Impedance-based and eigenvalue based stability assessment compared in VSC-HVDC system

This paper presents the comparison between the impedance-based and eigenvalue-based stability analysis methods for a VSC-based HVDC system. In order to apply the impedance-based method, an impedance model for the VSCs is analytically derived and the derived model is validated by comparing the frequency responses of the analytical impedance and the impedance measured in a detailed switching model of the VSC-HVDC system. To determine the stability from the eigenvalue based method, an analytical state-space small-signal model is developed and the model is validated by time domain simulations. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of impedance ratio indicates that the system can have harmonic oscillation; however the system still operates stably. A weakness of the impedance method is that the stability determined by this method is not a global stability assessment; and it is therefore necessary to investigate the stability at all possible sub/systems. On the other hand, the eigenvalue based method can determine the stability of the entire system; but it cannot predict harmonic oscillations caused by a PWM inverter operating in a stable point. A two terminal VSC-HVDC system has been developed analytically and the frequency domain stability analysis based on impedance and eigenvalues has been carried out. The theoretical analysis has been further validated by simulation and experiments.