Single Phase Inverters on 208 3 Phase

[jaggedben]

If I connect two 1200 W hair dryers, one to each 120 volt leg of 208, I consume 2400 watt at 10 amps. If I run that same 10 amps through both legs connected together I consume only 2080 watts. Is the primary side of the transformer using less current when 10 amps run through two legs individually than it does running two legs together?

Perhaps this will be helpful or perhaps not, but let me somewhat modify your example and ask you a question just about secondary currents, ignoring the primary.

First, two 1200W hair dryers on A-N and B-N.
A: 10A
B: 10A
C: 0A
N: 10A

2nd, one 2400W heater that runs on 208V, connected A-B.
A: 11.4A
B: 11.4A
C: 0A
N: 0A

And third, just to expand the point, three 1200W hair dryers, one on each L-N phase:
A: 10A
B: 10A
C: 10A
N: 0A

Does summing the currents in each list tell me how to rank the examples by power consumed? (It doesn't.) Are examples 1 and 3 consuming the same power judging by the sum of currents on conductors? (They're not.) That's the same mistake you are making by asking why a higher sum of primary line currents doesn't result in more power being drawn. When you don't know how to account for the vector math in three phase systems you make false assumptions.

 

[bellington]

As jaggedben said, power factor. In AC, Watts = Volts * Amps only when the voltage waveform and the current waveform are in phase. See, e.g. https://en.wikipedia.org/wiki/Power_factor

Cheers, Wayne

And power factor is the way we measure what?

 

[bellington]

Back feeding the single phase output of an inverter into two legs of 208 is similar to the situation discussed here: "When you run 120/208V single phase power into an isolation transformer, the two individual hot conductors of 120V are combined into one 208V waveform. Since most Motorola equipment requires 120V, we need to split the 208V back into two 120V conductors – but it can’t be done."

208V Single Phase Power: What You Need to Know

If you are designing a site that will have 208V single phase power to feed the UPS, you should be aware of the nature of 208V power

seps-inc.com

Regardless of the loss that occurs using two legs of 208 as single phase during consumption, it is impossible to create one sine wave to feed back into the two legs that will provide peak voltage to both legs at the correct time. I don't think anyone is suggesting that the 240 volt single phase inverter that is claimed to also work on 208 has the ability to recreate the two separate sine waves offset by 120 degrees and keep them separate as they enter the two legs of a 208 transformer, perfectly timed to match the peak voltage of each leg.

I think we all agree that the inverter will send one similar single resultant sine wave that will send some current back through the transformer at the correct time, but not an amount equal to what would be delivered through a single phase 240 to 480 transformer. Can one of you quantify the difference in energy transferred? That would greatly help me.

If it will work better than I am envisioning it, can one of you point me to a location where single phase inverters are working efficiently to back feed a 3-phase system and efficiently transferring the energy back through a transformer?

 

[Carultch]

I think we all agree that the inverter will send one similar single resultant sine wave that will send some current back through the transformer at the correct time, but not an amount equal to what would be delivered through a single phase 240 to 480 transformer. Can one of you quantify the difference in energy transferred? That would greatly help me.

If we assume a 100% efficient transformer for simplicity, there isn't any energy loss due to having unbalanced currents. And backfeeding through a transformer with mixed topologies will bring the currents closer to a balance on the primary. There will be phase shifts, and some products of current and voltage will not equal power. When you account for the real power with a dot product of vectors, instead of just a product of amplitudes, you'll see there is no difference.

If you have a balanced group of inter-phase inverters, the net result is no different than if you had a balanced group of phase-to-neutral inverters.

 

[winnie]

And power factor is the way we measure what?

In AC circuits, voltage and current are continuously changing values. When we say '10A 120V hairdryer' we are actually being lazy and using shorthand for '10A RMS, 120V RMS, 0 phase angles.

RMS means Root Mean Square and is a type of averaging which does a good job of describing a varying waveform as a single number. Specifically if you apply an alternating current of X RMS to a resistor, you will get the same time averaged heating as if you applied a DC current of the same X.

