HVDC technology

The conceptual design of the classic HVDC converter stations of today dates back from the mid 1970's, when thyristor valves were taking over in place of the mercury arc valves. But there has been a dramatic development in the performance of HVDC equipment and systems.




A HVDC converter station uses thyristor valves to perform the conversion from AC to DC and vice versa. The valves are normally arranged as a 12-pulse converter. The valves are connected to the AC system by means of converter transformers. The valves are normally placed in a building and the converter transformers are located just outside.

The 12-pulse HVDC converter produces current harmonics (11th, 13th, 23rd, 25th, 35th, 37th etc.) on the AC side. These harmonics are prevented from entering into the connected AC network by AC filters, i.e. resonant circuits comprising capacitors, inductances (reactors) and resistors. The filters also produce a part of the reactive power consumed by the converter.

The HVDC converter also gives rise to voltage harmonics on the DC side (12th, 24th, 36th etc.). A large inductance (smoothing reactor) is always installed on the DC side to reduce the ripple in the direct current. In addition, a DC filter is also normally needed to reduce the level of harmonic currents in the DC overhead line. The harmonics may otherwise cause interference to telephone circuits in the vicinity of the DC line.

The power transmitted over the HVDC transmission is controlled by means of a control system. It adjusts the triggering instants of the thyristor valves to obtain the desired combination of voltage and current in the DC system.

Several other apparatus are needed in a converter station, such as circuit breakers, current and voltage transducers, surge arresters, etc. The conceptual design of the classic HVDC converter stations remained unchanged until 1995, when ABB introduced HVDC with Capacitor Commutated Converters (CCC).