Single Phase Inverters on 208 3 Phase

[bellington]

Can someone suggest an efficient method of using single phase 240 v inverters for 134 kW of PV panels on one roof and 46 kW of PV panels on another roof to feed back into a 480/277 volt Y local grid with 208/120 Y panels? Connecting the inverters to feed into two legs of 208 seems like a huge loss of energy.

Thanks for your comments and suggestions.

 

[electrofelon]

Can someone suggest an efficient method of using single phase 240 v inverters for 134 kW of PV panels on one roof and 46 kW of PV panels on another roof to feed back into a 480/277 volt Y local grid with 208/120 Y panels? Connecting the inverters to feed into two legs of 208 seems like a huge loss of energy.

Thanks for your comments and suggestions.

Can you clarify what you mean by " feed back into a 480/277 volt Y local grid with 208/120 Y panels"? Best would be to have 480 inverters and connect to the 480 system. Also even if you need 240 inverters for some reason, why single phase? Need more information to give meaningful advice.

 

[bellington]

Can you clarify what you mean by " feed back into a 480/277 volt Y local grid with 208/120 Y panels"? Best would be to have 480 inverters and connect to the 480 system. Also even if you need 240 inverters for some reason, why single phase? Need more information to give meaningful advice.

Thank you. There is a main battery with 480 volt 3 phase two-way inverter/rectifier located in another building. Excess power from the two buildings mentioned will be fed to the battery through the 480 grid between buildings. Currently plans have 480 and 208 panels in those buildings. The roof PV supplier only offers single phase inverters with the roof.

So I'm asking for an efficient method to connect single phase inverters into a 3 phase system.

 

[jaggedben]

So I gather this is a micro-grid.
The question still stands: Why use 240V single phase inverters? The only semi-logical reason would be that you bought them already.

 

[jaggedben]

... The roof PV supplier only offers single phase inverters with the roof.
...

Sorry, I was reading too fast the first time.

My suggestion is to get a different supplier.

 

[bellington]

Sorry, I was reading too fast the first time.

My suggestion is to get a different supplier.

Do you happen to know of another provider of solar shingles?

 

[jaggedben]

Do you happen to know of another provider of solar shingles?

I think I catch your drift.

So, you need some transformers. Probably 480 to 240 delta.

You should try to balance the inverters around all three phases. You might need some kind of protection relay to drop all three phases if one fails.

 

[bellington]

I think I catch your drift.

So, you need some transformers. Probably 480 to 240 delta.

You should try to balance the inverters around all three phases. You might need some kind of protection relay to drop all three phases if one fails.

If I use one 3 phase transformer with 240 delta, how do I avoid confusion of one leg to ground expected to be 120?

 

[jaggedben]

Do the inverters need a neutral or not?

If they do then I guess you could use multiple single phase 480 to 120/240 transformers.

You're asking us for the most 'efficient' but without knowing the inverter spec all we can do is guess what's even possible.

 

[wwhitney]

Do the inverters need a neutral or not?

If they do then I guess you could use multiple single phase 480 to 120/240 transformers.

Using 3 of them to create a hexaphase secondary!

Cheers, Wayne

 

[Carultch]

If I use one 3 phase transformer with 240 delta, how do I avoid confusion of one leg to ground expected to be 120?

With clear labeling on each piece of equipment that clearly identifies the voltage of each leg to neutral, and your color code. You could add a diagram of the delta or wye shape and show where the ground is, relative to the phases.

It sounds like you'd either be building a high leg delta or corner-grounded delta grid, to get 240V between each phase.

It's common for single phase inverters to be dual-rated to work at both 208V and 240V, so check the datasheet to see if this is possible, if you'd prefer to use a 120/208V wye grid. It may be possible, but with a drawback that you lose some of your capacity. E.g. a 6 kW inverter meant for 240V might effectively become a 5 kW inverter at 208V. You'd connect the inverters across staggered pairs of phases, so that eventually they connect between all three possible pairs.

 

[tortuga]

Using 3 of them to create a hexaphase secondary!

Cheers, Wayne

If were going there then I gotta throw out a plug for the Scott-T transformer,
you'd save on grounding and bonding
one 200kVA scott-T could handle that 120/240V PV with two single-phase panelboards instead of 3
and you get a neutral with 120V to ground.

 

[jim dungar]

If were going there then I gotta throw out a plug for the Scott-T transformer,
you'd save on grounding and bonding
one 200kVA scott-T could handle that 120/240V PV with two single-phase panelboards instead of 3
and you get a neutral with 120V to ground.

