Stability Analysis of Droop-Based Converter Using SISO Method from DC Side Perturbation

Droop-based converter plays an important role in future multi-terminal DC (MTDC) system due to its control flexibility, which imposes a great challenge on the stability of the DC grid. Traditional input impedance and eigenvalue analysis methods fail to build the mathematical relationship between the system operating points and the related critical parameters that keep the system stable. The main objective of the paper is to examine the DC side stability of the single grid-connected VSC with the single input single output (SISO) method. The transfer function between the small perturbation of DC-link voltage and DC current at the end of transmission line is firstly constructed considering the control impacts of droop-based converter and the dynamics of DC networks. Based on SISO model, the stability criteria of single VSC converter under different control modes is further proposed by using the classical Routh Judgement. In addition, the sufficient stabilizing conditions and DC side stability boundary are accordingly obtained based on the derived stability criteria. It is worth mentioning that, a critical operation index, namely, DC power transfer limit (DCPTL) is firstly defined for the droop-based converter operating as an inverter, and DC system instability might occur when the transmitted power exceeds this limit. Finally, numerical simulation results of two-terminal HVDC system validate the accuracy of the proposed stability analysis and the criteria for the droop-based converter.