INTRODUCTION 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