Have you ever gotten pricing and delivery on a Scott-Tee transformer?
Back when they were somewhat common, a typical construction was the interconnection of two single phase pole mount units that had the proper tap locations. I can't imagine them being installed, new, outside of the Philadelphia area.

 

[bellington]

With clear labeling on each piece of equipment that clearly identifies the voltage of each leg to neutral, and your color code. You could add a diagram of the delta or wye shape and show where the ground is, relative to the phases.

It sounds like you'd either be building a high leg delta or corner-grounded delta grid, to get 240V between each phase.

It's common for single phase inverters to be dual-rated to work at both 208V and 240V, so check the datasheet to see if this is possible, if you'd prefer to use a 120/208V wye grid. It may be possible, but with a drawback that you lose some of your capacity. E.g. a 6 kW inverter meant for 240V might effectively become a 5 kW inverter at 208V. You'd connect the inverters across staggered pairs of phases, so that eventually they connect between all three possible pairs.

Thanks for the input. Yes, the inverters will work on 208. The best specs I can find show the inverters are 7.6 kW on 240, and only provide 6.6 on 208. My greater concern is the loss that happens to that 6.6 kW AFTER passing through two legs with two phases of a 208/120Y to 480/277Y transformer. Even if the numerous individual inverters could be perfectly divided among the three phases, there has to be significant energy loss. Unless the inverters can provide the two phase output that perfectly emulates the two phases offset by 120 degrees for each pair of legs, it appears we are throwing away a significant amount of solar energy collected. I don't know how to calculate those loses of the single phase inverters trying to emulate the two phases on each pair of legs as they back feed the power grid's 480/277Y to 208/120Y transformers. Any idea of how to calculate, or even estimate, the energy loss?

 

[jaggedben]

Thanks for the input. Yes, the inverters will work on 208. The best specs I can find show the inverters are 7.6 kW on 240, and only provide 6.6 on 208. My greater concern is the loss that happens to that 6.6 kW AFTER passing through two legs with two phases of a 208/120Y to 480/277Y transformer. Even if the numerous individual inverters could be perfectly divided among the three phases, there has to be significant energy loss. Unless the inverters can provide the two phase output that perfectly emulates the two phases offset by 120 degrees for each pair of legs, it appears we are throwing away a significant amount of solar energy collected. I don't know how to calculate those loses of the single phase inverters trying to emulate the two phases on each pair of legs as they back feed the power grid's 480/277Y to 208/120Y transformers. Any idea of how to calculate, or even estimate, the energy loss?

Well, you're on the wrong track here.

First, if you have the same or similar DC to AC ratio for the output rating at 208 as you would at 240, then energy production and losses won't be meaningfully, different. You will just need more inverters. For example on your 46kW array if you have (5) 7.6kW inverters that's a 1.21 DC to AC ratio, whereas with (6) 6.6kW inverters that's a 1.16 ratio, so you'll get slightly higher production. Those are both very reasonable ratios. For a pretty good discussion of clipping see here. https://forums.mikeholt.com/threads...ipping-of-pv-production.2573365/#post-2826236
I don't know the intracacies of design with your supplier but something like this has to be possible.

There's a second part to your concern that seems to involve confusion about how energy passes through transformers. There is energy loss any time you pass through a transformer, but different configurations won't meaningfully change how much is lost from your solar output. If you already have 208/120 available and the inverters work at 208, that's probably what you should use. Putting additional transformers in the mix will have more losses. The portion of solar energy that goes straight to the loads at 208 will have fewer loses.

 

[winnie]

There is one additional energy 'conversion' that I think is being misunderstood.

When you change from 120V L-N. 240V L-L to 120V L-N, 208V L-L, you don't get a huge inherent loss of energy. You don't have 32V (15%) at whatever amps being lost as heat.

The inverter will operate with 208V output, and have a loss of capacity because the inverter can only output so much current. There is also likely a small loss in efficiency because the output losses (which scale with output current) are counted against 15% lower output (so a 99% efficient inverter might drop to 99.7% efficient); however this might be balanced by an increase in efficiency of other parts of the system.

If you have 208V inverters supplying a system with 120V L-N loads, you can see an additional capacity loss associated with power factor issues. However this is negligible if everything is balanced and looks like 3 phase loading.

-Jon

 

[bellington]

Well, you're on the wrong track here.

