Robust multivariable control design for HVDC back to back schemes

摘要

A robust multivariable control design procedure for HVDC back-to-back schemes is presented. HVDC links are commonly controlled with a decentralized structure, whereby in normal operation the rectifier may track direct current and the inverter may track direct voltage, firing angle or extinction angle. In the case of a back-to-back HVDC link the measurements of control variables of both sides are physically close, hence it is possible to apply a true multivariable controller whereby current and voltage are controlled by rectifier and inverter together. H-infinity control theory is used for a systematic tuning procedure which incorporates robustness against uncertainty in the short-circuit levels of both rectifier and inverter side AC systems. The linear control transfer functions for rectifier and inverter controllers are arrived at simultaneously thereby removing iterative design steps to find the best combination of control parameters for both rectifier and inverter controllers. The decoupling effect resulting from the multivariable controller structure reduces the interaction between tracking of direct current and direct voltage. It is verified by nonlinear EMTDC simulation that the designed controller is robust and can contribute to good fault recovery when integrated with other control loops and nonlinear control functions as implemented in manufactured controls.