Ground fault monitoring in Alcator C-MOD

The Alcator C-MOD tokamak has a complex AC power distribution system to power diagnostic equipment, magnet power, vacuum, heating, and cryogenic control. During a 9 Tesla pulse, the existence of ground loops between these systems can cause damage or compromise the performance of the associated equipment. The real-time detection of these ground loops in the grounding system is crucial to the productive operation of the tokamak. The ground fault monitor is capable of detecting ground loops with an impedance as high as 10 KΩ. This monitoring system utilizes current transformers and a multiplexed driver/sensor architecture to monitor each of the 20 ground lines that are connected to a star point ground in the C-MOD cell. Continuous monitoring is accomplished without breaking ground wire connections. The star ground cable to earth ground is monitored to detect ground leakage paths that circumvent the star ground point. A synchronous demodulator circuit is utilized to detect the driver signal in a noisy environment and in the presence of 60 Hz leakage current. The detector output is linear for fault impedance values in the range of 100 KΩ to 100 Ω. The detection of 60 Hz leakage current is possible with the use of an alternate measurement mode. The monitor runs continuously to detect faults as they occur. Outputs are monitored in the control room via a fiber optic interface. Control room functionality includes data logging of fault conditions, overrides, and alarms. A manual operating mode and instrument rack display panel can be utilized by the operator for tracking down ground faults. A diagnostic circuit is available to perform a “self-test” function, which verifies proper monitor operation and locates defective circuitry down to the component level