[RE-wrenches] Add-On to DGT, string inverter, w/Different Modules on 2nd string

Tue Jun 21 13:35:05 PDT 2016

Ash:

With the climate in your region, 14 Solar World SW 280 modules in series
will generate over 600 VDC.  14 SW280s would produce 3.92 kW STC.  14 280s
cannot be added to this system.

As a designer, you have to balance every aspect of a proposed project,
including electrical, mechanical, financial, code compliance, AHJ approval,
etc. etc.  Time marches on and products change.  You may get lucky and find
a source for modules to fill out the capacity of this inverter or you may
not.  That is part of the challenge.

William

[image: Gradient Cap_mini]
Lic 773985
millersolar.com <http://www.millersolar.com/>
805-438-5600

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *Brian Mehalic
*Sent:* Tuesday, June 21, 2016 1:00 PM
*To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
*Subject:* Re: [RE-wrenches] Add-On to DGT, string inverter, w/Different
Modules on 2nd string

An inverter with a single MPPT can only operate at a single DC voltage at
any one time; with two different PV source circuits of different voltages,
then the inverter will be unable to operate at the MPPT of the combined
array (the operating voltage may be the actual Vmp of one string, but
probably not both, and maybe not either!).

Try to match the voltages if possible - I believe the 175s are 72 cell
modules (i.e. 864 cells in series for the 12 module source circuit) and the
280s are 60 cell, so 14 or 15 of them in series would best match the
voltage of the old mods (maybe 14 assuming there has been some voltage
degradation of the older mods).

Brian Mehalic
NABCEP Certified Solar PV Installation Professional™ R031508-59

Solar Energy International
http://www.solarenergy.org

SEI Professional Services

http://www.seisolarpros.com

On Tue, Jun 21, 2016 at 1:47 PM, Ash Bowersock <Ash.Bowersock at rgsenergy.com>
wrote:

Good day RE Wrench crew,

I’ve got a client whom installed a 2kW array with a 3kW SunnyBoy inverter a
number of years back. Due to the age of the system I assume it is single
MPPT. The client oversized his inverter in the hopes of using it for this
future expansion (and unfortunately his installer went along with it). The
original modules were 12 SolarWorld 175’s I believe, and we would be
looking at adding SolarWorld 280’s more than likely. To me it is clear that
this is poor design practice and not something I am willing to do. My
recommendation is a micro-inverter system that we tie in w/his existing
using an AC Combiner. But the client wants to know exactly why we don’t
want to just add a second string of 12 SolarWorld 280’s to his 6 year old
SunnyBoy inverter.

Can someone please give me a technical reason why mixed modules on a string
inverter is “poor design practice”?

The more technical and dry an explanation, the better ;)

If you can’t convince them, bore them to tears with the details.

Ash Bowersock
Senior System Designer

[image: RGS Energy] <http://www.rgsenergy.com/>

833 W. South Boulder Road | Louisville, CO 80027
tel 303.222.8387

*RGSEnergy.com* <http://www.rgsenergy.com/>

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