Single Phase Inverters on 208 3 Phase

[jim dungar]

To be fair, the individual vectors may be considered sine waves....

Yes vectors are used to represent waveforms. Was my advice about studying vectors ill advised?

 

[jaggedben]

The graph did not help. My brain says if we start at 0 volts A-N,
at 0.0041667 of a second A-N reaches + peak,
at 0.0055556 of a second B-N reaches 0,
at 0.0083333 of a second A-N reaches 0,
at 0.0097222 of a second, B-N reaches + peak.

How can the results of two wave peaks that happened between time 0 and .0097222 of a second create a resulting wave + peak 0.0013889 of a second before the first wave started?

Okay let's try this then...
Black is the voltage from A to N.
Dashed red is the voltage from B to N, shown with the conventional 120 degree shift.
Because we are measuring the series voltage A-N-B, we are measuring the sum (A to N)+(N to B).
N to B is the opposite (negative, inverse) of B to N, so I've shown N to B in solid red. Its formula is simply B to N with a minus sign in front.
Now we add (A to N)+(N to B) to get the purple line. (Since N to B is the negative of B to N, this is the same as subtracting B to N from A to N. Hence the formula for the purple line is either subtracting the dashed red from the black, or added the solid red to the black.)
As you can see, the solid red and black lines are 60deg shifted, the purple peak is in the middle of them, and it's amplitude is 1.73 times.
Hope this helps.
Try picking any vertical line on the x-axis and visually adding the instantaneous values of solid red and black to see how they add up to purple.

 

[wwhitney]

Then something is wrong if the resultant peak positive voltage of the two happens before the two reach their peak.

In post #98, you listed when Van = Voltage A-N reaches a positive peak, and when Vbn = Voltage B-N reaches a positive peak.

But for Van - Vbn, we need Vbn to reach a negative minimum, not a positive peak. So at what time does Vbn reach its negative minimum?

Cheers, Wayne

 

[jaggedben]

It seems to me the resultant waveform of A-B as seen by a water heater would be a waveform of 208 volts, seen by the load as 60 degrees after phase A and 60 degrees before phase B. If I inject that same resultant wave, 208 volts at 60 degrees after A and 60 degrees before B, into the two A-B coils of the Y secondary, they can't subtract the individual desired waves from the one.

Leaving aside the correct phase shift math, I think I understand what you are saying, although I think you mean something like 'extract' two waveforms from one, not subtract.

You're correct that the transformer coils can't and don't do this. But you know what can and will? The BESS charging at the other end of your 'grid'.

Once you understand how the two L-N legs of the wye add up to a single L-L voltage, you may be able to understand how two battery chargers connected in series and programmed to charge using 60deg phase shifted waveforms can charge from a single AC waveform that is in sync with them. It's the same math, in the opposite direction. Subtract the solid red line from the purple and you get the black (or vice versa, the black from the purple to get the red.)

(In your grid it will actually be more complicated because your delta-wye transformer will transfer the wave form to two coils of the delta, and then all three L-N phases of the BESS will draw current to extract the energy from two coils of the transformer. But it should still all work out, because...)

Remember, your BESS is responsible for maintaining all three phase waveforms at all times. The solar inverters (and the transformers) just follow.

 

[bellington]

Okay let's try this then...
Black is the voltage from A to N.
Dashed red is the voltage from B to N, shown with the conventional 120 degree shift.
Because we are measuring the series voltage A-N-B, we are measuring the sum (A to N)+(N to B).
N to B is the opposite (negative, inverse) of B to N, so I've shown N to B in solid red. Its formula is simply B to N with a minus sign in front.
Now we add (A to N)+(N to B) to get the purple line. (Since N to B is the negative of B to N, this is the same as subtracting B to N from A to N. Hence the formula for the purple line is either subtracting the dashed red from the black, or added the solid red to the black.)
As you can see, the solid red and black lines are 60deg shifted, the purple peak is in the middle of them, and it's amplitude is 1.73 times.
Hope this helps.
Try picking any vertical line on the x-axis and visually adding the instantaneous values of solid red and black to see how they add up to purple.
View attachment 2570083

Yes, thanks. I am a very visual learner. Your dotted and solid red lines made my brain grasp the A-N and N-B series relationship that Wayne has been trying so hard to get me to see. Thanks.

Now, can you show me how to take that purple wave as an output of an inverter and put the black line and the red line back into A-N and N-B to send back to the A-N and N-B on the primary of the transformer?

 

[jaggedben]

Now, can you show me how to take that purple wave as an output of an inverter and put the black line and the red line back into A-N and N-B to send back to the A-N and N-B on the primary of the transformer?

