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

marco marco at pvthawaii.com
Sat Aug 9 12:47:10 PDT 2008


 
Amigo Bill,

What I understand from your words below: under most circumstances, there
should not be a problem with having, say, a single 7 kW inverter feeding
power (two hot conductors for 120/208 or one hot conductor for 277/480) into
a three-phase service.  Or, for that matter, two such inverters feeding into
a three-phase service.  Correct?  To play this out one step further,
assuming that the Fronius 12 kW inverter has a two hot conductor ouput and
is three-phase compatible, would that be pushing it since the possible
imbalance could be more than 10 kVA?

Marco


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
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Amherst, MA 01002
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