In a DC circuit, power delivered to a load is simply volts times amps. This is also true for the instantaneous power delivered in an AC circuit. But in general this rule fails when using RMS values for AC circuits. In general RMS current time RMS voltage does not equal power delivered to a load.

Power factor is on way of describing this difference.

Power factor is not a loss term. It results in loss of capacity and second order losses because higher current is needed to deliver the same power.

 

[jaggedben]

And power factor is the way we measure what?

Not efficiency, if that's what you're trying to say.

 

[jaggedben]

Back feeding the single phase output of an inverter into two legs of 208 is similar to the situation discussed here:

It's really not similar though. Because in your case you have a BESS primary source providing three phases in all situations (either as a source or as a *load*) and the solar inverter is just following the frequency and injecting current. The BESS will adjust to the injection of current at 208V much the same way as it would when there is a 208V L-L load that goes away.

Regardless of the loss that occurs using two legs of 208 as single phase during consumption, it is impossible to create one sine wave to feed back into the two legs that will provide peak voltage to both legs at the correct time.

Your BESS when it is charging would provide the peak conductance at the right times to make it happen. In your setup the solar inverter is always following.

I don't think anyone is suggesting that the 240 volt single phase inverter that is claimed to also work on 208 has the ability to recreate the two separate sine waves offset by 120 degrees and keep them separate as they enter the two legs of a 208 transformer, perfectly timed to match the peak voltage of each leg.

Your BESS primary source is the component of your setup that does this for you. You keep asking the solar inverter to do it, when it doesn't need to because your BESS will do it.

I think we all agree that the inverter will send one similar single resultant sine wave that will send some current back through the transformer at the correct time, but not an amount equal to what would be delivered through a single phase 240 to 480 transformer. Can one of you quantify the difference in energy transferred? That would greatly help me.

Okay: for an ideal transformer and conductors and the same power output in KW, it's zero. Perhaps Wayne will indulge you with the math, it will invole the product of phase shifted current and voltage sine waves. And yes, the current is different because the voltage and power factor are different.

If it will work better than I am envisioning it, can one of you point me to a location where single phase inverters are working efficiently to back feed a 3-phase system and efficiently transferring the energy back through a transformer?

I imagine @ggunn did a few of those back before three-phase inverters became common. Also Enphase supported it with the IQ7 until quite recently, see here:

https://cdn.myced.com/images/Products/ZZ0000/ZZ0106/00000/ZZ010600102_DS.pdf

 

[bellington]

It's really not similar though. Because in your case you have a BESS primary source providing three phases in all situations (either as a source or as a *load*) and the solar inverter is just following the frequency and injecting current. The BESS will adjust to the injection of current at 208V much the same way as it would when there is a 208V L-L load that goes away.



Your BESS when it is charging would provide the peak conductance at the right times to make it happen. In your setup the solar inverter is always following.



Your BESS primary source is the component of your setup that does this for you. You keep asking the solar inverter to do it, when it doesn't need to because your BESS will do it.



Okay: for an ideal transformer and conductors and the same power output in KW, it's zero. Perhaps Wayne will indulge you with the math, it will invole the product of phase shifted current and voltage sine waves. And yes, the current is different because the voltage and power factor are different.



I imagine @ggunn did a few of those back before three-phase inverters became common. Also Enphase supported it with the IQ7 until quite recently, see here:

https://cdn.myced.com/images/Products/ZZ0000/ZZ0106/00000/ZZ010600102_DS.pdf

"The IQ 6 and IQ 6+ Microinverters have a 97% CEC efficiency for split phase applications..."

Where is the efficiency rating for non-split phase applications?

 

[winnie]

"The IQ 6 and IQ 6+ Microinverters have a 97% CEC efficiency for split phase applications..."

Where is the efficiency rating for non-split phase applications?

