02. chapter 1

I

General Aspects of DC Transmission

and Comparison of it with

AC Transmission

1-1 mSTORICAL SKETCHB 17,lS$

Early Discoveries and Applications

Both electrical science and the practical applications of electricity began with direct current. Alternating current came later.

The basic discoveries of Galvani, Volta, Oersted, Ohm, and Ampere pertained to direct current. The first widespread practical application was telegraphy powered by electrochemical batteries and using ground-return circuits.

Electric lighting and power also began with direct current powered by dynamos. First came carbon arc lamps operated in series at constant current and fed from series-wound generators. Later came carbon-filament incandescent lamps operated in parallel at constant voltage and supplied with current from shunt-wound generators.

The first electric central station in the world, on Pearl Street, in New York, was built by Thomas A. Edison and began operation in 1882. It supplied direct current at 110 V through underground tubular mains to an area roughly 1 mi (1.6 km) in radius. It had Edison bipolar dc generators driven by steam engines. Within a few years similar stations were in operation in the central districts of most large cities throughout the world.

In view of the supremacy of direct current it is interesting to see it was almost completely superseded by alternating current and why direct current is again some high-voltage transmission lines.

* Superior followed of the r"hOlr.i',p?"

including this one, superior to groups of·items in the

2 GENERAL ASPECTS OF DC TRANSMISSION

Later Ascendancy of }\Jternating Current

The advent of the transformer, polyphase circuits, and the induction motor in the 1880s and 1890s led to ac electric power systems.

The transformer, simple, rugged, and efficient, made possible the use of different voltage levels for generation, transmission, distribution, and use. In particular, it made long-distance, high-voltage power transmission possible. The exploitation of water power, usually available at sites distant from major load centers, gave impetus to such transmission.

The induction motor, especially the polyphase induction motor, is also simple, rugged, and cheap and serves the majority of industrial and residential purposes. The commutators of dc motors and generators, in addition to requiring maintenance, impose limitations on the voltage, speed, and size of these machines. The voltage per bar of the commutator should not exceed about 22 V lest excessive sparking occur. Thus a high voltage per commutator requires many bars, resulting in a large diameter. A large diameter requires a low speed in order that the commutator and windings may withstand the centrifugal force. And a low-speed machine is heavier and more expensive than a high-speed machine of equal rating. The advent of steam turbines, which are best at high speed, gave a great advantage to ac generators.

When ac systems first appeared, there were heated arguments between the proponents of dc and ac systems. Advocates of dc branded ac dangerous because of the high voltages used. As a result of their advantages, however, ac electric power systems became almost universal. Power was generated, transmitted, distributed, and used as alternating current. If direct current was needed for some particular purpose, such as adjustable-speed motor dnves or electrolytic processes, alternating current was converted to direct current locally by motor-generator sets or synchronous converters or, later, by mercury-arc rectifiers.

The last vestiges of dc distribution were the low-voltage networks in the centers of large cities and electric traction (streetcar, trolley bus, rapid transit, interurban and suburban railways, and some long tunnels or mountainous sections of main-line railways). Finally, ho.wever, low-voltage ac networks replaced low-voltage dc networks, diesel locomotives replaced steam locomotives and many electric locomotives, and gasoline or diesel buses replaced most of the streetcars and interurban lines. Some dc rapid-transit systems still remain. The victory of alternatin'g over direct current, however, was almost complete.

Status of DC

Despite the general forgot the obvious

some engIneers never Section 1-4).

1-1 HISTORICAL SKETCH 3

They proposed, however, not to replace ac but to supplement it with dc. Specifically, they would superpose a dc transmission link on an ac system or interconnect two ac systems by a dc transmission tie line. Generation, use, and even most transmission and distribution, would remain by ac.

Such a dc transmission scheme requires that ac be converted to dc at the sending end of the dc link and that dc be converted to ac at the receiving end. The feasibility and advantageousness of the scheme depended on the development of suitable converters for the required high voltage and power. The development of suitable converters is considered shortly_ First, however, let us turn aside to describe the Thury dc system.

The Tbury SystemB18

A system of HV dc transmISSIon designed by a French engineer, Rene Thury, came into use at a time when ac systems were in their infancy, and it persisted well into the era of ac predominance. This system is interesting both as an engineering achievement and because of certain similarities to modern HV dc systems. At the sending end of the transmission line a number of series-wound dc generators, driven by prime movers, were connected in series to generate the required high voltage, and at the receiving end, a comparable number of series-wound dc motors, connected in series, drove lowvoltage dc or ac generators. The system operated at constant current. The voltage of each machine in the HV series circuit was regulated by shifting the brushes.

Since the series circuit was normally grounded at only one point, many of the machine windings had a high potential with respect to ground. It was not feasible to provide insulation between windings and frame for such voltages; instead, the frames were insulated from ground by setting them in a floor of asphalt over asphalt concrete, and were insulated from the driving or driven machines by insulated couplings.

Switching and instrumentation were very simple. Each machine was pro .. vided with a short-circuiting switch. A machine was taken out of service by reducing its terminal voltage to zero and then short-circuiting it. It was brought into service by the reverse of this procedure. An ammeter and a voltmeter were the only instruments required.

From 1880 to 1911 at least 19 Thury systems were installed in Europe, principally the use of water POWtf. The most important these was from Moutiers, in the French Alps, to Lyons, B3 installed in 1906 with a

112 mi (180 km) of which 2.8 mi (4.5 being open-wire line.

at 57.6 kV, 75 A. This line was ac system and was integrated


had four water turbines, each driving four generators of 3.6 kVeach. At Lyons the greater part of the power received by HV direct current was converted to alternating current and the remainder to 600 V dc for the street railway. The over-all efficiency was 70.5%, which was considered satisfactory for a hydroelectric system.

In 1911 a second hydroelectric plant at La Bridoire, situated about halfway along the line and rated at 6 MW, was added (in series). The line current was then doubled (to 150 A). In 1912 a third hydro plant, located at Bozel, 7 mi (11 km) beyond Moutiers, and rated at 9 MW, was added, raising the total generating capability on the line to 19.3 MW. The maximum circuit voltage became 125 kV and the route length 140 mi (225 km). Operation of the line continued until 1937, when it was dismantled. Thury himself died in 1938.

The Thury system performed reliably in spite of the large number of commutators in series. The limitations of dc machines, already mentioned, however, made it unsuitable to the larger amounts of power that had come to be required. Further development of HV dc transmission required better converters than motor-generator sets.

Development of a Practical Converter

A converter is basically nothing more than an assemblage of controlled switches. The commutator of a de motor, generator, or synchronous con-· verter is such a device. The vibrating reed is an even simpler switching device, used for stepping up direct voltage from a storage battery to a value suitable to plate supply in automobile radios or as a chopper in stabilized dc amplifiers. In these two applications the input and output are dc, with ac in the intermediate circuit, which is just the opposite of dc transmission interconnecting two ac systems.

Two of the more serious attempts to develop a switching converter suitable to HV de transmission are the transverter and the Marx atmospheric-arc converter.

The transverter, patented in 1920 by two British enineers, W. E. Highfield and J. E. Calverley, consisted essentially of polyphase transformers commutated by synchronously rotating brush gear. It performed the three basic operations of voltage transformation, phase multiplication, and commutation and could be used either as a rectifier or as an inverter. Since the commutators were stationary and only the brush gear rotated, the problem of centrifugal force was mitigated. Several experimental transverters were built, the largest of which was at 2 MW, 20 A, 100 kV on the de side, but none has been used

The 1932, is a

Marx of Braunschweig in arc between two like water-cooled


main electrodes is ignited by a high-frequency spark getween auxiliary electrodes in the path of the main arc and is extinguished after a current zero by a blast of air or mixed gases that continually plays on the arc path. At one time such converters could handle more power (40 MW) than any other converter then available. The life of the electrodes, however, was short, and the voltage drop across the arc was high (500 V). The loss in the arc, together with the power required for ignition, air blast, and cooling, amounted to 2.5 to 3% of the transmitted power at each terminal. This is considerably greater than the corresponding loss (about 0.3%) in mercury-arc converters.

Valves

The synchronously controlled switches heretofore described for use as converters can conduct in either direction, and the actual direction of current depends on the controlled instants of closing and opening and on the EMFS in the circuit. Generally, unidirectional conduction is desired. Devices having inherent unidirectional conduction are herein called valves. Among such devices are vacuum and vapor or gas-filled tubes having thermionic cathodes, the mercury-vapor tube with mercury-pool cathode, and various solid-state devices. In their simplest form, as diodes, they can be used as rectifiers but not as inverters.

The mercury-arc rectifier with pool cathode is the most suitable for handling large currents. It was invented by Peter Cooper Hewitt about 1903 and initially was made with a glass envelope. The steel tank appeared about 1908 to 1910.

order for a valve to be used as an inverter, it have a control electrode that can prevent the valve from conducting, although the anode is positive with respect to the cathode. Such an electrode (the grid) was added to the vacuum tube (Fleming valve) by de Forest in 1906. The grid was added to the mercury-vapor valve in 1928. It was then applied to the hot-cathode glass-envelope valve, and the resulting triode was thyratron. Later some thyratrons were made with steel jackets. Control grids were added to mercury-arc valves with pool cathodes about 1930. Although the grid in the vacuum tube can start, stop, and modulate the current through the tube, the grid of a mercury-arc valve can only prevent conduction starting. After it has started the control grid can neither stop the nor s:ontrol its magnitude. Conduction does not cease until becomes negative with respect to the cathode.

