Modeling and Stability Analysis of VSC Internal Voltage in DC-Link Voltage Control Timescale

With increasing application of voltage source converters (VSCs), electromagnetic stability, characterized by fast dynamics and controls of electromagnetic elements such as inductors and capacitors, becomes challenging for power system stability. This paper investigates modeling and stability analysis of VSC internal voltage in dc-link voltage timescale. Concepts of VSC internal voltage inertia, damping power, and synchronizing power are introduced to provide a physical insight into VSC stability issue. Internal voltage stability can be identified by analyzing damping and synchronizing powers. Insufficient of either damping power or synchronizing power will result in loss of stability. Furthermore, internal voltage stability affected by short circuit ratio (SCR), operating point, and phase-locked loop (PLL) bandwidth is analyzed by investigating their effects on damping and synchronizing powers, respectively. It is found that internal voltage stability becomes deteriorated due to the insufficient damping power with low SCR values, high operating powers, and PLL bandwidth getting close to dc-link voltage control bandwidth in a weak grid. Simulation results and experimental results verify the analysis.