Non-Net Metered PV system- Grid Tied

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bwaryjasz

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Pittsfield MA
I am in the process of designing up around a 25kw pv system and customer may want to expand in the future. Current Utility standards state that there can only be 1 net metered facility per parcel of land. In my design I am trying to come up with a cost effective way to track the current on the Line side of the main disconnect so that if the PV system were to ever try to "sell back" the power the CT's would trigger a Contactor on the Combined output of the solar before the interconnection point and turn the system off. Additionally I want to make sure the system can be turned back on automatically when this situation is no longer in play. Has anyone run into this before? any insight would be greatly appreciated.
 
So essentially a reverse power relay?

Will the utility let you backfeed them but without net metering? So just give to utility free power to avoid the reverse power relay?


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pcanning87 said:
So essentially a reverse power relay?

Will the utility let you backfeed them but without net metering? So just give to utility free power to avoid the reverse power relay?
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That's what I was thinking. Does the utility prohibit backfeeding in this case, or is it just because doing so makes no money for your customer? If the latter, why does the customer care? Giving it to the utility and turning off the system have the same effect to the customer (it's not like he is saving any money), and gear like you are asking about costs money.
 
GGUNN I just sent in an RFI to the Utility company to ask this very question again. I completely agree with you, a loss of PV output is a loss of PV output to the customer. whether the system is off or the power is just "given" to the grid.
 
Might have to do with the Metering of said power. if they change out the meter to the Typical Net meter that tracks imports and exports. the billing would be a nightmare. then if they didn't change out the meter the customer would be getting charged for the power they are exporting.
 
It could also be an issue with the capacity of the utility feeder line to handle Distributed Generation. If you never backfeed the grid, it may alleviate technical issues on the utility side.

That said it seems more likely that it's a commercial/Metering concern like bwaryjasz said.


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pcanning87 said:
It could also be an issue with the capacity of the utility feeder line to handle Distributed Generation. If you never backfeed the grid, it may alleviate technical issues on the utility side.

That said it seems more likely that it's a commercial/Metering concern like bwaryjasz said.


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AHJ's frequently designate a "downtown network" where no one is allowed to export to the grid at all, no matter what. To the OP: is this such a situation?
 
I understand that this is a common problem in Hawaii wherein the grid can not accept more PV input at this time. I attended a Solaredge presentation that showed they have a metering option that communicates to some of their inverters and eliminates backfeed to the utility. No direct experience.
 
BillK-AZ said:
I understand that this is a common problem in Hawaii wherein the grid can not accept more PV input at this time. I attended a Solaredge presentation that showed they have a metering option that communicates to some of their inverters and eliminates backfeed to the utility. No direct experience.
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So, what do the customers do with all that solar? Are they getting paid for simply having the panels on their roofs??
 
gadfly56 said:
So, what do the customers do with all that solar? Are they getting paid for simply having the panels on their roofs??
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Hawaii is a special case because they have their own very small grid which is subject to instability if too much DG gets fed into it. I don't know if solar owners are getting paid for anything but they have to use all the kWh that they produce.
 
ggunn said:
Hawaii is a special case because they have their own very small grid which is subject to instability if too much DG gets fed into it. I don't know if solar owners are getting paid for anything but they have to use all the kWh that they produce.
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Basically a GTI in Hawaii must now be able to throttle back its power output to prevent backfeeding the grid either by configuration or when given that command by POCO. When that happens the GTI simply does not draw full power from the panels, so there is no power looking for a load. The potential solar PV energy is just not used.
Nobody gets paid for power not produced.

Some homeowners may choose to turn on elective loads, such as resistance water heating, to get some benefit from the potential power.
 
Design it for output

Design it for output

Personally, I would design my system to output 24k and oversize the array.

What I mean is max the inverters output at 24k, or 2 inverters most likely. But don't stop there, oversize the array, so that you are putting out 24k more often, thus maximizing your output. If you don't oversize the array it hardly ever will produce max except at the peak of the day (think bell curve). Oversized think bell curve with cut off, instead of just at peak time, you could get hours of maxed output. I have designed several this way with no problems.

Sorry, I read further, I didn't realize that you can not feed back to the grid at all, that would be a problem, short of going pure off grid.
 
double check

double check

I would double check with the utility and make sure you understand them clearly.

The entire point of net metering is that your excess power goes to the grid. If you are grid connected your net metering, now the pay structure is a whole different thing. I mean that is basically the definition of it. At night or whatever then the meter turns the other way. Sorry ranting that is where your are stuck.

So if a system is not net metering, basically your off grid or a hybrid, which still involves an off grid component and storage.

There are situations I have run into with various funny rules, in some cases the utilities did not want to buy excess power, but that was on a yearly basis, not real time. If you can not feed back at all, then that is not net metering, and your stuck. The switch idea you propose won't work well, your system will be going on and offline all the time, as it would need to scale back production intelligently and then start production.

Much better to build an off grid system, even if you have the grid to fulfill shortages. In that ways you produce, store and use your power, but you won't give it back to them. It will also make much better use of your power, and will lower if not eliminate your bill.

Just my opinion. Let me know though, we can figure out a way to make it work well for you.
 
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seaton said:
I would double check with the utility and make sure you understand them clearly.

The entire point of net metering is that your excess power goes to the grid. If you are grid connected your net metering, now the pay structure is a whole different thing. I mean that is basically the definition of it. At night or whatever then the meter turns the other way. Sorry ranting that is where your are stuck.

So if a system is not net metering, basically your off grid or a hybrid, which still involves an off grid component and storage.

There are situations I have run into with various funny rules, in some cases the utilities did not want to buy excess power, but that was on a yearly basis, not real time. If you can not feed back at all, then that is not net metering, and your stuck. The switch idea you propose won't work well, your system will be going on and offline all the time, as it would need to scale back production intelligently and then start production.

Much better to build an off grid system, even if you have the grid to fulfill shortages. In that ways you produce, store and use your power, but you won't give it back to them. It will also make much better use of your power, and will lower if not eliminate your bill.

Just my opinion. Let me know though, we can figure out a way to make it work well for you.
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There are situations where you are not allowed to backfeed the grid at all, ever. Around here such is the case if your interconnection is on the downtown grid. If that is your situation then you must come up with a way to power only your local loads (partially or fully) from solar. Off grid? Well, yes, that is one way to do it, but off grid power is much more expensive than grid power, usually on the order of 10X as much, so you'll have to look at the economics and decide if it is worth it.
 
electrofelon said:
Out of curiosity, how does that fit in with net metering laws? Is there no net metering there or were the utilities allowed exceptions for certain areas?
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Net metering isn't a law, at least not in Texas. It's a tariff administered by the AHJ and they are free to amend/implement it as they see fit. In (at least) Austin and San Antonio, in the part of the city designated as the Downtown Network you cannot export power to the grid at any time.
 
ggunn said:
In (at least) Austin and San Antonio, in the part of the city designated as the Downtown Network you cannot export power to the grid at any time.
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So on a technical level, what's different about the Downtown Network that makes it harder for the POCO to handle backfeed there?

Thanks, Wayne
 
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