Research on the selection of low-voltage control reference and differential protection adaptability for offshore wind power

After a fault occurs in the offshore wind power AC collection line, the offshore MMC converter station can adopt a low-voltage current-limiting strategy to reduce the voltage reference value, thereby limiting the rise of fault current. However, there is currently a lack of research on the electrical characteristics of fault quantities in the AC collection line under the low-voltage current-limiting strategy. Based on this, this paper investigates the strategy of low-voltage current limiting adopted by offshore MMC converter stations after a fault, and establishes the physical circuit equations along with the control equations considering low-voltage current limitation. To ensure the equations are analytically solvable, the fast-response inner current control loop and the q-axis electrical quantities with small variation amplitudes are neglected, reducing the order of the equations and obtaining the analytical solution of fault electrical quantities for three-phase short-circuit faults. Finally, an electromagnetic transient simulation model is constructed in PSCAD/EMTDC, verifying the accuracy and feasibility of the proposed calculation method considering the low-voltage current-limiting strategy, with an error in the fault current of approximately 5%.