[RE-wrenches] Three-phase off-grid

William Miller william at millersolar.com
Sun Oct 23 21:30:38 PDT 2011


Dan:

We are not desiring 120 degrees.  We are desiring 180 degrees, and it can 
be derived, and without a Scott-T.  I have done this recently.

William Miller


At 08:26 PM 10/23/2011, you wrote:
>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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Miller Solar
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