Iterative Solving and Morphological Analysis of Small-signal Stability Region for Hybrid AC/DC System with MMC-HVDC

With the development of HVDC transmission technology, regional power systems will expand into large-scale hybrid AC/DC systems. Therefore, it is necessary to build a large-scale hybrid AC/DC system model that is able to accurately depict both AC and DC dynamics to study the dynamics and stability of the hybrid system. Considering the detailed dynamics from both AC and DC sides, this paper establishes a state-space model of the asynchronously interconnected hybrid AC/DC system with modular multilevel converter based high voltage direct current (MMC-HVDC). The specific analytic solution form is given and a multi-layer iterative trust-region algorithm (MITRA) is derived. The proposed algorithm builds the mapping relation between the points in the decision variable space and the equilibrium points. Combining with the boundary identification based on the Hopf bifurcation, this paper proposes a multi-iteration method for solving the small-signal stability region boundary. After verifying the model accuracy by using the Simulink simulation, the stability region of the hybrid AC/DC system is plotted with the generator output as the decision variable. By using the eigenvalues and participation factors, the mechanisms of DC instability and AC instability as well as the influences of control strategies and parameters on the region boundary are all quantitatively studied. The stability regions of the multi-region hybrid system and the AC system are compared and analyzed. The results show that the introduction of the converter station and the DC network may cause the stability region boundary of the multi-region system to present an approximate piecewise linear topology characteristic determined by multiple constraints. © 2024 Automation of Electric Power Systems Press. All rights reserved.