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

[Exeltech]

Eric,

I checked my magic spreadsheet for the BP SX170.

Using 800W/m^2 irradiance, and a presumed ambient of 37C (98F),
everything appears to be working within specified tolerances.  The
presumed conditions create exactly your measured Voc, which is
why I used them.  Other temperature and irradiance combinations
will also work...

The voltage delta you measured between Voc and Vmpp is within
BP's specifications.  No PV has a perfect "fill factor".  Fill factor is the
aspect that determines the shape of the V/I curve established when
Voc and Vmpp are plotted.  The BP you mentioned have a fill factor of
0.769, which is very typical of polycrystalline PV.  A perfect fill factor
would be 1.0 (Vmpp equal to Voc), which obviously doesn't happen.
Thus, all PV have an operating voltage at maximum power that's below
the open circuit voltage -- some more than others.


Measured:
375Voc
280V at inverter at mpp
14-15A (fluctuating) at mpp

Calculated:
375Voc
301V mpp
14.96A mpp

Assumed:
800 w/m^2
37C (98F) amb

4,503W array power under above conditions

280V measured at the inverter
14.5A

Presuming 301V is correct Vmpp, this equates to:
 
1.38 ohms total system R, and 290W loss

290W/4503W = 6.44%

6.44% total loss --higher than a 2.5-3% best-practices target, and likely
due to slightly oxidized and/or loose connections in the DC-side of the
system.  Could also be due to long conductor runs, slightly undersized
conductors for the amperage and length, or both.

As Jeff Quackenbush mentioned, Vmpp is slighly lower than one might
expect, but doesn't appear to rise to the level of something that would
yet be indicative of something wrong.


Dan




--- On Sat, 7/21/12, SunHarvest <eric at harvesthesun.com> wrote:

From: SunHarvest <eric at harvesthesun.com>
Subject: [RE-wrenches] Low power production at a winery: DC voltage drop upon SB6000 start-up
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Saturday, July 21, 2012, 3:19 PM



 
 



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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