Converter System Nonlinear Modelling and Control for Transmission Applications—Part II: CSC Systems

The high-power self-commutated voltage-source converter (VSC) and current-source converter (CSC) are the key control devices in high-voltage direct current, flexible ac transmission systems, and distribution flexible ac transmission systems. To achieve the expected control objectives, suitable control strategies must be implemented based on the available devices, system models, and control techniques. The self-commutated ac/dc converters control the electrical power by generating controllable ac fundamental and dc average outputs. These controllable outputs are controlled by the conducting state combinations of the converter switching devices, driven by their gate signals. The gate signals are specified by fundamental parameters of frequency, amplitude, and phase angle. The converter system model for describing the relation between the system-state variables and the gate signal parameters is essential for the converter system control strategies. The companion paper (Part I) derives the state variable equations for the transmission systems using voltage-source-type converters. Part II is for the transmission systems using current-source-type converters. The self-commutated converter systems provide control flexibility of active and reactive powers, but their nonlinearity makes their control difficult. The linearized state equations using feedback linearization are presented to enable the controller design by using linear control theory.