Design of PLL gains to increase the stability margins of three-phase converters connected to weak grids

This paper investigates the stability of three-phase grid-tied converters using impedance based and eigenvalues methods. The stability analysis of the system with the first method is carried out using the generalized Nyquist criterion for MIMO systems, which provides a notion of the stability margin. The second one is used to give guidelines on the choice of the PLL gains by means of its damping factor and crossover frequency. It is proposed to increase the damping factor to improve stability margins of the system. Results in the time domain were obtained through a Hardware-in-the-loop platform, presenting a good correlation with the mathematical analysis in the frequency domain.