Analysis of sub-synchronous interaction and damping characteristics of weak grid-connected high inertia energy storage synchronous condenser system

Due to the fixed rotor speed and insufficient inertia of conventional DC synchronous condenser (SCs), the high inertia energy storage synchronous condenser (HIESSC) is considered as a promising solution for providing both voltage and frequency support for the grid. However, as a new synchronous condenser, the HIESSC may suffer sub-synchronous oscillations (SSO) because of the interaction between the HIESSC and weak grid. Moreover, there are limited literatures addressing the analysis of internal sub-synchronous interaction within the system. For this issue, the sub-synchronous interaction and damping characteristics of the weak grid-connected HIESSC system are investigated in this article. Firstly, a complete closed-loop transfer function block diagram of the system is developed. Then, in the phase-locked loop (PLL)-dominated SSO mode, the damping paths of the system and dynamic interaction processes in the HIESSC grid-connected system are analyzed. On this basis, the damping contribution of HIESSC and sub-synchronous interaction are evaluated quantitatively by drawing on equivalent damping coefficients in damping torque analysis (DTA). Finally, the effectiveness of the theoretical analysis is validated by simulations and experiments.