The Enphase document linked does not give different specifications for 120/240V or 208/120V applications. Both are described in the document.

Either the document considers both 120/240V and 208/120V 'split phase' (which is a somewhat non-standard use of the term) and the answer is '97%' for both, or they are only giving the efficiency information for 120/240V use. If the latter is the case then based on the 97% efficiency number for 120/240V use, I'd expect an efficiency greater than 96.5% for 208/120V use.

-Jon

 

[wwhitney]

I don't think anyone is suggesting that the 240 volt single phase inverter that is claimed to also work on 208 has the ability to recreate the two separate sine waves offset by 120 degrees and keep them separate as they enter the two legs of a 208 transformer, perfectly timed to match the peak voltage of each leg.

Somewhere behind this statement is your misconception that has been bedeviling this thread. A 2-wire inverter can't possibly "recreate the two sine waves offset by 120 degrees and keep them separate." Doing that would require a 3-wire inverter, so that the inverter can generate two separate voltage waveforms in the first place. Moreover, for grid-following inverters there is no need or advantage for the individual single phase inverters to "recreate the two sine waves." [Grid forming inverters are another story, but these PV inverters are not grid forming, and as you have battery energy storage, it would be up to the battery inverters to be grid forming if you want power during a blackout.]

So maybe you have the impression that when using multiple single phase inverters on a 3 phase system, there is a need or advantage for the inverter connection arrangement (which will always be delta with 2-wire inverters) to match the transformer secondary configuration (which may be delta or may be wye). There is no need or advantage. If you installed a transformer to create a 240V 3-phase system to allow you to run your inverters at 240V instead of 208V, it would be fine to use either a 240V delta secondary configuration or a 240Y/139V wye secondary configuration (from a technical point view; you'd have to check whether the firmware on these inverters supports such non-standard voltage systems.)

The only advantage of 240V over 208V for this application is that 240V > 208V. As the inverters are limited to 32A, the result is that the CAPACITY of the inverter is reduced from 240V*32A = 7.68 kW to 208V*32A = 6.66kW. So you will need more physical units to get the same total inverter capacity. Once you do that, all other things being equal, there will be no significant power generation difference between the two cases. [A more detailed analysis might show a 1% difference either way due to small secondary details, but I'm going to ignore such differences.]

Cheers, Wayne

 

[winnie]

So maybe you have the impression that when using multiple single phase inverters on a 3 phase system, there is a need or advantage for the inverter connection arrangement (which will always be delta with 2-wire inverters) to match the transformer secondary configuration (which may be delta or may be wye). There is no need or advantage. If you installed a transformer to create a 240V 3-phase system to allow you to run your inverters at 240V instead of 208V, it would be fine to use either a 240V delta secondary configuration or a 240Y/139V wye secondary configuration (from a technical point view; you'd have to check whether the firmware on these inverters supports such non-standard voltage systems.)

Cheers, Wayne

As a specific example in the other direction, the Enphase microinverters linked by someone above have a power limitation rather than an output current limitation (I expect that all inverters have both, but in this specific case the power limitation is the limiting factor) and thus can output greater current at 208V than at 240V.

Additionally these specific inverters are only rated for 120V L-N, and so cannot be used on a 240V high leg, nor could they be used on a 240Y/139V system.

But that is specific to the Enphase inverters linked later in this thread, not the original inverters being discussed. As in all cases, check the datasheet/manual!

-Jon

 

[jaggedben]

"The IQ 6 and IQ 6+ Microinverters have a 97% CEC efficiency for split phase applications..."

Where is the efficiency rating for non-split phase applications?

On the datasheet.
Also that's entirely a measure of DC-to-AC conversion efficiency and has nothing to do with anything that happens once power leaves the inverter plug connector.

