PV Overcurrent!

CALLD
CALLD Solar Expert Posts: 230 ✭✭
in Off Grid Solar & Battery Systems #1
Without wanting to depress my northern hemisphere counterparts too much - I have a little dilemma that's bugging me. Too much PV current on partly cloudy days!!

I installed my system just before the depths of winter and have yet to see full summer potential.

The charge controller is not MPPT and as such has no ability to transform a higher voltage into a lower voltage with higher current.

The cumulative rated PV current is 25.0 Amps. Cumulative short circuit PV current is 26.5 Amps. All specifications are for 1000w/m2, AM1.5 irradiance conditions. At my latitude irradiance levels can exceed that for more than half the year, peak summer irradiance levels can be 15% more. However this can be exacerbated by the edge of cloud effect.

Observed PV current levels have been as follows:

May:

Clear day peak Amps: 23.5
Partly Cloudy day ECE surge Amps: 27.0

June:

Clear day peak Amps: 20.0
Partly Cloudy day ECE surge Amps: 25.0

July:

Clear day peak Amps: 22.0
Partly Cloudy day ECE surge Amps: 27.8

August:

Clear day peak Amps: 24.5
Partly Cloudy day ECE surge Amps: 29.2

September:

Clear day peak Amps: 25.5
Partly Cloudy day ECE surge Amps: 31.8

October:

Clear day peak Amps: 26.1
Partly Cloudy day ECE surge Amps: 34.3


Now the problem is the charge controller is rated at 30Amps, which at the time I thought was ok because it was more than the cumulative iSC of my panels. It has an automatic disconnect function that cuts off charging for 60 seconds if the PV current goes over 31.5 Amps for more than 60 seconds, 37.5 Amps for more than 5 seconds or 45 Amps for more than 1 second. It has no limit on how many times it can disconnect and reconnect the PV. I just don't feel comfortable with it having to go through this sort of punishment.

What kind of charge controller should I aim for with my setup? Realistically that is - don't want to go overkill unnecessarily (like getting a 60 Amp or 100 Amp etc.)
Or is my 30Amp OK?

D
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Comments

  • Plowman
    Plowman Solar Expert Posts: 203 ✭✭✭✭✭
    #2
    Re: PV Overcurrent!
    CALLD wrote: »
    Or is my 30Amp OK?
    I've been using the 125% controller sizing rule. Isc of array * 1.25. I think the US National Electric Code specifies 1.56, but not sure.

    26.5 * 1.25 = 33.1
    26.5 * 1.56 = 41.3

    I've also seen edge of cloud effects, >10 amps coming from a panel with an Imp of 8.47. These seem to be transient.
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #3
    Re: PV Overcurrent!

    Yes the effect is transient, mostly lasting a less than a minute, but I have very occasionally seen it being sustained for longer periods - up to 5 or 10 minutes. I've seen the charge controller cut-out twice in the last month - this happening when the current holds above 31.5 Amps for over 60 seconds. If one could safely rely on the controllers protection circuits to keep it from burning out I'd have no worries at all accepting this, but in the ideal world one would like equipment that can handle every type of weather condition without activating protection circuits.

    If I had to replace my charge controller I'd go with an MPPT. 40 Amp MPPT was my first thought, but 34.3 Amps at 36.6 volts (vmp) is 1255watts, which if the MPPT converts to 24 volts could be 52.3 Amps. It seems I may need to consider a 60 Amp MPPT at least, or maybe even more to throw in a good safety factor? 60Amp MPPT is expensive though and I'm not sure if it's overkill...
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: PV Overcurrent!
    CALLD wrote: »
    Yes the effect is transient, mostly lasting a less than a minute, but I have very occasionally seen it being sustained for longer periods - up to 5 or 10 minutes. I've seen the charge controller cut-out twice in the last month - this happening when the current holds above 31.5 Amps for over 60 seconds. If one could safely rely on the controllers protection circuits to keep it from burning out I'd have no worries at all accepting this, but in the ideal world one would like equipment that can handle every type of weather condition without activating protection circuits.

    If I had to replace my charge controller I'd go with an MPPT. 40 Amp MPPT was my first thought, but 34.3 Amps at 36.6 volts (vmp) is 1255watts, which if the MPPT converts to 24 volts could be 52.3 Amps. It seems I may need to consider a 60 Amp MPPT at least, or maybe even more to throw in a good safety factor? 60Amp MPPT is expensive though and I'm not sure if it's overkill...

    The amount of power a charge controller can put out depends on the amount of power put in.
    880 Watts on an MPPT controller would be about 28 Amps output on average.
    The additional panel current occasionally available from increased insolation will not effect an MPPT controller at all, as they have the ability to 'throttle back' which PWM type controllers do not (they mere connect and pass available current). Even a 30 Amp MPPT controller can handle this situation better than a PWM type.

    Personally I wouldn't worry about it so long as the PWM controller doesn't fry, and any good one won't (they are more Voltage sensitive than current sensitive; most any conductor can stand higher than rated current for short periods of time - it's mainly about heat dissipation).

    At 880 Watts, however, I would have spec'd an MPPT controller to begin with. As it is your charge rate is a tad on the low side (your 26 Amps high divided by 400 Amps hours = 6.5%, 20 Amps is only the 5% minimum). If I were you I'd be looking at a 60 Amp controller and another 360 Watts of PV (hopefully you can get more of the same panels) for that 400 Amp hours.
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #5
    Re: PV Overcurrent!

    Thanks Cariboocoot, I think I will go along with your advice in due course.

