Grid tied Solar

Solar, wind, hydro and other non-fossil sources of electricity.
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Hillhater   100 GW

100 GW
Posts: 9239
Joined: Aug 03 2010 10:33pm
Location: Sydney ..(Hilly part !) .. Australia/ Down under !

Grid tied Solar

Post by Hillhater » Nov 23 2017 9:50pm

Can someone clarify/explain what happen to the surplus solar power generated from a roof top system , that gets "fed back" into the grid ?
I understand that the GT inverter will increase the voltage marginally over the supply voltage to feed the power back to the local supply lines, but that is still only 240/380 volts (110v/phase) and no where near the much higher (10kV ?) of the local distribution grid feeding the step down transformers at the local sub station.
Will my "feed back" power go any further than that sub station , being redistributed on the local lines to my neighbours, or is there some mechanism for pushing my keen little kWs back past the substation, up to experience some National travel to far away places ? :? :D
This forum owes its existence to Justin of ebikes.ca

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jonescg   1 GW

1 GW
Posts: 3304
Joined: Aug 07 2009 9:22pm
Location: Perth, Western Australia

Re: Grid tied Solar

Post by jonescg » Nov 23 2017 11:39pm

Most of the current you push back into the grid will be consumed by your neighbours who don't have solar, or are running higher loads as their local voltages will be lower. If your whole neighbourhood has solar, you will all be pushing it back to the transformer. At this point, the voltage on the HV side will rise and feed current back towards the next neighbourhood.

If you end up with so much PV and wind adding current at the same time, the current will flow past all the substations and into the transmission lines to the next town. Some days South Australia powers parts of Victoria through the two inter connectors. So basically, it all gets used, and as the generating power increases, the generators will turn their output down so as to maintain a constant voltage on the lines.

craneplaneguy   10 kW

10 kW
Posts: 758
Joined: Nov 27 2015 9:15am
Location: S.E. Idaho USA

Re: Grid tied Solar

Post by craneplaneguy » Nov 24 2017 11:14am

Exactly, well explained, and I've pointed this out to my rural non grid tied neighbors, not that they care. In fact, if I knew their months power bill, I could tell them exactly how much of "their" power my system supplied. In my case this is pretty straightforward, as my meter is on the end of the line, with only two other places on the same line before it branches into others, where the power goes then gets a bit murky.

billvon   1 MW

1 MW
Posts: 1754
Joined: Sep 16 2007 9:53pm
Location: san diego

Re: Grid tied Solar

Post by billvon » Nov 24 2017 8:55pm

Hillhater wrote:
Nov 23 2017 9:50pm
Will my "feed back" power go any further than that sub station , being redistributed on the local lines to my neighbours, or is there some mechanism for pushing my keen little kWs back past the substation, up to experience some National travel to far away places ?
A few notes on this issue:

1) On a physical level your electrons aren't going anywhere. A 100 amp load will cause electrons to move a few millimeters per second in a copper wire. (And since the current reverses 120 times a second, those electrons never really go anywhere.) But what you are creating is potential; you are pumping electrons into that wire and that raises the potential of the wire, and that potential can force electrons out of the wire in other places. (Like your neighbors.)

2) That potential can be used by your neighbors, or by the whole grid. Distribution transformers are two-way, so any energy you generate can propagate through the grid. However, a simpler way of looking at it is that you are really powering the nearest loads. (And via an accurate analysis, slightly more current ends up in local loads than in the grid as a whole, since the effective impedance is lower to those loads.)

3) Although the grid is bidirectional, many grids cannot sense bidirectional current - and thus cannot control the grid once the current reverses. This is a problem with subgrids like Hawaii where there is so much solar installed that the current starts flowing back through the distro transformers to the larger grid. (Which is why new installations in Hawaii are currently prohibited.) Upgrades under the general heading of "smart grids" can avoid this problem.
--bill von

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