Steady state stability of HVDC systems

This paper presents the concept of a “Control Sensitivity Index” for the analysis of the stability of HVDC converters connected to weak AC systems. The index can be defined for a particular control mode as the rate of change of the controlled quantity with the controlling quantity. The index is calculated based on the steady state equations of the combined AC-DC system and provides valuable information about the stability of the system and the design of the controller. For example, a sign change in the index normally indicates onset of instability. The index also plays an important role in defining nonlinear gains. The theoretical results derived from these indices are verified in this paper against a detailed electromagnetic transients simulation of a system based on the First CIGRE Benchmark for HVDC systems