[RE-wrenches] Three-phase off-grid

[Exeltech]

Mac,

You can't get a 120 degree phase shift from a transformer with windings on a common core.  Two transformers can be combined in a "Scott-T" configuration" to synthesize a 120 degree shift, but special transformers are required.  Also, a Scott-T configuration must be used with closely balanced loads between phases.

There's an explanation of how a Scott-T topology works at this link:

http://en.wikipedia.org/wiki/Scott-T_transformer



Dan



--- On Sun, 10/23/11, mac Lewis <maclewis1 at gmail.com> wrote:

From: mac Lewis <maclewis1 at gmail.com>
Subject: Re: [RE-wrenches] Three-phase off-grid
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Sunday, October 23, 2011, 9:02 PM

Wrenches,
Is it a "special" transformer that will give you the 180 deg phase shift from the original 120 deg shift?
Thanks
On Fri, Oct 21, 2011 at 5:52 PM, William Miller <william at millersolar.com> wrote:




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"
<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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Mac Lewis



"Yo solo sé que no sé nada." -Sócrates





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