Current Regulators for Large Rectifier Power Supplies Used on Electrochemical Processing Lines

Typical large silicon rectifier dc power supplies for electrochemical lines have ratings of 20 000 to over 300 000 amperes, at voltages of 50 to 1000 volts. The operating efficiency of such installations is of major economic importance, and experience has shown that the use of high-accuracy current regulation can substantially improve efficiency. Regulation is required to compensate for ac supply fluctuations and electrochemical cell or potline characteristics variations. Underload tap changing transformers provide an economical and highly efficient means of varying the rectifier dc output voltage. Control resolution, however, is relatively limited due to the finite voltage steps between taps, and precautions must generally be taken to avoid unnecessary tap changer operations in order to achieve reasonable operating periods without contact maintenance. Several techniques for achieving high-accuracy current regulation with tap changer controlled silicon rectifier power supplies are described. One approach utilizes partial range saturable reactors to provide vernier control between tap changer steps and a fast response feedback amplifier-driver to give essentially continuous high-accuracy (0.1 to 0.5 percent) current regulation. Another method controls the tap changer by means of an error integrating or reset amplifier technique which ignores minor short-tenn current fluctuations but maintains the average value over several hours of operation to within a fraction of a percent of the preset value.