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In order to have a unity power factor, you have to sync current with grid voltage. You must define your control system specs' Phase error, lock time before going online. All the PLL parameters, lock time, jitter, THD with a step load, steady state, harmonic grid noise rejection ( because phase detectors are harmonic, a current sense feedback filter must reject the harmonic inputs fundamentals without any phase shift. Huawei (2000 model) make the best GTI's so copy their Eng specs. if you can get the detailed ones.
As overvoltage will appear as a short from the grid, the current loop for amplitude and phase must be very stable and have remote control parameters. Your transformerless connection to the grid has an impedance which will define your step load regulation error.