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A GTI has a great many technical characteristics to behave as a voltage source like the grid with  unity pf current under specifications from utilities, safety agencies and other regulatory bodies. This improves the load regulation errors shared by others on the grid and yet follows the phase and frequency drift of the grid to regulate overall load that increases with frequency from rotational loads.  There would be a great amount of disturbances of all types in the short and long term that require sensible detection of what to react to and what to ignore. This is necessary to have accurate references and measurements and transform the error to make incremental corrections, both to protect itself and drive the grid and meet all the specifications.

In order to prevent oscillations and accurate way to measuring grid impedance and dampen the current measured that is injected.  There are many types of PLLs and control loops such as inverse Park Transform,  Hilbert Transform, and Double Frame of Reference are samples.

A chapter starting p300 and appendix C focus on damping resonance.

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