Actually the first control element used in a mercury-arc valve with pool cathode was not a grid but an igniter, introduced' resulting valve

The' IS a from an auxiliary source is

IS igniter, like the grid,


Present-day mercury-arc 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 ITlaintaining the arc, and a control grid that prevents the arc 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 and the anode for obtaining a more uniform potential gradient than would otherwise exist. The grading electrodes are kept at the desired potentials by connecting them to taps on an external resistance-capacitance potential divider the ends of which are connected to the anode and control grid. This system of grading electrodes, invented by U. Lamm in 1939, has considerably increased the peak inverse voltage that the valves can withstand.

Valves for HV dc transmission are invariably of single-phase construction, in contrast to the polyphase valves with mercury-pool cathode formerly used extensively in low-voltage rectifiers for industrial and railway application.

The development of valves for HV dc transmission has been carried out since World War II principally by engineers in the U.S.S.R. and by the Swedish firm of Allmanna Svenska Elektriska Aktiebolaget (ASEA), with which Lamm is connected. A noteworthy feature of ASEA valves is the use of several, usually four, anodes in multiple on single-phase valves. The current ratings are 200 to 300 A per anode. Russian engineers have concentrated on single-anode valves, which so far appear to have been less successful than the ASEA valves.

About 1960, control electrodes were added to silicon diodes, giving siliconcontrolled rectifiers (SCRs), also 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 de use.

Experimental DC Transmission Projects and First Commercial Lines

The initiative in exploring the use of mercury-arc valves for dc transmission was taken by the General Electric Company. After two smaller experimentsB1

,2 they proceeded in December 1936 to use direct current on a 17-mi (27-km) line between the Mechanicville hydroelectric plant of the New York Power & Light Corporation and the General Electric factory in Schenectady.B15 The line carried 5.25 MW at 30 kV, 175 A. The converter at each end of the line had 12 hot-cathode glass-envelope thyratrons in 6 series

Mechanicville was at a frequency of 40 Hz, and was 60 Thus was demonstrated a

important in several subsequent i


The line initially operated at constant current, the conversions from constant alternating voltage to constant current 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 could not withstand the high short-circuit currents expected to occur on a constant-voltage system. After the more rugged steel-envelope mercury-pool ignitron became available, however, the line was converted in 1940 to constant-voltage operation. The circuitry then used was basically the same as that of modern dc transmission systems, fault currents being limited by control of valve ignition. The operation of the line was discontinued in 1945 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.

Meanwhile, two 25/60-Hz frequency changers using controlled mercuryan; valves were installed in steel mills in the United States in 1943. The larger of these, rated at 20 MW, was installed at the Edgar Thompson plant of the Carnegie-Illinois Steel Company near Pittsburgh. 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, in 1939, at the Fifth Swiss National Exhibition.B4

,5 Power of 0.5 MW at 50 kV, 10 A, was sent 19 mi (30 km) from Wettingen power plant near Baden to Zurich over a circuit of one conductor, partly overhead and partly in underground cable, with earth return. In 1946, Brown-Boveri discontinued their work on HV dc transmission.

Two HV dc experiments were in Germany during World War II at the instance of the German Secretariat for Aviation. B6,19 A 400-kV three-phase line from the Alps to the Ruhr had already been planned, but the Secretariat intervened in favor of a HV dc cable line, which, it felt, would be less vulnerable to air-raid damage. The Siemens-Schuckertwerke A.G. began experiments in preparation for such a line. They transmitted 4 MW at 110 kV a distance of 3 mi (5 km) over an existing line from a station in the Charlottenburg district of Berlin to one in the Moabit district. B6 A second, larger experiment was to be the transnlission of 60 MW by means of a 70-mi (II0-km) 400-kV dc cable from the Elbe (near Dessau) to Marienfelde (near Berlin).B6,9 This experiment was to be conducted jointly by Siemens and the A.E.G. The fortunes of war prevented completion of the project, and in 1945 such plant and pertinent documents as were taken to the U.S.S.R. as reparations.

In Sweden, where the principal new principal loads are in the

of the development of valves aroused in the possibility of a HV

sites are in the north and ; and, because

interest was as an alternative to


ac. An experimental transmission between Mellerud and Trollhattan (36 mi) began operation in 1944. It aided further development of valves by permitting them to be tested under service conditions.' The Swedish State Power Board decided to use alternating current for the north-to-south transmission already mentioned. The results of the Mellerud-Trollhattan transmission, however, encouraged the Board to proceed with HV dc transmission by submarine cable from the Swedish mainland to the island of Gotland, 96 km (60 mi) offshore.G This system, built by ASEA, began service in 1954 and may be considered the first commercial HV dc transmission system. The line transmits 20 MW at 100 kV through a single-conductor cable, with return path through the sea and earth. Each converter has two valve groups rated 50 kV, 200 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 constructing additional thermal power plants on the island. The distance is far too great for ac cable transmission.

Power flow is normally from the mainland to Gotland but is sometimes in the opposite direction. Much of the time when power is delivered to Gotland, there are no generators in operation there, the only synchronous machine being a condenser. Power is adjusted automatically to maintain rated frequency (50 Hz) in Gotland.

The link is still in operation (1970) and has a good performance record. One of the mercury-arc valves was replaced by an air-cooled thyristo{ assembly, 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.

In the U.S.S.R., where even greater distances than in Sweden separate the potential hydroelectric sites from the principal industrial load areas, the use of BV dc transmission was considered necessary, B8 and an extensive program of research and development was undertaken, begun as a part of the 5-yr plan of industrial development for 1946 to 1950.

experimental line between Moscow and Kashira (112 km or 60 mi, 30 MW, + 100 kV) began operation in December 1950.B10 ,11,13,16

was basically an underground cable line, but at times sections of overhead line were put into the circuit. Both bipolar metallic operation and monopolar, ground-return operation were tried. Practical ground electrodes were developed, and various kinds of valves and converter control were tested.

A Direct Current InstituteB12,14 was established in Leningrad, which since

1 has published approximately one volume per· year of articles on its A3

(294-mi) overhead line between a called Stalin grad , and the Donets

1-2 CONSTITUTION OF EHV AC AND DC LINKS 9

at reduced voltage and power in 1962 and, beginning in 1965, was operated at its full rating of + 400 kV, 900 A, 720 MW. I Other dcJines of lengths of 2000 to 2500 km and voltage of + 750 kV are planned.Q6

1-2 CONSTITUTION OF EHV AC AND DC LINKS

EHV transmission links, superposed on a lower-voltage ac network, or interconnecting two such networks, or connecting distant generating plants to an ac network, are compared as to their principal components and the arrangements thereof, according to whether the line operates on ac or dc. The phrase "transmission link" denotes the transmission line proper together with its terminal and auxiliary equipment.

Figure 1 a shows a single-circuit three-phase ac line. In general, such a line in the categories already mentioned, one which might be competitive with a dc link, requires transformers at both end~-step-up transformers at the sending end and step-down transformers at the receiving end- although in some cases they can be omitted at one or both ends. If the transformers are opera ted as an integral part of the link, only low-voltage circuit breakers are required.

