Fault Current Elimination for Modular Multilevel DC/DC Converter Based on Hybrid HBSM and TDM

As key equipment in medium voltage DC (MVDC) systems, modular multilevel AC/DC and DC/DC converters (MM-AC/DC, MM-DC/DC) have drawn marvelous attractions. However, research on DC fault ride-through focuses on MM-AC/DC, and the fault current elimination for MM-DC/DC remains a research gap, which limits the wide application of the MVDC system. To fulfil this research gap, the contribution of this paper is revealing the fault current characteristics of MM-DC/DC based on half-bridge and full-bridge submodules (HBSM and FBSM) and proposing a novel MM-DC/DC based on hybrid HBSM and thyristor-diode module (TDM). By integrating TDM in the upper bridge arm of one phase and the down bridge arm of the other phase in MM-DC/DC, the MM-DC/DC achieves self-elimination of fault currents. The basic concept is using the energy at the healthy side to modulate a reverse voltage source (RVS) at the faulty side of MM-DC/DC, forcing fault current through TDM pass across zero. TDM can extinguish the resulting fault current. The parameter design and control strategy of the novel MM-DC/DC are discussed. Simulation is carried out for verification, and the results show that fault current can be eliminated within several milliseconds without causing excessive operating losses and costs.