#1 June 26, 2014 13:51:00

wilding2004
Registered: 2013-10-25
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performance drop


Hi all,


I'm a little confused about a recent performance drop in my system, and I'm hoping someone can help me interpret the readings off the inverter.


I have 15 x 250w panels for a 3.75kW system. They are arranged into 2 strings, 1 of 8 panels and 1 of 7 panels fed into a single Afore HNS3600TL inverter.


The system was installed mid August 2013 and I regularly saw output at 3.4-3.6kW. We had some really sunny days towards the end of March and again I saw output around the 3.5kW mark.


As the weather continued to improve I started to look a bit closer, hoping to see brief periods of output nudging the system limits, but what I've actually seen is a decrease. The weather in June has been amazing - but I've yet to see any output above 3.1kW and more generally at about 2.8kW.


I went into the loft to confirm the readings directly from the inverter, and saw that one of the strings appears to be at a much lower voltage than the other. It generally has a value of Vpv(a)~260, and Vpv(b)~170


Is this lower voltage on the 2nd string the course of the lower output? And what could be causing it? There is no shading on any of the panels, and I've checked for pesky seagull droppings.


Any advice much appreciated.


Dave











Edited wilding2004 (June 26, 2014 13:51:00)

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#2 June 27, 2014 06:32:00

Dave Stevens
Registered: 2014-05-24
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performance drop


Hi Dave,


I'm not the most knowledgeable around here, but in the absence of other replies, here's my thought:


1. This is why it is good to monitor outputs, so these things are detected early.


2. My first thought was inverter overheating as its in the loft during a hot summer spell, but I then discounted that as you have 2 strings and the other one seems to be performing ok.


3. Have you checked the panel specs for temperature range performance? The panels on the string underperforming could be getting hot - a bit surprising in the UK, but we have had a nice streak of hot weather. It probably isn't this given the size of the drop, but it is worth checking. Different panel makes will have different temperature performance characteristics, so it might be worth letting us know your make, and is it the 7 or 8 panel string having the issue?


4. It could be an issue with a single panel, rather than the entire string. All the panels in a string are wired in parallel, which means a drop in one, forces a drop in all. Do you have a maintenance contract with your installer that allows you to get the panels checked?


Hope something I've said helps and that you get back to peak performance soon!


good luck!


Dave. URN 6737, 3.27kwp, 174deg facing, 30deg slope, no shade, installed 15/05/2014





Edited Dave Stevens (June 27, 2014 06:32:00)

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#3 June 27, 2014 11:31:00

wilding2004
Registered: 2013-10-25
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performance drop


Hi Dave,


Thanks for replying.


I don't know which string is underperforming, they aren't labelled on the inverter which is which. At least not that I can tell.


As to the temperature, I thought something along those lines - but all the panels are the exact same spec, and they are all on the same side of the roof. One string shouldn't be affected more than the other, I wouldn't have thought.


link to specs: The panels I've got are the ZDNY-250P60


http://www.solar-sunny.com/download/20140226ustesi.pdf


We've had a shed load of rain over night and this morning, so I'll see what happens between 1 and 2pm. Maybe the panels were dirty and I couldn't see it!





Cheers





Edited wilding2004 (June 27, 2014 11:31:00)

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#4 June 30, 2014 12:18:00

SteveRogers
Registered: 2011-12-08
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performance drop


Hi Dave,


Typical panels should produce around 30 to 35V DC each.  So your strings (8 and 7 panels) should produce 240-280 and 210-245 V.  These values are for unshaded strings.  Shading will make a big difference, even if it's just part of one panel.


Assuming no shading, it looks like one of your strings has a problem.  it could be damage to a panel or two or to a cable or maybe a loose connection.


Higher temperatures in the Summer do reduce the peak output - see the discussion in the General section (on “minus temperatures” but the calculation applies to high temps too) - http://www.microgen-database.org.uk/forums/general-discussion/minus-temps_-what-happens-to-your-power?pn=2 . For my panels, an increase of 10 deg C reduces the output wattage by about 5%.  In fact, panels can be a lot warmer than ambient, partly because being dark they absorb infrared and partly because they are inefficient and a lot of solar energy is converted to heat instead of electricity.  So 10-15% reduction might be found on warm days with no wind cooling.


You should get your installer back to check your panels and cables - probably on the 7 panel string.


Steve


URN 215 North Yorkshire 3.96 kWp, S facing, 40 deg tilt, minor shading





Edited SteveRogers (June 30, 2014 12:18:00)

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