Buck-Boost Transformer / Magnetizing Current

[KY Solar]

Help! - Transformer did not work!

Help! - Transformer did not work!

Guys, sorry for the delay. We had to special order the buck-boost transformer and just got it installed today….

Here's the "NEW" problem……
As some of you previously pointed out, the single autotransformer approach created a voltage imbalance. We started at 125/125. After the (10% buck) transformer install we ended up with 113/108 volts. Now the solar array is completely dead. None of the inverters will turn on with the new voltage and related imbalance.

We used a Schneider 3kVA with Phase A passing straight through. Phase B is tapped down from one end (10%?).
If you have access to the Schneider online buck-boost calculator, here are the input parameters:
Input voltage: 264
Load: 130A
Buck: 10% (I really wanted 5% buck but transformer was not available)
Schneider SquareD part #: 3S43F
Wiring diagram: 4

YES!! - These micro-inverters use a neutral to calculate the phases. Something is bad wrong - Please help!

Do we need to install TWO 5% buck boost transformers?
Thanks for any suggestions.

 

[GoldDigger]

Yes. You need two autotransformers. Either add a second identical one or replace the one with two 120V 5% transformers.

Sent from my XT1585 using Tapatalk

 

[topgone]

Guys, sorry for the delay. We had to special order the buck-boost transformer and just got it installed today….

Here's the "NEW" problem……
As some of you previously pointed out, the single autotransformer approach created a voltage imbalance. We started at 125/125. After the (10% buck) transformer install we ended up with 113/108 volts. Now the solar array is completely dead. None of the inverters will turn on with the new voltage and related imbalance.

We used a Schneider 3kVA with Phase A passing straight through. Phase B is tapped down from one end (10%?).
If you have access to the Schneider online buck-boost calculator, here are the input parameters:
Input voltage: 264
Load: 130A
Buck: 10% (I really wanted 5% buck but transformer was not available)
Schneider SquareD part #: 3S43F
Wiring diagram: 4

YES!! - These micro-inverters use a neutral to calculate the phases. Something is bad wrong - Please help!

Do we need to install TWO 5% buck boost transformers?
Thanks for any suggestions.

With your connection, you will have your inverter voltage pushed down to 265 X (240/(240+24)) = 240 volts.
If you wanted a 5% bucking transformer (265 *0.95 = 251.75 volts) you can parallel the LV windings (2 X 12v) and achieve what you wanted. The inverter side voltage will then be = 265 X 240/(240 + 12) = 265(240)/252 = 252.4 volts.
I_fl @ 12V secondary is = 3,000/12 = 250 amperes, so your new autotransformer capacity = (250 + 12.5) X 240 = 63kVA.
Edit: Please see page 9 on the attached link
Here

 

[GoldDigger]

A fine suggestion, topgone, except that it does not do anything to solve the OP's problem of unbalanced voltage between the two line conductors and the neutral.

Sent from my XT1585 using Tapatalk

 

[topgone]

A fine suggestion, topgone, except that it does not do anything to solve the OP's problem of unbalanced voltage between the two line conductors and the neutral.

Sent from my XT1585 using Tapatalk

Just gave the guy the idea of how to get 5% bucking.
He can choose to use 2 X 3kVA split and have a neutral. He doesn't have to look for a 120, 5% autotransformer because he can rewire the primary (parallel the windings) of 3S43F unit.

 

[wwhitney]

So the 3S43F transformer is called out as a "120 x 240V primary, 12/24 V secondary" transformer by Square D. Am I correct in understanding that means it has two 120V HV coils, and two 12V LV coils, and both ends of each coil are brought out?

If that's the case, then instead of wiring the transformer as shown in "Wiring Diagram 4" below, couldn't one of the LV coils be moved to the left side of the diagram, to ensure that the neutral will remain neutral? Not sure if it matters which LV coil is moved, if not then starting at the diagram shown, disconnect X3 from X2, and reconnect X3 to H1. Then X2 and X4 would be connected to the utility ungrounded conductors, and H1 and H4 connected to the PV ungrounded conductors, for a buck configuration.

Except I may have swapped X3 and X4, as I don't understand the polarity nomenclature. And likewise, maybe it is necessary to move the X1/X2 coil instead of the X3/X4 coil.

Also, in this configuration, should the neutral common to both voltage system be landed at the H2/H3 connection?

Cheers, Wayne

 

[gar]

170507-2331 EDT

KY Solar:

You were originally told along the way that you would probably required the equivalent of a neutral, and then you ignored that information. Very likely you do not actually need a new neutral point. What you need are two balanced voltage points.

If there are 4 indpendent windings on you transformer, then unconnect X3 from X2, and connect X4 to X1. Connect X3 and X2 to your 240 V lines. Now X4 and X1 should be your solar connection, and the X4 to neutral, and X1 to neutral should be about equal in voltage.

H2H3 does not need to connect to neutral, and I don't really know if there is a good reason for one choice or the other.

.

 

[gar]

170507-2414 EDT

KY Solar:

Assuming the connection I suggested to you is correct, then before you ever connect it to your solar system you must check that it produces the expected voltages.

Whenever you test something like this it would be desirable to test it at some much lower voltage initially and see if the results are as expected. For example, power it at 24 V. But based on your problem so far I am not sure you know how to do this, and interpret the results.

