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    #16
    Originally posted by pv_n00b View Post
    There's nothing in the NEC that says you can't connect a generator in parallel with a PV system, which is good because there are cases where it is the desired design. Remember 90.1(B), the purpose of the code is safety, not keeping the equipment working. That's the job of the system designer or engineer.

    Having a generator and PV in parallel is not a guarantee that one will damage the other, it just creates the condition under which one can damage the other. Under the right conditions these two can get along fine, but those conditions are not usually maintained if they are not designed in.
    Remember that PV already operates in parallel with a generator. It just so happens to be called “The Grid”. Remember also that all grid-interactive PV is designed to disconnect when “The Grid” goes outside of specification, which is actually rather tight. Very few non-inverter generators can hold frequency and voltage well enough to keep an inverter running.
    Julie in Austin

    Born to brew, forced to work ...

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      #17
      But, unlike a small discrete generator, the grid can sink significant power when needed. It gets redistributed to non-local loads and/or causes the generators feeding the grid to reduce their output.

      Sent from my XT1585 using Tapatalk

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        #18
        Originally posted by tallgirl View Post
        Remember that PV already operates in parallel with a generator. It just so happens to be called “The Grid”. Remember also that all grid-interactive PV is designed to disconnect when “The Grid” goes outside of specification, which is actually rather tight. Very few non-inverter generators can hold frequency and voltage well enough to keep an inverter running.
        That hasn't been my experience. I used to teach an off grid lab at ImagineSolar with a Sunny Boy, a Sunny Island, and a run-of-the-mill Generac generator. The SB ran just fine when the SI was in pass-through mode with the genny as the grid.

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          #19
          Originally posted by ggunn View Post
          That hasn't been my experience. I used to teach an off grid lab at ImagineSolar with a Sunny Boy, a Sunny Island, and a run-of-the-mill Generac generator. The SB ran just fine when the SI was in pass-through mode with the genny as the grid.
          If the generator is able to produce a stable enough voltage and frequency signal the anti-islanding logic in the Sunny Boy won't know it doesn't have an actual grid. It's definitely possible, especially with larger generators where the rotational mass helps to stabilize things, but extremely unlikely with crappy "alternator generators" like what you buy in a Blue or Orange store for a few hundred bucks.

          I don't know how SMA's anti-islanding logic works, but the usual approach is to try changing the grid frequency. If successful, the inverter assumes it's islanded and MUST trip. If not, the inverter will continue to run so long as the generator is within voltage and frequency for the rest of the intertie requirements.
          Julie in Austin

          Born to brew, forced to work ...

          Comment


            #20
            Originally posted by GoldDigger View Post
            But, unlike a small discrete generator, the grid can sink significant power when needed. It gets redistributed to non-local loads and/or causes the generators feeding the grid to reduce their output.

            Sent from my XT1585 using Tapatalk
            So long as the PV system's output is less than the loads, the generator will back off the same as if the connected loads were reduced.

            Automatic grid controls are actually pretty trick -- the more the instantaneous imbalance is too much supply, the faster (in very tiny amounts) the generators rotate, converting excess power into additional rotational energy, increasing frequency. The rise in frequency is detected causing some of the generators to reduce output. When the imbalance goes the other way, the deficiency is met by rotational energy in the generators being converted to electricity and a lower frequency. That frequency drop is detected and those same generators increase their output.

            Each grid has its own "inertia" -- in the Texas (ERCOT) grid, a change of supply or demand of 450MW produced a frequency change of 0.1Hz under normal conditions.
            Julie in Austin

            Born to brew, forced to work ...

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              #21
              I talked to Generac. Their point of view is that the model being used is not "designed" to be used with solar.

              My recommendation was to rearrange the setup to a typical arrangement with the solar separated from the generator since there was no engineering oversight or approvals.

              Comment


                #22
                Originally posted by electrofelon View Post
                I remembered you said that in a previous thread - hence, I didnt say the inverter would not synch to it What I said was the inverter would likely shut down before any damage was done. That is, even if the inverter did synch up to the genny, if demand went below production, the inverter would drive up frequency and/or voltage and THEN shut down. That is my hypothesis anyway. Anyone want to take one for the team and try it?
                Originally posted by ggunn View Post
                I think it would be a race. I wouldn't count on the inverter shutting down before frying control circuits in the generator.
                Assumption:
                Generator is residential grade, 10KW - 25KW, 120/240. Gen has no over-frequency, under-frequency, over-voltage, or under-voltage protective relays. Gen has over current protection.

