outback-generator-on grid [RE-wrenches]

[Eric Youngren]

Somebody correct me if I am wrong but it seems to me that the underlying
problem here is that we have good battery based (voltage source) inverters
and good grid-tied (current source) inverters but it is difficult to do both
jobs effectively with a single unit.  Yes the GTFX is capable of it but the
problem with generators, limit of 2 in a stack and the lower conversion
efficiency for grid-tie makes it a problematic choice.

I am designing a good sized GT w/batt system right now and I am leaning
towards an AC coupling approach using two different inverter types to
accomplish the different goals (grid-tie and battery back-up).  I have not
done this yet, however, so I am hoping to throw it out for discussion and
advice from those that have been there.

My thinking right now is to use a quad stack of FX3048 on a ~1600 AH
industrial battery and approx 8 kW of PV on two Sunny Boy 3800s.   The Sunny
Boys will be wired into backfed breakers in a 200A panel that will be fed by
the Outback stack.   During normal operation the Sunny Boys will synch with
the grid and pass current through the Outback stack and out through the
utility meter.  When the grid fails the Outbacks will take the load and use
any surplus incoming AC from the Sunny Boys to charge the batteries.  The
existing 15kW diesel standby generator will be there on its currently
installed auto transfer switch to feed the main panel and the chargers in
the Outbacks when battery voltage drops. I know that I'll need a diversion
controller and dump load to prevent overcharging by the unregulated charging
from the Sunny Boys.  Is there anything else I should be thinking of?

It seems to me that this could be a relatively elegant, efficient and robust
way to accomplish multiple goals. (The Sunny Island is another option that
may be a bit more elegant, but I do like those Outback inverters.) There is
also a long distance wire run between the array location and the battery
room which makes this arrangement appealing.  

Am I correct in my thinking that it will be more effective and efficient to
use a grid-tied inverter for selling PV power to the grid and a battery
based inverter to back up loads and keep the batteries charged? Will they
play together well?  Or can both of these jobs be done effectively with a
single machine, like the GTFX?  Or is the GTFX a product that the market
wants but basic inverter topology makes difficult to accomplish?  Or is it
the UL listing required tolerances for grid-backfeed that make it difficult?

Thanks,

Eric Youngren
Rainshadow Solar
Orcas, WA 


-----Original Message-----
From: Todd Cory, Mt. Shasta Energy Services
[mailto:toddcory at finestplanet.com] 
Sent: Thursday, April 05, 2007 5:58 AM
To: RE-wrenches at topica.com
Subject: Re: outback-generator-on grid [RE-wrenches]


IF you get the Outback to accept the power from an inverter generator, 
you will still need an external transfer switch to change the AC input 
to the inverter from grid to gen and someone (the customer) to manually 
(using the mate) change the AC input from grid to gen.

An AC battery charger plugged into the generator will not produce much 
current due to the lower peak to peak power output of most traditional 
generators. There is also the possibility of selling the generator 
output (via battery charger) to the grid if you continue to leave it 
running after the grid has been restored.

To my knowledge, there is no elegant way out of this as of yet.

Todd

Brad Bassett wrote:

>If one of the inverter generators doesn't do it for you, it's easiest to 
>just use a separate AC battery charger plugged into the generator (or 
>two of them if they are 120v and the gen is 240v). A lot easier than 
>most DC generators, except maybe one of those small auto generator 
>setups. 
>
>Brad Bassett
>
>
>
>Todd Cory wrote:
>  
>
>
>  
>
>>I think the DC genny kludge is unfortunately at present the only real 
>>usable option. 
>>    
>>


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