Poloidal-field thyristor amplifier with improved power factor, for TEXTOR

Rapid current control in highly inductive poloidal-field coils requires a regenerative converter whose output voltage is many times larger than the drop across the ohmic resistance. During the flat-top phase of the current pulse the converter is phased back, resulting in a relatively large ripple and high reactive power. Controlled freewheeling, using thyristors between the d.c. terminals of the three-phase thyristor bridge and the neutral of the transformer winding, improves the output waveform and reduces the reactive power drawn from the mains without reduction of the dynamic performance. This circuit is used in two 12-plus thyristor amplifiers 2500 V-15 kA for the vertical field and for ohmic heating in the Euratom/KFA-Juelich Textor experiment