Stability of a Voltage Source Converter subject to Decrease of Short-Circuit Capacity: a Case Study

The subject of this paper is the investigation of the stability of a Voltage Source Converter (VSC) undergoing a sudden decrease of the Short Circuit Capacity of the AC system to which it is connected. A case study is reported on a simple system including an HVDC terminal and a Thévenin equivalent. First, a small-signal stability analysis is performed showing that the PLL-based vector control may become unstable at powers lower than the theoretical static stability limit. Dynamic simulations demonstrate that the stability limit may further decrease depending on the disturbance. The need for a fast instability detection method is highlighted.