The series diode rectifier-modular multilevel converter (DR-MMC) based DC transmission system has significant potentials for application in large-scale offshore wind power integration. However

the integration system encounters challenges in starting directly from land due to the unidirectional conductivity of DR and the difference in DC voltage between offshore and onshore MMCs. Therefore

a startup strategy comprising three stages

such as uncontrolled charging

semi-controlled charging

and fully controlled startup

is proposed. Firstly

the topology and control strategy of the integration system are introduced. Then

for the uncontrolled charging stage

the charging characteristics of each sub-module (SM) are analyzed based on the equivalent circuit

and the calculation method for the current limiting resistor is proposed. Aiming at the issue of some SM being unable to deblock due to insufficient energy during the uncontrolled charging stage

a semi-controlled charging strategy for SM energy balancing is proposed. For the fully controlled startup stage

a coordinated startup control strategy of AC wind farms

offshore MMC and onshore MMC is proposed on the basis of the bidirectional power transmission characteristics of MMC and low voltage ride-through capability of AC wind farms. Finally

the effectiveness of the proposed startup strategy is validated through PSCAD/EMTDC simulations.