[RE-wrenches] Three-phase off-grid

[William Miller]

Friends:

I posted a question here about three weeks ago on the subject of connecting 
a dual SW inverter stack to a three phase generator.  I have learned a lot 
since then.  Here are the most important points:

Most large, three phase generators are allegedly field convertible to be 
either 208 or 240 three phase.  The 208 version is probably wye configured 
and the phase angle between any two phases is 120 degrees.  If you connect 
a 120/240 inverter system (even numbers of Outbacks, two SW's or XWs 
configured single phase, and maybe other models) across two of the phases, 
the 120 degree phasing will cause the inverters to puke.  If you purchase 
and connect the correct transformer across two of these phases and you will 
provide the 180 degree phase angle you need but you will imbalance the 
generator.  For a large (30KVA or larger) generator, the 10 KVA of 
imbalance is not a big issue.  Your transformer will cost about $1,000 
delivered and will weigh about 200 pounds.

You can swap some connectors and have a 240 configuration which will 
probably have a "stinger" leg.  If it does, the windings are probably a 
delta with one winding center tapped.  If so, the phase angle between the 
two phases on either side of the neutral is 180 degrees.  You can connect 
your Outbacks (or others) across these two phases and neutral without a 
transformer.  You'e saved your client $1,000 and yourself a hernia.

If the customer can live with a stinger then you have your cake and eat it 
too:  Three phase for 240 VAC loads and a 120/240 single phase inverter 
system for 120 or 240 VAC loads.

This realization was huge to me.  Think about it...

William Miller

PS:  Just replacing a 3 phase panel with a single phase panel does not give 
you 120/240 with 180 degree phase angle.

Wm



At 08:23 AM 10/21/2011, you wrote:
>Do they actually have 208v loads? If not, replace with 120/240 panel. Just 
>a thought... Although a transformer would be required to use an existing 
>208v generator.
>
>Jason Szumlanski
>
>On Oct 21, 2011, at 11:17 AM, "Allan Sindelar" 
><<mailto:allan at positiveenergysolar.com>allan at positiveenergysolar.com> wrote:
>
>>Wrenches,
>>We have been asked for a design for an off-grid system to reduce 
>>generator runtime for a mountain resort. Initial projections (consumption 
>>measurement results not yet available) suggest 15-20 kW of PV and around 
>>30 kW of inverter capacity. However, the generators and existing wiring 
>>are three-phase 120/208.
>>
>>My assumption is that our only options are a set of six Sunny Islands, or 
>>9-12 Outback GVFX3648s. Are there any other options or approaches that 
>>I'm missing?
>>
>>Thank you, Allan
>>--
>>A
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