[RE-wrenches] Low power production at a winery: DC voltage drop upon SB6000 start-up

[JRQ]

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 <eric at harvesthesun.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org> 
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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