 

[wwhitney]

the inverter connection arrangement (which will always be delta with 2-wire inverters)

This is not quite right. Rather, a delta inverter arrangement will work with either a delta or wye voltage system, while a wye inverter arrangement is only possible if you have a wye voltage system. All other things being equal, delta is probably preferred because the higher L-L voltage means a lower current for a given power output. But if you have a 2-wire single phase inverter that could be configured to output 208V, 240V, or 277V with sufficient flexibility, you could use it delta connected on 208Y/120V or a 240Y/139V or a 240D, or a wye connected on a 480Y/277V or 416Y/240V.

Cheers, Wayne

 

[bellington]

This is not quite right. Rather, a delta inverter arrangement will work with either a delta or wye voltage system, while a wye inverter arrangement is only possible if you have a wye voltage system. All other things being equal, delta is probably preferred because the higher L-L voltage means a lower current for a given power output. But if you have a 2-wire single phase inverter that could be configured to output 208V, 240V, or 277V with sufficient flexibility, you could use it delta connected on 208Y/120V or a 240Y/139V or a 240D, or a wye connected on a 480Y/277V or 416Y/240V.

Cheers, Wayne

Thanks for the extended discussion. My brain is fine with a single phase inverter connected to a delta secondary, as each pair of L-L can be seen by the inverter as a single phase. I'm also good connecting to each single leg of 3-phase Y with a balanced set of inverters. You have stimulated my interest in seeking to determine if the intended single phase 240 inverters may function at 277 and tie into existing legs of 480. The main drawback would be providing for adequate current carrying capability on the neutral leg.

Sorry that I am simply not able to make my brain accept connecting to two legs of Y and making it work efficiently.

Thanks for your patience with me and your explanations.

 

[wwhitney]

You have stimulated my interest in seeking to determine if the intended single phase 240 inverters may function at 277 and tie into existing legs of 480. The main drawback would be providing for adequate current carrying capability on the neutral leg.

Neutral ampacity is generally not an exceptional issue here, as with balanced L-N loads or sources (equal across all 3 choices of L), the way neutral current adds in 3 phase results in zero current on the neutral. This concurrent thread addresses the issue: https://forums.mikeholt.com/threads/current-carrying-conductors.2579160/

Sorry that I am simply not able to make my brain accept connecting to two legs of Y and making it work efficiently.

OK, a few comments: don't rule out delta-arranged 2-wire inverters on your existing 208Y/120V system because of this difficulty. It will work efficiently, and you should consider it among the various solutions for your customer. I bet it will be the simplest, most cost effective one, even though it uses a little more wire and a few more inverters due to the lower voltage.

Second, on this question of delta connection on a wye secondary, there is nothing special about inverters, it is the same with loads. We use delta connected loads on a voltage system supplied with a wye transformer secondary all the time. Both 3 phase motors and groups of three 2-wire loads in a delta arrangement. It works efficiently.

Lastly, as a thought experiment that might help you: suppose I give you 3 conductors and a battery operated oscilloscope, in a room comprising all insulating materials with no ground reference present. With the oscilloscope you can simultaneously measure the voltage waveforms between each pair of conductors and determine that you have a 480V 3 phase 3 wire supply.

Then for an idealized source, it is impossible to tell what the upstream equipment supplying those 3 wires is, whether it's a transformer with a 3W 480V delta secondary, or a transformer with a 4W 480Y/277V secondary from which only 3 conductors are brought into the room, or some 3 phase rotary generator. And not only is it impossible for you to tell, any electrical equipment connected to just those 3 wires can't tell the difference either.

And to the extent that that idealization doesn't quite model the real world, it will fail to do so by a matter of a percent or two, not, say, the 15% difference between 208V and 240V. [Assuming you've probably sized and rated the equipment, you obviously can't push 180 kW of solar through a 45 kVA transformer.]

Cheers, Wayne

 

[jaggedben]

Thanks for the extended discussion. My brain is fine with a single phase inverter connected to a delta secondary, as each pair of L-L can be seen by the inverter as a single phase. I'm also good connecting to each single leg of 3-phase Y with a balanced set of inverters. You have stimulated my interest in seeking to determine if the intended single phase 240 inverters may function at 277 and tie into existing legs of 480. The main drawback would be providing for adequate current carrying capability on the neutral leg.

