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Backfeeding transformers with grid interactive micro-inverters

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    #46
    Originally posted by PVfarmer View Post
    [I]
    But besides that....if the grid side voltage was high, the inverters would adjust and put out less current?
    Within their operating voltage range, the GTI will try to put out the same amount of power (whatever is available from the PV or limited by the GTI's rated output.
    So if the grid voltage rises the GTI can deliver that same amount of power at a lower current, yes.

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      #47
      Originally posted by GoldDigger View Post
      whatever is available from the PV or limited by the GTI's rated output.
      Isn't there a limit on the DC input of the inverter also?

      Anyway, say you have a 10kW inverter with the max recommended 12.5kW DC of panels.
      It puts out 41.7A max at 240V.
      But the idea with the 12.5kW of panels is that while you lose some due to clipping midday, the inverter is running at max output for more minutes per day.

      So if the inverter is putting out its max 41.7A at 240V and then the grid voltage drops below 240, the inverter will match the lower voltage, but continue with the 41.7A, it can't go above 41.7A.

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        #48
        Originally posted by PVfarmer View Post
        So if the inverter is putting out its max 41.7A at 240V and then the grid voltage drops below 240, the inverter will match the lower voltage, but continue with the 41.7A, it can't go above 41.7A.
        And conversely if the voltage goes higher than 240V then the inverter can put out slightly more power and still not exceed 41.7A. The DC input might have a power limit that puts a cap on this behavior, but that depends on the inverter.

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          #49
          Originally posted by PVfarmer View Post
          Isn't there a limit on the DC input of the inverter also?

          Anyway, say you have a 10kW inverter with the max recommended 12.5kW DC of panels.
          It puts out 41.7A max at 240V.
          But the idea with the 12.5kW of panels is that while you lose some due to clipping midday, the inverter is running at max output for more minutes per day.

          So if the inverter is putting out its max 41.7A at 240V and then the grid voltage drops below 240, the inverter will match the lower voltage, but continue with the 41.7A, it can't go above 41.7A.
          The VA output profile changes when A equals IMAX.
          [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

          Comment


            #50
            Originally posted by PVfarmer View Post
            ggunn-
            I meant Sunny Island and SI, of course.


            But what about the Tripowers? Don't they do the same, so if there was some kind of frequency problem in the grid (or xfmrs), it would affect the inverters?
            Sunny Boys have the capability (and you have to get into the firmware to enable it) to ramp down output as frequency increases or decreases from 60Hz to (I believe) 58Hz on the low side and 62Hz on the high side. At those points they shut down entirely. This is so they can operate off grid with Sunny Island inverters. In off grid mode, the SI's control the frequency of the AC bus and they monitor the state of charge of their batteries as well as the load they are supplying; as the batteries approach 100% SOC the SI's shift the frequency up or down (they alternate so that clocks on the AC bus have a fighting chance of staying nearly accurate) so that the batteries don't get overcharged. The SB's respond by ramping down their output. It's pretty slick.

            Otherwise (if you don't have the off grid capability enabled) they just run full out and shut down when the frequency drifts out of the window they can operate in. No ramp.

            Tripowers definitely have a frequency operating window but I do not know if they have an off grid capability whereby they would ramp down instead of just shutting down when the frequency drifts.

            BTW, a transformer cannot affect frequency.

            Comment


              #51
              Originally posted by PVfarmer View Post
              ...Smart$
              Someone would have to run the numbers, but I can't fathom a double step down scenario as being more efficient than a single transformer step down scenario.


              For instance-
              If the POCO was going to provide a standard 12470V to 480/277 transformer(s) that were 96% efficiency, but instead the customer bought a 4160V to 480/277 that was 98.75% and then the POCO's was 4160 to....7200V?
              If not 7200V, 12.47KV....but if that POCO one happened to be 97.5% due to....some reason, the 98.75% and 97.5% together would be better than a single 96%, wouldn't they?..
              You have to consider that the efficiency rating is given at kVA throughput... not at zero or points in between. Efficiency % goes in the dump at zero throughput.

              I doubt your setup will have full-rated throughput 24/7.
              [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

              Comment


                #52
                Originally posted by Smart $ View Post
                You have to consider that the efficiency rating is given at kVA throughput... not at zero or points in between. Efficiency % goes in the dump at zero throughput.

                I doubt your setup will have full-rated throughput 24/7.
                The thing that bugs me is that efficiency curves aren't really published by the xfmr makers.
                I stumbled on a chart accidentally somewhere, it looked like under 20% of nameplate, the curve just takes a nosedive, so there's nothing you can really do about that aside from making sure you don't oversize the xfmr(s).

                This one here says 50%....and then some lower voltage ones (same maker) are super-efficient and are rated so between 35-65%.
                http://www.temcoindustrialpower.com/...al/HT5515.html

                So...say this 75kVA model xfmr- if you had 60kW of inverters that were also 60kVA (3 phase), the 60kVA is 80% of 75kVA- you'd be above the "efficiency zone" when the inverters are at max, but then you'd be right where you want to be when the inverters are putting out 37.5kVA.

                But then again, you might want 90kVA of xfmrs, not really sure there, that's the POCO's call anyway.

                If the load you were running off the 60kVA of inverters was 20kVA max, you'd want 30kVA of those 35-65% models for stepping down to run the load, I guess 66% is close enough to 65%?
                Product features

                [LIST][*]U.S. DOE 10 CFR Part 431 (and/or TP1) efficiency standards and the Canadian Energy Efficiency Regulations SOR/94-651 (and/or C802.2) efficiencies at 50% of rated load [*]UL Listed and CSA Certified [*]Efficiencies are calculated under a linear load profile [/LIST]

                Comment


                  #53
                  I don't get into design much nowadays, but you get the gist of what I was trying to point out. Good luck with your endeavors...
                  [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

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