Stable operating area evaluation of grid-forming MMC-HVDC system with renewable energy integration

This paper presents a method to obtain the operating area of the grid-forming modular multilevel converter based high voltage direct current (MMC-HVDC) system with photovoltaic and hydroelectric power integration. First, the steady-state mathematical model and small-signal model of the whole system are established and the stable operation constraints are analysed. Then the method to obtain the operating area of the system is proposed and the operating areas of MMC-HVDC under different shortcircuit ratio (SCR) are analysed. The results show that with the decrease of SCR, the stable operation area is mainly reduced due to the enhancement of voltage constraints, and the feasible range of reactive power of MMC-HVDC decreases. However, when SCR increases to 5, the system cannot transmit rated power due to small-signal stability constraint, which means that the voltage-frequency (VF) droop control is not suitable for strong AC system. Finally, the accuracy of the theoretical analysis is proved by time-domain simulation results.