Control and Impedance Modeling of Offshore Wind Power Hybrid Cascaded DC Transmission System Based on VSC and DRU

Offshore wind power integration system applying hybrid cascaded DC transmission has the advantages of high voltage, large capacity, and low investment cost, but its stability has gained many concerns. And the system impedance modeling is the basis for analyzing stability. In this paper, a detailed mathematical model of VSC-DRU hybrid cascaded valve station is established. In order to realize active power distribution and AC voltage control of the DC transmission system, the power-voltage control strategy of VSC with filter capacitor current feedback is studied. Considering the influence of output phase-angle adjustment of phase-locked loop, active damping control, and power control, the detailed impedance model of VSC-DRU hybrid cascaded valve station is derived and analyzed by using small-signal disturbance method, which provides a basis for studying stability. Finally, the simulation results in Matlab/Simulink show that the proposed control has good performance of power-voltage regulation. And the correctness of the established detailed impedance model is verified by the frequency-scanning method.