A Soft-switched Hybrid DC Circuit Breaker for the Protection of Fusion Power Plant Electrical Systems

In recent years, the Danish nuclear fusion energy research programme (DANfusion) has been supporting the ITER project in France, and contributing to EUROfusion activities, for a DEMOnstration fusion power plant. DEMO aims to showcase advanced technologies for controlling powerful fusion plasma and safely generating electricity within the range of 300 to 500 megawatts to the external grid and to supply its own electrical power systems. DC circuit breaker is the core device to protect the fusion power plant electrical systems. It is used to interrupt the DC current to avoid overheating. In the planning phase, the designed operation current for the central solenoid and poloidal field coils in DEMO is 45 kA, and the current of toroidal field coils is 74.6 kA. In the planning phase, DEMO is actively investigating cutting-edge protection technologies that not only meet industrial viability but also ensure long-term reliability and high performance. This paper presents a soft-switched hybrid DC circuit breaker (CB) for FDUs in DEMO. The proposed CB comprises a mechanical bypass switch, an integrated gate-commutated thyristor (IGCT)-based static branch, and a thyristor-based counter pulse circuit. This design ensures a reliable zero current switching (ZCS) turn-off of the parallel and series connected IGCTs and thyristors, which avoids the risk of gate voltage oscillations and asymmetrical operations. Moreover, the IGCT-based static CB can interrupt the current under low-current schemes rapidly. The proposed DCCB offers two current interruption schemes for the FDU according to quench conditions, to improve reliability. A temperature-dependent discharge resistor made of mild steel is investigated in this design. Compared to constant-resistance solutions, the resistance of the discharge resistor is reduced, in turn, the peak voltage on coils is significantly reduced. The theoretical analysis and feasibility study of the proposed hybrid DCCB regarding each main component are presented. Simulation tests are conducted in PLECS/Plexim under various conditions to verify the effectiveness of the proposed DCCB.