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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.
There are also several grading electrodes placed between the control grid e.w.mimbark 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 divider the ends of which are connected to the anode and control grid.
– Direct Current Transmission, Volume 1 by Edward Wilson Kimbark
This system of grading elec trodes, invented by U. Lamm inhas considerably increased the peak inverse voltage that the valves can withstand.
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. A noteworthy feature of ASEA valves is the use of several, usually four, anodes in multiple cureent single-phase valves.
The current ratings are to A rirect anode. Russian engineers have concentrated on single-anode valves, which so far appear to have been less successful than the ASEA valves. Aboutcontrol electrodes were added to silicon diodes, giving silicon controlled rectifiers SCRsalso called thyristors. At present these are not capable of handling the highest voltages and powers required for HV dc transmission. 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.
The initiative in exploring the use of mercury-arc valves for dc transmission was taken by the General Electric Company. After two smaller experi 2’mentsB1. B1S The line carried 5. The converter transmissiob each end of the line had 12 hot-cathode glass-envelope thyratrons in 6 series pairs. The ac input at Mechanicville was at a frequency of 40 Hz, and the output at Schenectady was at 60 Hz.
Thus was demonstrated a feature of dc transmission that has been important in several subsequent instalIations: The line initially operated at constant current, the conversions from con stant alternating voltage to constant cureent and vice versa being made by an LC bridge circuit called the monocyclic square. 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.
After the more rugged steel-envelope mercury-pool ignitron became available, however, the line was converted in to constant-voltage operation. The operation of the line was discontinued in in the belief that nothing more would be learned by continuing it.
Perhaps an additional belief was that there was no future in dc transmission. The larger of dirfct, rated at 20 MW, was installed at the Edgar Thompson plant of the. 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. B4,S Power of 0. 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.
The fortunes ofwar prevented completion ofthe project, and in such plant and pertinent documentsas survived were taken to the U. It aided further development ofvalves by permitting them to be tested under service conditions. The Swedish State Power Board decided to use alternating ‘-eurrent for the north-to-south transmission already mentioned.
The line transmits 20 MW at kV through a single-conductor cable, with transmossion path through the sea and earth. Each converter has two vaIve groups rated 50 kV, A, 10 MW, the groups being in series on the dc side. Each valve has two anodes working in parallel. Building the dc link was judged more economical than construeting transmissioh thermal power plants on the island. The distance is far toa great for ac eable transmission. Power fiow is normally from the mainland to Gotland but is sometimes in the opposite direetion.
E.w.ki,bark uch of the time when power is delivered to Gotland, there are no generators in operation there, the only synchronous maehine being a condenser. Power is adjusted automatically to maintain rated e.w.imbark 50 Hz in Gotland.
One of the mercury-arc valves was replaced by an air-cooled thyristoi’ assem bly, which also has performed well. 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 doubling the voltage.
9780471475804 – Direct Current Transmission, Volume 1 by Edward Wilson Kimbark
Practical ground electrodes were developed, and various kinds of valves and converter control were tested. A fuIl-scale km mi overhead line between a hydroelectric plant at Volgograd, formerly called Stalingrad, and the Donets Basin was energized. 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 as to their principal components and the arrange ments thereof, according to whether the line operates on ac or transmissiob.
The phrase “transmission link” denotes the transmission line proper together with its terminal and auxiliary equipment. Figure la shows a single-eircuit three-phase ac line.
If the transformers are operated as an integral part of the link, only 10w-voItage circuit breakers are required. Most long overhead ac tines require series compensation of part of the induetive reaetanee. In the figure, one bank of series capacitors for this pur pose is shown at the middle of the tine. Three-phase tines eannot be operated, except for a diret 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.
Therefore three-pole switching is always used to clear permanent faults, although such a fauIt may involve only one conductor. This being so, two parallel three-phase circuits are required for reliable transmission see Fig. Long two-cireuit ac links are usually sectionalized by means of intermediate switching stations for severa reasons.
Among these are a limiting the deerease in stability power Iimit attributable to switching direcf one circuit to clear a fault or for line maintenance, b Iimiting the overvoltage when a line is energized from one end, c providing a place for the connection of grounding transformers to limit the overvoltages of the unfaulted phases with respect to ground when one phase is faulted to ground, and d for con nection of intermediate loads or generation.
Intermediate generation raises transmiswion stability limit of the link. On many long EHV lines, shunt reactors are required for limiting the voltage, especially at light loads, but they may be required even at full load.
Direct current transmission kimbark
These reactors are usually placed at intermediate switching stations and are so indicated in Figure Cirrent. A representative single-circuit dc link is shown in Figure le. The tine itself usually has two conductors, aIthough some Iines have only one, the return path being in the earth or seawater or e.w.kimnark.
At both ends of the tines are converters, the components of which transmisskon transformers and groups of mercury arc valves. The converter at the sending end is called a reetifier, and that at the receiving end an inverter. Either converter, however, can function as rectifier or inverter, permitting power to be transmitted crurent either direction.
The ac line, of course, also has this reversibility. 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. The breakers are required, however, for clearing faults in the transformers or for taking the whole dc link out of service. Large inductances ealled de. One pole of a four conductor line is shown in Figure Id, with digect converters per terminal.
The bus-tie switches I are normally open. 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. Then switches 3 would be opened, isolating the faulted line. Next the converter voltages would be raised to equality with those of the respective adjacent converters, e.wk.imbark which switches E.w.jimbark would be closed.
The capability of ali converters would then be usable, and the power normally carried by two conductors would then be carried by one. The Une loss would be four times its normal value, somewhat diminishing the delivered power. The whole switching operation would take about 0.
Each pole would be switched independently of the other. Direet-current links are classified as shown in Figure 2.
The rnonopolar link has one conductor, usually of negative polarity, and ground or sea returno.
Direct Current Transmission – Kimbark
Each terminal has two converters of equal rated voltages in series on the dc side. The neutral points junctions between converters are grounded at one or both ends. If both neutrals are grounded, the two poles can operate inde pendently. Norrnally they operate at equal current; then there is no ground current. In the event transmiszion a fault transmiission one conductor, the other conductor with ground return can carry up to half of the rated load.
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.
In a bipolar scheme reconnection of the whole converter to one pole of the line is more complicated and is usually not feasible because of graded insulation. In this respect a homopolar line is preferable to a bipolar tine in cases where continuai ground current is. Parte 1 de 6. Livro – Whitebook Livro – Whitebook. Exp1 queda livre Experimento de queda livre.