High-current variable-voltage (HCVV) rectifiers are used in the metal and chemical industries. Typical power ratings vary from tens of kW to hundreds of MW. Even with the advancement of the power factor correction rectifiers, accepted choices of high-current AC to DC converters remain 6, 12 or 24-pulse thyristor/diode rectifiers, because of high reliability, efficiency and availability of suitable semiconductor devices. The main issues with these rectifiers are poor input power factor, high current harmonic distortion, high-maintenance cost, high weight and large volume. Moreover, to achieve low output voltage ripple, large capacitive filters are required. To tackle these issues, a two-pronged approach is taken in this thesis. First, the power quality issues of thyristor rectifiers are addressed with the help of passive and hybrid filters. Second, completely different, medium-frequency transformer-based topologies are proposed for the HCVV applications.