Stator Field Oriented Control of Cycloconverter Doubly Fed Induction Motor-Generator System for Plasma Fusion Operation

Experimental plasma fusion machines require large amounts of pulsed de power to produce strong magnetic fields for plasma confinement and shaping. The power system currently used for these machines is based on typical power-plant equipment: a motor-generator unit (MG) feeds a set of AC-DC rectifiers. The MG contains an induction motor and a synchronous generator, mounted on the same shaft. The MG operates like a mechanical flywheel: during charging, the motor accelerates and absorbs energy from the utility; during discharge, the energy stored is sent to the loads. For that, the output voltages of the generator are rectified and the resulting dc power is injected into the coils of the plasma machine. A doubly fed induction motor (DFIM) is used; this is equipped with 3-phase windings on both the stator and rotor; the rotor winding is accessible through slip rings. At startup, the rotor is connected to a rheostat (to reduce the starting current) and the motor accelerates to the steady-state sub-synchronous speed. To increase the energy stored in the rotating mass, a 3-phase cycloconverter (CCV) is then connected to the rotor circuit. The CCV injects additional energy into the system and the motor accelerates to supra-synchronous speed. The actual control algorithm used for supra-synchronous acceleration is based on the V/Hz principle and is implemented using the analog circuitry of the late 1970's. This paper presents a control approach that could be used to modernize this system - the method presented is based on stator field-oriented vector control. The paper discusses the control of a 12 pulse CCV that is a part of the system. Simulation results are shown.