The ITER superconducting coils of the Poloidal and Toroidal field circuits are protected by several energy discharge units (20 in the toroidal circuit and 24 in the poloidal circuit), devoted to extract the energy stored in the windings in case of quench and to perform an ultimate protective action in other abnormal conditions for which the fast extraction of the energy from magnets is mandatory (e.g. unrecoverable faults of the cryogenic system, overcurrent in the coil, arcs in the DC circuit). These Fast Discharge Units (FDU) are basically composed of a bypass switch rated for the full current in parallel to a Vacuum Circuit Breaker (VCB), a counterpulse capacitor bank and a discharge resistor (temperature dependent). The full current is normally carried by the bypass switch when the FDU is operated, the bypass switch opens and the current is diverted by arc voltage drop to the vacuum switch when the current is completely diverted, the VCB opens and the counterpulse bank is fired to force current zero in the VCB the current is then transferred to the discharge resistor. The counterpulse bank is also equipped with a changing-polarity switch system to operate FDU in both current polarities. Finally a pyrobreaker is connected in series for a further back-up protection. The FDUs intervention has nevertheless a severe impact on the machine operation, and can produces hazards in the circuit components: the most important hazardous conditions are:
- Plasma disruption (unrecoverable by control action)
- Induced currents in the Vacuum Vessel and Back Plate Structure.
- Overvoltage across the ac-dc converters.
Furthermore, failures in the FDU operation can lead both to hazards for circuit components and for FDU itself, e.g.:
Misfiring (premature or failure in firing) of one (or more) FDU which produce stresses on the FDU and overcurrent in the circuits. FDU operation in presence of coil section or internal coil short circuits, which produce the uncontrolled growth of the current in the shorted branch. The paper deals with the design criteria adopted to determine the parameter of the discharge resistors (nominal energy and initial resistance) and analyzes the various operating and fault conditions to determine the current scenarios which can be discharged safely by the FDU system.