Interoperability assessment of MMC and DRU connected offshore windfarms in meshed multi-terminal DC grids

In the last decade, a number of academic and industry studies have identified diode rectifier unit (DRU) as a potential replacement for offshore modular multilevel converter (MMC) of DC connected offshore windfarms. However, side-by-side operation of DRU and MMC connected windfarms in multi-terminal DC grid will present new operational challenges. Therefore, this paper will study the interoperability of a minimum meshed DC grid, which includes MMC and DRU connected offshore windfarms. To identify any potential issues that may arise from introduction of DRU, the system performance during onshore AC faults are simulated using PSCAD models. Simulation results show that the DRU connected windfarm exhibits different behaviours with the MMC based equivalent, but does not adversely impact the DC grid performance. Instead, the use of DRU improves DC grid performance with its inherent sensitivity of active power transmission to DC voltage variation.