Smoothing reactors are vital components in HVDC systems. The purpose of the reactor is to reduce the so called current ripple on the DC side of the system.
The direct current that comes from the rectifier in the DC system has superimposed harmonic components, also called ripple. The smoothing reactor is connected in series with the rectifier and the whole load current flows through it. The purpose of the reactor is to provide high impedance to the flow of the harmonic currents, reduce their magnitude and thus making the DC current more smooth.
ABB provides oil immersed reactors for all HVDC applications. A smoothing reactor in a HVDC system can be much larger than even the largest shunt reactor. ABB has supplied several different sizes up to the largest ones needed in HVDC lines for transmission of high power and on high voltage levels.