Amplitude design of perturbation signal in frequency-domain analysis of grid-connected systems

The rise of renewable electricity production has driven the power grid to a remarkable transformation, where a large share of the electricity production is integrated to the grid through power-electronic inverters. The inverters have fast internal dynamics and no inherent inertia, which makes the power grid prone to stability issues. The stability analysis to ensure system robustness can be performed based on the impedance ratio of the inverter and power grid. The grid impedance is often an unknown parameter, and methods for grid impedance measurements are required. Past studies have presented a number of measurement methods based on a broadband perturbation, such as pseudo-random binary sequence (PRBS), and Fourier techniques for obtaining the grid impedance. However, only a little attention has been paid to the injection-amplitude design, and most often, the amplitude has been selected based on trial and error. This work presents an algorithm based on the total harmonic distortion (THD) levels of grid currents and voltages for choosing a suitable perturbation amplitude. The proposed method makes it possible to fully automate the stability analysis of a grid-connected system. Experimental results based on a three-phase grid-connected inverter are presented and used to demonstrate the effectiveness of the proposed method.