Impact of Primary Frequency Droop Coefficients on Lyapunov Stability of Linearized Asynchronous AC-MTDC System

In this paper, requirement on frequency droop coefficients for the stability of AC-MTDC system is studied. Two models, each with a certain level of modeling detail are employed. One is a simplified Nth-order model and the other is detailed full-order model. Using the Nth-order model, stability boundaries for several cases are numerically constructed in the space of frequency droop coefficients. Based on observation made form these boundaries, necessary and sufficient condition for stability is hypothesized. The ‘Stability Hypothesis’ is then proved for a couple of specific cases using Lyapunov method. Finally, the detailed model is used to validated the hypothesis in a practical setup through time-domain simulations.