Direct current, which was once the main means of distributing electric power, is still widespread today in the electrical plants supplying particular industrial applications.
The advantages in terms of settings, offered by the employ of d.c. motors and by supply through a single line, make direct current supply a good solution for railway and underground systems, trams, lifts and other transport means.
In addition, direct current is used in conversion plants (installations where different types of energy are converted into electrical direct energy, e.g. photovoltaic plants) and, above all, in those emergency applications where an auxiliary energy source is required to supply essential services, such as protection systems, emergency lighting, wards and factories, alarm systems, computer centers, etc.. Accumulators - for example – constitute the most reliable energy source for these services, both directly in direct current as well as by means of uninterruptible power supply units (UPS), when loads are supplied in alternating current.
This Technical Application Paper is intended to explain to the readers the main aspects of the most important applications in direct current and to present the solutions offered by ABB SACE products.
The main purpose is giving precise information by means of tables providing a rapid choice of the protection/disconnection device and paying particular attention to the installation characteristics (fault typologies, installation voltage, earthing arrangement).
There are also some annexes giving further information about direct current, and more precisely:
· Information about the distribution systems in compliance with the international Standard IEC 60364-1;
· Calculation of the short-circuit current in d.c. in compliance with the international Standard IEC 61660-1;
· Circuit-breakers and disconnectors for applications up to 1000Vd.c.
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