Ac system

Ac system

(c)

~~~ I o~~ ~_lL~ 1

Ac system

Ac system

Fig. 1. Constitution by single-line diagrams.


Most long overhead ac lines require series compensation of part of the inductive reactance. In the figure, one bank of series capacitors for this purpose is shown at the middle of the line.

Three-phase lines cannot be operated, except for a very short time (less than 1 sec) with one or two conductors open, because such operation causes un balanced 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 fault may involve only one conductor. This being so, two parallel three-phase circuits are required for reliable transmission (see Fig. 1 b). Long two-circuit ac links are usually sectionalized by means of intermediate switching stations for several reasons. Among these are (a) limiting the decrease in stability power limit attributable to switching out one circuit to clear a fault or for line maintenance, (b) limiting 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 connection of intermediate loads or generation. Intermediate generation raises the 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. These reactors are usually placed at intermediate switching stations and are so indicated in Figure 1 b.

A representative single-circuit dc link is shown in Figure 1 c. The line itself usually has two conductors, although some lines have only one, the return path being in the earth or seawater or both. At both ends of the lines are converters, the components of which are transformers and groups of mercuryarc valves. The converter at the sending end is called a rectifier, and that at the receiving end an inverter. Either converter, however, can function as rectifier or inverter, permitting power to be transmitted in 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 of the valves, for these faults can be cleared more rapidly by grid control of the valves. The breakers are required, however, for clearing faults in the transformers or for taking the whole dc link out of service. .

Harmonic filters and shunt capacitors for supplying reactive power to the converters are connected to the ac sides of the converters. Large inductances called dc smoothing reactors are connected in series with each pole of the dc line.

Some writers claim that a two-conductor de line provides the same re-as a two-circuit three-phase line either

of the dc line can be used or for periods, say, a few days per year.

1-3 KINDS OF DC LINKS 11

If higher reliability is required of a dc line than that provided by two conductors, three or four conductors may be provided. One pole of a fourconductor line is shown in Figure Id, with two converters per terminal. The bus-tie switches 1 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, after which switches 1 would be closed. The capability of all converters would then be usable, and the power normally carried by two conductors would then be carried by one. The line loss would be four times its normal value, somewhat diminishing the delivered power. The whole switching operation would take about 0.3 sec, a time as short as that required for rapid reclosure on an ac line. Each pole would be switched independently of the other.

Comparison of the ac and dc links shows that (a) the dc line proper is simpler, having one or two conductors instead of three, but that (b), on the other hand, the terminal equipment is more complex, having the groups of valves and some auxiliary equipment that the ac line does not need.

1-3 KINDS LINKS

Direct-current links are classified as shown in Figure 2. The monopolar has one conductor, usually of negative polarity, and

ground or sea return. The has two conductors-one positive, the other negative.

Each terminal 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. they operate at equal current; then there is no ground current. In of a fault on one conductor, the other conductor with ground return can carry up to half of the rated load.

The rated of a bipolar link is expressed as + 100 kV, for example, pronounced and minus 100 kV.

The homopolar has two or more conductors all having the same polar-ity, usually negative, and always operates with ground return. In the event of a fault on one conductor, the entire converter is available for connection to the remaInIng or conductors, which, having some overload capability, can carry more than half of the rated power, and perhaps the whole rated power, of increased line loss. In a bipolar scheme reconnection

usua.lly line is

one of line is more complicated graded insulation. In this respect a homopolar

in cases where continual ground current is


Rectifier Inverter

-1Hii ______ ~~ __ ----f1t-(a)

+

(b)

.----tll - -- - ~ ~ - - - 11t-----.

(c)

Fig. 2. Kinds of de links.

not deemed objectionable (see Chapter 9). An additional minor advantage is the lower power loss due to corona. Negative polarity is preferred on overhead lines because of its smaller radio interference.

Cascaded Groups

In each of these kinds of links there are usually several converters connected in parallel on the ac side but in series on the dc side for obtaining the desired level of direct voltage from pole to ground. Each such converter consists of a transformer bank and a group of valves.

1-4 HV DC PROJECTS FROM 1954 TO 1970

The successful operation of the transmission in other countries.

(

construction in 1970 is given 1.

awakened interest in dc in operation or under

links are situated in nine

Table 1. DC Transmission Links in Operation or under Construction in 1970

Length (km, mi) Direct

Voltage Power Overhead Scheme Terminals Year (kV) (MW) Line Cable Total Remarks

Vastervik, Sweden 1954 100 20 0 96(60) 96(60) Monopolar Visby, Gotland Sea return

Channel Lydd, England 1961 ±100 160 0 64(40) 64(40) Echinghen, France

Volgograd-Donbass Volgograd, U.S.S.R. 1962- 100 90 474(294) . 0 474(294) Donets Basin, U.S.S.R 1965 ±400 720

New Zealand Benmore, South Id. 1965 ±250 600 570(354) 40(25) 610(379) Haywards, North Id.

Konti-Skan Goteborg, Sweden 1965 250 250 88(54) 87(54) 174(108) Monopolar Alborg, Denmark Sea return

Sardinia Codrongianos, 1966 200 200 290(180) 116(72) 406(252) Via Corsica Sardinia, Homopolar

San Dalmazio, Italy Vancouver Island Arnott, British 1968 130 78 41(26) 32(20) 73(46) First stage

Columbia Vancouver Island Ultimate ±130 312 Final stage

Pacific NW-SW The Dalles, Oregon 1970 ±400 1440 1372(853) 0 1372(853) Three bridges Sylmar, California per pole

Kingsnorth Kingsnorth power plant ±266 640 0 59(37) 59(37) Each pole has a Beddington and 82(51) 82(51) different desti-

Willesden (London) nation River, Radisson (Kettle Ultimate ±450 3240 895(555) 0 895(555) To be built in

Manitoba Rapids) stages Dorsey (Winnipeg)


different countries. Brief comments are made on these links, all of which except Volgograd-Donbass were based wholly or mainly on ASEA techniques.

English Channel CrossingH

The next link to go into service after Gotland was an interconnection between the ac systems of England (Central Electricity Generating Board) and France (Electricite de France) through two single-conductor submarine cables. The distance (42 mi or 64 km) is shorter than that of the Gotland scheme, but the rated power (160 MW) is eight ti!11es as great. Each valve has four anodes, and each of two bridges (one per pole) is rated at 800 A, 100 kV, 80 MW. Like Gotland, the Channel Crossing scheme involves crossing water; but, unlike Gotland, it does not use the sea as a return conductor. Because of concern with the effect of the direct current on ships' compasses in a channel having much shipping, two cables were laid close together, one operating at + 100 kV with respect to ground and the other at -100 kV. The midpoint (neutral) of the converters is grounded at one terminal only, so that ground. current cannot flow except briefly in the event of a cable fault.

This link interconnects two large ac systems but has a small power rating compared with the capacity of either system. An ac link of this kind would have been feasible except that it would be difficult to control. The British power system has no autolnatic load-frequency control. Installation of such a control for the sake of the interconnection would have been very expensive. The dc link is an asynchronous tie between two systems of the same nominal frequency (50 Hz). Its power flow is readily controlled to a set value.

The purpose of the interconnection is to take advantage of time-zone and generation diversity. The direction of power flow varies. The French system has a considerable amount of hydroelectric generation; the British system has practically none. In seasons in which the supply of water to the hydro plants is ample, power can be exported to Great Britain. When water is scarce, power can be imported from there.

The Channel link was plagued by troubles in its first few years of operation. One of the transformers in the French terminal failed. The submarine cables were broken several times by trawlers, and they could not be repaired soon because of bad weather and rough seas. Since then the link has operated with very little trouble.

Volgograd-Donbass Line I

When built, this was hydroelectric power and mining district in

dc line. usually carries power from a River at Volgograd to an industrial . For such an operation generators

1-4 HV DC PROJECTS FROM 1954 TO 1970 15

in the hydro plant may be disconnected from the ac bus and connected only to a valve group of the rectifier. For power flow in the opposite direction the inverter valve groups are connected to the ac bus.

It seems that the link did not offer any advantage in cost compared with an ac link, but it was built to gain experience in dc transmission for longer higherpower lines that will be built in the future.

Each terminal has eight valve groups (four per pole), using single-anode valves of Russian design, with two valves in series in each arm.

The year 1965 was called "the dc year" by the editor of Direct Current. Not only was the Volgograd-Donbass link brought up to its designed voltage and power, but also two additional dc transmission schemes (New Zealand and Konti-Skan) and a frequency changer at Sakuma, Japan, went into operation. A third transmission scheme (Sardinia) was expected to go into operation, but it was delayed until the following year.

New Zealand LinkJ

To meet the growing demand for power on the North Island, either additional steam-electric power plants would have to be built there, or hydroelectric power plants would have to be built on the South Island, from which the power would be transn1itted electrically to the North Island. Submarine cables 24 mi (39 kIn) long would be required across Cook Strait, which separates the two islands. The hydroelectric alternative was more economical and it was chosen. Direct-current transmission was selected as being more feasible than ac for this long water crossing. Three de cables are used (one for each pole and a spare), but 11 ac cables would have been required (for three three-phase circuits and two spares), which would have occupied a wide belt of sea bed. The decision was made even before the English Channel scheme was in operation.

