[RE-wrenches] Actual losses from dissimilar orientations on single inverter

[Darryl Thayer]

Hi all
There are a couple of principals here
1) NEVER NEVER PUT TWO STRINGS OF DIFFERENT ORIENTATION IN SERIES!!
2)The currents are dependent upon irradiance, which varies greatly.
3) The voltage and the Vmp is rather close for varying irradiance, and therefore if forced non coincident orientation in parallel is not so great.  In that the currents will add and both strings will be near MPP.
4) of course the caveats that parallel strings must always be of the same modules and same count.
Darryl 


--- On Fri, 9/12/08, August Goers <august at luminalt.com> wrote:

> From: August Goers <august at luminalt.com>
> Subject: [RE-wrenches] Actual losses from dissimilar orientations on single inverter
> To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
> Date: Friday, September 12, 2008, 9:38 AM
> Wrenches  -
> 
> The correspondence below brings up a question I've been
> pondering... What
> are the actual losses due to differing string orientations
> on a single
> inverter? Are there significant differences between the
> major inverter
> manufacturers? What if we have a very steep roof facing
> East - West, would
> we be better off with two inverters? 
> 
> The well known Fronius white paper
> (http://www.fronius-usa.com/worldwide/usa.solarelectronics/downloads/fronius
> _ig_reaction_to_non_optimal_conditions.pdf) seems to
> indicate that losses
> will be in the ~1% range. I took a SMA class a couple years
> back and they
> indicated that the losses were probably closer to the 3%
> range, maybe even
> more. Maybe things have changed by now, it would be great
> if some
> manufacturer reps would chime in.
> 
> Does anyone have any data or solid info on this issue?
> 
> Looking forward to hearing the latest.
> 
> Best,
> 
> August
> 
> August Goers
> 
> Luminalt Energy Corporation
> O:  415.564.7652
> M:  415.559.1525
> F:   650.244.9167
> www.luminalt.com
> august at luminalt.com
> 
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
> Behalf Of William
> Miller
> Sent: Thursday, September 11, 2008 4:34 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] trouble shooting system with Sun
> Tie
> 
> Dana:
> 
> The failure mode of the original ST inverters were a
> failure in the MPPT 
> algorithm.  The inverter would sweep the impedance too high
> until the 
> voltage would collapse.  The inverter would then take
> several minutes until 
> it allowed the PV voltage to rise again.  This would repeat
> throughout the 
> day resulting in a significant decrease in overall output. 
> You could 
> witness this over any given half hour period.
> 
> Regarding arrays in dissimilar orientation:  The loss due
> to differing 
> orientations should be about 1%, IF all modules in each
> string are facing 
> the same direction.  This is because the different IV
> curves are of a 
> similar shape when superimposed on each other.  The ST
> inverter might be 
> less able to average these curves and more susceptible to
> collapse.
> 
> Did you check the array fuses?
> 
> William Miller
> 
> 
> At 03:23 PM 9/11/2008, you wrote:
> >Hi all,
> >
> >I was called out this week to trouble shoot a system I
> didn't install.
> >It's a five year old 1.44kW system with the
> following: Sun Tie ST 1500
> >inverter and 12 Astro Power modules. It only produced
> 634kWh last
> >year. With our sun here, the shading, orientation,
> equipment, etc. it
> >should have produced around 1123kWh.
> >
> >I've heard a lot about Sun Tie failures, but
> don't know the details.
> >What are the common failure modes? Is it likely that a
> failure of the
> >Sun Tie would leave it operating, but only producing
> half what it
> >should?
> >
> >When I showed up the system was producing 950W on a
> relatively sunny
> >day - seemed pretty reasonable. After restarting the
> inverter it only
> >produced around 150W. Is the MPPT very, very slow to
> find that sweet
> >spot, or what was going on there?
> >
> >The system is comprised of three strings of 4 modules
> each. When I was
> >there all three strings were producing about the same
> current, so that
> >leads me to believe it's not a module failure.
> >
> >To make things more fun, the modules were installed at
> two different
> >orientations. One string is at 38d slope, 30d east of
> true south,
> >while the other two are at 36d slope, 75k east of true
> south. This
> >certainly has a detrimental impact on the performance,
> but it doesn't
> >seem to me that it could account for all of that energy
> loss.
> >
> >Thoughts?
> >
> >Thanks a lot!
> >
> >Dana
> 
> 
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