[RE-wrenches] Imbalanced Legs in 3-Phase systems

[Jeff Clearwater, Village Power Design]

Hi Bill and all,

Thanks for the great input.  That was my sense but it's nice to get 
confirmation!

I imagine that determining imbalanced phases is more a matter of 
analyzing what loads are on what phases over time with a a peak 
current (or low voltage) recorder -  rather than one spot measurement.

Best,

Jeff




>Jeff and Ryan,
>
>The 6kVA value comes from the California Rule 21 that I worked on. It only
>applies to split-phase 240V systems (6kVA on 120V). There is no specific
>limit for imbalance on a 3-phase service.
>
>A basic rule for single phase generation on a 3=phase service is to rotate
>among the phases in a balanced manner. This means the largest imbalance
>possible right now is 7kVA because of the SMA SB7000US. There is a
>single-phase SatCon that is 30kVA, but it would be silly to buy a
>single-phase Satcon when the major on 3-phase units.
>
>The best way to place the imbalanced single-phase generators on the service
>is to test the current on all three phases and put the inverter(s) on the
>most heavily loaded phase(s). This reduces the site imbalance by reducing
>the imbalance on the serving transformer (lowering current on the most
>heavily loaded phases). It is common for 3-phase services to have imbalances
>of 10 kVA or more, so single-phase inverters can reduce the problem rather
>than make it worse.
>
>Make any sense?
>
>Bill.
>
>
>
>-----Original Message-----
>From: re-wrenches-bounces at lists.re-wrenches.org
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Ryan
>Mayfield
>Sent: Friday, August 08, 2008 3:40 PM
>To: 'RE-wrenches'
>Subject: Re: [RE-wrenches] Imbalanced Legs in 3-Phase systems
>
>Jeff,
>
>My memory also recalls something about a 6kW value.
>
>I've talked to one of our local utilities about this as well and they base
>the accepted imbalance by the size of the transformer serving the building.
>I have never read their white paper on the subject though. When I described
>a similar situation for an install we were working on, the engineer
>acknowledged that he would not have any concern with such an arrangement.
>You may approach your utility and see if they have a standard protocol you
>could follow for your system.
>
>Ryan
>
>
>
>-----Original Message-----
>From: re-wrenches-bounces at lists.re-wrenches.org
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jeff
>Clearwater, Village Power Design
>Sent: Thursday, August 07, 2008 10:14 AM
>To: RE-wrenches
>Subject: [RE-wrenches] Imbalanced Legs in 3-Phase systems
>
>Hi Wrenches,
>
>How does one determine how much imbalance can be tolerated on a 3 phase
>system?
>
>I'd like to feed 2 SMA SB5000s at 277 VAC into a 480 VAC panel
>instead of 1 13 KW Solectria at 480VAC as the PV system is only 9500
>Watts STC.  The main on the backfed service panel is 600 A.  But I'm
>not that sure of the loads as it's new construction and hasn't been
>characterized.
>
>What are the appropriate calcs to do here?
>
>Thanks!
>
>Jeff C.
>--
>~~~~~~~~~~~~~
>Jeff Clearwater
>Village Power Design
>Sustainable Energy & Water Solutions for Home & Village
>http://www.villagepower.com
>gosolar at villagepower.com
>NABCEP (tm) Certified Solar PV Installer
>
>Voice: 413-259-3776
>Fax: 413-825-0703
>65 Schoolhouse Rd
>Amherst, MA 01002
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~
>_______________________________________________
>List sponsored by Home Power magazine
>
>RE-wrenches at lists.re-wrenches.org
>
>http://lists.re-wrenches.org/listinfo.cgi/re-wrenches-re-wrenches.org
>
>List rules & etiquette:
>http://www.re-wrenches.org/etiquette.htm
>
>Check out participant bios:
>www.members.re-wrenches.org
>
>No virus found in this incoming message.
>Checked by AVG - http://www.avg.com
>Version: 8.0.138 / Virus Database: 270.5.12/1597 - Release Date: 8/7/2008
>5:54 AM
>
>_______________________________________________
>List sponsored by Home Power magazine
>
>RE-wrenches at lists.re-wrenches.org
>
>http://lists.re-wrenches.org/listinfo.cgi/re-wrenches-re-wrenches.org
>
>List rules & etiquette:
>http://www.re-wrenches.org/etiquette.htm
>
>Check out participant bios:
>www.members.re-wrenches.org
>
>_______________________________________________
>List sponsored by Home Power magazine
>
>RE-wrenches at lists.re-wrenches.org
>
>http://lists.re-wrenches.org/listinfo.cgi/re-wrenches-re-wrenches.org
>
>List rules & etiquette:
>http://www.re-wrenches.org/etiquette.htm
>
>Check out participant bios:
>www.members.re-wrenches.org


-- 
~~~~~~~~~~~~~
Jeff Clearwater
Village Power Design
Sustainable Energy & Water Solutions for Home & Village
http://www.villagepower.com
gosolar at villagepower.com
NABCEP (tm) Certified Solar PV Installer

Voice: 413-259-3776
Fax: 413-825-0703
65 Schoolhouse Rd
Amherst, MA 01002
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~