I think we were typing at the same time. Try my last post on for size. That's all I've got for tonight.

 

[bellington]

Leaving aside the correct phase shift math, I think I understand what you are saying, although I think you mean something like 'extract' two waveforms from one, not subtract.

You're correct that the transformer coils can't and don't do this.

That has been my concern. My head has no problem using a single phase inverter connected to a single phase leg of a transformer secondary, be it 240 delta, or 416/240Y. My head does have issues with a single phase inverter attempting to back feed two phases.

 

[jaggedben]

That has been my concern. My head has no problem using a single phase inverter connected to a single phase leg of a transformer secondary, be it 240 delta, or 416/240Y. My head does have issues with a single phase inverter attempting to back feed two phases.

And your quote of me cuts off the rest of my post that addresses that concern.

 

[jim dungar]

That has been my concern. My head has no problem using a single phase inverter connected to a single phase leg of a transformer secondary, be it 240 delta, or 416/240Y. My head does have issues with a single phase inverter attempting to back feed two phases.

It doesn't back feed into two phases.
It back feeds into two phase conductors. The conductors are connected to neutral point/conductor.

Imagine a triangle. The ends of hypotenuse are the two phase conductors A and B. The vertex opposite the hypotenuse is the Neutral point/conductor.
You know the AB voltage and all three angles so you can use sine and cosine function to determine the voltages across AN and BN and thus determine what their waveforms would look like.

 

[bellington]

And your quote of me cuts off the rest of my post that addresses that concern.

I left that part off because I don't have any delta/wye transformers and it sounds like you are talking magic. If I get a single resultant wave coming out of the inverter and going into the two legs, needing to see both phases, nothing AFTER that transformer can make that adjustment.

 

[bellington]

Are

It doesn't back feed into two phases.
It back feeds into two phase conductors. The conductors are connected to neutral point/conductor.

Imagine a triangle. The ends of hypotenuse are the two phase conductors A and B. The vertex opposite the hypotenuse is the Neutral point/conductor.
You know the AB voltage and all three angles so you can use sine and cosine function to determine the voltages across AN and BN and thus determine what their waveforms would look like.

If each inverter were connected to the single phase 120 legs A-N, B-N, C-N, then I'm good. These are not connected to a single phase leg, but each one connected to a two phase leg, A-B, BC, CA. Unless the inverters can reproduce the two original source waves, and not the resultant wave as seen by L-L load, then I can't see it working well.

 

[wwhitney]

If I get a single resultant wave coming out of the inverter and going into the two legs, needing to see both phases

This is just not true. The two coils do not need to "see" both phases separately.

A single current waveform, in phase with the A-B voltage, will only be 30 degrees out of phase with the N-B voltage, and also only 30 degrees out of phase with the A-N voltage. That single current waveform provides power transfer to both coils (for an inverter; or from both coils, for a load).

See post 89.

Cheers, Wayne

 

[jim dungar]

Are

If each inverter were connected to the single phase 120 legs A-N, B-N, C-N, then I'm good. These are not connected to a single phase leg, but each one connected to a two phase leg, A-B, BC, CA. Unless the inverters can reproduce the two original source waves, and not the resultant wave as seen by L-L load, then I can't see it working well.

What part of using sine and cosine functions to solve for unknowns is causing you problems?
The angular relationship of AN, NB, and AB is determined by the grid you are connecting to, not by your single 2-wire piece of equipment.

 

[jaggedben]

I left that part off because I don't have any delta/wye transformers and it sounds like you are talking magic. If I get a single resultant wave coming out of the inverter and going into the two legs, needing to see both phases, nothing AFTER that transformer can make that adjustment.

First of all, if anything can make the adjustment before the transformer, it can just as easily make the transformation after the transformer; the transformer just changes the voltage. But okay, let's forget about a delta-wye transformer (I just assumed that because it's typical) and simplify to a wye system, with or without transformer. It simplifies the discussion of how the BESS can extract two waveforms from one.

I realize that ordinary loads could never extract two waveforms from a single source waveform, and I gather that's what's tripping you up. But that's exactly what is special - or magic, if you want to use that word - about your BESS. It's a core part of what the BESS is designed for.

Go back to my graph in post #102 which helped you understand how two out-of-phase voltage source waveforms in series create one load voltage waveform at their end terminals. For a purely resistive L-L load the graph is the same for voltage and current, so consider them now for a minute as representing current graphs, not voltage graphs. Got it? Okay now ... flip the source and the load. Make the two smaller amplitude graphs loads that are programmed to absorb current in the correct phase relationship. That is what your BESS will do when it charges. Do you see how the math for the currents is the same regardless of whether the smaller amplitude sine waves are the source or the load?

