Nonlinear Control of Asynchronous BTB-Link

The high power self-commutated VSC are the key control devices in HVDC, FACTS and DFACTS systems. The self-commutated voltage source converter back to back (VSC BTB) link connects two ac power systems asynchronously, can control the active power exchanged between the two ac system and the reactive powers at the both terminals independently. To achieve the expected control objectives suitable control strategies for control of the VSC-BTB link system must be implemented based on the available devices, system modeling and control techniques. The VSC-BTB link system model, describing the relation between the system state variables and the switching device states, is essential for control the VSC-BTB link system. An average state model for the VSC-BTB link is derived, presenting affine nonlinearity. The self-commutated converter systems provide control flexibility of active and reactive powers, but their nonlinearity makes their control difficult. The linearized state equations transformed by using feed back linearization are presented to allow the controller design by using linear control theory. The linearization is valid globally to insure the four quadrant controllability and high performances.