Eric,
This doesn't sound so bad. The Voc seems a little low (10 modules in series at
44.2 Voc would be 442 Voc), but you haven't provided irradiance or temperature
information, so that's hard to assess. As far as the drop from Voc to Vmp,
consider:
35.4 Vmp / 44.2 Voc = 80%
280 Vmp / 375 Voc = 75%
Given that the array could be 8 years old, and that you haven't provided
temperature or irradiance information for when you took these measurements, a
5% variance from the label at STC seems pretty reasonable. The modules
themselves may have a +/–5% power rating.
Also, if you're getting 14-15A (DC) when the rated Imp for four strings is
19.2A -- that would be a good amount of production during the middle of the day
in most conditions. How often are you getting 1000W/m2 at 25˚C?
If you're still concerned, you could verify the voltage drop on the DC wiring
and look for hot spots or maybe a loose connection in the array wiring.
Jeffrey Quackenbush.
________________________________
From: SunHarvest <[email protected]>
To: RE-wrenches <[email protected]>
Sent: Saturday, July 21, 2012 4:19 PM
Subject: [RE-wrenches] Low power production at a winery: DC voltage drop upon
SB6000 start-up
Hello Wrenches,
I am helping a client troubleshoot a
complaint about underproduction of his on-grid system installed about 6-8 years
ago. At first I thought I might encounter the bad Kyocera KC120 issue but
it turns out they have BP170's, 40 of them for an STC power rating of 6.8kW.
Ratings of the BP modules: PTC=150.7, Vmp=35.4,
Voc=44.2, Imp=4.8, Isc=5
The system has four panels of ten modules wired in
series for a nominal rating of about 350v, 4.8a per string.
At the DC disconnect (first accessible combiner) I
measured: All strings right at about 375v(oc), 4.7a(mp). After the strings
are combined, at the DC input of the SB6000 (with the AC power to the inverter
OFF) I measured about 375V as expected. Once the AC
power is connected, the SB6000 starts up, and MPP operation is
initiated, the DC voltage drops to about 280V, and amps sit between 14
& 15A. I didn't think the voltage was supposed to drop upon MPP
tracking...especially this much. This voltage drop would account for the
observed power loss between actual and rated production values. I'm going to
check with SMA but I wanted to ask the experts here too, as someone may advise
something like, "Oh yeah, BP modules have a similar defect as the
Kyocera..."
My questions:
Is the DC voltage supposed to drop significantly in
MPP mode on these inverters?
If not, does this indicate a bad
inverter?
Anyone know of problems with BP modules
manufactured around 2006-2008?
The Kyocera modules showed good volts and amps
until a load was connected. Seems like a similar issue here.
Eric Stikes
SunHarvest Solar
A Sustainable
Energy Group Partner
+1 (530) 798 - 3738
www.harvesthesun.com
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