Again, the key here is that it is the BESS' job to maintain these waveforms, not the solar inverter's job. It is true (as Wayne said somewhere way upthread) that if you had three 208V grid-forming voltage sources then they wouldn't form a neutral point. But they don't need to, because the BESS forms the grid and creates that neutral point. It keeps its three phases in the correct relationship to each other, either as a source or as a load.

That's the 'magic' here: it's the AC coupled BESS that maintains the waveforms while seemlessly transitioning from being a source to being a load. Trust me, I install these and watch them do it. (Okay mine are single phase, but still.) They maintain the waveform for the solar inverters and the regular load while seamlessly transitioning themselves as needed from source to load and back again.

 

[bellington]

What part of using sine and cosine functions to solve for unknowns is causing you problems?
The angular relationship of AN, NB, and AB is determined by the grid you are connecting to, not by your single 2-wire piece of equipment.

And that's my problem. I can't see how that single 2-wire inverter can replicate those angular relationships and match the grid.

 

[wwhitney]

And that's my problem. I can't see how that single 2-wire inverter can replicate those angular relationships and match the grid.

It can't, but it doesn't need to.

Cheers, Wayne

 

[jim dungar]

And that's my problem. I can't see how that single 2-wire inverter can replicate those angular relationships and match the grid.

Do you agree with these statements?

The inverter does not do anything except create a single waveform output, because it only has two conductors.

The grid has multiple sets of conductors which are interconnected and separated by some phase angle, be that angle 0°, 120°, or 180°.

If multiple waveforms can be combined into a single waveform, then conversely a single waveform can be 'broken down' into multiple waveforms.

 

[bellington]

Do you agree with these statements?

The inverter does not do anything except create a single waveform output, because it only has two conductors.

I agree.

The grid has multiple sets of conductors which are interconnected and separated by some phase angle, be that angle 0°, 120°, or 180°.

I also agree.

If multiple waveforms can be combined into a single waveform, then conversely a single waveform can be 'broken down' into multiple waveforms.

This is where I don't agree, at least for one wave passing through two phase angle legs of a transformer's secondary coils. If we are using our resultant wave and sending it back through A-B, then we are pushing electrons backwards through B-N most of the time.

 

[bellington]

First of all, if anything can make the adjustment before the transformer, it can just as easily make the transformation after the transformer; the transformer just changes the voltage.

The exception to this statement is that your purple resultant waveform, even reversing the polarity of measurement of B-N to N-B on your graph, the resultant wave still has 60 degrees of time that the resultant wave being fed back into the secondary is pushing electrons in the opposite direction of A-N and another 60 degrees of time the resultant wave is pushing electrons in the opposite direction of N-B. The primary side of the transformer doesn't replicate this battle, only the results. Those results feeding back through the primary will be less than what we put into the secondary.

But okay, let's forget about a delta-wye transformer (I just assumed that because it's typical) and simplify to a wye system, with or without transformer. It simplifies the discussion of how the BESS can extract two waveforms from one.

I realize that ordinary loads could never extract two waveforms from a single source waveform, and I gather that's what's tripping you up. But that's exactly what is special - or magic, if you want to use that word - about your BESS. It's a core part of what the BESS is designed for.

Go back to my graph in post #102 which helped you understand how two out-of-phase voltage source waveforms in series create one load voltage waveform at their end terminals. For a purely resistive L-L load the graph is the same for voltage and current, so consider them now for a minute as representing current graphs, not voltage graphs. Got it? Okay now ... flip the source and the load. Make the two smaller amplitude graphs loads that are programmed to absorb current in the correct phase relationship. That is what your BESS will do when it charges. Do you see how the math for the currents is the same regardless of whether the smaller amplitude sine waves are the source or the load?

Again, the key here is that it is the BESS' job to maintain these waveforms, not the solar inverter's job. It is true (as Wayne said somewhere way upthread) that if you had three 208V grid-forming voltage sources then they wouldn't form a neutral point. But they don't need to, because the BESS forms the grid and creates that neutral point. It keeps its three phases in the correct relationship to each other, either as a source or as a load.

That's the 'magic' here: it's the AC coupled BESS that maintains the waveforms while seemlessly transitioning from being a source to being a load. Trust me, I install these and watch them do it. (Okay mine are single phase, but still.) They maintain the waveform for the solar inverters and the regular load while seamlessly transitioning themselves as needed from source to load and back again.

 

[ruxton.stanislaw]

If it helps to think about it this way, some of these concepts are similar to how a zig-zag or grounding transformer work; to derive a neutral from delta wiring.