The transmission system includes, in addition to the submarine cables, 335 n1i (535 kn1) of overhead bipolar transmission line on the South Island and 25 mi (40 knl) on the North Island. It extends froln Bennlore power plant on the South Island to Haywards Substation on the North Island, near the city of Wellington. The power rating of 600 MW is considerable compared with the aggregate generation on either island then (1400 MW on the North Island and less on the South Island) and slightly exceeds the rating of the Benmore plant. The cost of the dc-transmission scheme was about two-

. of that of the ac-transmission schenle that was considered as an

return is used in en1ergencies when one circuit is faulted.

or the


The valves, manufactured by ASEA, are rated at 1.2 kA, 125 kV, and have four anodes.

Konti-Skan LinkK

This is an interconnection between Sweden and Denmark and thus, through previously existing ac connections, between Germany and the rest of Western Europe and the Scandinavian countries. It crosses the Kattegat by way of the island of Laesa and has two cable sections and overhead sections on the island and at each end.

The dc scheme was compared with an ac scheme having a shorter cable. The cost of the two schemes was approximately equal, but the dc scheme presented two advantages over the ac:

1. The dc line provides an asynchronous tie. The stability limit of the ac scheme was estimated as 350 MW; the ultimate power capability of the dc link was 500 MW. The need for expensive load-frequency regulation is avoided.

2. The dc scheme can be built in two stages, and thus almost half of the investment can be postponed. The first stage operates monopolarly with one submarine cable and sea return at a power capability of 250 MW. In the second stage the line will be a bipolar, metallic circuit for 500 MW, with sea return used only in emergencies.

Four-anode, 1.1-kA, 125-kV va~ves are used.

Sakuma Frequency Changer

This station was put into operation in 1965, interconnecting the 50- and 60-Hz systems of Japan. It can transmit 300 MW in either direction. There is no dc transmission line, the. dc circuits being confined to the station. With minor exceptions, the equipment and circuits are like tho'se of a transmission scheme. The valves are similar to those of the New Zealand and Konti-Skan links.

Sardinian SchemeL

In order to use large deposits of low-grade coal on the Italian island of Sardinia, a thermal power plant was built there, and a dc link was built connecting it, by way of the French island of Corsica, to the Italian mainland near La Spezia. This link consists mainly of submarine cable, with some overhead line on the ends. A peculiarity of this the line has two the same polarity, with sea return. is negative is transmitted from Sardinia to the

1-4 HV DC PROJECTS FROM 1954 TO 1970 17

is the usual direction, although the opposite direction holds when the Sardinian plant is shut down. Power flow is regulated so as to keep constant frequency on the Sardinian ac system. The valves are similar to those of several other schemes, are rated at 1.0 kA, 100 kV, and have four anodes.

Vancouver Island SchemeM

This provides a dc connection between the mainland of the Canadian province of British Columbia at Arnott, south of the mouth of the Fraser River, and Vancouver Island. It is being built in stages of 78 MW each, with an expected final power of 312 MW. It crosses the Strait of Georgia by submarine cable and Salt Spring Island by overhead line. The four-anode valves are rated at 1.2 kA, 130 k V.

This is the first scheme in which a dc link operates in parallel with an ac link.

Pacific Northwest-Pacific Southwest IntertiesN

The purpose of this scheme is to take advantage of seasonal diversity in load and generation' between the northwest area, comprising the states of Washington and Oregon 5 and the southwest area, comprising southern California and Arizona. The entire scheme includes two 500-kV ac circuits with a total length of 905 rni (1450 km) from the Columbia River to the vicinity of Los Angeles and two + 400-kV bipolar dc circuits. The first dc circuit is fronl Celilo substation near The Dalles, Oregon, to Sylmar substation, near Los Angeles. The second dc circuit is planned to be built from Celilo to Mead substation near Hoover Darn at Boulder City, Nevada. The power ratings of the ac lines are 1000 MW each and those of the dc lines 1440 MW each. A third dc line, the so-called "dc cross tie," from Sylmar to Mead, about 270 TI1i (430 km), has been discussed, but there is no definite plan for building it.

Each of the two dc lines exceeds any previous dc line in length and in power rating, although the rated voltage is equal to that of the VolgogradDonbass line. The valve ratings are also greater, being 1.8 kA, 133 kV, 240MW per group, with six anodes per valve.

The dc lines operate in parallel with a 60-Hz ac systen1. Because of the great length of the ac lines, the stability of the ac system poses a considerable problenl, and it was necessary to use a high degree (average 65%) of series

·on. A permanent bipolar dc line is one of the severest disturbances that the ac , although the occurrence of such a fault is believed


Kingsnorth 0

The Central Electricity Generating Board of Great Britain is interested in the use of dc links for reinforcing an ac system in areas of high load density without increasing the interrupting duty of ac circuit breakers. A trial installation of this kind is the transmission of power by underground dc cable from the Kingsnorth thermal power plant, situated on the south shore of the Thames River estuary, to two substations in London. This is a bipolar scheme having three cables: one for each pole and a neutral cable. Each pole goes to a different substation, with the result that, although the whole scheme has three terminals, each pole has only two terminals. The Beddington substation is 37 mi (59 km) from Kingsnorth, and the Willesden substation is 14 mi (23 km) beyond Beddington. Whenever the loads of the two substations are unequal, there will be neutral current. This current is not allowed to flow in the ground for fear of damage by electrolytic corrosion to some of the many buried metallic structures found in a metropolitan area.

The rating of this scheme is + 266 kV, 1.2 kA, 640 MW. There are four groups of valves at Kingsnorth and two groups at each substation, each group being rated at 133 kV, 1.2 kA, 160 MW.

Nelson River, Manitoba, SchemeP

The Nelson River has a potential hydroelectric power development of about 6500 MW, including S011le diversion of water from other streams. It has been decided to develop this power and to trans111it it to Winnipeg by direct current. Bipolar + 450-kV overhead dc lines were judged more economical than 500-kV ac lines. Ultimately there will be several such bipolar circuits as the development proceeds by stages. With two such lines, the translnission capacity will be 3240 MW.

In response to the invitation for bids on terminal equiplnent for the first stage, three proposals were received for thyristor converters and two for mercury-arc-valve converters. The proposal for 111ercury-arc equipment by the English Electric Company was accepted. Each valve group will operate at 1.8 kA, 150 k V, 270 MW. There will be three groups in series per pole.

Eel River (New Brunswick)

This station provides an asynchronous tie between the 60- ac systems of Quebec and of New Brunswick. As at Sakuma, Cl are

to the station. In contrast to Sakuma, the ac systems are equal, although one can drift

1-5 LIMITATIONS AND ADVANTAGES OF AC AND DC TRANSMISSION 19

other. The distinctive feature of the Eel River station is that it is the first large converter station designed to use thyristor valves initially and exclusively. The rating of the station is 320 MW, 80 kV dc, 230 kV ac.

1-5 LIMITATIONS AND ADVANTAGES OF AC AND DC TRANSMISSION

Noting the universal use of alternating current for electric power transmission, as well as for generation, distribution, and use, one naturally asks what limitations ac transmission has that have led to the use of dc transmission in some projects.

The limitations may beither technical-something cannot be done-or economic-it can be done more cheaply some other way. In most practical cases the technical limitations are not reached, and economic limitations dictate the final choice of design.

We are interested in limitations on the amount of transmitted power and on the distance over which it can be transmitted. More exactly, we are interested in the cheapest method by which a certain amount of power at a certain load factor can be transnlitted reliably over a certain distance. The power depends on the current, voltage, power factor, and number of conductors.

Current Limit

The temperature of a conductor must be limited in order to avoid damage to the conductor itself (permanently increased sag) or, in case of a cable, to the insulation in contact with it. Hence the current in the conductor must be limited in accordance with its duration and the ambient temperature. The limiting current is seldom reached on long overhead ac lines because of other limitations' being reached first, but on cables the current limit due to heating is important, as sho\vn later.

The ac resistance of a conductor is somewhat higher than its dc resistance because of skin effect, but the difference is not important in nonmagnetic conductors of the usual diameters at the usual power frequencies.

Voltage Limits

The normal working voltage and the overvo] tages caused by switching surges and lightning must be Iinlited to values that wi]] not cause puncture or flashover of the insulation. overhead lines, switching surges, rather than lightning, have serious transient overvoltages, and on ac lines attelnpts are to peak values of two or three times nornlal crest voltage. on . nes are lower than this, say,


1.7 times normal voltage. On overhead lines, the maximum working voltage or the minimum conductor size is limited also by loss and radio interference due to corona. In current ac practice, radio interference during foul weather (rain, snow, or fog) is usually the limiting factor. Here dc lines have adistinct advantage in that radio interference is slightly decreased by foul weather, while interference due to ac lines is greatly increased by foul weather. In cables, where the limiting factor is usually the normal working voltage, the insulation will withstand a direct voltage higher than the crest of alternating voltage, which is already 1.4 times the rms value of the alternating voltage.

Reactive Power and Voltage Regulation

On long EHV ac overhead lines and on much shorter ac cables, the production and consumption of reactive power by the line itself constitutes a serious problem. On a line having series inductance L and shunt capacitance C per unit of length and operating voltage V and current I, the line produces reactive power

(1)

and consumes reactive power

QL == wLl2 (2)

