D.C. power control for aluminium and electrolytic loads

The paper discusses critically the control of large d.c. power blocks, often exceeding 100 MW in one unit. Only power supply from silicon rectifiers is considered, since all other power supplies are now obsolete and many existing installations using mercury arc, mechanical rectifiers or machines are likely to change to silicon in the future. The large aluminium line is taken as an example of a difficult control problem, and the choice of control range, stability of current and method of initiating tap changes are considered in turn. A new master-local control scheme is described which brings all the rectifiers of one installation under a single master control, keeps total load current/power or voltage (as desired) constant, maintains maximum possible efficiency with minimum harmonic current in the supply, minimises tap changes and widens effective control range. The scheme permits any desired combination of rectifiers to operate under local or master control within chosen power-factor limits and is applicable to transductor-controlled installations or thyristor installations. The three possible control aims of constant current, unregulated voltage and constant power are discussed and their consequences are examined.