Control sensitivity indices for stability analysis of HVDC systems

This paper presents a new concept called the “Control Sensitivity Index” or CSI, for the stability analysis of HVDC converters connected to weak AC systems. The CSI for a particular control mode can be defined as the ratio of incremental changes in the two system variables that are most relevant to that control mode. The index provides valuable information on the stability of the system and, unlike other approaches, aids in the design of the controller. It also plays an important role in defining nonlinear gains for the controller. This paper offers a generalized formulation of CSI and demonstrates its application through an analysis of the CSI for three modes of HVDC control. The conclusions drawn from the analysis are confirmed by a detailed electromagnetic transients simulation of the AC/DC system. The paper concludes that the CSI can be used to improve the controller design and, for an inverter in a weak AC system, the conventional voltage control mode is more stable than the conventional γ control mode