Conventional offshore wind farm (OWF) integration systems use 35kV or 66kV AC cables to collect the wind energy to the offshore modular multilevel converter (MMC) platform. When the scale of OWFs increases

the number of collection cables and the cost of offshore platforms increase significantly. Aiming at the 2GW and above offshore wind power integration scenario

the hybrid medium frequency AC/DC collection and high-voltage direct current (HVDC) transmission system for OWF integration is proposed

and the optimal AC collection voltage and medium frequency are designed. Firstly

the topology of the system is introduced. The OWFs near the offshore converter platform utilize medium-frequency AC collection

while remote OWFs use DC collection. The AC/DC power is then transmitted through the diode rectifier (DR)/MMC. Next

the operation characteristics of the system are analyzed

and the coordinated control strategy of the system is designed. Then

the effects of medium-frequency AC collection voltage and frequency on system performance are analyzed. Furthermore

the economic analysis model of the integration system is established to calculate the system investment and loss cost. Finally

the system operation characteristics under different AC collection voltages and frequencies are compared through PSCAD/EMTDC simulation analysis. The results show that the optimal medium frequency AC collection voltage and frequency are 110kV/175Hz.