Design and construction of thyristor DC circuit breakers for Alcator C-MOD

The detailed design of the Alcator C-MOD thyristor DC circuit breakers required extensive simulation and careful consideration of parasitic elements as well as device recovery and turn-on limitations. Simulations were performed both on commercially available packages and with direct solution of circuit equations with custom programs. The need for high reliability and minimum device count resulted in the choice of 77-mm devices normally used in phase control applications. The circuit uses up to 12 parallel paths to carry pulsed currents as high as 50 kA. Maximum interrupting voltages of 2 kV are supported by devices rated 4.4 kV. Each SCR is shunted by a diode in series with an air core decoupling reactor. The counterpulse capacitor is discharged through one or two SCRs and utilizes a pulse forming network to quickly reduce the main SCR current to zero, and then provide a relatively low and stable reverse voltage during the commutation interval. All devices are mounted with radial symmetry. The diode reactors are subject to modest forces under normal conditions but large lateral forces when adjacent paths are lost.<>