per unit of length. The reactive power produced by the line equals that consumed by it, with no net production or consumption, if

hence if

wCV2 == wLI2

V == (L)1/2 == Z, I C S

(3)

In this case the load impedance has the value ,known as the surge impedance of the line. The surge impedance of an overhead line with single conductors is about 400 Q, and with bundle conductors, about 300 n; that of cables is only 15 to 25 Q.

The power carried by the line so loaded is

V2

P == VI == -n Z

is called the surge impedance loading of distance and depends

overhead lines are as

s

(4)

load. is indefor


voltage (k V) 132 230 345 500 700

surge impedance loading (MW) 43 130 300 830 1600

On a line carrying its natural load, the magnitude of voltage is the same everywhere, as shown in curve 2 in Figure 3, and the reactive power is zero

V E

1.05 ....----..-------r--1.-N-:-"o--:l:--oa-d---,----.---,

2. Natural load P . 1.00 ~~-----=:.:....:..:.~:.:::.:..;:.;;;.;;;:.;;~~-----~

0.95

o 10 20 30 Distance from sending end (elec deg)

Fig. 3. Voltage profiles of one-twelfth-wavelength low-loss line with equal terminal voltages E. Length at 60 Hz is 258 mi (416 km).

everywhere (curve 2 in Figure 4).

+0.5r---~----~----~----~----~----~

O~-------------~-----~F---------------------~

P+jQ

-0.5~ __ ~~ __ ~ ____ ~ ____ ~ ____ ~ ____ ~ o 10 20 30

Distance from sending end (elec deg)

Fig. 4. Flow of reactive power Q on the line in Figure 3.

Most lines cannot be operated always at their natural loads, for the vary with time. The most economical load on an overhead line is usually greater than the natural load. If thc load is greater than the natural load, reactive power is line and must be supplied from both ends. are nlaintained at both ends of the line, amounts of supplied from both ends (curve 3 in


and the voltage elsewhere sags, being least at the center of the line (curve 3 in Figure 3). If the load on the line is less than the natural load, net reactive power is produced by the line and is delivered to one or both ends. With eq nal voltages at both ends, equal amounts of reactive power are delivered to both ends (curve 1 in Figure 4), and the voltage everywhere else is higher than at the ends, being greatest at the middle (curve 1 in Figure 3). In all cases, the flow of reactive power through series inductive reactance is "downhill," that is, in the direction of de~reasing voltage.

Thus, to maintain constant equal voltages at both ends, reactive power must be absorbed at light load and supplied at heavy load. The reactive power required for a given variation of load increases with distance (see Figure 5, curves 3 and 4).

4.-~--~------~----~------~------~--~~

3

2

o ~------------------------I

-1

-2~ ____ ~ ______ ~ ____ ~ ____ ~ ______ ~ ____ ~ o 30 60 90 120 150 180

Length of line (elec deg)

Fig. 5. Characteristics of lossless line with equal terminal voltages E (except curve 2) versus length up to one-half wavelength. Curve 1. Maximum power/natural power, Pm/Pn. Curve 2. Voltage at open end/sending-end voltage. Curve 3. Reactive power from both ends/natural power for P = 1.5Pn . Curve 4. Reactive power from both ends/natural power for no load (P = 0). Curve 5. Voltage at middle/terminal voltage for P = 1.5Pn.

If we stipulate, instead of constant voltages at both ends, fixed voltage at the sending end and fixed power factor, say, unity, at the receiving end, the receiving-end voltage varies with load. For a unity-power-factor load, the voltage is high at light load and load. The arTIount of variation increases with the 1i a long line the variation of voltage and it becomes necessary


to supply or consume the reactive power required for maintaining approximately constant voltages. On lines up to 250 mi long, reactive power is ordinarily supplied at the terminals. In the past, synchronous condensers were commonly used for this purpose. They can control the voltage by either supplying or consuming reactive power, as required. Nowadays shunt static capacitors and reactors are found to be more economical. They are switched in blocks.

Figure 5 shows some other disadvantageous characteristics of long, uncompensated transmission lines up to one-half wavelength (180°). One is their power limit. For any given length I the maximum power that can be transmitted on such a line is shown by curve 1. It is

where /- 2nl

0= IvLC =- = PI A

(5)

(6)

is the electrical length of the line, I being the actual length and A the wavelength. For a greater load than the natural load, ~here is a maximum distance; for example, for P = 1.5Pn this distance corresponds to a line angle of sec -1 (P mlPn) = sec -1 1.5 = 41.8° and is (41.8/360))" or 360 mi at 60 Hz. As the length of line approaches this value, the reactive power that must be supplied to the line increases rapidly, as shown by curve 3, and the voltage at the middle of the line drops rapidly, as shown by curve 5.

Another limitation of long lines is the high voltage at an open end (the Ferranti effect), shown in curve 2. This is important when a line is being put into service by first connecting one end of it to the main ac system, for it is not feasible to close both ends at exactly the same moment.

Long .. distance ac power transmission is feasible only with the use of series and shunt compensation, applied at intervals along the line, as illustrated in Figure 6. Series compensation of degree s reduces the effective series inductance from L by sL to (1 - s)L and thus decreases the electrical length-

Eq. (6)-from f3l to 131 J 1 - s and at the same time decreases the surge impedance-Eq. (3)- and increases the natural load by the same factor. The reactive power produced by, shunt capacitance of the line at light may still be excessive, requiring shunt compensation of part h of it. The effective shunt capacitance is then reduced from C to (1 - h)C, and the electrical length

is reduced by the additional factor J 1 - h or by the total factor

J (1 - s)(l - h).

impedance is altered by the factor if h 1'./ s.


Fig. 6. Series and shunt reactive compensation for 750-mi (1200 km) 500-kV 60-Hz single-circuit line delivering 1000 MW and having two 1780-MCM A.C.S.R. conductors per phase. Sending-end voltage, 525 kV /0; receiving-end voltage, 500 kV -29.5°. The

series capacitors have an aggregate loading of 1507 M var.; shunt reactors, 1275 Mvar.; reactive power entering line from sending end, 193 Mvar.; from receiving end, 355 Mvar. Degree of series compensation, 73%; of shunt compensation, 58%. (Ref. D4, Appendix B, Figure 7.)

In Figure 6 and in similar schemes shown in the Ref. D4 for other voltages and distances, the compensation was chosen to limit the angle between terminal voltages to 30° and to limit the voltages at the ends and at compensating points to not more than 1.05 times non1inal voltage, except that the voltage at an open end is limited to not more than 1.1 times normal voltage.

On representative long overhead EHV compensated lines operating at full load, the total reactive power furnished from both ends of the line and from intermediate series capacitors plus the reactive power consumed by shunt

5----~--~--~--~--~--~---,----,-------.

200 400 600 800 1000 Length of line (mi)

Fig. 7. Reactive power requirements of long EHV overhead ac and dc lines at full load as a function of the length of line. (That of ac lines is from data in Ref. D4.)

reactors varies almost linearly with distance, approximately 4.4 MvarjMW . 8

A dc line itself requires no reactive power, itself is merely the resistive drop

as shown in Figure 7 . It is . Mm.

drop on the line at both ends of the line,


however, draw reactive power from the ac systems. It varies with the transmitted power and is approximately half of the latter at each end. It is independent of the length of line. Usually shunt capacitors or synchronous condensers are installed for supplying this reactive power.

Both ac and dc lines have the disadvantage of requiring adjustable supplies (or sinks) of reactive power. For distances of more than 400 km (250 mi), however, Figure 7 shows that the dc line requires less than the ac line.

On submarine or underground cables, the situation is different from that on overhead lines. Cables are always operated at a load much below the surge impedance load in order to avoid overheating. Consequently the reactive power produced by charging the shunt capacitance greatly exceeds that consumed by the series inductance. D2 In a 50- or 60-Hz cable, 25 to 50 mi (40 to 80 km) long, the charging current alone equals the rated current, leaving no margin for load current. Shunt compensation theoretically could correct this situation.D3 Shunt reactors, however, would be required at, perhaps, 10-mi (16-km) intervals. Since it is difficult to lay and repair submarine cable to which shunt reactors are connected, the practical length of ac submarine cables is only about 20 mi (30 km). Dc cables have no such limitation.

Stability

By the stability of an ac system is meant its ability to operate with all synchronous machines in synchronism. If a long ac line is loaded to a certain value, known as its steady-state stability !irnit, the synchronous machines at the sending end accelerate and go out of synchronism with those at the receiving end. This condition is analogous to a slipping -belt or clutch in a mechanical transmission system. The slipping electrodynamic system not only fails to transmit the power that it should but also gives rise to objectionable fluctuations in voltage.

Even if a line is operated below its steady-state' the machines at the sending and receiving ends may lose synchronism after some large disturbance, notably a short circuit, unless the line is operated below its transient stability limit, which is always lower than the steady-state limit. Practically speaking, the steady-state stability limit is the transient stability limit for very small disturbances.

The problem of stability or synchronous operation constitutes the most serious limitation of a long ac transmission system.

The power transmitted from one machine to the other in a two-machine ac system is given by

(7)


where E1 and E2 are the internal voltages of the two machines, <5 is the phase difference of these voltages, and X is the reactance of the architrave of the ,,~quivalent n circuit of the system joining the internal points. Each machine

j ~~presented by an internal voltage "behind" an internal reactance. The reactance X is very nearly the sum of the inductive reactances inside the two machines, of the transmission line, and of the step-up and step-down transformers. An actual power system involving a long interconnecting line with many generating stations at each end of the line may be represented reasonably well by a two-machine equivalent system in which all the machines at the receiving end of the line are replaced by one equivalent machine and all those at the sending end by another.

A graph of power P as a function of phase difference <5 between internal voltages is a sine wave. Maximum power occurs at <5 = 90° and is

(8)

