Protection of Modular Multilevel Converter against DC fault current in MTDC systems and its reliable operation under internal switch failures and unbalanced AC faults

• Applicant: Professor Dr.-Ing. Hossein Imaneini, Ph.D.
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540869154

This project investigates the protection of MMC (or hybrid MMC) against DC fault current in multi-terminal DC (MTDC) systems and the fault-tolerant operation of MMC against internal switch failures and unbalanced AC faults. Particular emphasis is put on developing new solutions to reduce the peak of fault current in MMC arms and hybrid circuit breakers (CBs) in DC faults. The adoption of a reliable fault detection algorithm in MTDC systems, the coordination of hybrid CBs with MMC stations, utilization of hybrid MMCs with fewer full-bridge submodules, and modification of the MMC control system are steps to reduce the breaking duty of hybrid CBs and current rating of MMC stations and to develop technically and economically feasible solutions for future MTDC systems. In addition to protecting against external DC faults, the fault-tolerant operation of hybrid MMCs under internal switch failures and unbalanced AC faults is investigated to maximize the availability of MMCs. Each fault condition imposes a particular imbalance on the hybrid MMC. This imbalance affects the MMC in terms of power/energy distribution into the converter arms, increasing the 1) voltage ripples (and overvoltages) in SMs, 2) energy imbalance among half-bridge and full-bridge submodules, 3) circulating current in MMC arms, and 4) fluctuations in DC- and AC-side currents. All these variables should be kept within certain limits to maintain operational reliability, especially when the modulation index is greater than one. Finally, we will use simulations and experiments to verify all developed methods and confirm their validity and applicability.

• DFG Programme: Research Grants