Power System Stability Impact Assessment for the Current Limits of Grid Supporting Voltage-Source Converters

A Thevenin network impedance at the point of connection and the impedance of a power electronic converter can be used to evaluate the stability of a power converter under test. With a surge in the power electronic converters in the power systems, the Thevenin network impedance itself is influenced by the power converter dynamics. This paper explains the small-signal stability impact of parallel-connected power converters present in the power system on the rest of the network using an intuitive impedance-based approach. Firstly, the paper compares the impedance of the power converter with a synchronous machine. The comparison study reveals that unlike a synchronous machine's impedance, the converter's impedance is highly dependent on the operating point. Furthermore, it is also shown that the power converter's dynamic impedance exhibits a discrete jump when an active or reactive current limit is triggered. In a converter dominated power systems, such features of the power converter's dynamic impedance can reduce the system's reliability, increase the Thevenin network impedance seen by other components, and potentially result in instability. Finally, the electromagnetic transient simulation results which support the findings of the paper are provided.