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Most long overhead ac tines require series compensation of part of the induetive reaetanee. The breakers are required, however, for clearing faults in the transnission or for taking the whole dc link out of service.
The line transmits 20 MW at kV through a single-conductor cable, with return path through the sea and earth. After the more rugged steel-envelope mercury-pool ignitron became available, however, the line was converted in to constant-voltage operation. Kimbqrk la shows a single-eircuit three-phase ac line.
EHV transmission links, superposed on a lower-voltage ac network, or inter eonnecting two such networks, or connecting distant generating plants to an ac network, are compared kombark to their principal components and the arrange ments thereof, according to whether the line operates on ac or dc. B1S The line carried 5. One pole of a four conductor line is shown in Figure Id, with two converters per terminal.
Each valve has two anodes working in parallel. The neutral points junctions between converters are grounded at one trannsmission both ends. Intermediate generation raises the stability limit of the link.
Aboutcontrol electrodes were added to silicon diodes, giving silicon controlled rectifiers SCRsalso called thyristors.
Lamm inhas considerably increased the peak inverse voltage that the valves can withstand. The tine itself usually has two conductors, aIthough some Iines have only one, the return path being in the earth or seawater or both. If both neutrals are grounded, the two poles can operate inde pendently. Circuit breakers are installed only on the ac sides of the converters.
These breakers are not used for clearing faults on the dc line or most misoperations ofthe valves, for these faults can be cleared more rapidly by grid controlof the valves. This system of grading elec trodes, invented by U.
The fortunes ofwar prevented completion ofthe project, and in such plant and pertinent documentsas kimbzrk were taken to the U. The line initially operated at constant current, the conversions from con stant alternating voltage to constant current and vice versa being made by an LC bridge circuit called the monocyclic square.
In the figure, one bank of series capacitors for this pur pose is shown at the middle of the tine. They transmitted 4 MW at kV a distance of 3 mi 5 km over an existing line from a station ia the Charlottenburg district of Berlin to one in the Moabit district.
Practical ground electrodes were developed, and various kinds of valves and converter control were tested. If the transformers are operated as an kimbatk part of the link, only 10w-voItage circuit breakers are required. The ac input at Mechanicville was at a frequency of 40 Hz, and the output at Schenectady was at 60 Hz.
Exp1 queda transmissoon Experimento de queda livre. The initiative in exploring the use of mercury-arc valves for dc transmission was taken by the General Electric Company.
A demonstration of dc transmission using grid-controlled steel-tank mercury-arc conversion was given at Zurich, Switzerland, inat the Fifth Swiss National Exhibition. Kimbbark 1 de 6. The ac line, of course, also has this reversibility. Then switches 3 would be opened, isolating the faulted line.
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.
Each converter has two vaIve groups rated 50 kV, A, 10 MW, the groups being in series on the dc side. In the event of a fault on one conductor, the other conductor with ground return can carry up to half of the rated load. On trznsmission long EHV transmissoon, shunt reactors are required for limiting the voltage, especially at light loads, but they may be required even at full load.
Each terminal has two converters of equal rated voltages in series on the dc side. The current ratings are to A per anode. Either converter, however, can function as rectifier or inverter, permitting power to be transmitted in either direction.
Direct Current Transmission – Kimbark
A noteworthy feature of ASEA valves is the use of several, usually four, anodes in multiple on single-phase valves. If a permanent fault occurred on the lower conductor, the converters connected to it would be controlled so as to bring the voltage and transmissoin on it to zero.
One of the mercury-arc valves was replaced by an air-cooled thyristoi’ assem bly, which also has performed well. Each pole would be switched independently of the other. Norrnally they operate at equal current; then there is no ground current.
Kimvark distance is far toa great for ac eable transmission. Power is adjusted automatically to maintain rated frequency 50 Hz in Gotland. Thus was demonstrated a feature of dc transmission that has been important in several subsequent instalIations: Plans have been announced for doubling the voltage and power on the existing cable by the addition of a new thyristor valve group to each terminal, thereby transmisison the voltage.
Direct Current Transmission – Kimbark – Livro de HVDC do Kimbark
Perhaps an additional belief was that there was no future in dc transmission. Present-day mereury-are valves for high-voltage transmission, known as excitrons, have, in addition to the anode 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 conduct.
Long two-cireuit ac links are usually sectionalized by means of intermediate switching stations for severa reasons. The Une loss would be four times its normal value, somewhat diminishing the delivered power.
Constant-current operation was chosen because the hot-cathode tubes then used couId not withstand the high short-circuit currents expected to occur on a constant-voItage system.
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 hornopolar Iink has two or more conductors ali having the same polar ity, usually negative, and always operates with ground returno In the event of a fault on one conductor, the entire converter is available for connection to the remaining conductor or conductors, which, having some overload capability, can carry more than half of the rated power, and perhaps the whole rated power, at the expense of increased tine loss.
These reactors are usually placed at intermediate switching stations hgdc are so indicated in Figure Ih. Russian engineers transmisaion concentrated on single-anode valves, which so far appear to have been less successful than the ASEA valves.