What happens if ?

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What happens if ?

Postby tytower » Thu Jan 05, 2017 5:39 pm

If I have a grid connect system ready to supply 240V 3Kilowatts to the grid and instead of wiring it into the grid I wire it into the output of an inverter run by batteries and take that output power source off to run my house .

Will it work?
Will it blow anything? remembering the grid tie inverter adjusts its output to suit what it finds from the grid and it is fee to export all it produces ( Zero Export Off).


Please explain why you think what you do as much as possible.
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Re: What happens if ?

Postby Gordon-Loomberah » Fri Jan 06, 2017 12:42 pm

Welcome to the Energy Matters Forums Tytower :)

I suspect that wiring the output of a grid connect inverter to a stand alone inverter could potentially let the magic smoke out of one or both units, or perhaps nothing at all will happen when the anti-islanding of the grid-connect inverter shuts it down.
You don't mention what brands or models of inverters, but what you are proposing should not be attempted by someone who has no understanding of how it all works, and I expect it will end in tears.

There are inverters, such as from Selectronic and SMA that are designed to work as grid-connected and stand alone, but not by just connecting all the outputs from 2 or more inverters together.
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Re: What happens if ?

Postby australsolarier » Sat Jan 07, 2017 7:44 am

it is amazing how many time that question comes up. mind you, i was thinking of that very question too about 13 years ago. i rung up the manufacturers of the grid feed-in inverter. what the engineer told me was something like this: the grid fee-in inverter will pump as much energy as possible into the stand alone inverter and will definitively burn it out. sooner later than later.
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Re: What happens if ?

Postby tytower » Sat Jan 07, 2017 8:20 am

Hmm yeh thanks for that . I am looking for someone who has definitely done it if possible or someone who runs a set up in this configuration . The small inverter converts DC to AC by running a switched DC first one way through one pair of diodes of a diode bridge and then the other way through the other pair of diodes in a diode bridge thus producing AC and the frequency of switching is set to equal 50 cycles per second .

No current can flow in reverse through the diode pair that is being used at any instant so current cannot flow into the small inverter in any way that I can see.
The grid tie inverter is geared to match the a/c frequency and phase that it is being pumped into at all times and shut down if out of whack or no a/c power is available . At least that is my understanding so far .

Thats why I ask the question here. perhaps there is a qualified techie on here who knows for sure.


It is also interesting to me that some grid tie inverters have a "zero export" facility and I wondered if that were set "on" then can current be drawn from any other part of the machine to feed an in-house suppy ?
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Re: What happens if ?

Postby APR » Sat Jan 07, 2017 3:42 pm

tytower wrote:Will it work?


Export limiting is done by the grid tied inverter measuring the current going to the grid, and adjusting it's power output so the export limit is not exceeded. With grid tied inverters, export limiting relies on a connection to the grid to be in place so the inverter can monitor the flow of current, even if the export limit is set to zero.

Typically, when PV grid tied inverters see the AC frequency rise above a specified frequency they see this as an error condition and are designed to shut down. But, some grid tied inverters are designed to ramp down their output as the AC frequency rises.

When a grid tied inverter is pushing energy that has nowhere to go, the output voltage of the inverter will rise.

If you have a battery connected off grid inverter that senses voltage rise on it's output and increases it's frequency with that voltage rise, it will control the output of a grid tied inverter that ramps down it's output with frequency rise. The off grid battery inverter has to have AC output characteristics that will operate the grid tied inverter, and not all of them do.

I would recommend you don't play around with incompatible hardware thinking you can make it work.
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Re: What happens if ?

Postby tytower » Tue Jan 10, 2017 5:28 am

If I approached life like that I would never achieve anything new. Not the way I do things.
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Re: What happens if ?

Postby davidg » Tue Jan 10, 2017 9:19 am

tytower wrote:If I have a grid connect system ready to supply 240V 3Kilowatts to the grid and instead of wiring it into the grid I wire it into the output of an inverter run by batteries and take that output power source off to run my house .

Selectronic SP-pro can do, but with caveats.

SP-Pro can be retro fitted into an existing grid-tie system.

However, it may not be able control excess power of the grid-tie inverter, that is not really a problem as the existing grid-tie system can already export excess power to the grid.

It will ramp to kick any GTI if there is no other choice or control over the output of the inverter is lost. There's more to it, but it would take quite a while to explain and it would turn into a very long post, I'll skip that, and say, but wait there's more .................... :mrgreen:
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Re: What happens if ?

Postby tytower » Thu Jan 12, 2017 5:38 pm

Thanks thats one to look at anyway .

No I will not have the "grid" attached at all . By "grid " I mean the powerco line in.

Mine will be totally in house , my network , no outside connections to anyone or anything.

It will ramp to kick any GTI
I did not understand what you meant by this?
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Re: What happens if ?

Postby davidg » Fri Jan 13, 2017 9:57 pm

tytower wrote:
It will ramp to kick any GTI
I did not understand what you meant by this?

Grid Tie Inverter. Ramp = Frequency Ramp typically to 55.2 Hz which forces the GTI to go offline for at least one minute depending on the programming of the GTI.

However if the Grid inverter is a KACO, ABB PVI, or Fronius Primo (must all be the same brand GTI on any one SP-Pro) then the SP-Pro can with extra control devices mange them/it directly, from 100% to 0% dynamically to match load and charge ability for the batteries, these can all be used in full off-grid systems and I have installed exactly that in various systems. Fronius is new they have only been added recently.
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