Unified Power Flow Calculation for Multi-terminal Direct Current (MTDC) Considering Droop Control and Modulation Index

Power flow calculation of AC/DC hybrid power grids with multi-terminal voltage source converters (VSC-MTDC) is the basis for stable operation of hybrid power grid. This paper incorporates different VSC control strategies into the power flow, and considers adaptive droop control coefficients, modulation index as variables of power flow model. Then droop coefficient value and modulation of VSC under different states of hybrid power grid are obtained. The article establishes direct relation between VSC control parameters and output power, through transferring the circuit of VSC stations which connects to AC power grid. It unifies the power flow calculation methods of the VSC-MTDC in hybrid power grid under various control strategies. In order to prevent the VSC modulation index (MI) from exceeding the limit, the modulation of VSC as a variable of power flow equation can be obtained from power flow results. MI value can be analyzed through power flow to avoid overmodulating of VSC. Finally, an example verifies the validity of the model and fast convergence of the method.