AC coupled recommendation?

400bird

Solar Enthusiast
Joined
May 23, 2020
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394
Setting/limiting EVSE current yes, but I can't graph the actual current/wattage.

The EVSE broadcasts amperage, but not measured voltage or wattage. I could multiply current that by 240 to get a rough wattage number. But I already had a current sensor and one spare input on the PI.

Graphing the command wouldn't work either as it's always sending the available current even when the car is fully charged or isn't connected.

Edit: added words to clarify...
 
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fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,456
Setting/limiting EVSE current yes, but I can't graph the actual current/wattage.

The EVSE broadcasts amperage, but not measured voltage or wattage. I could multiply that by 240 to get a rough number. But I already had a current sensor and one spare input on the PI.

Graphing the command wouldn't work either as it's always sending the available current even when the car is fully charged or isn't connected.
Oh, your just adding another CT sensor to the Pi (for actual EV charging power). I thought you meant you were going to connect a CT sensor directly to the EVSE charger so that it could determine its own charge power…
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,456
Setting/limiting EVSE current yes, but I can't graph the actual current/wattage.

The EVSE broadcasts amperage, but not measured voltage or wattage. I could multiply current that by 240 to get a rough wattage number. But I already had a current sensor and one spare input on the PI.

Graphing the command wouldn't work either as it's always sending the available current even when the car is fully charged or isn't connected.

Edit: added words to clarify...
Hey, one question I have about your Conext XW-based rig is how it handles balancing.

I know it is a low-frequency inverter with a large Autotransformer, but is that Autotransformer always connected in a way that export or import to grid is always balanced?

Specifically, I the case that 240VAC solar power is more than enough to offset mains load, but that mains load is 75% on one leg and 25% on the other, will the XT put out power in a 75%/25% L1/L2 split so that both L1 and L2 are neither importing or exporting?

I don’t have any questions about the function of the transformer when grid is down and backup power is being used for critical loads panel, but I’m confused about whether it is always balancing house/mains load when the grid is connected.
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,311
I think an autotransformer will only balance to some extent when it sees a voltage differential.

A transformer typically has 10% voltage sag between open circuit and rated wattage, so if L1 is 250V, I think L2 would have to be 225V for transformer to transfer its rated watts from one leg to the other.

This of course implies power dissipation when transformer output voltage sags. Some types (toroid) may be rated at higher efficiency, which would suggest less voltage sag.
 

fafrd

Photon Sorcerer
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Aug 11, 2020
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I think an autotransformer will only balance to some extent when it sees a voltage differential.
I thought Autotransformers balanced based on the Neutral voltage.

As long as Neutral is at Ground / midpoint, no balancing occurs but as soon as one leg has greater load and starts dragging Neutral closer to it, the Autotransformer provides balancing current from the other leg to get Neutral closer to Ground / midpoint.
A transformer typically has 10% voltage sag between open circuit and rated wattage, so if L1 is 250V, I think L2 would have to be 225V for transformer to transfer its rated watts from one leg to the other.
If loads are dragging L2 down to 225V, I believe that means Neutral is being pulled 12.5V up in the direction of L2 (so L2 to Neutral is 235.5V and L2 to Neutral is also 235.5V, at least for your example of 250/225V).

So what you are saying is that an Autotransformer delivering it’s full rated wattage will have something like a ~10% difference between the peak-to-peak differential voltage of the loaded leg (lower) versus the unloaded leg (higher).

I’m just trying to understand what that means in terms of import / export.

If you are just consuming from grid, imbalance can cause one leg to be lower than the other but peak voltage of the unloaded leg can never exceed the voltage leaving the transformer at the pole.

If you are self-consuming 240VAC power you are generating from solar, with no Autotransformer, the unloaded leg will increase above pole transformer voltage by the same amount that the loaded leg will decrease below pole transformer voltage (so kW measured at 240V by the meter will be zero).

From what you are saying, connecting an Autotransformer to that same solar-with-unbalanced-load will reduce the voltage difference to ~10% of what it would be without any Autotransformer, but the meter will still see an unloaded leg at a higher voltage than it was leaving the pole transformer…

This of course implies power dissipation when transformer output voltage sags. Some types (toroid) may be rated at higher efficiency, which would suggest less voltage sag.
My main interest is in understanding whether the utility smart meter can detect any difference between a customer consuming only a small amount of power versus a customer self-powering from generated solar energy.

Sounds like there will always be some distinguishable difference with a low-frequency inverter (Autotransformer) and that is one benefit to a dual-120V high-frequency architecture…

Of course, if the smart meter can only measure L1-versus-L2 voltage, and not L1-versus-pole-Neutral and L2-versus-pole-Neutral voltages, the difference is immaterial.
 

400bird

Solar Enthusiast
Joined
May 23, 2020
Messages
394
Hey, one question I have about your Conext XW-based rig is how it handles balancing.

Specifically, I the case that 240VAC solar power is more than enough to offset mains load, but that mains load is 75% on one leg and 25% on the other, will the XT put out power in a 75%/25% L1/L2 split so that both L1 and L2 are neither importing or exporting?
I'm pretty sure when connected to the grid the "auto transformer" isn't going anything. I'm pretty sure I have seen one leg pulling power from the grid while I'm pushing power out on the other leg.

It doesn't matter to me, the power company just looks at L1 + L2. So, +500 watts and -500 watts, would be grid zero.

I don’t have any questions about the function of the transformer when grid is down and backup power is being used for critical loads panel, but I’m confused about whether it is always balancing house/mains load when the grid is connected.

I'm pretty sure I've seen this situation work great too. Again positive on one leg and negative on the other.
And it will start my central AC.


I say "auto transformer" because I'm not sure it's quite the same. It's good coils in there, but I think it's just the actual transformer.

I'm not familiar with auto transformers on a grid connected system, but I'd think you would not want to connect it to the grid or you might end up balancing the entire neighborhood!
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,311
If loads are dragging L2 down to 225V, I believe that means Neutral is being pulled 12.5V up in the direction of L2 (so L2 to Neutral is 235.5V and L2 to Neutral is also 235.5V, at least for your example of 250/225V).

My bad, nominally 120/240V, so if one phase is 125 relative to neutral and the other is 112.5V (180 out of phase), that's a 10% difference and I would expect balancing transformer to carry about its rated current.

Yes, this would try to balance the neighbor's imbalance as well. Wire resistance to loads will determine which causes how much balancing.

If you instead used an isolation transformer, 240V on grid side and 120/240V split phase on your side, grid current would be perfectly balanced.
 

fafrd

Photon Sorcerer
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Aug 11, 2020
Messages
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My bad, nominally 120/240V, so if one phase is 125 relative to neutral and the other is 112.5V (180 out of phase), that's a 10% difference and I would expect balancing transformer to carry about its rated current.

Yes, this would try to balance the neighbor's imbalance as well. Wire resistance to loads will determine which causes how much balancing.
Al least on my pole, the pole transformer puts out L1, L2, and a grid neutral tied to ground at the pole.

So I believe any imbalance by your feed and your neighbor’s feed is being balanced by the pole transformer.

If you have an Autotransformer, your house is putting out a balanced 240V load and leaving less work for the pole transformer to do.

But your puny Autotransformer is unlikely to balance your neighbor’s imbalance if the much larger pole transformer wasn’t able to achieve that beforehand…
If you instead used an isolation transformer, 240V on grid side and 120/240V split phase on your side, grid current would be perfectly balanced.
Yes, with an isolation transformer, you’d be perfectly balanced. As you would also be with two 120V inverters that each maintain zero export / zero import…
 
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