If you only had a two wire 24 V supply, then it would be necessary to use H2H3 as your neutral reference point.

.

 

[topgone]

So the 3S43F transformer is called out as a "120 x 240V primary, 12/24 V secondary" transformer by Square D. Am I correct in understanding that means it has two 120V HV coils, and two 12V LV coils, and both ends of each coil are brought out?

If that's the case, then instead of wiring the transformer as shown in "Wiring Diagram 4" below, couldn't one of the LV coils be moved to the left side of the diagram, to ensure that the neutral will remain neutral? Not sure if it matters which LV coil is moved, if not then starting at the diagram shown, disconnect X3 from X2, and reconnect X3 to H1. Then X2 and X4 would be connected to the utility ungrounded conductors, and H1 and H4 connected to the PV ungrounded conductors, for a buck configuration.

Except I may have swapped X3 and X4, as I don't understand the polarity nomenclature. And likewise, maybe it is necessary to move the X1/X2 coil instead of the X3/X4 coil.

Also, in this configuration, should the neutral common to both voltage system be landed at the H2/H3 connection?

Cheers, Wayne

Good point/s.

 

[Smart $]

...couldn't one of the LV coils be moved to the left side of the diagram...

Yes. Exactly what I was thinking while reading KY's post.

I'd move X1-X2 to left side. Keep all orientations the same, diagrammatically.

HV1 • X1-X2 • LV1 • H1-H2 • N • H3-H4 • LV2 • X3-X4 • HV2​

 

[kwired]

Yes. Exactly what I was thinking while reading KY's post.

I'd move X1-X2 to left side. Keep all orientations the same, diagrammatically.

HV1 • X1-X2 • LV1 • H1-H2 • N • H3-H4 • LV2 • X3-X4 • HV2​

Do you pass the neutral through, or do you establish a new neutral point at the H2/H3 junction. If a new point is established it is no longer a grounded conductor (should not be using white or gray for identification, should be providing overcurrent protection I would think) you can't ground this point either, you need a separately derived system to ground a new point in the system.

Also should there be high unbalance for some reason, if you didn't pass the neutral through, the H windings have to carry that unbalance and are not rated for nearly as much current as the X windings.

Just some thoughts that came to mind here.

 

[Smart $]

Do you pass the neutral through...

The neutral is connected and passes through but requires no special consideration. The connections are really no different than using two buck transformers to do the job. The electrical difference is the shared core.

 

[kwired]

The neutral is connected and passes through but requires no special consideration. The connections are really no different than using two buck transformers to do the job. The electrical difference is the shared core.

I should have maybe mentioned that Gar said in post 27 that H2H3 doesn't need to connect to neutral, which sort of got me thinking about all this.

I think it does need to be connected if you want 120/240 on the "output" side and have any potential for neutral current to exceed the H winding ratings.

 

[Smart $]

I should have maybe mentioned that Gar said in post 27 that H2H3 doesn't need to connect to neutral, which sort of got me thinking about all this.

I think it does need to be connected if you want 120/240 on the "output" side.

IMO connecting the neutral is not necessary as Gar stated, but I believe connecting it helps to stabilize the voltage.

 

[kwired]

IMO connecting the neutral is not necessary as Gar stated, but I believe connecting it helps to stabilize the voltage.

Then comes what I mentioned earlier, if you don't connect it to "neutral" of both sides, your "derived" side is still neutral to that side, but is not a grounded conductor, and you can't create another connection to ground with it, or you potentially have parallel paths through grounded objects for neutral current to flow. This point will normally operate "near ground potential" but can vary from ground potential as it is not solidly connected to ground. Also since it is not a grounded conductor should you be required to provide it with overcurrent protection?

Connecting neutral of both sides of this together makes those concerns all go away.

 

[wwhitney]

NEC 215.11 (or 210.9) requires the grounded conductors on each side of the autotransformer to be connected. Whether that grounded conductor should be connected to H2/H3 is open for discussion.

Cheers, Wayne

 

[Smart $]

...
Connecting neutral of both sides of this together makes those concerns all go away.

I going to stick with this and let others, if any, handle the speculation. :happyyes:

 

[Smart $]

NEC 215.11 (or 210.9) requires the grounded conductors on each side of the autotransformer to be connected. Whether that grounded conductor should be connected to H2/H3 is open for discussion.

Cheers, Wayne

Where else would you connect it and get essentially a balanced 120/240 output?

 

[wwhitney]

Where else would you connect it and get essentially a balanced 120/240 output?

Perhaps my comment was poorly worded--I just meant to say that the utility side grounded conductor must be connected to the PV side grounded conductor by 215.11, but that section doesn't offer any guidance on whether that common grounded conductor should be landed at H2/H3. It makes sense to me to do it, but I'm not a transformer expert, which is why I asked the question in my first post in this thread.

Cheers, Wayne

 

[Smart $]

Perhaps my comment was poorly worded--I just meant to say that the utility side grounded conductor must be connected to the PV side grounded conductor by 215.11, but that section doesn't offer any guidance on whether that common grounded conductor should be landed at H2/H3. It makes sense to me to do it, but I'm not a transformer expert, which is why I asked the question in my first post in this thread.

Cheers, Wayne

Well, let's make it definitive.

Both line- and load-side neutral conductors must connect at the H2, H3 junction.