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Timbergetter

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Joined: 08/10/2018
Location: Australia
Posts: 55

Posted: 11:37pm 04 Sep 2019

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I understand that grid connected solar inverters must detect when the grid goes down and then immediately shut the inverter itself off. This is necessary to avoid injury to linesmen working on the grid during outages. I have heard that the inverter does this by continuously sensing the presence of a 50 Hz voltage on its output to stay alive. Would it therefore be technically possible to devise a black-box which outputs a high impedance 50 Hz signal to enable the on-mode of the inverter? If this were possible and if it were combined with a fail-safe slow response changeover switch, then one may be able to safely power a house with solar during an extended mains outage. Of course it would be prudent to manage the power consumption during the event such that it was well below the expected instantaneous generated power.

I did ask a related question in another thread but I cannot now track down that thread, so apologies if I seem to be repeating myself.

Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 1578

Posted: 12:55am 05 Sep 2019

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There are GTI inverters now that will run from solar or with the assistance of batteries. The batteries can be fairly low voltage and small capacity ( like a couple of car batteries) and it is better to go this way so there is some reserve for passing clouds, birds sitting on panels etc.

As far as the black box ideea, You would have to feed 230V to the thing as well as the frequency.

You can use a generator with the panels. The trick is to make sure the load is greater than the solar output. The GTI will try to push back all it can and if it's got excess power it will try to backfeed the generator and thats where the problems start and quickly end. The genny otoh will happily supply only what is required. If you have say, a 2Kw load and 1.8 Coming from the inverter, The GTI will max out and the genny will back off to only supply the required 200W to get the voltage up where it wants it.

I believe the same can be done with an inverter. Use that to supply the Voltage and frequency but the GTI must be effectively lower than the GTI's output and the load to be driven. As long as there is somewhere for all the GTI power to go, not a problem. You cannot have the GTI trying to dump it's power into the inverter..... although I recently watched something that suggested if there were batteries an inverter in some circumstances could back feed and charge the batteries.

I would suggest playing on the safe side as unless you had the knowledge to understand exactly what was going o9n in the inverter you'd be taking a BIG risk.

I'm not sure it's possible to have a regular GTI running on it's own supplying power but having it supply the bulk of the power along with another generation source certainly is doable.

Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 12:58am 05 Sep 2019

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Timbergetter said I understand that grid connected solar inverters must detect when the grid goes down and then immediately shut the inverter itself off. �This is necessary to avoid injury to linesmen working on the grid during outages. �I have heard that the inverter does this by continuously sensing the presence of a 50 Hz voltage on its output to stay alive.

That is partially correct, in that the inverter uses the grid voltage to initially synchronize itself in both phase and frequency.
But once hard connection to the grid has occurred, it should be realized that the inverter would then only be monitoring its own output voltage, and that would not indicate if the inverter was operating into the grid, or into a complete open circuit.

So, voltage measurement is only useful during the initial synchronization and lock up stage.

Quote
�Would it therefore be technically possible to devise a black-box which outputs a high impedance 50 Hz signal to enable the on-mode of the inverter? �

Yes that would enable the initial on mode, but as soon as the inverter tried to transfer any power it would shut down.

The grid tie inverter "must" be connected to a solid power source, either to the grid or to another inverter which has a power rating significantly higher than the grid tie that is attempting to connect.

Attempting to connect to a high impedance source or a much lower powered inverter will cause the grid tie to see an open circuit, and it will shut down.
It will work fine with a high powered inverter under load, or if there is a battery to back feed into.

Why this is so, depends on the measured current coming out of the grid tie inverter back to the source. If the grid tie tries to feed current back into an open circuit, the expected load will not be there and it will shut down.

Another scenario is the grid blacks out, and the grid tie tries to power every other house in the street, which it obviously cannot do. So again it shuts down from over current.

Its all a bit tricky, but basically if the grid shuts down, a normal grid tie system will also shut down, unless you can completely disconnect from the grid and run a complete off grid system, and use the grid tie to supplement that.

Cheers, �Tony.

Timbergetter

Regular Member

Joined: 08/10/2018
Location: Australia
Posts: 55

Posted: 03:45am 05 Sep 2019

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Many thanks Davo99 & Tony for your very comprehensive consideration of this issue. It looks like I'll continue to drag the Genny across the yard to the house on these occasions. I should really build a weatherproof hutch for the Genny near the house.

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