A Grid Impedance Estimation Method Robust Against Grid Voltages Unbalance and Harmonic Distortions

In this paper, a grid impedance estimation procedure based on the injection of active and reactive powers through a grid-tied inverter is proposed. The estimation is based on Newton-Raphson algorithm acting on the phasors of the voltages and currents at the point of common coupling between the inverter and the grid. Unlike, similar procedures, based on power injections, the method is implemented in the alpha beta frame, avoiding Park transformations. This is an advantage because dq quantities are prone to be too noisy, or oscillating in unbalance grid conditions or in the presence of voltage harmonics. Moreover, the proposed method allows variations of active and reactive to be imposed simultaneously, differently of methods relying on dq voltages and currents. This feature speeds up the estimation. The voltages and current phasors used to estimate the impedance are determined with the aid of a positive sequence extractor and Fourier algorithm providing the method robustness against unbalance grid conditions and harmonic distortions. Besides the impedance estimation, this paper proposes a technique in which the impedance information is applied to enhance the inverter's control. Here, the impedance value is used with a Routh criterion to assure the system's stability. The propositions are verified with PSIM simulations and with a Hardware-in-the-Loop real-time platform.