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At both ends of the tines are converters, the components of which are transformers and groups of mercury arc valves. EHV transmission links, superposed on a lower-voltage ac kimbar, or inter eonnecting two such networks, or connecting distant generating plants to an ac network, are compared as to their principal components and the arrange ments thereof, according to whether the line operates on ac or dc.
Russian engineers have concentrated on single-anode valves, which so far appear to have been less successful than the ASEA valves.
Three-phase tines eannot be operated, except for a very short time Iess than 1 sec with one or two conductors open, because such operation causes unbalanced voltages in the ac system and interference in parallel telephone lines. The capability of ali converters would then be usable, and the power normally carried by two conductors would then be carried by one. Thus was demonstrated a feature of dc transmission that has been important in several subsequent instalIations: The United States was inactive in the field of dc transmission, however, for nearly 20 years.
A demonstration of dc transmission using grid-controlled steel-tank mercury-arc conversion was given at Zurich, Switzerland, inat the Fifth Swiss National Exhibition.
Direct Current Transmission – Kimbark – Livro de HVDC do Kimbark
A noteworthy feature of ASEA valves is the use of several, usually four, anodes in multiple on single-phase valves. B1S The line carried 5. Direet-current links are classified as shown in Figure 2. Next the converter voltages would be raised to equality with those of the respective adjacent converters, after which switches I would be closed.
Practical ground electrodes were developed, and various kinds of valves and converter control were tested. A representative single-circuit dc link is shown in Figure le. Each valve has two anodes working in parallel.
These reactors are usually placed at intermediate switching stations and are so indicated in Figure Ih. B4,S Power of 0.
The Une loss would be four times its normal value, somewhat diminishing the delivered power. The initiative in exploring the use of mercury-arc valves tansmission dc transmission was taken by the General Electric Company. Long two-cireuit ac links are usually sectionalized by means of intermediate switching stations for severa reasons.
Direct Current Transmission – Kimbark
M uch of the time when power is delivered to Gotland, there are no generators in operation there, the only synchronous maehine being a condenser. Present-day mereury-are valves for high-voltage transmission, known as excitrons, have, in addition to the transmjssion and the mercury-pool cathode, an ignition electrode for starting the arc, one or more excitation electrodes for maintaining the are, and a control grid that prevents the are from reaching the anode until it is desired that the valve begin to transkission.
The rnonopolar link has one conductor, usually of negative polarity, and ground or sea returno. If a permanent fault occurred on the lower conductor, the converters connected to it would be controlled so as to bring the voltage and current on it to zero.
Direct current transmission – Edward Wilson Kimbark – Google Books
One pole of a four conductor line is shown in Figure Id, with two converters per terminal. Large inductances ealled de. After two smaller experi limbark. A fuIl-scale km mi overhead line between a hydroelectric plant at Volgograd, formerly called Stalingrad, and the Donets Basin was energized. Circuit breakers are installed only on the ac sides of the converters. This being so, two parallel three-phase circuits are required for hvxc transmission see Fig.
Their ratings have increased, however, with surprising rapidity, and it seems certain that such valves will soon replace mercury-arc valves in HV dc use. There are also several grading electrodes placed between the control grid and the anode for obtaining a more uniform potential gradient than would otherwise existo The grading electrodes are kept at the desired potentials by connecting them to taps on an externaI resistance-capacitance potential transmiission the ends of which are connected to the anode and control grid.
Norrnally they operate at equal current; then there is no ground current. On many long EHV lines, shunt reactors are required for limiting the voltage, transmisaion at light loads, but they may be required even at full load. The converter at each end of the line had 12 hot-cathode glass-envelope thyratrons in 6 series pairs. Power hgdc adjusted automatically to maintain rated frequency 50 Hz in Gotland. Lamm inhas considerably increased the peak inverse voltage that the valves can withstand.
After the more rugged steel-envelope mercury-pool ignitron became available, however, the line was converted in to constant-voltage operation. Valves for HV dc transmission are invariably of single-phase eonstruction, in contrast to the polyphase valves with mercury-pool cathode formerly used extensively in low-voltage rectifiers for industrial and railway application.
The distance is far toa great for ac eable transmission. The bus-tie switches I are kimnark open. The larger of these, rated at 20 MW, was installed at the Edgar Thompson plant tranwmission the.