    If I was to go with a 60Amp MPPT I would add another 2x140 watt panels. My PV array would then consist of 2 parallel strings of 144 cells yielding 73.2 Vmp. Currently they are 3 parallel strings of 72 cells yielding 36.6Vmp. It would then consist of my 2x300watt panels in series, 4x140 panels in series and then paralleled before going to the charge controller.

    Seems like the obvious route to take given the size of my inverter and the fact that I'm aiming to increase my battery bank to 520Ah when it's up for replacement.

    D
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: PV Overcurrent!

    If you're planning on increasing the battery bank to 520 Amp hours @ 24 Volts you'd want approximately 1620 Watts of panel.
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #7
    Re: PV Overcurrent!
    Cariboocoot wrote: »
    If you're planning on increasing the battery bank to 520 Amp hours @ 24 Volts you'd want approximately 1620 Watts of panel.

    I hear you, and do not disagree with the advice, however I have found that with a PV to battery ratio like that I end up with a huge surplus of PV power on sunny days. This maybe due to my usage profile and latitude. 1620watts would see my batteries out of bulk by 10am with my current usage profile. I have also found that anything over 20 amps gets my batteries up to absorb no problem by lunchtime provided loads are off or less than 30watts. In lieu of getting more PV I'd consider getting a boost charger to get them to 100% using grid power if needs be. Yes I know I'm using batteries when grid tie wound be more effective - but this system is sort of more of a pastime than means of saving money...
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: PV Overcurrent!

    I have a power surplus in Summer too. That's when I run the pumps and power tools. :D

    It may be that you don't need even as much battery as you have, but yes usage patterns make a difference.
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #9
    Re: PV Overcurrent!
    Cariboocoot wrote: »
    I have a power surplus in Summer too. That's when I run the pumps and power tools. :D

    It may be that you don't need even as much battery as you have, but yes usage patterns make a difference.

    Haha yes it's so easy to manage surplus power when I'm at home! Trouble is my system lacks the automation needed when I'm at work during the day. There are days when the forecast has said gloomy miserable rainy zero production weather, so that I purposefully don't use batteries the night before - then as my good friend Murphy would have it there is more sun than I could ever imagine and the charge controller enter's float for the whole day while I'm at work. Of course the opposite happens all to often aswell. Just shows you who the boss is...........

    But my latest dilemma is this: When the batteries reach 100% SOC well before midday, and with very light loads on the inverter, fast moving clouds cause a surge in power so sudden the charge controller doesn't have time to react quick enough - this causes a split-second spike in battery voltage that causes the inverter to shut down in over-voltage protection mode. It doesn't self reset. Really annoying when you get home and find everything's been off for perhaps most of the day!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #10
    Re: PV Overcurrent!

    Some of the newer inverters are now using 32 volts or more as the cutoff voltage... For many inverters the 30 VDC (or 15 vdc for 12 volt systems) is just too low.

    You do have a good size battery bank... But you may need to use a different charge controller. A faster PWM (there is really no excuse for charge controllers to "over voltage" the battery bank, but many do), or possibly a Midnite Classic with a Wizbang Jr. -- You can program a limiting current which may help keep battery voltage from spiking (a guess--Not really sure).

    A new inverter with higher input voltage rating would help... As would Inverter companies that would give you the option of automatic low voltage/high voltage reset--Why cannot the inverter turn on when the battery voltage is back in operational range???:cry:

    http://www.solar-electric.com/lib/wind-sun/PST-2000-specs.pdf
    Wide operating DC input range:
    10.7 - 16.5 VDC / 21.4 - 33.0 VDC

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #11
    Re: PV Overcurrent!

    Unfortunately, Samlex makes 2 different sine wave inverters that are 2000 watt, this version only has a 21 to 30 volt input range!

    I think you said that your PWM CC has a monthly equalizing that you can't turn off? I might be confused with someone else, but equalizing allows for a slightly higher voltage and that might be what's shutting your inverter down.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #12
    Re: PV Overcurrent!

    BB: I think the culprit here is my PWM charge controller.

    Photowhit: Yes it does have a monthly EQ setting.

    After checking I've found it does re-set after a while.
    The second culprit is probably the batteries, silver calcium grid alloy requires a higher charge voltage during absorb and also has a very low absorb current - less than 4 amps is needed to push them over 30v once they get to 100% SOC.

    My inverter has a 21.6v to 31.0v range. I had absorb set at 30.4v today (normally 29.8v) because it had been 5 days since they last saw 100%. 30.4v absorb, coupled with a very light inverter load (40watts), a howling gale blowing, light patchy cloud in crystal clear, cold air with the sun at 60° altitude caused some very intense surges in PV power.

    I think in future I will switch off the inverter before bringing the batteries over 29.8v for whatever reason - EQ or boost-charging.
    I will certainly never get Silver Calcium batteries again, the higher absorb voltage wastes power & this battery chemistry is not suitable for cycling applications either. So we learn...
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #13
    Re: PV Overcurrent!
    CALLD wrote: »
    I think in future I will switch off the inverter before bringing the batteries over 29.8v for whatever reason - EQ or boost-charging.

    If you have a need, or even a strong desire to keep some AC loads operating off the inverter during the EQ, there is another solution.
    Instead of opening the battery connection to the inverter, put a suitable high power diode or diode array in series with the inverter input. Depending on the diode type this will drop the input voltage by a (relatively) stable voltage somewhere between .2V and 1.0V. You do need some pretty high power diodes, since even .2V at 100A is 20 watts.
    SMA SB 3000, old BP panels.
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