  This paper proposes a computational method to efficiently and quickly
estimate stability regions of droop control slopes for modular multilevel
converter (MMC)-based multiterminal dc (MTDC) systems. The proposed method is
based on a general small-signal model consisting of a dc grid with arbitrary
topology and MMCs with dq controllers. The general small-signal model developed
by a systematic way can be used for small-disturbance stability analysis. To
verify the developed small-signal model, a comparison between the developed
model calculated in MATLAB and the detailed switching model simulated in
PSCAD/EMTDC is conducted, which demonstrates the accuracy of the developed
small-signal model. Based on the eigenvalues sensitivity and the Taylor Series
of eigenvalues, a set of inequality constraints are derived and used to
efficiently estimate the stability regions of all coupled slopes of the droop
characteristics. It is helpful for efficiently designing and adjusting the
droop controller parameters for the MMC-MTDC systems. The effectiveness of the
proposed method is demonstrated by the several examinations including the
supremum test and the stability region sketch on accuracy and feasibility.
