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 <[email protected]> wrote: From: SunHarvest <[email protected]> Subject: [RE-wrenches] Low power production at a winery: DC voltage drop upon SB6000 start-up To: "RE-wrenches" <[email protected]> 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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