Relocate Current Sensors

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Are the CTs even being used to determine how much the PV system outputs, or not?
Regardless, there is no meaningful difference between putting the CTs on the line side of the service disconnect and putting all the loads through them on the load side. Either way they are measuring all the loads.
 
Ravenvalor said:
He is concerned that if I do not move the 2 - solar taps from the load side of the service disconnect to the line side of the service disconnect that the home will get power from the grid before getting it from his solar panels. Does this sound feasible? Why would the home choose the solar power before the grid power?
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It doesn't choose. The electrons don't come with serial numbers to keep track of which ones feed the on-site loads, and which ones backfeed the grid. The system is feeding both the on-site loads and the grid simultaneously. We just say "it first feeds the on-site loads" as a matter of bookkeeping.

The utility service meter, or any power meter in general, cannot tell the difference between 4 kW consumption-only vs 6 kW consumed with simultaneous 2 kW production. The meter sees this as a net import of 4 kW either way. You'd need to meter at separate locations in the circuit, to tell the difference. On an instant-by-instant basis, all it can meter is the net flow of power, and assign a sign to it, to keep track of direction.

How it accumulates power (Watts) into energy (kilowatt-hours), depends on the specifics of the meter. As in whether it is a net meter, a secure-forward meter, or another type of meter.
 
ggunn said:
The PV is a current source while the grid is a voltage source, and current can only flow in one direction at a time through a conductor. The upshot is that at any point in time if the household load is consuming more than the power the PV is producing, the house takes all of the PV production and the grid makes up the difference. If the load is less than what the PV is making, the PV provides all the power for the load and the excess goes into the grid.
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Very informative. Thanks....
 
ggunn said:
The upshot is that at any point in time if the household load is consuming more than the power the PV is producing, the house takes all of the PV production and the grid makes up the difference. If the load is less than what the PV is making, the PV provides all the power for the load and the excess goes into the grid.
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Just like the alternator and battery in a vehicle. If the vehicle needs more power than the alternator is producing, it comes from the battery. If there is surplus power, the battery gets recharged.

ggunn said:
Frankly, that is above the heads of most folks who are not in the biz, so Larry's explanation should suffice.
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I try to keep it simple, especially if the reader has to pass it on.
 
LarryFine said:
Just like the alternator and battery in a vehicle. If the vehicle needs more power than the alternator is producing, it comes from the battery. If there is surplus power, the battery gets recharged.


I try to keep it simple, especially if the reader has to pass it on.
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I have to admit that I have never considered PV as a current source. :)
 
Ravenvalor said:
I have to admit that I have never considered PV as a current source. :)
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That's why net metering works like it does. The grid is a voltage source; the amount of current flowing through a load connected to a voltage source is determined by its effective resistance, and the voltage at the terminals is constant within the source's linear range. It's the same for a battery, but not so for a current source like a PV system; PV's output current is determined by the amount of sunlight falling on the array.
 
I agree, the concept of a PV system as a current source is a very useful approximation, and as a simple answer to the the question in post is better than the one that I gave.

"The grid is designed as a voltage source. It will supply current as needed by the loads. The PV system is designed as a current source. It will push out all that it can, first going to the loads in the home then going out to the grid."

Of course both are approximations. A PV _cell_ is pretty well approximated as a current source with the current level set by the sunlight falling on it and a compliance voltage set by the open circuit voltage (not quite, but close; the IV graph is a curve not a rectangle!) A PV _system_ is more of a constant _power_ source with the power set by the sunlight hitting the array (looks like a current source when connected to the constant voltage grid), however this is because of inverter design decisions. A PV system can certainly be designed to provide a constant voltage output, this would be the case of just about any off grid PV system.

-Jon
 
Ravenvalor said:
Hello,

The customer posed an interesting question to me today.
He is concerned that if I do not move the 2 - solar taps from the load side of the service disconnect to the line side of the service disconnect that the home will get power from the grid before getting it from his solar panels. Does this sound feasible? Why would the home choose the solar power before the grid power?

I have to admit I am stumped. :)

Thanks.
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To put it simply, it's not possible for the utility to be sending power to the home and for the PV system to be sending power to the utility at the same time. The same utility service conductors cannot be moving power in both directions at the same time.
 
