DC filter

For overhead line HVDC transmissions the DC filter takes care of telephone interference.

The smoothing reactor, which is installed also for other reasons, is an important element in the filtering of d.c. side harmonics. For overhead line transmissions, it is normally necessary to install additional filter circuits between the pole bus (outside the smoothing reactor) and the neutral bus. Capacitors or filter circuits may also have to be installed between the neutral bus and ground. The filter types used on the d.c. side are essentially the same as those used on the a.c. side, i.e. series resonance filters and high pass filters.

The largest item of a DC filter, the capacitor, is often suspended in ABB's HVDC projects, especially in seismic areas.

ABB has developed an active DC filter, that enables efficient filtering using a small-size filter.