DC Fault Current Estimation in a Multi-Terminal Hybrid MMC-HVDC System Considering Fault Ride Through Control

The DC fault current estimation is of importance to predict the worst scenario and optimize an HVDC system. Based on the hybrid modular multilevel converter (MMC) with the fault ride through (FRT), this paper investigates its DC fault current estimation method for a multi-terminal HVDC system. Instead of the conventional RLC model, this paper models the hybrid MMC as a circuit of RL and time-dependent controlled voltage source in series. The proposed model can reflect the control dynamics of the hybrid MMC in the DC fault transients. Moreover, this paper has established the relationship between the control parameters and the equivalent circuit model under two different FRT control methods. Furthermore, a method based on complex frequency domain analysis and mesh analysis is proposed for the fault current estimation of the HVDC systems. Subsequent simulations based on a four-terminal bipolar hybrid MMC-HVDC system validate the availability and accuracy of the proposed method.