Sorry that I am simply not able to make my brain accept connecting to two legs of Y and making it work efficiently.

Thanks for your patience with me and your explanations.

My final comment is to suggest that you hire an electrical engineer for this project. It seems like it has the budget for it.

 

[bellington]

Second, on this question of delta connection on a wye secondary, there is nothing special about inverters, it is the same with loads. We use delta connected loads on a voltage system supplied with a wye transformer secondary all the time. Both 3 phase motors and groups of three 2-wire loads in a delta arrangement. It works efficiently.


Cheers, Wayne

Again, thanks for the patience and time spent. If I am any closer to grasping the single phase inverters becoming a 3-phase generator, the delta configuration for the inverters connected to the wye secondary was the key. Would I connect inverter A to breakers 1 and 3, inverter B to 3 and 5, and inverter C to 5 and 1 to create a group of delta connected inverters feeding back into the 208 wye transformer?

 

[jaggedben]

Again, thanks for the patience and time spent. If I am any closer to grasping the single phase inverters becoming a 3-phase generator, the delta configuration for the inverters connected to the wye secondary was the key. Would I connect inverter A to breakers 1 and 3, inverter B to 3 and 5, and inverter C to 5 and 1 to create a group of delta connected inverters feeding back into the 208 wye transformer?

Conceptually I think you are on the right track, but you would not connect two inverters to the same breaker, and your numbering is off for a conventional 3 phase panel.

Here's a breaker space diagram color (found with google) coded by leg for a typical panel:

https://www.pinterest.jp/pin/7459155627111786/

So for your first group of three inverters, the first would be in spaces 1&3, the second 4&6, and the third 3&7. The next group would start with 8&10, and so on.

An engineer could calculate the line currents for you. Your BESS manufacturer ought to have support engineers who can tell you how important it is to have multiples of 3.

 

[wwhitney]

Again, thanks for the patience and time spent. If I am any closer to grasping the single phase inverters becoming a 3-phase generator,

The phrase "3-phase generator" is a bit of a misnomer here, as it suggests that (3) of these inverters could be used to create a stand-alone 3-phase grid. Which they can't as grid-tied inverters. They rely on the existing 3-phase grid for all the of "3-phaseness" of what they are doing. They just play follow the leader--each one just sees a single compliant voltage waveform and then pushes out some current in phase with that waveform (for the default case where they are configured to have power factor 1).

the delta configuration for the inverters connected to the wye secondary was the key. Would I connect inverter A to breakers 1 and 3, inverter B to 3 and 5, and inverter C to 5 and 1 to create a group of delta connected inverters feeding back into the 208 wye transformer?

If breaker 1 is on leg A of the supply, breaker 3 is on leg B of the supply, and breaker 5 is on leg C of the supply, then precisely. [Edit: sounds like that's not the usually breaker numbering convention on 3-phase panels, per jaggedben's simultaneous post.]

A practical question here that I don't know the answer to, but I'm sure the other commentators in the thread do: if delta-connecting (3) single phase 2-wire inverters, would you able to do that with a single 3-wire feeder from a 3-pole breaker? Or does each inverter need to be protected on the AC side with a double pole breaker in accordance with its rating, requiring the use of (3) double pole breakers?

Cheers, Wayne

 

[wwhitney]

but you would not connect two inverters to the same breaker

Just to be concrete here, if the inverter's maximum continuous output current is 32A, then putting (3) of these in a delta configuration directly on a single 3-pole 70A breaker (70A = 32A * 125% * sqrt(3)) is not an option? You'd need to use (3) double pole 40A breakers?

I guess even if it isn't required by the NEC or the manufacturer's instructions, using (3) double pole 40A breakers would be more convenient for maintenance and troubleshooting, so you could turn off just one inverter.

Thanks,
Wayne