[RE-wrenches] 3 Phase Answers

[David Brearley]

Jeremy,

You might find the information on electrical service types in John Berdner¹s
article useful:

http://solarprofessional.com/article/?file=SP1_1_pg32_Berdner&search=

Also, here¹s the link to the 3-phase line currents article that Rebekah Hren
mentioned:

http://solarprofessional.com/article/?file=SP3_1_pg66_Hamon&search=

This sounds to me like an issue with an inexperienced utility that doesn¹t
know what to require or expect. It is not at all uncommon for first time
projects to elicit requests of this nature. For example, I¹ve worked on
small PV pilot project designs‹like solar for schools projects‹with
registered professional electrical engineers who want to specify a 3 or 4kW
3-phase inverter. Because the site has a 3-phase service, right?

It makes perfect sense from their point of view. But that inverter product
isn¹t out there & doesn¹t need to be. There are single-phase appliances on
3-phase electrical systems, after all.

A small grid-tied PV system is basically a dedicated appliance circuit, only
it exports current. It is a load reduction appliance with anti-islanding
protection...and maybe even backup power capabilities. While a battery
backup system might raise more flags with the utility, the system may still
be small enough to interconnect at 120Vac without creating or exacerbating
any existing phase imbalance issues. Typically the phase imbalance limit is
set at 6kW by the utility. If you are backing up 3-phase loads, well then
you need 3-phase power from the system.

BTW: Did you know that there are three power stages in those 3-phase Fronius
IG Plus inverters? Sounds an awful lot like stacking single-phase inverters
onto a 3-phase service, doesn¹t it? I suspect that these units still output
3-phase power even with only one power stage operating, which the units will
do to reduce tare losses. But the product platform is primarily single-phase
and only the 3-stage units are available with 3-phase output. The smaller
units don¹t need to have a 3-phase output. You can configure them granularly
as needed to avoid phase imbalance.

Of course, the AHJ can set their rules however they see fit, but you can
also educate them about national industry standards with line diagrams and
case studies or by bringing in a professional engineer with PV systems
expertise. You shouldn¹t have to use a 11.4kW 3-phase inverter just to
interconnect a small array at a facility with 120/208 service. Similarly,
you shouldn¹t have to scrap plans for a utility-interactive battery backup
system just because none of the available inverters offer 3-phase power in a
single box. They don¹t have to. If the system size is small enough, you¹re
not creating any meaningful phase imbalance. If the system is larger, you
can stack inverters to achieve a 3-phase output from the inverter system.

If this is a real project, you might want to start monitoring the line
currents at the site. Show the utility the existing load profile. It¹s
probably not nearly as simple and balanced as they think it is. If one phase
always (or usually) has a higher current than the others, you may be able to
use your load reduction appliance to improve an existing phase imbalance.

Hope this helps.

Best regards,

David Brearley, Senior Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer 
david.brearley at solarprofessional.com


On 2/16/10 10:49 AM, "All Solar Inc. Jeremy Rodriguez"
<allsolar at scswifi.net> wrote:

> Thanks to all for the help.  Its going to have to be an IG Plus. 208V Delta:
> 120V WYE config. 
> Wait til June for it...................
>  
> Jeremy
>  
>  
>  <mailto:allsolar at live.com>
> 
> 
> _______________________________________________
> List sponsored by Home Power magazine
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> List-Archive: 
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
> 
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
> 
> Check out participant bios:
> www.members.re-wrenches.org
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20100216/7e0693ff/attachment-0004.html>