Direct current switchgear
The direct current equipment often includes a coil (called a reactor) that adds inductance to help smooth the direct current. The inductance amounts to between 0.1 H and 1 H. The smoothing coil can have either an air-core or an iron-core. Iron-core coils look like oil-filled high voltage transformers. Air-core smoothing coils resemble, but are considerably larger than, carrier frequency choke coils in high voltage transmission lines and are supported by insulators. Air coils have the advantage of generating less acoustical noise than iron-core coils, they eliminate the potential environmental hazard of spilled oil, and they do not saturate under transient high current fault conditions. This part of the plant will also contain instruments for measurement of direct current and voltage.
Special direct current filters are used to eliminate high frequency interference. Such filters are required if the transmission line will use power line carrier techniques for communication and control, or if the overhead line will run through populated areas. These filters can be passive LC filters or active filters, consisting of an amplifier coupled through transformers and protection condensers, which gives a signal out of phase to the interference signal on the line, thereby cancelling it. Such a system was used on the Baltic Cable HVDC project.
