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

[Brian Mehalic]

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/>
>
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