Design and testing of Crowbar Protection System for the JT-60SA superconducting magnet power supplies

The Crowbar System is a key protection system for the Super Conducting Magnet Power Supplies (SCMPSs) of JT-60SA, it is a Safety Relevant component and it is designed to be seismic resistant. The Crowbar System is operated in case of faults, quench of superconducting coils, plasma disruption or earthquake event, and it has to carry the current flowing through the superconducting coils in order to protect them and the SCMPS from over-voltages. The circuit configuration is based on a hybrid-making-switch, composed by a bidirectional-static-switch with a mechanical-switch and a varistor in parallel. The static-switch is composed by eight thyristors in bidirectional configuration (four connected in parallel and four in antiparallel), and it is designed to be able to safely intervene even if one thyristor or the mechanical-switch is not operating. The mechanical-switch (rated 50 kA making capacity) takes some tens of milliseconds to close, limiting the conduction time of thyristors. The overvoltage protection is performed by a voltage transducer, a Break-Over-Diode (BOD) and a varistor assuring a fast intervention with a high redundancy protection level.