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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