The fault current characteristics of DC faults in offshore wind power systems where diode rectifier unit (DRU) is connected in parallel with modular multilevel converter (MMC) were researched to clarify the time-domain evolution mechanism of DC inter-pole short-circuit fault currents in DRU-MMC parallel offshore wind power transmission systems. The topological structure and operational characteristics of parallel DRU-MMC offshore wind power system were taken into consideration

and the mechanism of fault current injection was analyzed. Moreover

a method for fault current calculation was proposed on the basis of operational circuits after considering the continuous current injection by the DRU at the sending end and the discharge process of the MMC capacitor at the receiving end. The time-domain analytical expressions for the fault current were derived after considering the dominant factors and individual current components in fault loop before and after the MMC locking. Finally

the proposed analytical method was validated by comparing with simulations on the PSCAD/EMTDC platform. The influences of varying parameters and coupling relationships of bridge arm reactance

sub-module equivalent capacitance

DRU transmission power

and line equivalent resistance to the DC fault current were analyzed. The results indicate that the proposed analytical method can be adopted to accurately describe the variation patterns of DC pole-to-pole short-circuit fault currents in paralleled DRU-MMC system

with an error margin of less than 10%. This study provides a reference for the rapid calculation of fault currents

the design of DC breaker parameters

and the value of protection settings for DRU-MMC systems in offshore wind farms.