Stability limitation and analytical evaluation of voltage droop controllers for VSC MTDC

This paper presents a systematic analysis of DC voltage stability of a multi-terminal VSC-HVDC (MTDC) system, with the emphasis on a comparative study of the most ubiquitous droop control configurations. The paper introduces a general framework for the analysis of various droop control configurations employed in MTDC systems. This framework is then used to compare leading droop control configurations in terms of their impact on the relative stability, performance and robustness of the overall MTDC system. A generalized analytical MTDC model that contains detailed models of AC and DC system components is derived. Limitations imposed by DC power flow, DC inductor, cable modeling and AC network impedance on DC system stability are identified. Classical and multivariable frequency response analysis and eigenvalue analysis are applied to open-loop and closed-loop models to compare the stability and robustness of five leading droop controllers, with the focus on feedback signal selection and controller parameterization. This paper also proposes an active stabilizing controller, which takes the form of a modified constant power control, to enhance the controllability and robustness of the DC voltage control.