How Are My Panels Connected?

a0128958a0128958 Solar Expert Posts: 316 ✭✭✭

I hope this is the right forum to post this.  My question is for grid-tie, batteryless systems that already exist, that's of a troubleshooting nature.

A few months ago my roof was 'totaled' by a devastating hail storm.  The storm was a pretty news worthy event, particularly in the Dallas area.  My PV system survived just fine.  It was a good lesson on why you want to install, or have installed, class A UL listed panels, and done so properly on a roof using a racking system.

My PV system is the one that's illustrated in the 'sticky' posting at the top of the Beginners Forum here.  Here's a picture of the system:  http://www.welserver.com/WEL0043/ .

It's worked fine over the years, within the design limitations noted in the postings, with zero maintenance ever needed.  It's performance remains unchanaged and was a good lesson on grid-tie batteryless solar PV design tradeoffs.  Last year it averaged about 67% efficiency, about half way between PV Watts 77% default and utility company minimum performance expectation of 62%.  See http://www.welserver.com/perl/plot/WEL0043/SolarPVefficiency.png .

Fast forward to now.  Looking at http://www.welserver.com/perl/plot/WEL0043/DailySolarEnergyProduction2.png for a picture of the daily kWh for the past 13 months, we see where the PV system was taken down for the roof to be replaced.  It's between the -4 and -5 marks on the X-axis (starting 5 months ago, 'restored' 4 months ago).

The system is a 3 string design, with 12 panels each per string.

Included in the re-roofing job was outsourcing of the removal of the whole system, and then later re-installation.  Since there are no 'as built' documents supplied by the installer when installed in 2009, the roofer was instructed to include effort to mark, document and photograph everything such that each panel would be put exactly where it was, and ground and string connections would be connected just as before removal.

You know what's coming.  The roofer's subcontracted electrician didn't do any the expected 'as built' documentation.  As a result, the system is virtually non-operating.

At the moment:

String 1 = 551 VDC OC - it's disconnected because it's close to the 600 DC maximum system voltage.

String 2 = 294 VDC OC - when connected as the only circuit, in the morning it's cool enough such that it gets above the Sunny Boy 7000W inverter's minimum 300 VDC starting voltage and stays running all day.

String 3 = 271 VDC OC - when connected as the only circuit, it doesn't get high enough to trigger start up of the inverter.

Total panels = 35 (one's removed to fix wire damage).

Here's my key question:  is it possible to determine how many panels, and which ones, are connected to each string?

Here's an image of each panel's specs:  http://www.pbase.com/neukranz/image/119532803 .  For example, Voc = 37.33 .

Is this a reasonable approximation of what's connected to each string?

String 1 = 17 panels

String 2 = 9 panels

String 3 = 9 panels

 Many thanks!

Best regards,

Bill

Comments

  • BB.BB. Super Moderators Posts: 26,872 admin
    For some reason, your Voc seems to be around 31.886 volts right now (add up all the voltages and divide by 35 panels). Sounds way low (assuming you have reasonable sun on the panels).

    But, your guesses on existing panel connections does seem about correct.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    edited April 9 #3

    Thank you.

    All measurements made with all panels in full sun.  I waited until afternoon to ensure all 35 have no shade on them.

    I don't know how they're connected amongst each other.  Coming in to the inverted the 3 strings are not connected - i.e., my string voltage measurements are OC.

    So I'm confused.  I'm not smart enough to know if:

    1. panel Voc degrades over time, such that after 8 years of use Voc declines for each panel from labeled 37.33V to 31.9V. 

    2. some panels are at labeled Voc still (37.33) but others are not for unknown to me reason.  Can some panels be at zero but for the whole string Voc averages 31.9 V?

    Thanks!

    Best regards,

    Bill

  • PhotowhitPhotowhit Solar Expert Posts: 4,280 ✭✭✭✭
    I don't know what happened, but the numbers would support the panels being wired with each orientation as a single string. The picture appears to show 8 panels on the left, 19 in the middle (one out for repair) and 9 on the right

    So you would have strings of 19, 9 & 8  if you divide the voc numbers by the rough 30v you get 551/30.6=@18, 294/30.6= @9 (9.6), and 271/30.6@ 8 (8.8) if the center array was angled more directly at the sun, it might be hotter, also a longer string would be limited by the lowest panel, so the numbers aren't very far off. I could put in a higher number and discount the longer string and 'make then numbers work'.