Pm is the steady-state stability limit. It is approximately equal to the square of the operating voltage divided by the series reactance. In a long-distance transmission system most of the reactance is in the line itself, and a much smaller part is in the two terminal systems, consisting of machines, transformers, and local lines. The inductive reactance of a single-circuit 60-Hz overhead line with single conductors is about 0.8 n/mi (0.5 Q/km); with double conductors, about i as great. The reactance of the line itself is proportional to the length of the line, and thus the power per circuit of a given voltage, as limited by steady-state stability, is inversely proportional to the length.

The transient stability limit is lower than the steady-state limit, and, as a rough guide, we may take the former as half of the latter, corresponding to a phase difference of sin - 1 0.5 = 30° in the initial steady state. (This value was assumed in Figures 6 and 7.)

In an uncompensated line operating at its natural load the phase ,of the voltage varies directly with the distance, going through one cycle (360°) per wavelength. A 30° difference, then, corresponds to -/2 wavelength. On a 60-Hz line this is 3100/12 = 258 mi (416 km). On a 50-Hz line it is 310 mi (500 km). A lighter load can be transmitted farther: a heavier load not so far.

The distance to which the natural load can be transmitted stably can be extended considerably by placing synchronous condensers or, better yet, synchronous generators at various intermediate points of the transmission system. If both generators loads are scattered along the transmission system, this method long-distance transmission is called transJ11ission displacement. As shown in 8, the over-all transmission can be regarded


500 MW 500 MW 500 MW 500 MW 500 MW

(a) O~~----) ~ ~

500 MW 500 MW 500 MW 1000 MW

(b)

500 MW 500 MW 500 MW 1000 MW

Fig. 8. Long-distance transmission by displacement regarded (a) as several short transmission systems in tandem and (b) as a long-distance straightaway transmission system supported by several intermediate generators each having its local load. Losses are neglected.

either as several short transmission systems in tandem or as a long transmission line supported by several intermediate generating stations, each having its own local load.

Perhaps the most economical method of increasing the distance of straightaway ac transmission is by use of series capacitors. whose reactance compensates a part of the series inductive reactance of the line itself. The maximum part that can be compensated feasibly or economically has not yet been determined. Probably it is about 75%. By use of this assumed maximum series compensation, the distance for stable 60-Hz transmission of the natural load of an overhead line could be extended to 258/(1 - 0.75) = 1030 mi (1660 km). Such amounts of series compensation have not yet been used: 35 to 50% is more usual. For straightaway transmission of 1000 mi (1600 km) dc transmission would prove more economical than ac.

Another method of making very long ac power lines operate stably has been proposed: it is to make the line electrically somewhat longer than onehalf wavelength.D1

,5 -7 It will then behave as if it were a half-wavelength shorter than it is. If the actual distance is less than one-half wavelength, the electrical length may be artificially increased in either of two ways: (a) by adding lumped LC sections at the ends or (b) by connecting shunt capacitors at intervals along the line.

A dc transmission link in itself has no stability problem. Two separate ac systems interconnected only by a dc line do not operate in synchronism, even if their nominal frequencies are equal, and they can operate at different nominal frequencies, for example, one at 50 and the other at 60Hz. Each of the separate ac systems may have its own internal stability problems. The sustained interruption of the power on the dc line constitutes a mild threat to

GENERAL ASPECTS OF DC TRANSMISSION

stability equal to that caused by loss of a large load in the sending-end system and to loss of a generator in the receiving system. Alternating-current systems are designed so as to be stable under such mild shocks.

the two ac systems are interconnected by one or more ac lines in addition to a dc line of comparable rating, sudden and sustained interruption to the power on the dc line may result in a loss of synchronism between the two ac systems. Therefore parallel operation of one dc line and one or more ac lines is inadvisable unless the ac lines are strong enough to withstand the loss of the dc line.

If, however, there are two or more dc lines in parallel with one or more ac lines, the dc lines can be so arranged that if one of them is lost, the other dc line or lines assumes its load. In such a case, there is no great stability problem.

In this regard each pole of a bipolar line may be considered a separate line.

Circuit Breakers

Alternating-current circuit breakers take advantage of the current zeros that occur twice per cycle. They are designed to increase the breakdown strength of the arc path between contacts so rapidly that the arc does not restrike. Direct-current circuit breakers do not have this natural advantage and therefore have to force the current to zero. So far no successful dc circuit breaker has been built for the high voltages and high currents used in dc transmission.

In simple two-terminal dc transmission, such as all projects in operation to date have been, the lack of dc circuit breakers has not been felt, because faults on the dc line or in the converters are cleared by using the control grids of the converter valves to block the direct current temporarily. Experience with ac transmission, however, has shown that most lines that initially operate radially later became incorporated into an ac network. The lack of a de circuit breaker is a handicap to the tapping or networking of dc lines. Reasonable proposals have been made for the operation of a three- or fourterminal line in which a faulted line section can be switched out by running the voltage of the whole system to zero, opening switches to isolate the faulted section, and then raising the voltage back to normal. The time of the whole sequence of events would be approximately equal to that now required for rapid reclosure of ac circuit breakers.

Nevertheless, the lack of HV dc circuit breakers must be regarded as a present limitation of HV dc transmission. It is likely that such circuit breakers will be developed. *

* Some development work is described in Section 7-7.


Short-Circuit Current

The interconnection of ac systems through an ac line raises the shortcircuit currents, sometimes to an extent that exceeds the interrupting capability of existing circuit breakers and requires their replacement by more capable breakers. The interconnection of ac systems by a dc link, however, does not increase short-circuit currents of the ac systems nearly so much, for the dc line contributes no current to an ac short circuit beyond its rated current.

On the other hand, the proper operation of a dc line terminal requires that the short-circuit power of the ac system at the point of installation be several (now at least five) times the rated power of the dc line, and sometimes this requirement dictates increase of the ac short-circuit power by the provision of synchronous condensers or additional ac connections.

The current in a short circuit on the dc line, after a momentary transient due to a discharge of the shunt capacitance of the line, is limited by automatic grid control to twice rated current. Nor do faults on the dc line draw excessive currents from the ac systems.

Power per Conductor and per Circuit

Let us assume that an ac line and a dc line using the same conductors and insulators are built. How does the power per conductor compare on the two lines?

Assume that in each case the current is limited by temperature rise. Then the direct current equals the rms alternating current.

Assume also that the insulators withstand the same crest voltage to ground

in each case. Then the direct voltage is -)2 times the rms alternating voltage. The de power per conductor is

Pd == Vd1d

and the ac power per conductor is

P a == Vafa cos¢

(9)

(10)

where fd and fa are the curents per conductor, Vd and Va the conductor-toground voltages, and cos ¢ the power factor. The ratio is

Pd Vd1d Vd ld 1 -)2 (11 )

Taking cos ¢ == 0.945, Pd/Pa = 1.5. Now compare a three-phase, three-conductor ac line with a bipolar two-


conductor dc line. The power capabilities of the respective circuits are

and the ratio is

Pd 2 Pd 2 3 -=-·-=-·-=1 Pa 3 Pa 3 2

(12)

Both lines can carry the same power. The dc line, however, is simpler and cheaper, having two conductors instead of three. Consequently an overhead line requires only t as many insulators, and the towers are simpler, cheaper, and narrower. A narrower right of way could be used.

Both lines have the same power loss per conductor. The percentage loss of the dc line is only two-thirds that of the ac line. If the basis of comparison is equal percentage loss, the power of the three-phase ac line is decreased to

-J 2/3 that of the two-conductor dc line. If cables are used instead of overhead line, the permissible working stress

(voltage per unit thickness of insulation) is higher for direct current than for alternating current, and, in addition, the power factor for direct current is unity and, for alternating current, considerably lower than that assumed above. Both changes further favor direct current over alternating current by increasing the ratio of dc power to ac power per conductor. The resulting ratio might be from 5 to 10.

Because the power limit of overhead ac lines is often determined by factors other than conductor heating, the ratio of dc power per conductor to ac power per conductor may be as high as 4.

Ground Return

A two-conductor bipolar dc line is more reliable than a three-conductor ac line, because, in the event of a fault on one conductor, the other conductor can continue to operate with ground return during the period required for repairing the fault. The operation of an ac line with ground return is not feasible on account of the high impedance of such a circuit and the telephone interference caused by such operation. Further information on dc ground return is given in Chapter 9.

A monopolar de line with earth return is still simpler than a three-phase ac line and is equally reliable. It is especially submarine cable. A line can be built in stages with monopolar operation initially, later changed to

operation with doubling of the power rating.


Terminal Equipment

The converters required at both ends of a dc transmission link have proved to be reliabL: but expensive. They also constitute a bottle neck to the power transmissible, for the valves have but little overload capability.

Other terminal equipment ~n either ac or dc lines may limit the voltage or current, hence the power; for example, the voltage and continuous current rating of circuit breakers and the seal-off voltage of lightning arresters.

Harmonics

