Buck Boost Transformer Question

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Minor disclaimer: Excluding small buck-boost, I can count the number of autotransformers that I have touched on one hand.
Only two issues I know of:
210.9 Branch circuits​
215.11 feeders​
Some exceptions apply
 
200110-1118 EST

This new forum software is a bigger piece of junk than anything before. Can read the forum on MAC, can not enter a post, but can save a blank post.

George:

On the line side, what you have labeled LV, there must be a neutral conductor from the source wye transformer neutral point to the neutral point of the three boost transformers.

A tightly coupled transformer reflects its secondary load impedance to its primary.

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drcampbell said:
You answered your own question: The load does not require a neutral.
Click to expand...
gar said:
On the line side, what you have labeled LV, there must be a neutral conductor from the source wye transformer neutral point to the neutral point of the three boost transformers.

A tightly coupled transformer reflects its secondary load impedance to its primary.
Click to expand...

I love picking easy questions. :p
 
George Stolz said:
If my load does not require a neutral, do I need to bring a neutral to the line side of this series of buck-boost transformers for proper operation?
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Making some guesses about context, where this might be used, transforming:
600/346Y to 480Y​
480/277Y to 208/120Y​
240/130Y to 208/120Y​
No line to neutral loads​
​
Following Gar's post.
Looking at the physics:
If the load is well balanced, there is no neutral current. So, the Wye point would stay close to Zero volts (defined as the HV neutral potential)

However, if not well balanced, the Wye point will move. I'm not sure what that will do the output. (See the disclaimer in my first post)
My inclination would be to install the primary neutral and stabilize the Wye point.

Or, unless you are trying to transform 240D to 208/120Y, and you don't have a 240V neutral.
If this is the case, I've never seen that before. (Again, see the disclaimer in my first post.)
My inclination here would be to get the right transformer

Then again, maybe it is something else of which I have no clue.
 
Autotransformer configured as buck-boost, per the OP, do not really behave the same way as wye-wye isolation transformers.
For this specific case, there is no load neutral so there cannot be any L-N imbalance caused by the load, the line side L-N will behave the same as wye connected motor windings (which also do not have neutral connections).

However, I would probably provide the line side neutral, particularly if the conductor was a #10 or smaller (above that I would weigh the conductor cost versus the risk of a transformer replacement).
 
More detail: this is using three buck-boost transformers, boosting 208V 3Ø to 240V 3Ø for a sensitive 3Ø load. There is no neutral required at the load.

I hadn't thought about a significantly reduced neutral just for the sake of voltage reference (no real current flow) - that sounds like the best approach.

Thanks!
 
200110-1330 EST

To repeat a closely coupled power transformer is both a voltage transformer and a current transformer. This is simultaneous. The transformation ratio is a function of the turns ratio. In some respects it is possibly better to view the transformer as an impedance transformer based on the turns ratio squared.

As a first order approximation you can assume that the transformer alone has a shunt impedance of infinity.

In George's application if the delta load on the output of the boost transformers is unbalanced, and the wye point of the boost transformers primaries is left floating, then the boost transformer primaries will not have equal voltages, and thus the boost voltages will not be equal.

If the boost primaries are fed from source line to line voltages, then the system will work. This will introduce some phase shift, but for most applications no problem.

This new forum software has significant problems. Thus, falls in my category of junk.

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As an example let's assume the line side of the autotransformer bank is fed from a 208V/120V wye secondary with its neutral grounded, but this neutral is not connected to the common connection of the three autotransformers . Then if the output of one of the boost auto-transformers has a bolted fault to ground, this common connection could experience a short duration overvoltage as much as 120V x 208/(240-208) = 780V rms depending on when the autotransformer saturates and/or the overcurrent device opens. Finite source impedance, loads, etc. would limit the voltage to some extent, and the peak value of the overvoltage likely depends on where in the AC cycle the ground fault is initiated. But in any case, this could be a reason to bring the neutral to the autotransformers.
 
George Stolz said:
More detail: this is using three buck-boost transformers, boosting 208V 3Ø to 240V 3Ø for a sensitive 3Ø load. There is no neutral required at the load.

