DC Voltage Compensation-based Inertia Coordinated Control of VSC-MTDC Integrated Offshore Wind Farms

To realize the frequency support of voltage source converter-based multi-terminal DC (VSC-MTDC)-integrated offshore wind farms (OWFs) system to the main grid, this paper proposes a DC voltage compensation-based inertia coordinated control (VICC) scheme. The VICC establishes the frequency connection between the offshore grid and the main grid by changing the MTDC DC voltage so that the wind turbines can emulate the synchronous generators' inertia response through its additional supercapacitors charging/discharging after sensing the main grid frequency dynamics. According to the P-u_dc-f droop control established in the grid-side VSC, the nonlinear coupling characteristics of DC voltage and main grid frequency under the influence of OWFs inertial power are analyzed. Then, a DC voltage compensation control scheme based on local information measurement is proposed to offset the influence of OWFs inertial power on frequency transmission so that the OWFs can accurately emulate the main grid frequency dynamics and provide accurate inertia support to the main grid. In addition, a small-signal model of VSC-MTDC-connected OWFs is established, and stability analysis is implemented. Finally, a simulation model of VSC-MTDC connected OWFs is established in MATLAB/Simulink. The VICC is verified and evaluated under load changes and grid faults. The results show that VICC can accurately couple the offshore grid frequency with the main grid frequency without communication, enable OWFs provide inertia support to the main grid through VSC-MTDC, significantly improving the main grid frequency stability.