Protection from internal faults in a special high power switching conversion system for thermonuclear fusion application

This paper compares different provisions to protect a special high power switching power supply from internal short-faults, in particular internal short-circuits. The power supply is based on a five three-phase NPC inverters, with common dc-link, rated for a total power of about 60MW and for an output voltage of 6.5kV, using IGCTs as switching components. A large, distributed, capacitor bank is connected at the dc-link side of the inverters, with huge amount of stored energy, as high as 760kJ during nominal operation. In case of internal short-circuit of one inverter leg, due for example to a failure of one IGCT, this energy is discharged into the faulty leg, potentially leading to explosion and fault propagation. Simulations have been performed in order to analyze, select and design the most suitable protection to avoid this risk.