I hadn't thought about a significantly reduced neutral just for the sake of voltage reference (no real current flow) - that sounds like the best approach.

Thanks!
Click to expand...
I'm pretty sure you only need two. When you increase or decrease Vab and Vbc, Vca will take care of itself.
 
200112-1241 EST

You people do not define what you are talking about.

George defined a circuit in the first post. I think his post was clear and defined. His question related to a question on whether or not he needed to run a neutral wire to the center point of his wye connected primaries of his three boost transformers. The answer is yes.

ggunn:

Your reply "I'm pretty sure you only need two. When you increase or decrease Vab and Vbc, Vca will take care of itself."

"Two" what? Remove one of his boost transformers, and now you have two boost transformer primaries connected in series across a source voltage. These transformers operate as impedance transformers, and the primary voltages divide the source voltage based on the reflected impedances which vary with load.

But you were really suggesting changing the circuit in more ways than just removing one boost transformer. This you did not state. Really you were suggesting that the boost transformers be reduced to two, and also changing from where the primaries were fed without stating the change in primary wiring. This new circuit will work. But makes the output line to earth voltages unequal. For a balanced load it makes the source currents unequal. All of this may be OK. We don't know,

,
 
gar said:
200112-1241 EST

You people do not define what you are talking about.

George defined a circuit in the first post. I think his post was clear and defined. His question related to a question on whether or not he needed to run a neutral wire to the center point of his wye connected primaries of his three boost transformers. The answer is yes.

ggunn:

Your reply "I'm pretty sure you only need two. When you increase or decrease Vab and Vbc, Vca will take care of itself."

"Two" what? Remove one of his boost transformers, and now you have two boost transformer primaries connected in series across a source voltage. These transformers operate as impedance transformers, and the primary voltages divide the source voltage based on the reflected impedances which vary with load.

But you were really suggesting changing the circuit in more ways than just removing one boost transformer. This you did not state. Really you were suggesting that the boost transformers be reduced to two, and also changing from where the primaries were fed without stating the change in primary wiring. This new circuit will work. But makes the output line to earth voltages unequal. For a balanced load it makes the source currents unequal. All of this may be OK. We don't know,

,
Click to expand...
We installed a 480/277V wye connected PV inverter through a transformer on a 208/120V service, but our installers improvised on the inverter placement and put it a lot farther away from the transformer (an autotransformer) than I designed the conductors for. The added voltage rise in combination with the fact that the POCO was operating in the top of its voltage range kept knocking the inverter off line. We consulted with a transformer company, who prescribed a pair of (as in two) buck/boost transformers to bring the voltage back into range at the inverter. It worked. It may very well be that the fact that it was an autotransformer made a difference, but I won't be using them anymore, anyway.
 
ggunn said:
We installed a 480/277V wye connected PV inverter through a transformer on a 208/120V service, but our installers improvised on the inverter placement and put it a lot farther away from the transformer (an autotransformer) than I designed the conductors for. The added voltage rise in combination with the fact that the POCO was operating in the top of its voltage range kept knocking the inverter off line. We consulted with a transformer company, who prescribed a pair of (as in two) buck/boost transformers to bring the voltage back into range at the inverter. It worked. It may very well be that the fact that it was an autotransformer made a difference, but I won't be using them anymore, anyway.
Click to expand...
A three phase inverter can typically deliver its designed output power equally into the three ungrounded conductors even if the grid-derived voltages are unequal. A grid interactive inverter cannot be characterized as a constant impedance load nor as a simple voltage or current source. Now the monitoring software of the inverter may trip out on too high a voltage imbalance between the wye to neutral voltages, which is a different problem.
 
1. Since the three buck/boost transformers are all connected L-L, there is no reason for a neutral connection on the line side for the resulting autotransformers to work properly.
2. Since the load also does not require a neutral, there is no need for a line side neutral connection. NEC does not require an unused wye neutral to be present anywhere except at the service disconnect.
If the autotransformers were wired L-N, then a neutral would definitely be required, although a floating wye point would tend to float near ground.
I have a nagging suspicion that the configuration shown will result in a small phase shift in the line-neutral voltages, but it would not have any undesirable consequences.
 
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