Why Loss of Grid Tie Output, even though DC input is fine?

balee123
balee123 Solar Expert Posts: 86 ✭✭
in Grid Tie and Grid Interactive Systems #1
Hello all,

I have two grid tied inverters, a Xantrex STRX2500 and STRX1500 wired in parallel. Both have been functioning until today. Now the STRX1500 is outputting 60% less. This just happened today as I monitor the system daily and noticed the anomaly today. After taking some measurements, I have confirmed that the DC input, both voltage and current, is what is expected for the array. It is only after "being inverted" that the read out now shows 400W instead of the usual 1000W for this time of year.

Any idea what might be going on?

Panels:
100W AP-100
Voc 20.1V
Vmp 16.1V
Isc 7.2A
Imp 6.2A


Panels are set up as 4 in series, 4 paralleled into the STRX1500.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    If you really have 1000+ Watts going in to a GT inverter but only 400W coming out, the inverter is fried.

    It looks suspiciously like three of the parallel panel strings are not actually connected though. Obviously you should be seeing 64 Volts @ 24 Amps ins (+/-). Are you checking the input current with a clamp-on DC meter?
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #3
    Re: Why Loss of Grid Tie Output, even though DC input is fine?
    Cariboocoot wrote: »
    If you really have 1000+ Watts going in to a GT inverter but only 400W coming out, the inverter is fried.

    It looks suspiciously like three of the parallel panel strings are not actually connected though. Obviously you should be seeing 64 Volts @ 24 Amps ins (+/-). Are you checking the input current with a clamp-on DC meter?

    I actually thought the same thing, that three of the parallel panel strings were not connected, however, this did not seem to be the case. Yes, I checked the input current with a clamp-on DC meter at the point where the large gauge wire carrying DC current goes to the circuitry that inverts the DC to AC. I actually measured ~ 20 Amps @ 54 V. Is it possible that the display meter on the inverter is bad and there still may be more than 400W going to the grid but it's only displaying 400W?

    I will try and check the actual AC output, the next time I get a chance.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    Yes, that's the next thing to do; clamp on to the AC out and see what current is actually coming from the inverter.
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #5
    Re: Why Loss of Grid Tie Output, even though DC input is fine?
    Cariboocoot wrote: »
    Yes, that's the next thing to do; clamp on to the AC out and see what current is actually coming from the inverter.

    Did the above and this is what I see:

    2.65 AAC on each leg. At 240V this means 636 W after inverter, doesn't it?

    However, only 360 W is displayed on inverter's intergal meter.

    Seems to imply there is something wrong with the inverter's integral meter, doesn't it?

    Every hear of a display/meter going bad, but inverter still functioning to output the right amount?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    If your input is 54 V @ 20 A = 1080 Watts while at the same time the output is 240 V @ 2.65 A = 636 Watts then some 400 Watts is going missing regardless of any metering discrepancy. The 360 Watts shown by the inverter may be further indication of something wrong with it rather than the only thing wrong with it.

    Unfortunately these things are not very user-serviceable. No 5U4's to pull out and replace. :D
    And unlike an off-grid inverter you can't run in independent of the grid so you can't take a look at the output waveform and see if it really is dropping Voltage peaks (blown FET's).
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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #7
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    I forgot to mention that the latest AC current readings as well as the 360 W integral meter reading, were taken much later in the day compared to the 20 Amps @ 54 V measurement, which was close to midday. The normal output of 1080 W would be at midday, which is when I saw the first 400 W integral meter reading. The fact that I am still seeing something close to 400 W, could indicate that it is a meter problem, couldn't it?

