Help with converting off grid system to grid tie with backup.

[Rmanka88]

Hello all.

I will try to keep this as concise as possible.
I have been asked to help convert an off grid system to grid tie with battery backup.
Homeowner acquired a pair of DC solar trailers.
Each consisted of some panels, a Kubota genset and a pair of sunny island 6048's.

He paid an electrician to convert this to a grid tie system, buy the install was never completed.
Electrician installed panels and mounted both sunny islands to a wall on the detached garage. Batteries are wired to the islands already.
He then installed a sunnyboy sb7.01-41 inverter and tied it to grid with a 50a backfeed to the garage sub-panel.
This sunny islands are not connected to the house wiring yet.
My plan is to rs485 all the inverters together and configure the for grid-tie with storage.
Where I need help, is in regards to an automatic transfer switch that is smart.

The power meter is a good 75 feet from where the garage is located.

What I would like to do, is wire in a smart switch, such that that it just disconnects the grid from the main panel, and backfeed the sunny islands from the sub panel.
As this is a small house, with an expected power draw of around 8kw, I should be able to run the entire house from the pair of islands.

A smart transfer switch is needed, otherwise when the grid comes back, it'll cook the battery inverters.
All the smart switchs I've so far found, appear to only work with gensets, and lack a timer feature.

Could someone point me in the right direction, in regards to a proper switch?

Sorry for the long post, and thanks for your help.

Rich

 

[MPRanger]

I suspect the first thing to do is acquire the power company okay and verify they have upgraded the meter to a smart meter that allows feeding the grid. Otherwise, what is fed to the grid will be read as consumption and the power bill will be all consumption plus what you fed to the grid, which will look like consumption. Basically, the highest power bills they have ever seen.
Here is a link that explains the options at Eaton. https://www.eaton.com/us/en-us/prod...s/automatic-transfer-switch-fundamentals.html

 

[Rmanka88]

Already has a net meter installed.

I've found a transfer switch, and it is on order.

All I have to do now, is see how the timing parameters work on the islands in regards to grid/ backup changeover.

The autimatic transfer switch has a 15 second timer between states.

I'd like to know if that's enough time to shed the genset load, and close out the sunny island grid/gen relay.

My concern, is that the transfer switch kicks back to the grid setting, while the generator is still running. That would be a total disaster.

 

[MPRanger]

Yes, I have used a Generac ATS that switches the generator off after it switches back to the main, in such a fashion that the power from the generator gets stopped at the switch.

 

[Rmanka88]

Yes. But the genset will be wired directly to the sunny island, as it is in the same building as the inverters. The main panel is about 100ft away.

Seems redundant to run two transfer switches.

If need be, I'll run a high power contactor between the generator and inverters, but I'd rather not.

 

[MPRanger]

Redundant...yes that describes it. If the first ATS is not smart enough to change by program, you will need an additional switch, unfortunately.