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Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th Century, led to greater appeal and use of a.c. transmission. Through research and development in Sweden at Allmana Svenska Electriska Aktiebolaget (ASEA), an improved multi-electrode grid controlled mercury arc valve for high powers and voltages was developed from 1929. Experimental plants were set up in the 1930’s in Sweden and the USA to investigate the use of mercury arc valves in conversion processes for transmission and frequency changing.
D.c. transmission now became practical when long distances were to be covered or where cables were required. The increase in need for electricity after the Second World War stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km experimental transmission line was commissioned from Moscow to Kasira at 200 kV.
The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland.
Thyristors were applied to d.c. transmission in the late 1960’s and solid state valves became a reality. In 1969, a contract for the Eel River d.c. link in Canada was awarded as the first application of sold state valves for HVDC transmission. Today, the highest functional d.c. voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme in Brazil. D.c. transmission is now an integral part of the delivery of electricity in many countries throughout the world.
WHY USE DC TRANSMISSION?
The question is often asked, “Why use d.c. transmission?” One response is that losses are lower, but this is not correct. The level of losses is designed into a transmission system and is regulated by the size of conductor selected. D.c. and a.c. conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will generally result in lower losses but cost more.
When converters are used for d.c. transmission in preference to a.c. transmission, it is generally by economic choice driven by one of the following reasons:
1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length than an equivalent a.c. line designed to transmit the same level of electric power. However the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c. transmission is less than its a.c. transmission alternative. The d.c. transmission line can have a lower visual profile than an equivalent a.c. line and so contributes to a lower environmental impact. There are other environmental advantages to a d.c. transmission line through the electric and magnetic fields being d.c. instead of ac.
2. If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km but d.c. cable transmission systems are in service whose length is in the hundreds of kilometers and even distances of 600 km or greater have been considered feasible.
3. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is quite small. This occurs in Japan where half the country is a 60 hz network and the other is a 50 hz system. It is physically impossible to connect the two together by direct a.c. methods in order to exchange electric power between them. However, if a d.c. converter station is located in each system with an interconnecting d.c. link between them, it is possible to transfer the required power flow even though the a.c. systems so connected remain asynchronous.
The integral part of an HVDC power converter is the valve or valve arm. It may be noncontrollable
if constructed from one or more power diodes in series or controllable if
constructed from one or more thyristors in series. Figure 1 depicts the International
Electrotechnical Commission (IEC) graphical symbols for valves and bridges (1). The
standard bridge or converter connection is defined as a double-way connection
comprising six valves or valve arms which are connected as illustrated in Figure 2.
Electric power flowing between the HVDC valve group and the a.c. system is three
phase. When electric power flows into the d.c. valve group from the a.c. system then it is
considered a rectifier. If power flows from the d.c. valve group into the a.c. system, it is
an inverter. Each valve consists of many series connected thyristors in thyristor modules.
Figure 2 represents the electric circuit network depiction for the six pulse valve group
configuration. The six pulse valve group was usual when the valves were mercury arc.