The converters used with a dc line produce harmonic voltages and currents on both ac and dc sides. These harmonics, especially in the extensive ac networks, may cause interference with audio-frequency telephone lines. Filters are required on the ac side of each converter for diminishing the magnitude of harmonics in the ac networks. These increase the cost of the converter stations. Fortunately the capacitors used in the filters also supply part of the reactive power required by the converters. The cost of the filters and of the additional reactive power supply should be regarded as a part of the cost of a dc line terminal.

Control of Tie-Line Power

The power flow on tie lines interconnecting different areas under different ownerships must be controlled in conformity with contractual obligations. In addition, the frequency of the whole system, or the frequencies of the parts connected asynchronously, must be controlled.

The control system is a little simpler if the tie lines operate on dc than if on ac, but the difference is not important. This subject is discussed in Volume 2.

Generating Units

Some hydroelectric generating stations connected to a load center through long ac lines have generators with abnormally low transient reactance or abnormally high moment of inertia specified in order to raise the stability limit. These features raise the cost of the generators and would not be required if dc transmission were used, for there would be no stability problem with direct current. In addition, if such a station were connected to an ac system only through dc lines, the speed of the prime movers could be allowed to vary with the load or the head of water, perhaps giving a cheaper or a more efficient prime mover, and the nominal frequency of the generator, no longer confined to 50 or 60Hz, could be chosen for best economy. Perhaps also, in


such a station, less harmonic filtering would be required. (The Volgograd hydroelectric plant has no filters.) Altogether, the generating plant could be designed for best economy. To date, however, no such plant has been built.

1-6 SUMMARY OF ADVANTAGES AND DISADVANTAGES OF HV DC TRANSMISSION

Advantages

Greater power per conductor. Simpler line construction. Ground return can be used. Hence each conductor can be operated as an independent circuit. No charging current. No skin effect. Cables can be worked at a higher voltage gradient. Line power factor is always unity; line does not require reactive compensa

tion. Less corona loss and radio interference, especially in foul weather, for a

certain conductor diameter and rms voltage. Synchronous operation is not required. Hence distance is not limited by stability. May interconnect ac systems of different frequencies. Low short-circuit current on dc line. Does not contribute to short-circuit current of ac system. Tie-line power is easily controlled.

Disadvantages

Converters are expensive. Converters require much reactive power. Converters generate harmonics, requiring filters. Converters have little overload capability. Lack of HV dc circuit breakers hampers multiterminal or network operation.

1-7 PRINCIPAL APPLICATIONS OF DC TRANSMISSION

The foregoing discussion of and shortcomings of HV dc transmission indicates the

1. For cables crossing bodies of wider than 20 mi (32 km).

1-8 ECONOMIC FACTORS 33

2. For interconnecting ac systems having different frequencies or where asynchronous operation is desired.

3. For transmitting large amounts of power over long distances by overhead lines.

4. In congested urban areas or elsewhere where it is difficult to acquire right of way for overhead lines and where the lengths involved make ac cables impractical.

5. And, of course, combinations of these factors occurring in the same project.

Six of the first seven commercial installations, beginning with Gotland, involve submarine cables. All but the first two of these include great lengths of overhead line in addition to cables.

In the English Channel crossing and in the Konti-Skan scheme asynchronous operation was preferred because of the simplcity and economy of control. Some installations of converters similar to those used for HV dc transmission have been installed for frequency conversion with no dc line.

In the United States and the U.S.S.R. the principal interest in HV dc transmission is for long overhead lines.

In Britain there is much interest in dc transmission by underground cable through metropolitan areas, especially London. The first such scheme is Kingsnorth. It is likely that such applications will be considered in large cities in the United States in the future.

1-8 ECONOMIC FACTORS

The cost per unit length of a dc line is lower than that of an ac line of the same power capability and comparable reliability, but the cost of the terminal equipment of a dc line is much more than that of an ac line. If we plot the cost of transmitting a certain amount of power by one method or the other as a function of the distance over which it is transmitted, the resulting graph is similar to Figure 9. The vertical intercept of each curve is the cost of the terminal equipment alone. The slope of each curve is the cost per unit length of the line and of that accessory equipment which varies with the length. The curve for ac transmission intersects that for dc transmission at an abscissa called the break-even distance. If the transmission distance is shorter than the break-even distance, ac transmission is cheaper than dc; if longer, dc IS

cheaper than ac. Estimates of the break-even distance of overhead

technical literature, range from 500 km (310 mi) to 1500 great variation can be explained, at least in part, by a simple


...... If)

o <....>

Break-even ~-- distance, ----':10-1

500 mi

o 200 400 600 Distance (mi)

Ac

800 1000

Fig. 9. Comparative costs of ac and dc overhead lines versus distance.

of Figure 9, shown in Figure 10. Here the cost of each line is assumed to vary over a certain range, + 5% for the ac line and + 10% for the dc line. The true cost of each is assumed to be within the crosshatched area. (Greater variation is assumed for the cost of dc transmission than for that of ac because there has been less experience with dc than with ac.) It is now apparent that even such small variations in estimated costs make the estimated breakeven distance vary over a range of 2 or 3 to 1.

For cables the break-even distance is, of course, much shorter than for overhead lines, lying between 15 and 30 mi (24 and 48 km) for submarine cables and, perhaps, twice as far for underground cables.

The ordinate in Figures 9 and 10 might be either capital cost or annual

...... If)

o <....>

o 200

Fig. 10.

Ac

Range of break- even distance

I I I

I °E 1 I °EI ~I

0-1 81 col ;1 L.()I I gl I I

1 I 1 400 600 800 1000

Distance (mi)

of variation of costs on break-even distance.

1-9 THE FUTURE OF DC TRANSMISSION 35

cost; it might be for a given amount of power or per megawatt. In any case the curves would have the same form.

In view of the relative novelty of HV dc transmission, there is a prospect for a greater decrease in the unit cost of dc line terminals with increasing experience and volume of production than in the cost of ac equipment. The result would be to decrease the break-even distance.

An economic comparison between ac and dc transmission made by an international working party of C.I.G.R.E.C7 and based on 1965 costs showed average break-even distances of 1000 km (600 mi) for transmitting 1080 or 2160 MW on two overhead circuits and 77 km (48 mi) for transmitting 1080 MW on two shunt-compensated underground cable circuits. An assumed future 20% reduction in dc terminal costs reduced the break-even distance to 830 km (515 mi) for the overhead lines and to 64 km (40 mi) for the underground cables.

In the great majority of dc transmission schemes already ~uilt, other factors than the costs assumed in such comparisions play a significant role. These other factors are long water crossings, frequency conversion, and the advantage of asynchronous ties between large ac systems.

1-9 THE FUTURE OF DC TRANSMISSIONE

The increasing size and load density of metropolitan areas create problems of right of way for HV overhead lines. The increased public demand for the better appearance of electric lines and for the preservation of the natural environment is putting pressure on the electric power companies for placing transmission and distribution lines underground, out of sight, even where the load density is not high. Dc cables are cheaper and more compact than ac cables for the same power and are not so limited in the feasible distance of transmission.

As Greber discerningly points out,E3 the basic problem of ac transmission is that of inductive and capacitive reactance; the basic problem of dc transmission is switching.

It is the series inductive reactance of long overhead ac lines that causes the synchronous stability limit. It is the shunt capacitive reactance of long ac cables that overloads them with charging current. On long overhead lines, the presence of both kinds of reactance causes excessive variation of voltage with load.

Series and shunt compensation of reactance are used on long ac lines, but add to the cost and complexity of such Ii IS

not required on a de line itself, but only on ac converters. fact gives an advantage to long dc lines over ac and


simpler means of compensation were developed, however, the economic balance between ac and dc transmission would be shifted in favor of ac.

The switching problem on dc lines lies not only in the need for dc circuit breakers but also in the converters, which are essentially a group of synchronously controlled switches. If cheaper, simpler, and more reliable switches (perhaps, solid-state devices) were developed, not only would dc networks be feasible, but also the converters would be cheaper than they are now and more free from misoperations, such as arcbacks: Improved swit<;;hes would make the control of the reactive power of converters possible, permitting it to flow in or out of the converter, or neither, as desired. In addition, dc transformers would be possible. They could operate on either of two principles. One kind would be analogous to the vibrator power supplies now used with battery-operated radios, but, of course, at a much greater power level and with the vibrator replaced by a new kind of switch. The other kind would rapidly switch capacitors so as to be charged in parallel and discharged in series for voltage step-up or, vice versa, for step-down. Thus the development of superior switches could give great impetus to dc transmission.

Other impending developments could alter the picture in favor of direct current. The new methods of power generation-thermoelectric, magnetohydrodynamic,Es and by fuel cell-inherently generate direct current. There is some possibility that direct conversion from nuclear energy to HV direct current might be developed. E1 Cryogenic superconducting cables might transmit direct current long distances at low voltage and high current with no voltage drop and no power loss except that required to remove from the cable the heat that leaked into it from its surroundings. E9* Superconducting de generators and motors are being developed. E4

