Enhanced DC Voltage and Frequency Regulation for High Voltage AC/DC Power Grid

This work proposes a primary frequency support mechanism for a voltage source converter (VSC) based multi-terminal high voltage dc (MTDC) transmission system that interconnects asynchronous ac grids. For the ac disturbance case, an augmented control strategy composed of a coordinated droop control (CDC) and a supplementary controller is developed. In CDC, the droop constants are determined considering the desired disturbance sharing ratio among the ac areas and the loading of the converters. On the other hand, the supplementary controller makes the designed droop constants provide the expected performance independent of the dc network parameters and the VSCs power losses. Therefore, the augmented controller can make the interconnected ac power systems exchange power according to the specified disturbance sharing ratio. In addition, the work introduces a new power rerouting technique to enhance the frequency and the dc voltage regulation of the ac/dc power system when a VSC trips. Furthermore, the droop constants for the nominal operation are designed to make the ac grids share the disturbance due to wind farm power variation based on their strength. Case studies are conducted for evaluation, and the presented technique shows enhanced performance in frequency (for instance, 38% improvement when ac disturbance happens) and dc voltage (for example, almost zero deviation when converter outage occurs) regulations compared to the existing mechanisms in the literature.