Net damping criterion for stability analysis of grid-tied VSCs in DC voltage control timescale

Stability of grid-connected VSCs in DC voltage control (DVC) timescales (i.e., the frequency range of dynamics covering converter outer controls) has recently caught increased attention, while the existing approaches, such as eigenvalue analysis and dq-domain impedance analysis, have respective limitations on addressing these types of stability issues. This paper proposes an alternative net damping criterion dedicated for analyzing the DVC timescale stability in a multi-VSC system. This criterion is strictly mapped from the Nyquist stability criterion utilizing the gain margin concept, which preserves the advantages of the classical positive net damping criterion suggested by Canay [20]-allowing for decomposition analysis of a subsystem's contribution to the closed-loop stability in a single-input single-output (SISO) framework, but overcomes its deficiency of possibly erroneous prediction of system dynamic behaviors. Case studies show that the proposed criterion can correctly predict some unstable conditions (e.g., monotonic divergence) which cannot be identified by the classical net damping criterion. Additionally, the condition for when the classical criterion is available is also pointed out, the proposed criterion can also act as a complement of the classical criterion for stability examination.