For the centralized development of multi-GW offshore wind farm clusters

constructing a multi-terminal heterogeneous transmission topology based on modular multilevel converters and diode rectifier units shows significant cost-reduction potential. However

under conventional control strategy

to maintain the voltage and frequency stability of the DRU-side wind farms

the capacity of back-to-back auxiliary converter will reach 30% of the system's rated capacity. This requirement greatly weakens scheme's economic advantage. To address this issue

we first thoroughly analyzed the dynamic commutation characteristics of the DRU and revealed the magnitude-phase complementary relationship between DRU and auxiliary converter. Furthermore

the mechanism that how reactive power balance influences the frequency of wind farms was also clarified. On this basis

a dynamic-steady-state hierarchical coordinated frequency control strategy based on wind farms and auxiliary converter was proposed

which effectively lowers the capacity requirement of auxiliary converter to 10% and restricts the dynamic frequency deviation of the offshore wind power collection system to within ±0.5 Hz under typical power perturbations. Finally

an electromagnetic transient model of the diode-based multi-terminal heterogeneous transmission system was built on the PSCAD/EMTDC simulation platform. Comprehensive simulations of both steady-state and dynamic response characteristics were performed

validating the feasibility and effectiveness of the proposed strategy.