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How do modern grid-tie invertes which are commonly found in solar pv panels and wind turbines actually stop islanding from occurring when the utility grid is down? I know there are a lot of methods which are used to prevent islanding but which method is the most common?

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Answer from a non-electronics person who's been reading up on this over the last few days. Depends on standards and things (VDE-AR-N 4105 being a common one), but here is a non-exhaustive list:

  1. Voltage change
  2. Frequency Change
  3. Rate of Change of Frequency (RoCoF, dF/dt)
  4. Vector shift

Most of them makes intuitive sense I think, at least if your generator is mechanical in nature. A change in load usually causes your engine/power source to slow down, causing a drop in voltage, a change in frequency, and a vector shift. The vector shift is when a momentary slow-down causes a cycle to take sligthy longer than the previous cycle, so for example on a 50hz grid where a cycle is supposed to take 0.02 seconds, it might take 0.22 seconds for just two cycles before returning to normal, but that might be enough of an indication that islanding has occured.

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They use anti-islanding measure with the help of phase locked loop(PLL) to detect the frequency of the grid.In case of absence of grid the PLL output sensed that is passes through a phase detector and then compared to a dc offset allows the system to acknowledge the absence of grid.

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