Voltage Source Converters Connected to Very Weak Grids: Accurate Dynamic Modeling, Small-Signal Analysis, and Stability Improvement

The interfacing of the vector-controlled voltage-source converters (VSCs) into weak grid (WG) systems can induce severe instabilities. This is attributed to interactions between the vector control of the VSCs and the high impedance of the WG. The weak connection limits the amount of active power that can be injected by the VSC to the WG. In this work, the small-signal analysis is used to derive; first, a transfer function that helps to design the controller of the VSC output voltage based on the VSC-WG accurate dynamics; second, the full-order state-space model of the VSC-WG system. A modal analysis is then conducted to develop the participation factors to characterize the influencing states on the dominant modes of the system, followed by a sensitivity analysis to evaluate the influence of the vector control gains and other parameters on the dominant modes. The results of the modal analysis confirm that instabilities at the nominal power are inevitable for VSCs connected to very weak grids (VWGs). Inspired by this shortcoming, a novel compensation method is proposed to mitigate the dynamic instability of VSCs connected to VWGs. Finally, several offline time-domain simulations and hardware-in-the-loop (HIL) real-time experiments are conducted to verify the validity of small-signal analysis, validate the effectiveness of the proposed compensation method; and investigate the performance of the system under challenging scenarios such as sudden variations in the phase angle of the grid.