string voltage - how close to calculated value?

autoxsteve
autoxsteve Solar Expert Posts: 114 ✭✭✭✭
in Advanced Solar Electric Technical Forum #1
I've done a couple of tests on my PV system and the input voltage appears to be lower than the calculator suggests, but about 10%.

voltage of each panel at MPP is 26.7. With two strings of 11 panels in each string, that should be a strong input voltage of 293.7 volts, DC.

I'm seeing about 267 volts, dc a difference of 26.7 which appears too coincidental to one module short.

My understanding of panel outputs is the voltage will remain constant with sufficient light and the current will vary as the MPPT does its function.

I'm wondering if I've got a wiring problem on the strings that needs to be addressed.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: string voltage - how close to calculated value?

    What panels (mfg and model) are you using... And, what is the surface temperature of the panels (IR thermometer or instant read thermometer perhaps insulated with some "pre heated" paper towels...

    I have used the Xantrex GT sizing site as a quick way to get various solar panel mfg info from one place. I even back calculated Vmp@temp from Vmp-spec and TC and came up with very reasonable module surface temperatures... As long as I had no clouds, the numbers for my system (Xantrex GT 3.0 Grid Tied inverter) seemed to work out very well for me.

    Also, beside measuring voltage, take a look at Imp current. If you take a big hit here, then I would really suspect a problem with the panels or wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #3
    Re: string voltage - how close to calculated value?

    Steve,

    A PV string voltage of ~90% of the Vmp spec is pretty normal for this time of year in your location.

    A PV module’s STC Vmp spec is based on a PV cell temperature of 25 C (77 F). Since PV cell temperature under full solar insolation (1,000 W / m^2) runs ~35 C above ambient, ambient temperature may have to be ~ -10 C (~14 F) for the module to deliver STC Vmp. Higher ambient/cell temperature causes the Vmp to drop. The temperature coefficient of voltage for your modules is -0.47%/C.

    Your ES-190 modules’ Normal Operating Cell Temperature (NOCT) spec, which is based on 20 C (68 F) ambient and 80% insolation, is 23.8 V, or ~89% of the STC Vmp spec. This NOCT spec suggests that the Vmp from a string of 11 modules wired in series would be ~262 V, or pretty close to the 267 V you’ve observed. In fact, the values match if you factor in the module's +/-2.5% power tolerance spec.

    See: http://www.evergreensolar.com/upload/195W_Product_Datasheets/S195_US_010707.pdf

    Although aggravating, this lower operating voltage is normal. It’s also the main reason why the CEC uses the PTC (PVUSA Test Conditions) spec of 168.8 W for your ES-190 in calculating your system size and rebate eligibility.

    See: http://www.consumerenergycenter.org/cgi-bin/eligible_pvmodules.cgi

    The bad news is that PV systems rarely deliver meet their STC power specs. The good news is that this part of your system appears to be operating nominally.

    HTH,
    Jim / crewzer
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  • autoxsteve
    autoxsteve Solar Expert Posts: 114 ✭✭✭✭
    #4
    Re: string voltage - how close to calculated value?

    Thanks for the information, Jim.

    I asked the question innocently, with the expectation of a reasonable explanation.

    I'll learn more as I go, of course with the help of all of you.

    More to come as my 'go live' date awaits....
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #5
    Re: string voltage - how close to calculated value?

    :cool:

    Regards,
    Jim / crewzer
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