it simple.
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An open delta configuration, as shown in the diagram, only produces 57.7% of the kVA of a closed delta.
 
fastline said:
Closed would use 3 transformers with no sq rt factoring.
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Assuming equal sized single phase units.
A closed delta simply provides 3X the rating of a single transformer.
An open delta provides 57.7% the rating of a closed delta.
An open delta provides 86.6% of the rating of 2X a single transformer.
 
jim dungar said:
Assuming equal sized single phase units.
A closed delta simply provides 3X the rating of a single transformer.
An open delta provides 57.7% the rating of a closed delta.
An open delta provides 86.6% of the rating of 2X a single transformer.
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This is different than I provided? 2X the 75kva = 150kva*.866 = 129.9kva....
 
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