A Unified Current Control Scheme for Voltage Source Converter High Voltage Direct Current Under Unbalanced Grid Conditions

Abstract

This paper presents a novel unified dynamic model and vector current control strategy to improve voltage source converter (VSC) based high voltage direct current (HVDC) (VSC-HVDC) transmission system performance under unbalanced system conditions. Both the AC- and DC-side models are developed in the unique positive synchronously rotating reference (dq-p) frame with respect to both the positive- and negative-sequence components. Based on the model, the active power (or DC voltage) control is schemed with a proportional-integral regulator plus a resonant integrator, intended to eliminate the 2ω ripple in the DC-link, and reactive power control resorts to a resonant filter to mitigate the current harmonic distortion due to the ripple component. To prevent the sequential component decomposition, the current control of both the positive- and negative-sequence components is designed in a unified way in the stationary αβ frame. The proposed strategy can maintain rather low level AC current harmonics along with the DC-link 2ω ripple elimination, enhancing the VSC-HVDC operational ability under AC unbalanced conditions. The PSCAD/EMTDC simulation results verify the validity of the proposed strategy.