The establishment of the dual-carbon goal has promoted the rapid development of renewable energies.The access of large-scale power electronic equipment makes the power grid present a low inertia characteristic.Flexible DC transmission systems based on the modular multilevel converter(MMC)can provide inertia support for the new energy system by using virtual synchronous generator(VSG)control to simulate the mechanical characteristics of synchronous generators in the scenario of connecting to a weak grid.This can provide inertia support for the new energy system.To satisfy different working conditions

the converter station needs to switch between two control modes

including grid-forming control and grid-following control.In this paper

a seamless switching technique between grid-forming and grid-following control is proposed for MMCHVDC converter stations. Without modifying the inner-loop current control

a unified outer-loop reference current is constructed for both control modes

ensuring continuity of the output voltage's phase angle and amplitude during the switching process.Simulation results show that the proposed seamless switching technique can effectively reduce the impact on the system caused by the control switching process.