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Grid-tie inverters synchronize the phase and also check the quality (frequency, magnitude) and shutdown in case of errors to prevent what's called islanding, i.e. to prevent feeding current to network when there are some issues.

How is that done, exactly? I mean I understand how to measure voltage, phase, etc. from AC, but how can you feed current to the same line at the same time without messing up the measurement? Aren't you measuring what you feed instead of the original mains?

Is there a short off period to perform the measurement? How is it done?

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    \$\begingroup\$ You are feeding power and you have a wattmeter circuit that allows you to monitor the power you feed and make adjustments within a control loop. Why not look-up how generators come on line; it's the same issue but much much bigger and probably much better documented. \$\endgroup\$
    – Andy aka
    Oct 19, 2022 at 10:28
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    \$\begingroup\$ The inverter tries to shift the frequency... If it cannot then the grid is there, if it can then the grid is absent so it shuts down. Very basic description. \$\endgroup\$
    – Solar Mike
    Oct 19, 2022 at 11:08
  • \$\begingroup\$ Oh ok so it tries to drive a bit off phase. So if measure current from the A: grid line before the inverter, B:after the inverter and C: the wire that goes from grid to inverter. When inverter tries to shift phase and mains exists there is current in spot C which does not show up in A or B. Current that's occur when inverter tries to push grid out of sync. \$\endgroup\$ Oct 19, 2022 at 11:45
  • \$\begingroup\$ The PLLs take care of that. And they don't care about how polluted the grid is, only that they respect the fundamental's phase (whatever relative percentage it needs to have) -- and that's because their purpose is not cleaning the grid, it's synchronizing with it. As you may guess, the PLLs do not act instantaneously so, during transients, there are brief periods where the phases are misaligned, but that's only for the duration of the response of the inverter (there's more than the PLL in there). Since it's considered that the changes are slow relative to the period of the grid, it works. \$\endgroup\$ Oct 19, 2022 at 11:45
  • \$\begingroup\$ hmm of course it shows up in spot A. current from grid to inverter which does not show up after inverter. Is that how it works? \$\endgroup\$ Oct 19, 2022 at 12:00

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Usually the grid connection is a relatively strong source such that the current injected to the grid from the inverter does not significantly change the grid voltage that is being measured. However, if the grid connection is relatively weak such that this approximation is not good, then it can cause control problems for the inverter. Usually various phase-locked loop (PLL) designs are used for detecting the grid frequency. A new area of development is what are called "grid forming" inverters, which are able to provide a reference voltage to the system and help to regulate system frequency rather than just following whatever the system frequency is.

Here is a review article that covers the various methods of synchronization, and here is a magazine article about grid-forming inverters from a renewable generation perspective.

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