                Disclaimers:[LIST=1][*]Grid tied inverters are not an area of my expertise.[*]I don't know what happens if the gen is an inverter type. I have not seen any analysis, but inverters don't tend to sink power - unless they are designed as 4-quadrant vfd.[/LIST]

                I absolutely agree with Chamuit's solution. However:

                g -
                I recall a similar thread where (I think) you explained:
                [LIST][*]grid-tied the inverters act similar to a constant current source.[*]They output all the power available from solar panels DC input.[*]Frequency is controlled by the paralleled source (the grid/gen).[*]Voltage is controlled by the paralleled source.[*]The inverters will shut down if the voltage or frequency get out of tolerance (I don't recall exactly, but +,- 5% Hz, and +,- 10% V sticks in my mind.)[/LIST]

                Assuming any to this is true, here is what I am seeing:
                [LIST=1][*]If the system load goes under the PV system output, the gen throttle pulls back, and the inverters back-feed the gen, driving it as a synchronous motor, causing it to over-speed.[*]Frequency goes up, voltage goes up[*]Eventually either the over-voltage or over frequency shuts down the inverters.[*]63Hz, 132V should not hurt the gen.[/LIST]

                Questions:
                What would be the failure mode that could "fry" the generator control circuits?
                Which control circuits are you thinking are in danger?
                Voltage regulator?
                Start-stop controls?
                Some other clearly specified control circuit?
                Without data you’re just another person with an opinion – Edwards Deming

                Comment


                  #23
                  The standard frequency tolerance for a GTI is +.5hz, -3 hz for .16 seconds. Max voltage is about 264. I don't see those figures resulting in Genny damage, but that's total speculation on my part.
                  Ethan Brush - East West Electric. NY, WA. MA

                  "You can't generalize"

                  Comment


                    #24
                    Originally posted by electrofelon View Post
                    The standard frequency tolerance for a GTI is +.5hz, -3 hz for .16 seconds. Max voltage is about 264. I don't see those figures resulting in Genny damage, but that's total speculation on my part.
                    So the OF is +1% and the UF is -5%?
                    Without data you’re just another person with an opinion – Edwards Deming

                    Comment


                      #25
                      I read of one case in another forum where the generator shutdown and delivered a fault message. Evidently that generator had somewhat more sophisticated protection controls than what iceworm is positing. The thing is, even if your genny doesn't get damaged, if it shuts down, you've defeated the purpose of having it.

                      Comment


                        #26
                        Originally posted by jaggedben View Post
                        I read of one case in another forum where the generator shutdown and delivered a fault message. Evidently that generator had somewhat more sophisticated protection controls than what iceworm is positing. The thing is, even if your genny doesn't get damaged, if it shuts down, you've defeated the purpose of having it.
                        And of course, once the genny shuts down, so will the PV system, if it hasn't already.

                        Comment


                          #27
                          Originally posted by iceworm View Post
                          Questions:
                          What would be the failure mode that could "fry" the generator control circuits?
                          Which control circuits are you thinking are in danger?
                          Voltage regulator?
                          Start-stop controls?
                          Some other clearly specified control circuit?
                          We damaged a generator in the lab I spoke of, probably due to operator error on my part in programming the Sunny Island to protect the genny from backfeed from the Sunny Boy. I was told that it took out a/the "control board" in the generator. My areas of expertise lie elsewhere, so that's just about all I know, but it seems obvious to me that if you backfeed a generator that cannot sink power with a GT PV system, Bad Things can occur. Exactly which Bad Thing would happen first is not that important, IMO. My advice is to not do it.

                          Comment


                            #28
                            Originally posted by iceworm View Post
                            Questions:
                            What would be the failure mode that could "fry" the generator control circuits?
                            Which control circuits are you thinking are in danger?
                            Voltage regulator?
                            Start-stop controls?
                            Some other clearly specified control circuit?
                            Grid-tied inverters are designed to convert DC to AC and move it on. They are not designed to curtail the output based on the needs of the AC loads as an off-grid inverter will do. So if we put a grid-tied inverter in an off-grid situation and there is not enough load to absorb the inverter output that current will go somewhere it is not wanted. Where it goes and what it goes through depends on the weakest link on the AC side. If something on the AC side does not fail, eventually the inverter will raise the voltage high enough to trip itself off line as it tries to push current into the loads.

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                              #29
                              gg, pv -
                              I appreciate the effort. But without some physics, not much help
                              Without data you’re just another person with an opinion – Edwards Deming

                              Comment


                                #30
                                Originally posted by jaggedben View Post
                                I read of one case in another forum where the generator shutdown and delivered a fault message. Evidently that generator had somewhat more sophisticated protection controls than what iceworm is positing. The thing is, even if your genny doesn't get damaged, if it shuts down, you've defeated the purpose of having it.
                                Pretty much — unless there’s some control logic which starts disconnecting inverters as the load on the generator approaches zero, that’s the result. I tried that on a lark once, because I’m a nerd, and it’s hard to do with software. You’d have to know all the loads and always keep enough of a load on the generator to power the largest intermittent load.
                                Julie in Austin

                                Born to brew, forced to work ...

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