Operation of the Alcator C-MOD power system

The energy storage and power conversion system for Alcator C-MOD is designed to supply up to 500 MJ to the experiment at peak powers in excess of 300 MVA. The experiment requires flexibility in the control of magnet currents to achieve simultaneous shaping, ohmic drive, and position control with a small set of uncompensated PF magnets. The relatively modest budget and limited manpower make issues of maintenance and reliability extremely important to the eventual success of the experiment. These requirements resulted in a power system design with many unique features. A total of 11 conversion systems power the 14 magnets of the tokamak to provide nearly complete flexibility in plasma shaping and control. Five thyristor circuit breakers are used to initiate the plasma. Four of the PF converters utilize circulating current configurations to maintain plasma position control as the magnet currents pass through zero. The TF converter incorporates transformer tap-changing circuitry to reduce alternator loads during peak demand periods. Many converters have solid-state fuse loss and conduction monitor circuits that allow continued operation when only 1 device in a parallel bank fails. The same circuits detect commutation failures and protect the converters from these potentially destructive faults. Unique design features can sometimes lead to new and unforeseen problems in operation or maintenance. This report will discuss the initial operating experience of the Alcator C-MOD power system from preliminary acceptance testing through plasma operations. The performance of the thyristor circuit breakers, commutation and conduction monitor circuits, circulating current power supplies, and tap-changing TF supply will be presented and compared with expectations. The overall performance and reliability of the power system will be examined as well