Array DC Voltage drops 85- 90 volts DC

When the inverter cylces on the voltage drops.

This is a 5K system with an SMA 5000US inverter. 240 vac single phase

there are 11 canadian solar CS6P 230 watt modules in each of 2 strings, open voltage is about 352 volts DC on either string. when the inverter is activated the voltage slowly drops to about 260 volts at the inverter.

the same condition occours if either string is disconnected from the inverter and either of the strings are run separately on the system. the system has been installed for about 3 months.

does this seem normal or does there appear to be a problem with the inverter or array ? The daily KW production seems to have dropped about 8-10 kw hours

basic conditions are:

direct sunlight
Florida
90 degrees F
no shading

any help is appreciated

thank you

Comments

  • Solar GuppySolar Guppy Solar Expert Posts: 1,987 ✭✭✭
    Re: Array DC Voltage drops 85- 90 volts DC

    No, what you describe would be correct operation as your point of maximum power is typical ~70% of the VOC and you are just observing how the sunnyboys tracking works.

    Production falls as temperatures rise and in Florida the tropical weather with cloudy skys and afternoon rains lowers production

    Best month is typically April for generation when temperatues are moderate and no rain or clouds
  • System2System2 Posts: 6,290 admin
    Re: Array DC Voltage drops 85- 90 volts DC

    Thanks for the response, it just seemed a little low but the system still operates fine. I was getting around 31-33 kwh per day and now its down to 25 or 26 kwh per day on a good sunny day.

    The wattage still seems to be at a good level so I guess I should just stop worrying about it

    thanks for your help

    AK
  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: Array DC Voltage drops 85- 90 volts DC

    Even where I live--My GT system will average around 19+ kWH per day spring/fall and drop down to ~16 kWH per day in the summer on hot days.

    Temperature, humidity, clouds, wind, sun angle, (for me, the marine layer when it gets past the coastal hills) etc. all affect panel output. It is amazing to see the a 20% spread in current inverter output between days and trying to figure out why one day power output is more then the other...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Array DC Voltage drops 85- 90 volts DC

    Yeah my system, which saw a 77Kwh day last spring, is now producing 68Kwh days with 105F+ temps outside. We did have a pretty terrible day on Friday, but the temps were high and it was very cloudy all day with rain late in the day. The monsoon season is here and cloud cover is greatly enhanced.

    I suspect this fall when temps drop back to reasonable levels we will be back to great generation levels.

    if you look at my graphs on my link for minute data over 24 hours you will see the clouds roll in at 2:45 PM.
  • System2System2 Posts: 6,290 admin
    Re: Array DC Voltage drops 85- 90 volts DC

    Thanks for the information, I really thought the string voltage drop indicated a problem somewhere, it drops nearly 90 volts as I said from the time the inverter cycles on.

    free line voltage is about 350-352 disconnected at the array or at the connection block on the SMA inverter. as soon as it is connected to the inverter either string connected individulally or in parrallell will have the same results and after the inverter is activivated the voltage drops around 90 volts ,or to the bottom of the mininmum voltage limit.

    It just seems like a large drop, but all of the modules seem to operate individually to specifications with the few tests I have done on them.

    I need to check the current on each one, but it is time consuming and I need to understand the best way to load test each module.

    I have a DC amp clamp coming so I can dead short each one and record the amperage or, I suppose I could use a multimeter to check between the terminals on each moudule under the same conditions to see the current that way but I cant seem to get the advertised current flow that way.

    thanks

    Andrew
  • Solar GuppySolar Guppy Solar Expert Posts: 1,987 ✭✭✭
    Re: Array DC Voltage drops 85- 90 volts DC

    There is nothing to check with your meter, what I gather your missing is that solar panels put out zero energy at open circuit conditions and have to be loaded to about 70-75% of the open circuit voltage to produce maximum power.

    Now, if in hot conditions, this is below the inverters minimum input, yes, you have an issue, that would be not enough panels in a series string. This can be checked with a string calculator, the best one out there is the Xantrex site

    http://www.xantrex.com/support/gtsizing/index.asp?lang=eng

    I ran your panels, you should be fine, the SMA tracks down to 250VDC

    http://www.affordable-solar.com/admin/product_doc/Doc_SUNNYBOY5678-DUS094221_20100204151313.pdf
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