Hybrid Options for upgrade of the LHC energy extraction switchgear

The high current switching in the more than 230 energy extraction facilities for rapid discharge of the superconducting circuits in CERN´s LHC collider is presently based on specifically adapted, electro-mechanical DC switchgear. The experience from two years of operation confirms the high reliability of these systems with no observed opening failures and a calculated, single failure probability of 2.5 % for a 20-year operations period. However, meticulous adjustments and sustained maintenance of the DC breakers, required because of degradations occurring during commutation (even in the presence of capacitor commutation assistance), is an essential activity of every shutdown of the machine. Consequently, CERN is now considering a replacement of at least a part of the existing systems by new hybrid facilities in which IGCTs are used as static switches in combination with the more classical gear. The paper describes the philosophy behind the choices for topology and component selection for low- to medium current, mono- and bi-polar circuits (IGCT with separate blocking diodes and a series-connected, classical opening switch as back-up, only activated in case of an IGCT release failure) and for bi-polar, high-current applications (13 kA) with traditional DC breakers paralleled by an IGCT-based switch, which serve as an efficient snubber system. Driver circuits and protection features are also described.