Unified adaptive droop control design based on dynamic reactive power limiter in VSC-MTDC

A unified adaptive droop control strategy for voltage sourced converter based multi-terminal high voltage direct current transmission (VSC-MTDC) system is proposed in this paper. The proposed strategy designs VSC-MTDC control system in a unified way to both improve the performance of active and reactive power control. On the reactive aspect, the voltage support ability of voltage sourced converter (VSC) is enhanced by the introduced dynamic reactive power limiter. The dynamic limiter can improve the reactive power support ability of VSC and guarantee the necessary active power being transferred. On the active power aspect, the traditional droop control of VSC-MTDC is improved by unified adaptive droop control. By considering the headroom of the whole active and reactive power capacity of VSC, the unified adaptive droop control strategy can realize reasonable distribution of the unbalanced active power caused by system disturbance. Compared with the fixed coefficient droop control and the conventional adaptive control, the proposed strategy will not only release the potential reactive power control ability of VSC-MTDC, but also can distribute the unbalanced active power properly to each converter. The reactive power control and active power control are designed on local substation level and system level respectively; each control level has its independent function and will not interfere with each other.