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

[Bill Brooks]

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
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