Improved Vector Control Strategy for Current-Source Converters Connected to Very Weak Grids

This paper investigates the interconnection of current-source converters (CSCs) to a very weak grid using the conventional vector control in the rotating reference frame. It is shown that the system stability is degraded under weak-grid conditions due to the implementation of the synchronous reference frame phase-locked loop (PLL). Supplementary controllers are proposed and integrated to the outermost control loops of the CSC to alleviate the associated negative impacts of the PLL. The proposed compensators do not alter the dynamic characteristics of the conventional vector control. More importantly, they enable the CSC to inject 1.0 per-unit of active power at a unity short-circuit ratio. Small-signal stability analysis and controllers design procedures are presented in details. The design of the proposed compensators is independent of the conventional vector control and hence no extensive coordination and parameters tuning are needed. Time-domain simulations results are presented to validate the effectiveness of the proposed techniques.