I'm curious how this is done safely. Engine-driven generators are required to be on a transfer switch specifically to prevent the back-feeding of power so that utility workers are not at risk of electrocution. So how is that risk addressed with the approved back-feeding of solar-generated power?
Back-feeding solar-generated power?
- Thread starter Jon456
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The short explanation is that grid-tie inverter outputs are setup as a current source, rather than a voltage source. The utility system supplies the voltage. The inverter measures system voltage and cuts off its output when the voltage is not within its set nominal operating range.
Thanks for the explanation. But this leaves me wondering, how is the solar system able to backfeed unless it is at a higher potential (voltage) than the utility power? And if it's supplying power at a higher potential than the grid, how does the grid-tie inverter know when the grid loses power? Well, I suppose one way would be the massive load suddenly placed on the solar supply!
As you can tell, I really don't understand how the grid-tie inverter works. How is it setup as a current source, rather than a voltage source? Do you have a link to a page that explaines all the theory behind this?
There you go... you answered your own questions.
I only have a basic understanding myself. Don't have any links offhand... and assume you are just as capable of using web search engines as I. Suggest perusing manufacturer's websites for technical info.
jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
Aside from voltage, a GT inverter also syncs to the grid frequency, so if it loses that it doesn't know what frequency to operate at and it shuts down.
I'm no less fuzzy than Smart $ on the exact details. I think if you understand exactly how these things work you are probably working for an inverter company.
I'm no less fuzzy than Smart $ on the exact details. I think if you understand exactly how these things work you are probably working for an inverter company.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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It doesn't have to be at a higher potential; in fact it isn't except for whatever voltage drop is in the line between the inverter and the interconnect. The way to look at it is that your inverter is in parallel with the generation capacity of the grid and your household loads are in parallel with all the rest of the loads on the grid.
When the grid goes down, your inverter can tell. Electronically how that happens is not really important unless you are an inverter designer, but it's not merely a function of increased load because it will shut down when you open the AC disconnect switch to the grid. Anyway, the UL1741 standard dictates that the inverter shut itself down within a very short time (a few milliseconds) when the grid voltage goes away. That protects any utility line workers who are making repairs as well as your inverter which would (as you pointed out) experience something very close to a dead short when the grid supply is cut off and the loads are still connected.
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
My understanding - which may not be entirely correct - is that the inverter does initially operate at higher voltage than the grid in order to backfeed current. However, since the grid can easily handle such a difference in load without a major change in voltage, things will settle to an equilibrium that is not measurably different from grid voltage.
Is this close enough? Or wrong?
Your right in the sense that it doesn't have to be at a higher potential... but consider that equal [voltage] potential will always be achieved at some point(s) in the circuit. This will always be true at the local utility-inverter-load node(s).
Now consider that inverters are designed to maximize their power output. If the utility branch of the mentioned node(s) has the same amount of "push-pull" as the inverter branch, then each will provide only half the power utilized by the local load branch. The only way power output of the inverter can be at maximum is if adjusts to have more "push-pull", which equates with more current, than the utility branch. While it is said that GT inverters are essentially current sources, they cannot maximize their output without operating at a higher potential than the utility.
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