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

[SunHarvest]

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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