This is getting a bit off topic but I can't help myself. The idea that grid-tied inverters are current sources and stand-alone inverters are voltage sources did make it easy in the past to understand how they operated. Current sources just followed the voltage and did not attempt to change it, as long as it stayed in a range they just pumped out current with no care about where it went. Voltage source inverters in off-grid systems generated a voltage and maintained it by varying the output current. Add more load, the voltage goes down and the current supplied goes up to compensate and bring the voltage back up.
This kind of changed with the new advanced inverter designs that provided grid support functions. Now under some conditions, grid-tied PV inverters will try to change the grid voltage to provide voltage support when there is a grid fault that affects the voltage or frequency. So they are no longer just pure current sources. But while this mucks around with the pure current source model of the grid-tied inverter it makes no difference to the OP's question.
 
To that I would add that the statment 'PV inverters push out all the current they can' is also outdated. Many systems, both with and without batteries, are now configured to vary their output to match the load measured by CTs, either to determine battery charging and discharging, or to limit exporting to the grid. So depending on the OP's set up it may be important that the CTs measure the entire load of the house. (Although as I said above, it does not matter where they measure it so long as they measure it all.)
 
Outside of Hawaii, non-export residential systems are still rare. Or are there other states imposing non-export residential interconnections? That's really the only reason to curtail a grid-tied PV system. Even if you have a battery and low export compensation, once the battery is full and you have no other use for the energy on-site, export is going to net more than curtailment.
 
jaggedben said:
Many systems [...] without batteries are now configured to vary their output to match the load measured by CTs...
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Can you give me an example of one? I'm not saying they don't exist, just that I haven't seen one. Other than the SMA inverters with limited output lines, I mean.
 
pv_n00b said:
Outside of Hawaii, non-export residential systems are still rare. Or are there other states imposing non-export residential interconnections?
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Some AHJs discourage it by paying only a pittance for energy returned to the grid. We had a customer in such an area who wanted a non-export system because he didn't want to give energy to the utility for pretty much free, but when we ran the numbers it was obviously better for him to give away energy that didn't cost him anything than to pay significantly more for a system that wouldn't do that.

"Sticking it to the man" often means sticking it to yourself.
 
ggunn said:
Can you give me an example of one? I'm not saying they don't exist, just that I haven't seen one. Other than the SMA inverters with limited output lines, I mean.
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The Solaredge Storedge inverter does it. I used to set it up for systems with batteries before the utility gave permission to operate, then reconfigure without the export limit once PTO was granted. I think all Solaredge inverters can actually do it when you have a meter connected.

Also Enphase has some export limited (PEL) 'grid profiles' although I'm not super familiar with how they work. And I know they have been testing new products that do it too.

Finally, although it doesn't involve CTs, there's the 'frequency watt' and 'volt-watt' features of UL1741SA. Those are required now in California and are used both on-grid and (I believe) off-grid in AC coupled systems with batteries to curtail but not completely cut off PV output either when grid conditions are too hot or when batteries are full. I believe Enphase uses this for the off-grid functionality in conjunction with system commands, but that it can also be set up with other systems e.g. Powerwall.
 
pv_n00b said:
Outside of Hawaii, non-export residential systems are still rare. Or are there other states imposing non-export residential interconnections? That's really the only reason to curtail a grid-tied PV system. Even if you have a battery and low export compensation, once the battery is full and you have no other use for the energy on-site, export is going to net more than curtailment.
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I've heard it's a thing in Pennsylvania but I don't know much detail.

I didn't mean to imply than non-export is a meaningful percentage of existing systems, but I expect that situation will slowly but surely change.
 
Also the other reason you would do it, besides non-export rules, is Power Control Systems. Say your max system output is a little beyond what's allowed for by 705 for a main panel, but if there's load on a subpanel that your connected through, it would be fine. With CTs and a listed PCS you could throttle the output when the subpanel load isn't there. And if that load is usually there during the day, maybe you don't need to throttle it hardly ever and you can use almost all the energy from the larger system.
 
jaggedben said:
The Solaredge Storedge inverter does it. I used to set it up for systems with batteries before the utility gave permission to operate, then reconfigure without the export limit once PTO was granted. I think all Solaredge inverters can actually do it when you have a meter connected.

Also Enphase has some export limited (PEL) 'grid profiles' although I'm not super familiar with how they work. And I know they have been testing new products that do it too.

Finally, although it doesn't involve CTs, there's the 'frequency watt' and 'volt-watt' features of UL1741SA. Those are required now in California and are used both on-grid and (I believe) off-grid in AC coupled systems with batteries to curtail but not completely cut off PV output either when grid conditions are too hot or when batteries are full. I believe Enphase uses this for the off-grid functionality in conjunction with system commands, but that it can also be set up with other systems e.g. Powerwall.
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Generac also has a non export setting in their unit.
For some reason those are big here.
And the installers set them up like you do the solaredge to get them running before we give the PTO.
 
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