    It would make sense to some one who was ignorant of the original setup to wire them in that manner,  knowing there were 3 strings.. With the panels of each roof on a single string.

    Just guessing from afar...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • PhotowhitPhotowhit Solar Expert Posts: 4,280 ✭✭✭✭
    On the low VOC numbers, you do have some long leads, and warm/hot panels. Get them hooked back up properly and see what the working numbers are!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • PhotowhitPhotowhit Solar Expert Posts: 4,280 ✭✭✭✭
    HAHA!

    I just noticed the "how the panels should be wired is actually marked on your link!

    Look at the photo of the panels, the orange line shows breaks for each string! Show that to the electrician and I'll bet he'll get you fixed right up! They (or the insurance company) should still be responsible for making you whole.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    edited April 10 #7

    Thank you.

    I think what I've learned here is that:

    1. if each string is wired correctly, one panel after the next in series, string OC VDC (i.e. string not connected to inverter) will be maximized, when in full sun, afternoon, no shading.

    2. Panel Voc does not significantly degrade over time.

    On the back of each panel is a Tyco box with diodes in it.  But, since I don't have the strings connected to the inverter (and thus no current flowing through the panels):

    3. the diodes shouldn't affect anything.  And thus it's reasonable to expect to measure string Voc at 37.33 * number of panels.

    Thank you for the comment on how the panels should be wired.  At the moment I don't have any way of knowing if they indeed are.  I was simply trying to figure out what my situation is just from string Voc measurements.  I'm concluding I can't make much conclusion since it don't know how the panels are connected.

    Best regards,

    Bill

  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭
    edited April 10 #8

    I stand corrected on some of the comments above.

    Today I did a 'science experiment' with a panel I have in my garage (waiting for cables-eaten-by-squirrels to be replaced).

    It's a cloudy day.  I put the panel outside, and measured panel Voc.  It's 33.3 V.

    If I assume this measurement is reflective of all of my panels, then I have a standard of 33.3 V / panel when my panels are connected in a string.

    A key conclusion here is that It looks like Voc does degrade over time.  37.33 is label Voc.  Now it's 33.3.  That's a 4 V drop over about 8 years, or about 0.5 V/panel/year.

    Next I re-measured the String Voc voltages - cloudy conditions this time.  Almost identical to what I reported yesterday:  551 V string 1, 294 V string 2, and 271 V string 3.

    Another key conclusion here is that my string Voc values will be about the same, sunny (all sun on all panels without any shade) or shade.

    Next, if I sum all 3 string Voc measurements (1116 V) and divide by 33.3, I get 34 panels.  35 panels are actually on the roof, connected in a mystery manner.  34 is close to 35 so I think I've confirmed that all 35 panels on the roof are at least operating.  What I don't know yet is how any of them are connected, or if any panels are partially broken.

    Best regards,

    Bill

  • PhotowhitPhotowhit Solar Expert Posts: 4,280 ✭✭✭✭
    Bill, I would guess insurance is covering this. If it is it should be their problem.

    With your numbers, I would be surprised if I'm not correct in the manner they are currently wired. They had a blank canvas only knowing that they were wired in 3 circuits. You have 3 different clusters of panels and they closely match your measured output. 
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • a0128958a0128958 Solar Expert Posts: 316 ✭✭✭

    I think you're absolutely correct.  I appreciate you experts volunteering your time to comment and be so helpful!

    I enjoy the learning and the science project nature of what's in front of me.  Hence poking around on the subject to learn some more stuff, that may too be beneficial to others.

    WRT insurance: yes oes.  The roofer hired someone to remove, store, and re-install the panels.  After installation, output was about one-half to two-thirds what it was 12 months ago.  Fast forward 5 months, the system still isn't performing.  I'm starting to get personally involved.

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