Improved Vector Current Control for the VSC-HVdc Converter Connected to a Very Weak AC Grid

This paper proposes an improved vector current control (IVCC) strategy to integrate voltage source converter (VSC) based high voltage direct current (HVDC) to a very weak grid. In comparison to a constant value as used in the outer control loop in classical vector current control (VCC) strategy, the control references are adjusted according to the dynamic changes of system frequency. A small signal model including the system and the proposed control strategy is firstly developed. Based on the eigenvalue locus and the damping ratio analysis, the impact of adjusting different control loops is then investigated. Furthermore, an optimization model is built to coordinate the outer voltage control loop and the outer active power control loop of IVCC such that a wider range of phase-locked loop (PLL) gains can be applied, while reduced deviations of frequency and point of common coupling (PCC) voltage as well as a good enough fault-ride through capability can be achieved simultaneously. Finally, the effectiveness of applying IVCC in connecting VSC to a very weak grid is validated by detailed time domain simulations.