The aim of this paper is to provide some original and recent advances regarding the implementation of grid impedance estimation techniques based on additional inter-harmonic injection. The effect of the type of injection (i.e. current or voltage) is analysed first and the most convenient approach justified by means of simple qualitative considerations and simulations. A convenient injection pattern and a novel processing algorithm based on a moving window Discrete Fourier Transform (DFT) and averaging filter is introduced and analysed. The method provides a reduction of the number of injected bursts needed to obtain a certain impedance estimation accuracy with respect to state-of-art proposals, therefore leading to a reduction of both current THD and harmonic power losses. Finally the effect of the spectral leakage due to the non-integer ratio between processing sampling and the inter-harmonic injection frequencies, especially in case of natural variations of grid frequency, are fully analysed and a simple but efficient and original solution based on samples interpolation presented.
