A Backup Hybrid DC Circuit Breaker Design and Fast Discharge Units for DEMO Superconducting Magnets

The Plant Electrical System (PES) of the European DEMOnstration power plant (DEMO) is under conceptual design. The fast discharge units (FDUs) are vital components within the coil power supply (CPS) system. This paper proposes a tentative backup protection scheme for FDUs in DEMO. The proposed FDU consists of a main circuit breaker (CB) and a backup CB, which can rapidly discharge the coil energy in case of a quench into discharge resistors. The design of the backup CB is determined by the rating of superconducting (SC) magnets, the configuration of main CBs, and the ambient conditions of FDUs. Different DC interruption technologies in fusion devices are evaluated. The design criteria for the backup CB are discussed. Based on this, the paper proposes a backup hybrid CB which can discharge the stored energy rapidly and safely in the event of main CB malfunctions. The proposed backup CB comprises bypass switches, integrated gate-commutated thyristors (IGCTs) -based solid-state branches, a counter-pulse circuit. It facilitates solid-state branches zero current switching (ZCS) operation during the current interrupting process. A tentative backup protection trigger scheme is presented, which considers both the main CB and the backup CB operations. Simulation tests are conducted to validate the effectiveness of the proposed backup protection CB, ensuring the reliable and safe operation of the FDU during coil quench.