<table cellspacing="0" cellpadding="0" border="0" ><tr><td valign="top" style="font: inherit;">Eric,<br><br>I checked my magic spreadsheet for the BP SX170.<br><br>Using 800W/m^2 irradiance, and a presumed ambient of 37C (98F),<br>everything appears to be working within specified tolerances. The<br>presumed conditions create exactly your measured Voc, which is<br>why I used them. Other temperature and irradiance combinations<br>will also work...<br><br>The voltage delta you measured between Voc and Vmpp is within<br>BP's specifications. No PV has a perfect "fill factor". Fill factor is the<br>aspect that determines the shape of the V/I curve established when<br>Voc and Vmpp are plotted. The BP you mentioned have a fill factor of<br>0.769, which is very typical of polycrystalline PV. A perfect fill factor<br>would be 1.0 (Vmpp equal to Voc), which obviously doesn't happen.<br>Thus, all PV have an operating voltage at
maximum power that's below<br>the open circuit voltage -- some more than others.<br><br><br>Measured:<br>375Voc<br>280V at inverter at mpp<br>14-15A (fluctuating) at mpp<br><br>Calculated:<br>375Voc<br>301V mpp<br>14.96A mpp<br><br>Assumed:<br>800 w/m^2<br>37C (98F) amb<br><br>4,503W array power under above conditions<br><br>280V measured at the inverter<br>14.5A<br><br>Presuming 301V is correct Vmpp, this equates to:<br> <br>1.38 ohms total system R, and 290W loss<br><br>290W/4503W = 6.44%<br><br>6.44% total loss --higher than a 2.5-3% best-practices target, and likely<br>due to slightly oxidized and/or loose connections in the DC-side of the<br>system. Could also be due to long conductor runs, slightly undersized<br>conductors for the amperage and length, or both.<br><br>As Jeff Quackenbush mentioned, Vmpp is slighly lower than one might<br>expect, but doesn't appear to rise to the level of something that would<br>yet be indicative of
something wrong.<br><br><br>Dan<br><br><br><br><br>--- On <b>Sat, 7/21/12, SunHarvest <i><eric@harvesthesun.com></i></b> wrote:<br><blockquote style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px;"><br>From: SunHarvest <eric@harvesthesun.com><br>Subject: [RE-wrenches] Low power production at a winery: DC voltage drop upon SB6000 start-up<br>To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org><br>Date: Saturday, July 21, 2012, 3:19 PM<br><br><div id="yiv1919509689">
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<div><font face="Arial" size="2">Hello Wrenches,</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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.
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<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">Ratings of the BP modules: PTC=150.7, Vmp=35.4,
Voc=44.2, Imp=4.8, Isc=5</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">The system has four panels of ten modules wired in
series for a nominal rating of about 350v, 4.8a per string.</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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..."</font></div>
<div><font face="Arial" size="2"></font><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">My questions:</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">Is the DC voltage supposed to drop significantly in
MPP mode on these inverters?</font></div>
<div><font face="Arial" size="2">If not, does this indicate a bad
inverter?</font></div>
<div><font face="Arial" size="2"><br>Anyone know of problems with BP modules
manufactured around 2006-2008?</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">The Kyocera modules showed good volts and amps
until a load was connected. Seems like a similar issue here.</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">Eric Stikes<br>SunHarvest Solar<br>A Sustainable
Energy Group Partner<br>+1 (530) 798 - 3738<br><a rel="nofollow" target="_blank" href="http://www.harvesthesun.com">www.harvesthesun.com</a></font></div></div>
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