PV System Design Question

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gar

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philly:

It is the nature of the beast. It is a semiconductor device and it is not linear.

In an earlier post I provided you with an Internet link to a source that shows a simple equivalent circuit for a solar cell.

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jaggedben

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SunPower said:
So, what you are saying is that a PV system is not a continuous load?
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He is not saying that at all. That is a totally separate question from what is being discussed in this thread. And a PV system is considered to be a continuous load by the code when it comes to wiring methods, overcurrent protection, and so on.
 
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gar

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SunPower:

A PV system is really a source and when an inverter is included can not be backfed. It is not a load like a DC motor with a large amount of mechanical inertia where the motor can be a motor or generator.

Can you clarify what you mean by your question?

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tallgirl

tallgirl

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gar said:
110619-1811 EDT

SunPower:

A PV system is really a source and when an inverter is included can not be backfed. It is not a load like a DC motor with a large amount of mechanical inertia where the motor can be a motor or generator.

Can you clarify what you mean by your question?

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PV inverters can too be back fed -- that's one of my favorite questions to answer for clients. It depends on the inverter and how it works. For some vendors with battery-based inverters, back-feeding results in the inverter trying to down-regulate the AC voltage via AC-to-DC conversion. In those cases, you now have to take into consideration down-regulating the DC voltage. Which gets you into weird scenarios with obscenely huge DC currents to be managed. And if you manage those currents, you may find yourself with obscene amounts of heat to contend with. So ... never say "It doesn't happen."

You have to look at all the maximum currents in all possible directions and size accordingly. That's the short and correct answer.
 
tallgirl

tallgirl

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OutBack Power Technologies "V-Series" inverters. They are very handy for creating microgrids, but they have problems with down-regulating the power being backfed to them.

Also, the SMA SunnyIsland can be backfed, though it down-regulates the backfeed current differently by slewing the output frequency to the upstream inverteers.
 
K8MHZ

K8MHZ

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tallgirl said:
OutBack Power Technologies "V-Series" inverters. They are very handy for creating microgrids, but they have problems with down-regulating the power being backfed to them.

Also, the SMA SunnyIsland can be backfed, though it down-regulates the backfeed current differently by slewing the output frequency to the upstream inverteers.
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We have an Outback at school. I don't know the series. I will look today.

I take it that SunnyIsland is a bi-modal version of a SunnyBoy?
 
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jaggedben

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K8MHZ said:
I take it that SunnyIsland is a bi-modal version of a SunnyBoy?
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Not really, at least that's not what I would say...

The Sunny Island handles the batteries and grid tie, but does not handle the solar directly, it has no onboard MPPT. It's meant to be coupled with the Sunny Boy (or other sources), or used with with a separate PV charge controller, I guess (online documentation is hard to find on that).

Also it is single phase, and meant to be stacked to handle split phase or three phase operation.

In both respects it is really more comparable to an Outback or something similar than to the Sunny Boy itself.

For more details you can find it using Google. ;)
 
tallgirl

tallgirl

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K8MHZ said:
We have an Outback at school. I don't know the series. I will look today.
Click to expand...

If y'all would like some software to use to view it's performance and maybe play with it a bit, let me know.

I take it that SunnyIsland is a bi-modal version of a SunnyBoy?
Click to expand...

My understanding is that it's purely off-grid -- it won't sell back to a utility.

What it does is provide a grid-quality reference, then change the output frequency to make the grid-tied inverters stop producing power when the batteries are too full. I'm not at all fond of that approach for reasons of capacity and reliability, but that's not my decision ...
 
tallgirl

tallgirl

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Great White North
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jaggedben said:
I'm pretty sure that's incorrect. See page 131 of the manual:
http://files.sma.de/dl/7995/SI5048U-TUS101331.pdf
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It's listed as an off-grid inverter.

What the Sunny Island does is serve as a "grid replacement" for grid-tied inverters that would otherwise cease producing power when the grid is dropped.

Let's say you have a Sunny Boy and a nice little 6KW array. When the grid is lost for a power outage, the Sunny Boy shuts down and the system owner has no electricity.

Adding a Sunny Island would allow the Sunny Boy to continue producing power -- it provides a grid-quality reference that the Sunny Boy will use to conform to UL 1741. As long as the Sunny Boy is making enough power to meet the loads, the Sunny Island charges the batteries. When the batteries are full, it changes the output frequency and the Sunny Boy stops producing power and the Sunny Island is supporting the loads from the batteries.

What should become obvious right about now is that you need to have a 6KW inverter for the array, and a second inverter with a large enough output to support the loads. With a grid-interactive inverter (grid-tie: "no batteries", grid-interactive: "grid+batteries", off-grid: "batteries-only"), you size the inverter to the larger of the loads or the array. With the SMA solution, you size the inverters to the total of the two.
 
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