Smoothing reactors are intended to provide a high impedance to the flow of harmonic currents, and to reduce the rate of current rise on failures in direct current (d.c.) systems.

Two main application fields are defined as follows:

• The direct current has superimposed harmonic components. This situation occurs in d.c. systems for industrial applications. Since the system voltages are generally not higher than 10kV, these reactors are usually designed for indoor application.
• The direct current has small superimposed harmonic components. This situation occurs in HVDC transmission systems. The d.c. system voltages are generally 50kV or higher.

With regard to installation, these devices may be identified as:

• Cast resin dry type, conventional dry type or oil immersed
• With or without magnetic shield
• For indoor or outdoor installation
• With or without enclosure
• With fixed or variable reactance