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

This paper reports on the detailed design of the Alcator C-MOD thyristor DC circuit breakers which required extensive simulation and careful consideration of parasitic elements as well as device recovery and turn-on limitations. 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 though 1 or 2 SCRs and utilizes a pulse forming network to quickly reduce the main SCR current to zero, then provide an 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