AC transmission over long distances, especially via underground cable, requires frequent shunt compensation and causes stability problems. Among the many steps that can be identified with the development of modern HVDC transmission technology, the following are worth mentioning: (1) first attempt to combine the advantages of HVAC turbogeneration and HVDC transmission was made in the 1920s by Calverley and High field with the 'transverter'. (2) Hewitt's mercury-vapour rectifier, which appeared in 1901, and the introduction of grid control in 1928, provided the basis for controlled rectification and inversion. (3) Prior to 1940, experiments were carried out in America with thyratrons and in Europe with mercury-pool devices. (4) Countries with long transmission distances like America, the Soviet Union and Sweden, showed great interest in HVDC developments. (5) In Germany, the Secretariat for Aviation encouraged the development of HVDC technology during the war believing that underground transmission was less vulnerable to air raids.

Since the commutation reactance is low in relation to the DC smoothing reactance, an HVDC converter acts, from the AC point of view, as a source of harmonic currents (high internal impedance) and from the DC point of view, as a source of harmonic voltage (low internal impedance). The orders and levels of such harmonics have been discussed. Excessive levels of harmonic current must be prevented as they will cause voltage distortion, extra losses and overheating, as well as interfer ence with external services (e.g. telephone and railway signals). The obvious place to eliminate the harmonics is the source itself. In theory, characteristic harmonics could be eliminated either by some complex converter configuration (which would be uneconomical), or by the use of a series filter preventing the harmonics from arising (which would upset the correct operation of the converter). Therefore, accepting that the appearance of harmonics is an inherent property of the static-conversion process, it will be necessary to reduce their penetration into the AC and DC systems. Any solution which increases the pulse number reduces the harmonic orders penetrating into both sides of the converter and should be fully exploited. Beyond the economic range of higher pulse configurations, harmonic elimination will normally require the use of filters. These are now considered separately.

This chapter discusses the ideal control system for an HVDC converter.

A typical design sequence for an HVDC transmission scheme should include the following steps: (a) identify the main operational objectives to be met, i.e. energy considerations, MW loading requirements and maintenance; (b) identify any technical constraints which may have to be accepted, e.g. the maximum voltage and current ratings of submarine cables, limitations of earth return etc; (c) adopt voltage and current ratings; (d) decide the overall control requirements, e.g. constant-power control, short-term overload, damping characteristics, constant extinction-angle control, constant ideal (noload) direct voltage, etc.; (e) develop converter-station arrangements. (f) design the transmission line; (g) assess the capital equipment cost, the operating costs and the cost of losses. As there are still several schemes using mercury valves, this chapter starts with a brief exposition of their components and layout. Most of the chapter, however, is devoted to thyristor-converter technology.

This chapter discusses transient overvoltages and insulation coordination.

HVDC technology took a big step forward around 20 years ago when thyristor valves succeeded the mercury-arc valves previously used. The converter-station concept introduced at that time, however, has remained practically unchanged since then, even though great improvements in equipment and subsystems have taken place. At the same time there have been substantial advances in conventional AC technology and, particularly, in the application of power electronics to make power transmission more flexible and economical. Such competition is now exciting a continuous stream of new HVDC concepts and techniques with the aim of improving performance and reducing costs and delivery times.