[RE-wrenches] dual stacked inverters on 208V service

[Phil Undercuffler]

Which inverters are you using?   Do they share the same protected load
panel?  I'm not aware of any GT capable battery based inverters which can
sync and sell across only two phases of 208, especially when they're
intended for 240v output per your post.  Can you give us a little more
detail about what you have for equipment, and how you've got it set up?

If your facility truly has 120/208 with only two of the three phases
present, then your options are somewhat limited.

One option is to use a step-up transformer to take one leg of 120 up to 240,
then connect your inverters across that.  This will give you the 120/240
signal that your inverters are looking for.  However the transformer needs
to be sized for the full sell back current, you may have issues keeping the
OCP required for that one leg within the 690.64(B)(2) 120% limit of the
upstream loadcenter, and your utility might not allow the imbalance across
the phases.

Alternately, if this is a charge controller + battery system (ie, Outback)
you can split the system into two separate entities, each with their own PV
array, controller, battery bank, and protected load center.  If your
existing battery bank has two strings and all the protected loads are 120v
only with no shared neutrals, this might be the easiest solution.

If you absolutely positively must share one battery bank with two 120v
inverters that each must sell back on their own phase, then you'll need to
wire each to it's own separate protected load center, unstack them (so they
each think they're their own master) and disable all grid charging --
otherwise, you'll have cases where one inverter tries to charge the
batteries while the other tries to sell that power back.  Each inverter will
need it's own temperature sensor, if these are Outbacks.


Phil Undercuffler
Conergy





On Thu, Dec 31, 2009 at 2:15 PM, Nik Ponzio <nponzio at buildingenergyus.com>wrote:

> Hi Gang,
>
> I have a problem. We are installing a gridtie PV system with 2
> battery-based inverters for 240V. The service coming in to the condo is
> 120/208 split-phase fed from 3-phase 208 in main building. The master
> inverter pickups up L1 but the slave inverter will not sync. What am I
> missing?
>
> Thanks and Happy New Year to all!
>
>
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