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Grid Tie inverter still importing despite sufficient power from pv panel

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  • Grid Tie inverter still importing despite sufficient power from pv panel

    Hi. I have a 2kw grid-tie inverter (GTI). My GTI can zero out any import from grid for all my loads except my aircon. Without the aircon switch on, my GTI can support (does not import from grid) all my loads up to a maximum of 1.4kw.

    However, if i switch off all my loads and just switch on my aircon (inverter type 1hp split-type panasonic aircon), which just draws from 400w to 600w, the GTI only produces about 250w and maintains an import of about 150w to 250w despite the availability of excess pv power. I can say that I have excess pv power because if i add another load, the GTI's output will jump from 250w to 1kw immediately.

    This behavior of the GTI is somewhat limited to the aircons only. I have also tested the GTI with a second aircon in the house (another brand, LG inverter split-type 1hp), but this time, the GTI only maintains power import of 40w to 60w, which is lower than the panasonic aircon.

    Again, for all other loads (refrigerator, water heater, electric fans, electric kettles, television, computer, freezer), the GTI can maintain zero or minimal import. But when I switch on the aircon, grid import is mantained at 150w to 250w.

    Is it my GTI that is the problem here? Or are the aircons the cause of the GTI having difficulty in zeroing out grid import despite availability of excess pv power? If it is the aircons that is the cause, what do you think is the underlying cause of this problem? Power factor of the aircon? Are the aircons grounded? Any other cause?

    Thank you so much for any input on this problem that has been puzzling me ever since.

  • #2
    Most likely what you are seeing is the result of a low power factor on the A/C unit.
    Most grid tie inverters only produce output at close to unity power factor. That is, the current they source is proportional to and in phase with the grid voltage.
    If you are using a watt meter rather than just an ammeter and voltmeter you should see the GTI producing full power regardless of the load, since it will force power back into the grid unless it has specifically been configured for non-export operation.
    If the GTI has been configured for non-export operation (and relatively few models sold in the US can do that), then you could see the GTI delivering the real component of the volt-amps going into the A/C while the grid delivers the entire reactive component. You would see current from each source when the A/C is on.
    If you are using a true watt meter in both positions and are still getting the anomalous readings you describe, I would have to guess that the non-export sensing on the GTI installation is being confused by the low power factor of the A/C unit.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      Thank you for your extremely well-learned answer, sir.

      My GTI has been configured for non-export operation.

      I have been making some tests lately and I have observed that my AC (inverter split type) upon power on draws about 650watts and has a power factor of 0.85. With such a power factor, GTI can supply all of AC's power need.

      As the room cools down, AC power draw drops lower to about 450watts and power factor drops to about 0.70 to 0.75.

      When AC reaches "idle" speed, it draws about 320watts and power factor drops to 0.61 to 0.63, and that is the time that the GTI can't give the full power requirements of the AC and imports power from the grid of about 150w to 250w.

      I tried to place in parallel a capacitor across the AC but PF even dropped lower, so i think the over-all circuitry of the AC makes it a capacitive load instead of an inductive load.

      I think the 150w to 250w imported from the grid is not reactive power but real power. The electric meter turns.

      Do you know of any way on how to correct a low power factor of a capacitive load?

      What do you think of the behavior of my GTI? Do all GTI's require a high PF?

      Thank you sir inetdog!

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      • #4
        Originally posted by Erick View Post
        My GTI has been configured for non-export operation.
        Well that is just plain crazy talk. You just eliminated any chance of a payoff. You want to export every watt you can to recieve a credit and lower you bill. You just defeating the whole purpose of a GT System and loosing money.

        Originally posted by Erick View Post
        Do you know of any way on how to correct a low power factor of a capacitive load?

        What do you think of the behavior of my GTI? Do all GTI's require a high PF?
        Most can only go as low as .8 PF. Motors can go much lower than that all the way down to .4. Motor loads are inductive, not capacitive.
        MSEE, PE

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        • #5
          Originally posted by Erick View Post
          I think the 150w to 250w imported from the grid is not reactive power but real power. The electric meter turns.

          Do you know of any way on how to correct a low power factor of a capacitive load?

          What do you think of the behavior of my GTI? Do all GTI's require a high PF?

          Thank you sir inetdog!
          The best way to improve the power factor of a conventional motor (inductive) load is to parallel the right size capacitor with the motor downstream of the controls. That way the capacitor is not left connected to the line when the motor is not running, which would be a problem too.

          Note carefully that if the A/C uses a solid state motor controller to run the compressor at a lower speed when the demand is low, you must not connect anything between the motor and the variable frequency power source.

          Unless they are specifically manufactured to allow PF control, a GTI will have a minimum power factor with which it will work properly. And even when above that power factor there may still be reactive current coming from POCO.

          SunnyBoy 3000 US, 18 BP Solar 175B panels.

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