[RE-wrenches] large(ish) AC coupled battery backup

Fri May 3 14:54:48 EDT 2019

Yes, two radians combined can handle one 12 kW grid tied inverter.

The 6 kW limitation is not a limitation of how much power the radian can actually handle, it's that the grid tied inverters do an impedance/stability test, and some inverters larger than 6 kW will detect that the inverter is not actually the grid, and the GT inverter will shut down before it even fires up. If it weren't for that, the radian would be able to handle more PV in an AC coupled situation.

Is this an all new system? Sometimes, doing a mixture of AC and DC coupling makes the most sense financially if you don't actually need 4 radians worth of backup power. The following setup I'm talking about assumes that you have an external transfer switch, and it's the extreme setup, not necessarily a recommended setup. I just want to demonstrate that you would really only need 2 radians if the required instantaneous backup power needs were low enough.

If you do an AC/DC coupled mix, for each Radian, you can actually handle up to 6 kW AC of GT inverter(s), and it can export 7.2 kW AC from the DC coupled side. With a DC/AC ratio of 1.25, that would be 7.5 kW of AC coupled PV and 9 kW of DC coupled PV, so with the right design and external transfer switch, the radian could potentially handle up to 16.25 of PV per inverter. Now you have to tie the AC coupled PV into the load side of the transfer switch (or in the backed up loads panel). This way, if the grid is available, the AC coupled PV is not connected at all to the battery based inverter, but when the power fails, then the PV gets connected to the radian(s) through the transfer switch. Again, I want to stress that it is imperative that with this kind of system, it has to be tied through an automatic external transfer switch, and that you can't have the power flowing to the grid from both the AC and DC coupled PV simultaneously.

Hope this is somewhat helpful.

Thanks,
Kienan

Maxfield Solar
maxfieldsolar at hotmail.com<mailto:maxfieldsolar at hotmail.com>
(801) 631-5584 (Cell)

________________________________
From: RE-wrenches <re-wrenches-bounces at lists.re-wrenches.org> on behalf of Dave Tedeyan <dtedeyan at taitem.com>
Sent: Friday, May 3, 2019 7:43 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] large(ish) AC coupled battery backup

Thanks Darryl,
Yea, it used to be two separate 200A services. At some point, someone decided to turn one of those panels into a sub and run it all of a single service.
Regarding paralleling Radians, I know that the max solar input when AC coupled is only 6kw for an 8kw inverter. When they are paralleled, does that jump up to a single 12kw input, or are you just limited to two 6kw inputs?

Cheers,
Dave

Dave Tedeyan, PE
Senior Engineer | Taitem Engineering, PC
[https://www.taitem.com/signatures/logo.png]
110 South Albany Street | Ithaca, NY 14850
o. 607.277.1118 x121  f. 607.277.2119
www.taitem.com<http://www.taitem.com>

Solar • Sustainability • Energy • Design
B-Corporation Best for the World 2018 Honoree

On Fri, May 3, 2019 at 9:27 AM Darryl Thayer <darylsolar1 at gmail.com<mailto:darylsolar1 at gmail.com>> wrote:
the 2 each 200 Amp panels is to overcome the 42 circuit limit in one panel.  must be a big house.  This sounds like a whole house backup.  you will need a transfer switch much like a whole house generator switch unless your customer needs TOU or some other grid service.

For the AC coupled paralleled Radians or XW6848 and have a turn off for one or both of the SB inverters based on battery voltage (SOC).

On Fri, May 3, 2019 at 7:58 AM Dave Tedeyan <dtedeyan at taitem.com<mailto:dtedeyan at taitem.com>> wrote:
Hi All,

Does anyone have a good recommendation for an AC coupled energy storage solution for someone who has a 27.6kw array with two SE11400A-US inverters? The output of one of those inverters is more than a Sonnen or Radian can handle (which is what I am most familiar with).
I am not sure yet whether this will be a whole home backup or just a subset of their loads, but they have two 200A panels on a single 200A service.

Thanks,
Dave

Dave Tedeyan, PE
Senior Engineer | Taitem Engineering, PC
[https://www.taitem.com/signatures/logo.png]
110 South Albany Street | Ithaca, NY 14850
o. 607.277.1118 x121  f. 607.277.2119
www.taitem.com<http://www.taitem.com>

Solar • Sustainability • Energy • Design
B-Corporation Best for the World 2018 Honoree
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