High and ultra-high voltage direct current (HVDC / UHVDC) transmission schemes are used for bulk power transmission. The systems are also used for back-to-back and sea cable interconnections, as well as for the grid connection of renewable energy facilities such as offshore windfarms. Depending on the converter technology, LCC (line-commutated converters) and/or VSC (voltage source converters), these schemes include smoothing or converter reactors, AC and DC harmonic filter reactors and PLC-noise filter reactors.

Smoothing reactors reduce the ripple current magnitude that is present in a DC system.

Smoothing reactors are installed in HVDC schemes with system voltages up to 1100kV and may reach 250MVAr equivalent power ratings (50Hz equivalent power) per coil unit.