For electrolysis processes the DC current is supplied by an adapted and integrated transformer/rectifier combination. For large processes, like aluminum smelting, several units with individual and different phase shifting are connected in parallel operation to achieve the required process current. Various rectifier and transformer connections are utilized for the various applications. For DC voltages in the lower range, the interphase connection is often preferred, while for the higher voltages and for the large processes, like aluminum, the bridge connection is more economical. The rectifier can be a thyristor type or a diode type. The thyristor type requires only coarse (stepped), if any, voltage regulation in the transformer unit. This will often be done by a no-load tap changer (NLTC). The diode rectifier will require a continuous regulation of the secondary voltage of the transformer. This is done by a combination of a stepped voltage regulation, applying a coarse- or multicoarse-fine on-load tap changer (OLTC), and the fine tuning voltage regulation by applying saturable rectors on the LV side of the transformer. The regulation range of the transformer is frequently between 100% and close to zero voltage. Except for some thyristor applications with short voltage ranges, the stepped regulation is done by the use of a separate regulating transformer, either in a separate tank or in a common tank with the rectifier transformer itself. This process regulating transformer can be either an auto type or a double wound type.
Rectifier transformers in electrolysis operation have normally a steady, but continuously high loading. The high secondary currents and LV bushing arrangements for large units, together with the secondary current harmonics in general, are essential design requirements to manage.

Technical Data:

Unit ratings	:	Up to 1000 KVA
Secondary voltage	:	Up to 1500 VAC
Unit process	:	Current up to over 100 KADC
Total process current	:	Several hundred KADC