Voltage sequence control based high-current rectifier system

This paper presents a thyristor-based tap changing transformer and diode rectifier for variable-voltage high-current DC applications. Back to back thyristors are connected in series with the transformer tappings on the primary side. Thyristors are phase-controlled in a continuous manner to achieve variable AC voltage at the secondary side of the transformer. This leads to a variable DC voltage at the output of three-phase diode rectifier connected at the secondary side of transformer. A passive filter is added at the primary side to achieve power factor improvement over the range of output voltage. This scheme is optimized to determine optimum turns ratio of the transformer and optimum reactive power rating of the passive filter. Design guidelines are presented for various values of power factor along with optimum turns ratios and passive filter rating. A 62.5 kW experimental system with passive filter of 17.5 kVA (reactive) is developed to verify the system performance.