With the continuous increase in the penetration rate of new energy in our country

the trend of "hollowing out" the power grid is becoming increasingly evident. The decline in grid strength has led to insufficient reliability in the power transmission of the current DC system. Based on this

this paper proposes a full- controllable composite converter topology(F3C) based on reverse-blocking IGCT devices. On the premise of ensuring the reliable transmission of high-capacity active power

it realizes the active support for weak power grids. This paper begins by analyzing the topology of the converter

its power constraints

and operational characteristics

delineating the power operating range of the F3C converter. It proposes a coordinated decoupling control strategy for F3C-HVDC

which is then validated through simulation from three aspects: the steady-state response characteristics of the system

power decoupling characteristics

and AC fault ride-through characteristics. Finally

a comprehensive evaluation of F3C is conducted from an economic perspective. The results show that the steady-state step response speed of F3C is less than 0.5 s and DC system has the power decoupling control capability. When transmitting rated active power

it has a steady-state reactive power support capacity of more than 0.3 pu. Under AC faults

it can resist commutation failure. Moreover

by increasing reactive power during the fault

the fault recovery time can be reduced by 48%. When only considering the requirements for devices and capacitors

the cost of the F3C valve with the same capacity and the same support capacity is only about 63.7% of that of the IGBT-MMC. The work presented in this paper holds certain reference significance for the research and construction of ultra-high voltage direct current (UHVDC) transmission projects in our country.