Transient DC-link voltage oscillation stability assessment of VSC-HVDC systems using Lyapunov direct method.

• A reduced-order model for assessing the transient DC-link voltage oscillation stability of VSC-HVDC systems is proposed. • Lyapunov direct method is applied to assess the transient DC-link voltage oscillation stability of VSC-HVDC systems. • The conservative stabilizing conditions based on two kinds of Lyapunov functions are derived. • Limit cycle of the converter-tied DC system is found and the asymptotic stability boundary is mapped based on limit cycle theory. • The system stability can be improved by tuning the system parameters. Lyapunov direct method is utilized in this paper to reveal the transient voltage oscillation stability mechanism for VSC-HVDC systems. Firstly, a reduced analytical nonlinear model for transient voltage oscillation stability assessment is proposed, which is a Liénard system. Based on the proposed model, a novel Lyapunov function is constructed and the conservative stability boundary is depicted, which reveals that the transient voltage oscillation can be well damped when the operating points are within the domain of asymptotic stability. In addition, it is found that there exists one limit cycle for VSC-HVDC systems. Once any system operation point exceeds the limit cycle, the divergent oscillations are triggered, and the aperiodic instability will eventually appear if the perturbations failed to be cleared in time. Finally, according to the limit cycle theory, one sufficient instability condition is derived and the transient voltage instability risk of the VSC-HVDC systems can be well evaluated. [ABSTRACT FROM AUTHOR]