Advances on inter-harmonic variable-frequency injection-based grid-impedance estimation methods suitable for PV inverters

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.