New and stronger power quality requirements are issued due to the increase amount of photo-voltaic (PV) installations. In this paper different methods are used for continuous grid monitoring. By injecting a non-characteristic harmonic current and measuring the grid voltage response it is possible to evaluate the grid impedance directly by the PV-inverter, providing a fast and low cost implementation. This principle theoretically provides a correct result of the grid impedance but when using it into the context of PV integration, different implementation issues strongly affect the quality of the results. This paper presents a new impedance estimation method including typical implementation problems encountered and it also presents adopted solutions for on-line grid impedance measurement. Practical tests on an existing PV-inverter validate the chosen solutions.