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A better knowledge of the grid impedance is essential in order to improve power quality and control of the Distributed Power Generation Systems (DPGS) and also for a safe connection or reconnection to the utility grid. An LCL-filter associated to a Voltage Source Inverter (VSI) is usually used in DPGS based on renewable energy. The VSI controller is used to determine the grid impedance in both healthy operation and faulty modes. When grid faults occurence, the grid impedance variations have a considerable influence on the control and the stability of the system, even on the decision to disconnect the DPG systems of the network. This work presents a rapid and simple technique to detect the grid impedance variation and to determine the grid impedance before and after grid faults accurs. Implementation on FPGA control board, simulations and experimental results are presented to validate the described method.