First, if you have the same or similar DC to AC ratio for the output rating at 208 as you would at 240, then energy production and losses won't be meaningfully, different. You will just need more inverters. For example on your 46kW array if you have (5) 7.6kW inverters that's a 1.21 DC to AC ratio, whereas with (6) 6.6kW inverters that's a 1.16 ratio, so you'll get slightly higher production. Those are both very reasonable ratios. For a pretty good discussion of clipping see here. https://forums.mikeholt.com/threads...ipping-of-pv-production.2573365/#post-2826236
I don't know the intracacies of design with your supplier but something like this has to be possible.

There's a second part to your concern that seems to involve confusion about how energy passes through transformers. There is energy loss any time you pass through a transformer, but different configurations won't meaningfully change how much is lost from your solar output. If you already have 208/120 available and the inverters work at 208, that's probably what you should use. Putting additional transformers in the mix will have more losses. The portion of solar energy that goes straight to the loads at 208 will have fewer loses.

Perhaps I am confused on a couple of things. It appears to me that if I have X current and Y voltage coming from a group of PV panels to get 7.6 kW out of the inverter on 240 and connect the same configuration to 208, I just LOST 1 kW on each converter. Am I confused on that part?

On the transformer aspect, would a typical 7.6 kW single phase inverter provide one sine wave per 1/60th of a second, or two offset by 120 degrees to match the two phases of the two legs? I would think the same energy loss in combining two phases into one for consumption ((2) 120 legs = 208) would produce further losses as this process is attempted to be reversed, especially if the inverter is only sending one signal per 1/60th of a second.

 

[wwhitney]

Perhaps I am confused on a couple of things. It appears to me that if I have X current and Y voltage coming from a group of PV panels to get 7.6 kW out of the inverter on 240 and connect the same configuration to 208, I just LOST 1 kW on each converter. Am I confused on that part?

Yes. The inverter can take your X amps @ Y volts DC and either convert it to Z amps @ 240V, or Z * (240/208) amps @ 208V. Same power production, to first order.

[Although it is theoretically possible that the maximum DC voltage allowed with the inverter set to 208V AC output would be lower than the maximum DC voltage allowed when it is set to 240V AC output; not sure if any commercially available products have that limitation. That could require a different stringing arrangement.]

On the transformer aspect, would a typical 7.6 kW single phase inverter provide one sine wave per 1/60th of a second, or two offset by 120 degrees to match the two phases of the two legs?

A 208V inverter for use on a 208Y/120V doesn't need to act as two 120V inverters, it can just act a 208V 2-wire source. You seem to have some misconceptions about 3 phase power.

Cheers, Wayne

 

[winnie]

Perhaps I am confused on a couple of things. It appears to me that if I have X current and Y voltage coming from a group of PV panels to get 7.6 kW out of the inverter on 240 and connect the same configuration to 208, I just LOST 1 kW on each converter. Am I confused on that part?

You will have lost about 1 kW of _capacity_ in this case, and thus will need more inverters for a given number of panels. If you use the same number of panels per inverter, then with the lower capacity inverters you will have more clipping and you _will_ lose some production.

On the transformer aspect, would a typical 7.6 kW single phase inverter provide one sine wave per 1/60th of a second, or two offset by 120 degrees to match the two phases of the two legs? I would think the same energy loss in combining two phases into one for consumption ((2) 120 legs = 208) would produce further losses as this process is attempted to be reversed, especially if the inverter is only sending one signal per 1/60th of a second.

A single phase inverter outputs a single 60Hz sine wave. If you have 1 single phase inverter connected to two legs of a 208/120V system, the fact that you have different phase angles will result in a loss of capacity (the power factor issue I mentioned). This is not a loss of energy, just of capacity. However if you have a balanced 3 phase set of L-L single phase inverters feeding a balanced 3 phase set of L-N loads, everything balances out and this loss of capacity goes away.

-Jon

 

[wwhitney]

If you have 1 single phase inverter connected to two legs of a 208/120V system, the fact that you have different phase angles will result in a loss of capacity (the power factor issue I mentioned). This is not a loss of energy, just of capacity. However if you have a balanced 3 phase set of L-L single phase inverters feeding a balanced 3 phase set of L-N loads, everything balances out and this loss of capacity goes away.

I'm not seeing what you mean by the above. Agreed that a current-limited inverter will have lower capacity at 208V AC output than at 240V AC output. But I see no capacity loss differential between 1 such inverter vs 3 such inverters connected to a 208Y/120V system. What am I missing?

Cheers, Wayne