Control Strategy for VSC-HVDC Power Quality Improvement under Unbalanced Grid Conditions

This paper presents a novel unified dynamic model and control strategy to improve the power quality for VSC based high voltage direct current transmission system (VSC-HVDC) under unbalanced grid conditions. The unified models present the dynamic behavior of VSC-HVDC in the unique positive synchronously rotating reference (dq-p) frame with respect to both the positive- and negative-sequence components. Based on the unified model, a strategy to either eliminate the dc-link ripple or achieve the balanced currents, along with a rather low level harmonics in each grid is introduced by resorting to the resonant integrator and filter based scheme in the two cascaded control loops. The resonant integrator and filter scheme enables effective control of the positive- and negative-sequence currents, and avoid the sequence component decomposition. The simulation studies in PSCAD/EMTDC verify the improved performance of VSC-HVDC system regarding the power quality and the ride-through capability.