Stability analysis of interconnected AC power systems with multi-terminal DC grids based on the Cigré DC grid test system

This paper presents selected case studies of small-signal stability analysis for power systems consisting of interconnected AC and DC sub-systems. The analysis is based on the Cigré DC-grid test system, which is designed to resemble a generalized future configuration of multi-terminal HVDC (MTDC) systems. The small-signal stability of the system is investigated to identify critical modes associated with different parts of the interconnected AC and DC power systems. The analysis is presented in several steps in order to identify interaction problems and critical modes that can occur with different typical system configurations. Firstly, a case with point-to-point HVDC connection is analysed, followed by the analysis of a four-terminal DC system, before the stability of the entire Cigré DC grid configuration is studied. The state variables participating in critical modes, and the parameter sensitivities of these modes are identified and discussed. The obtained results illustrate particular challenges that can occur during a step-wise construction of a large-scale MTDC transmission system.