    Maybe I will take measurements at midday again. Maybe next weekend when I'm home at that time.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    You have to evaluate the inverter performance based on input and output at the same time. There are too many things that can affect solar panels to try and rely on output vs. "typical for the time of day". Otherwise given the variances you could have just about anything showing up.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #9
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    Just check the DC input and AC output about the same time--You should be looking at >90% conversion efficiency--including measurement errors (DC input to AC output).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #10
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    does your system log / track inverter temperature ? 1,000 w in - 400w out = 600w of heat !!
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • balee123
    balee123 Solar Expert Posts: 86 ✭✭
    #11
    Re: Why Loss of Grid Tie Output, even though DC input is fine?
    balee123 wrote: »
    I forgot to mention that the latest AC current readings as well as the 360 W integral meter reading, were taken much later in the day compared to the 20 Amps @ 54 V measurement, which was close to midday. The normal output of 1080 W would be at midday, which is when I saw the first 400 W integral meter reading. The fact that I am still seeing something close to 400 W, could indicate that it is a meter problem, couldn't it?

    Maybe I will take measurements at midday again. Maybe next weekend when I'm home at that time.


    So I just took the DC input and AC output measurements at the same time and this is what I see:

    Input
    50V 21.5A DC 1075W

    Output
    240V 4.2A AC 1008W

    93.7% efficient conversion

    However the STRX1500 inverter's LCD display was still only indicating 409W being produced. It appears the integral LCD meter is bad.

    I also noticed over that last week, on a couple of days, the integral LCD display indicated the a more reasonable value of close to 1000W.

    Other measurements at the Electric company's meter indicate that I am still producing the expected amount of electricity from the array, it just seems that the integral meter is not working properly. Seems to max out at ~ 400W even if producing close to 1000W.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #12
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    Probably the sensor/circuit that measures current is having a problem. One typically way is to use a small current transformer and put the output across a precision resistor--Then measure the voltage across the resistor.

    If the resistor or the voltage measuring traces/contacts/wires are bad, you can get a false reading.

    And the current sense transformer wiring can be very fine--And easy to break/fail. However, I am not sure how likely it would fail at 1/2 output.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #13
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    If you were truely having that difference in input-output power the inverter would be very hot.

    If not AC sense circuit problem, it could also be filter caps in inverter going bad. If the filter caps do not provide the peak AC sinewave power smoothing then there could be excessive ripple current showing up on the PV DC lines. Most meters will display incorrect number if there is a lot of ripple on the dc current giving you a V*A number much higher then actual. PV panels require smooth DC current loading at their MPP to yield their full output. Ripple current due to poor filter cap reserve on single phase AC inverter will reduce the effective PV yield.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #14
    Re: Why Loss of Grid Tie Output, even though DC input is fine?
    RCinFLA wrote: »
    If you were truely having that difference in input-output power the inverter would be very hot.

    If not AC sense circuit problem, it could also be filter caps in inverter going bad. If the filter caps do not provide the peak AC sinewave power smoothing then there could be excessive ripple current showing up on the PV DC lines. Most meters will display incorrect number if there is a lot of ripple on the dc current giving you a V*A number much higher then actual. PV panels require smooth DC current loading at their MPP to yield their full output. Ripple current due to poor filter cap reserve on single phase AC inverter will reduce the effective PV yield.
    That should be easy enough to check, right? Set your DVM to ACV and see what you have on the DC lines. The meter should ignore the DC component and show you the ripple.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #15
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    Measuring AC Ripple on a DC circuit--Here a "cheap" digital meter may be your friend. They typically just use a capacitor block DC and measure AC voltage (current, etc.).

    A more expensive True RMS (root mean square) reading meter will normally give you a DC+AC value of the wave forms--And will not display just AC ripple voltage component on DC.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #16
    Re: Why Loss of Grid Tie Output, even though DC input is fine?
    Cariboocoot wrote: »
    And unlike an off-grid inverter you can't run in independent of the grid so you can't take a look at the output waveform and see if it really is dropping Voltage peaks (blown FET's).
    You CAN however look at the current waveform (shunt or pickup coil) to get much the same information.
    SMA SB 3000, old BP panels.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #17
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    Probably the IC that measure the AC values is going south, its an odd ball part that is single sourced and from South Africa of all places. Shouldn't effect the units production but a 11+ year old Suntie is well past its expected life-time, so you should be considering replacement alternatives in the near future.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #18
    Re: Why Loss of Grid Tie Output, even though DC input is fine?

    Good to see you're still around, SG. :D
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