High voltage alarm

pictonrosepictonrose Registered Users Posts: 10 ✭✭
We added more panels and a second outback flexmax 80. We have panels wired in series/parallel. Panels are 245w, 30 v, 8 a. So I did the watts law calculations. 
30+30+30+30=120v
8+8+8+8=32a

120v x 32a = 3840w

The flexmax 80 specs says 150 VDC,  how do I convert voc to vdc?
We are getting high voltage alarm. Where have we gone wrong? 

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,425 admin
    edited February 21 #2
    Voc is the voltage from the solar panel(s) with no loads... Need to look at the exact specifications for your panels, but a "typical" Vmp~30 volt panel (voltage maximum power) has a Voc~37 volts (voltage open circuit).

    The FM80 (and a number of other MPPT charge controllers) have a maximum solar panel input voltage of 150 VDC maximum (to protect the input transistors from over voltage). How many panels you connect in series sets Voc-array and Vmp-array voltages.

    The other "gotcha" is that Vpanel rises in cold temperatures, and falls in hot temperatures (with "standard test temperature" ~77F/25C at the factory/marketing specifications).

    In warm/hot weather, the solar panel cell temperatures are (roughly) 20C hotter than ambient temperaturess. So "hot panels" have the Vmp-hot fall, and you need to make sure that Vmp-hot > Vbatt+ a couple degrees C (wiring and controller drops). Typically, we suggest that you take Vbatt * 1.3 = Vmp-array-std voltage. If you have a 48 volt battery bank:
    • 60 volts charging * 1.3 "hot array" fudge factor = 78 Volts Vmp-array-std suggested minimum for an MPPT controller
    For cold conditions, your specific condition... Voc rises when temperatures fall... In sub freezing conditions, the Voc-cold can be quite a bit higher. The math is not hard, but it is nice to have it automated (to avoid mistakes, give you multiple array configuration options, etc.). Outback's downloadable calculator is:

    https://outbackpower.com/resources/technical-support/string-sizing-tool

    I like the on-line tools--I don't have to download everybody's calculator and fire up the windows computer to run (typically use a Chromebook for posting). Midnite has a nice couple of websites. This one is for their "classic" larger controllers:

    I will make up some roughly accurate numbers for your array. And guessing you are at 0F (-18C) 245 Watt, Vmp-30.6 Voc=37.6, Imp=8. amps, Isc=10. amps, 4 panels in series:

    http://www.midnitesolar.com/sizingTool/index.php

    Rated PV Array Power:980Watts
    Anticipated Array Power @ 40C:914Watts
    Rated PV Array Current:8Amps
    Battery Charging Current @ 57.6 V:17Amps
    VMP (Maximum Power Point Voltage) :122.4Volts
    VOC (Open Circuit Voltage):150.4Volts
    VMP @ -18 C°:146Volts
    VOC @ -18 C°:171.6Volts

    At this point, it appears that your 4x panels in series is way to high of voltage--Especially at -18C... Your first fix (assuming I have the panel panel numbers close to correct, and your location/temperature (somewhere around Orrillia Ontario?)... A 3 string system would be:

    Rated PV Array Power:735Watts
    Anticipated Array Power @ 40C:685Watts
    Rated PV Array Current:8Amps
    Battery Charging Current @ 57.6 V:12.8Amps
    VMP (Maximum Power Point Voltage) :91.8Volts
    VOC (Open Circuit Voltage):112.8Volts
    VMP @ -18 C°:109.5Volts
    VOC @ -18 C°:128.7Volts

    Two panels in series is too low of working voltage for a 48 volt battery bank (but OK for 12 or 24 volt bank).

    And you would put 3 series and up to 8 parallel strings (at first glance).

    Details do matter--Please feel free to correct my guesses/assumptions if you need more help... But the first thing is to get to 3 series panels. Many of the MPPT controllers will record the max input voltage and void your warranty if you exceed their ratings (and obviously, damage can easily occur).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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