Grid impedance estimation for grid-tie inverters based on positive sequence estimator and morphological filter

This paper proposes a method for estimating the appropriate grid impedance for applications in which distributed generators are connected to the grid through three-phase inverters. The method is based on variations in active and reactive power injected by the inverters into the power system. These variations give rise to changes in the direct synchronous voltage $$v_{d}$$ and in the synchronous direct and quadrature currents $$i_{d}$$ and $$i_{q}$$ at the point of common coupling between the inverter and the power grid. These changes serve to compute the grid resistance and inductance seen by the inverter. This paper suggests the usage of a previously proposed technique for estimating the positive sequence from the grid voltage, so the method is robust against harmonic distortions. The technique is improved, employing a Fourier filter. To further enhance the method, a morphological filter is incorporated to filter out the synchronous currents. This filter is suitable to be applied in DC signals, like the ones related to synchronous reference frames. A complete evaluation of the proposed method is performed through simulations in MATLAB/Simulink environment. The results show that the positive sequence estimator and the morphological filter significantly increase the accuracy of the grid impedance estimation.