High Voltage Direct Current

The direct current equipment often includes a coil (called a reactor) that adds inductance in series with the DC line to help smooth the direct current. The inductance typically amounts to between 0.1 H and 1 H. The smoothing reactor 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 communication 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 capacitors, 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.

HVDC Converter

DC Equipments

The converter is usually installed in a building called the valve hall. Early HVDC systems used mercury-arc valves, but since the mid-1970s, solid state devices such as thyristors are used. Converters using thyristors or mercury-arc valves are known as line commutated converters. In thyristor-based converters, many thyristors are connected in series to form a thyristor valve and each converter normally consists of six or twelve thyristor valves. The thyristor valves are usually grouped in pairs or groups of four and can stand on insulators on the floor or hang from insulators from the ceiling.

Line commutated converters require voltage from the AC network for commutation, but since the late 1990s, voltage sourced converters have started to be used for HVDC. Voltage sourced converters use insulated-gate bipolar transistors instead of thyristors and can provide power to a deenergized AC system.

Almost all converters used for HVDC are intrinsically able to operate with power conversion in either direction. Power conversion from AC to DC is known as rectification and conversion from DC to AC is known as inversion.