The future of dc transmission looks bright.

BffiLIOGRAPHY

A. General

1. Power Transmission by Direct Current, by Ya. M. Chervonenkis, Moscow, 1948, translated from the Russian by N. Kaner and published for the National Science Foundation, Washington, D.C., by the Israel Program for Scientific Translation, Jerusalem, 1963. Elementary and out-of-date.

UC'V~H_~"" or resistive, are being actively investigated for dC. E2 ,7,8,10-16 Direct current, however has

BIBLIOGRAPHY 37

2. Direct Current, a magazine published by Direct Current, Ltd. (a subsidiary of Garraway Ltd.), London, from June 1952 to February 1967, quarterly, except from March 1961 to October 1963, when it was monthly. New series published by Pergamon, Oxford, beginning in April 1969, with editorial office at Manchester University, Department of Electrical Engineering and Electronics.

3. Nauchno-Izsledovatel'skii Institut Postoyannovo Toka, Izvestiya (Proceedings of the Direct Current Research Institute), Leningrad. Vol. 1 is dated 1957. Approximately one volume per year has been published since then. Contains articles on both ac and de transmission, in Russian. Cited hereinafter as N./.I.P. T.

4. "D.C. Power Transmission," a series of six articles published in Elec. Jour. (London), Vols. 163 and 164 (1959-1960). Part I, "Historical Development," by E. Openshaw Taylor, pp. 1227-1231, Dec. 4, 1959. Part II, "Basic Principles," by E. Openshaw Taylor, pp. 22-27, Jan. 1, 1960. Part III, "Rectifiers and Inverters," by R. Feinberg, pp. 294-299, Jan. 29, 1960. Part IV, "Transmission Circuits," by A. L. Williams, pp. 619-626, Mar. 4, 1960. Part V, "Operation ~nd Control," by Gunnar Engstrom, pp. 1048-1055, Apr. 8, 1960. Part VI, "Planning and Economics," by J. L. Egginton, pp. 1271-1280, May 6, 1960.

5. High Voltage Direct Current Power Transmission, by Colin Adamson and N. G. Hingorani, Garraway, London, 1960, xvi + 284 pp.

6. High Voltage Direct Current Convertors and Systems, edited by B. J. Cory, Macdonald, London, 1965, xiii + 269 pp.

7. Conference on High Voltage D.C. Transmission, held at Manchester, Sept. 19-23, 1966, I.E.E. Conference Publication 22, London, 2 parts. Part 1, Contributions, 454 pp. Part 2, Discussions, 143 pp.

B. History

1. "Constant-Current D.C. Transmission," by C. H. Willis, B. D. Bedford, and F. R, Elder, Elec. Eng., Vol. 54, pp. 102-108, January 1935. Disc., pp. 327-329 (March), 447-449 (April), and 882-883 (August).

2. "Power Transmission by Direct Current: Apparatus Used in 3000-kw 15,OOO-voIt, 200-amp Pump-back Test," by B. D. Bedford, F. R. Elder, and D. H. Willis, Gen. Elec. Rev., Vol. 39, pp. 220-224, May 1936. Tests preceding the MechanieviIleSchenectady experimental de transmission.

3. '.'D.C. Transmission in France," Elec. World, Vol. 106, No. 19, pp. 1341-1342, May 9, 1936. On 275-mi, 125-kV, 20-MW line from Moutiers to Lyon.

4. "The First Power Transmission at 50 kV D.C. with Mutators" (in French), by P. Egloff and J. J. Felix, Electricite, Vol. 23, No. 58-59, pp. 237-240, july-August 1939. Baden-Zurich transmission.

5. "The D.C. Power Transmission at the Swiss National Exhibition" (in German), by E. Kern, Bull. de I'Association Suisse des Electriciens, Vol. 30, No. 17, pp. 481-482, Aug. 18, 1939.

6. "H.V.D.C. Transmission," by , Elec. Times, Vol. 111, Nos. 2881,2883, 2885, pp. 36--40,98-101, 1 1 Experience in Germany during World War II, including Charlottenburg-Moabit experimental! IO-k~ de transmission and Elbe-Berlin project. English translation from German report.


7. "D.C. Power Transmission Developments by the Siemens-Schuckert Concern in Germany," by F. Busemann, B.E. & A.I.R.A., Report ZjT67, Nov. 24, 1947, 20 pp.

8. Item Al above.

Origin of the 440 kV D.C. H.V. Transmission Line Elbe-Berlin" (in German), by R. Troger, ETZ, Vol. 69, pp. 261-272, August 1948.

10. "Experience of High-Voltage Direct Current Transmission" (in Russian), by A. M. Nekrasov and M. R. Sonin, Elek. Stantsii, Vol. 26, No.7, pp. 26-32, July 1955. Kashira-Moscow experimental cable transmission.

11. "H.V.D.C. Transmission System" (in Russian), by V. P. Pimenov and M. R. Sonin, Elektrichestvo, No.7, pp. 93-99, 1955. Kashira-Moscow link.

12. "Institute's Activities in the Field of High-Voltage Direct Current Transmission of Energy" (in Russian), by V. P. Pimenov, N.I.I.P.T., Vol. 1, pp. 7-20, 1957. Work of of the Institute of DC Transmission.

13. "Results of the Operation of the Experimental Industrial Direct Current Transmission Line, Kashira-Moscow" (in Russian), by M. R. Sonin, N.I.I.P.T., Vol. 2, pp. 5-21, 1957. Report of operating problems, December 1950 to May 1956.

14. "The Work of the Direct Current Institute," by V. P. Pimenov, Direct Current, Vol. 3, No.6, pp. 185-191, September 1957. Translated from N.I.I.P.T., Vol. 1.

15. "D-C Transmission: An American Viewpoint," by G. D. Breuer, M. M. Morack, L. W. Morton, and C. A. Woodrow, A.I.E.E. Trans., Vol. 78, Part 3A, pp. 504-512, August 1959. Disc., pp. 512-515. Includes information on Mechanicville-Schenectady link.

16. "Work Done in the Soviet Union on High-Voltage Long-Distance D-C Power Transmission," by A. M. Nekrasov and A. V. Posse, A.I.E.E. Trans., Vol. 78, Part 3A, pp. 515-521, August 1959. Disc., pp. 521-522. Kashira-Moscow experimental transmission, development work and plans for the Stalingrad-Donbass line.

17. "D.C. Power Transmission," Part I of item A4 above.

18. "The History of D.C. Transmission," in Direct Current: Part I, Vol. 6, pp. 260-263, December 1961. Part II, Vol. 7, pp. 60-63, March 1962. Part III, Vol. 7, pp. 228-231 and 250, September 1962. Part IV, Vol. 8, pp. 2-5 and 27, January 1963. Part V, Vol. 8, pp. 88-93 and 115, April 1963. Includes a bibliography of 160 entries.

19. "Development of High Voltage D.C. Transmission at Siemens Schuckertwerke up to 1945" (in German), by M. Bosch and O. Schiele, Siemens Zeitschrift, Vol. 40, pp. 672-681, September 1966.

C. Comparison Between AC and DC Transmission

1. "Comparison of Transmission Costs for High-Voltage AC and DC Power Transmission in Japan," by Sadao Saeki, Appendix II to "Report on the Work of the Study Committee No. 10: D.C. Transmission at E.H.V.," C.I.G.R.E., 1956, Paper 507.

2. "Comparison of Transmission Costs for High Voltage A.C. and D.C. Systems" the name of Study Committee No. 10, D.C. at

by F. J. Lane, Bo G. Rathsman, U. Lamm, and K. S. Smedsfelt, Paper No. 417, 25 pp.

BIBLIOGRAPHY 39

3. Comparison of Direct and Alternating Current for High- Voltage Electric Power Transmission, Edison Electric Institute Publication No. 62-901, 1962.

4. "High-Voltage DC Transmission," Advisory Committee Report No. 20, published on pp. 289-313 of National Power Survey, a report by the Federal Power Commission, part II, "Advisory Reports," U.S. Government Printing Office, Washington, October 1964.

5. "Cost of Electrical Energy Transmission by AC and DC Extra High Voltage," Advisory Committee Report No. 16, pp. 189-203, loco cit.

6. "High Capacity D.C. Transmission in the U.S.S.R.," by A. Berkovski, N. ChouP-fakov, T. Izrailevich, A. Kolpakova, and S.Rokotyan, I.E.E. Conference Publication 22, H. V.D.C. Transrnission, Manchester, Sept. 19-23, 1966, Part 2, Paper No. 94, pp. 126-129. Also in Direct Current, Vol. 11, pp. 145-149, November 1966.

7. "A Technical and Economic Comparison between A.C. and D.C. Transmission," by W. Casson, C.I.G.R.E., 1968, Report 42/43-01, 42 pp.

D. Special Problems of AC Transmission

1. "On Normal Working Conditions of Compensated Lines with Half-Wave Characteristics" (in Russian), by A-. A. Wolf and O. V. Shcherbachev, Elektrichestvo, No.1, pp. 57-60, 1940.

2. "Charging Current Limitations in Operation of High-voltage Cable Lines," by C. S. Schifreen and W. C. Marble, A.I.E.E. Trans., Vol. 75, Part 3, pp. 803-812, October 1956. Disc., pp. 813-817.

3. "Long Cable Lines-Alternating Current with Reactor Compensation for Direct Current," by J. J. Dougherty and C. S. Schifreen, A.I.E.E. Trans., Vol. 81, Part 3, pp. 169-178, June 1962. Disc., pp. 179-182.

4. "EHV AC Transmission Line Compensation," Advisory Committee Report No. 14, prepared by a subcommittee of the Transmission and Interconnection Special Technical Committee, July 1963, published on pp. 141 72 of National Power Survey, a report by the Federal Power Commission, Part II, "Advisory Reports," U.S. Government Printing Office, Washington, October 1964.

5. "Half-Wavelength Power Transmission Lines," by F. J. Hubert and M. R. Gent, 1.E.E.E. Spectrum, Vol. 2, pp. 87-92, January 1965. Also in I.E.E.E. Trans. on P.A. & S., Vol. 84, pp. 965-974, October 1965.

6. "Analysis of Natural Half-Wave-Length Power Transmission Lines," by F. S. Prabhakara, K. Parthasarathy, and H. N. Ramachandra Rao, I.E.E.E. Trans. on P.A. & S., Vol. 88, pp. 1787-1794, Decen1ber 1969. Disc., p. 1794.

7. "Performance of Tuned Half-Wave-Length Power Transmission Lines," by authors of last item, ibid., pp. 1795-1800. Disc., pp. 1800-1802.

E. Future Prospects for DC Transmission

1. "Foreword," by Max Direct Current, Vol. 2, pp. 133-134, September 1955. "An 'Atomic Battery': Direct Conversion from Atomic Radiation to Electrical Engergy," ibid., pp. 135-137,

2. to the Generation and Distribution of Electric Power," by Eng., 81, pp. 122-129, February 1962. Superconductors, heat cables,generators and motors, fuses, circuit breakers, rectifiers, and refrigeration.


3. "Future Developments in H.V.D.C.," letter to the editor of Direct Current from Henry Greber, published in Vol. 9, inside front cover, August 1964.

4. "Superconducting DC Generators and Motors," by David L. Atherton, I. E.E.E. Spectrum, Vol. 1, pp. 67-71, December 1964.

5. "Survey of MHD Research," introduced by M. W. Thring, Direct Current, Vol. 10, No.1, pp. 40-59, February 1965. Survey of research in Britain, Australia, United States, France, Japan, Switzerland, Sweden, and Poland on magnetohydrodynamics.

6. "Future Possibilities of H.V.D.C.," by J. H. M. Sykes, Chapter 10 of Cory, Ref. A6 above, 1965.

7. "Superconducting Power Transmission May Be a Reality Within Ten Years," by D. Atherton, Elec. News and Engg. (Don Mills, Ontario), Vol. 74, No. 11, pp. 52-55, November 1965.

8. "Prospect of Employing Conductors at Low Temperature in Power Cables and in Power Transformers," by K. J. R. Wilkinson, I.E.E. Proc., Vol. 113, No.9, pp. 1509-1521, September 1966. Disc., Vol. 114, No. 12, pp. 1892-1898, December 1967. Estimate of power saved if the conductor in a 760-MVA, 275-kV ac cable were, alternatively, niobium at 4°K, aluminum at 20oK, or beryllium at 77°K. Summary of this paper and of discussion of it in Elec. Times, Vol. 151, No.5, pp.168-170, Feb. 2, 1967, under the title, "Prospect for Low Temperature Transmission."

9.' "Superconducting Lines for the Transmission of Large Amounts of Electrical Power over Great Distances," by R. L. Garwin and J. Matisoo, I.E.E.E. Proc., Vol. 55, No.4, pp. 538-545, April 1967. Preliminary design of 1000-km, 100-GW, 200-kV, 500-kA, dc line with Nb3 Sn conductors refrigerated to 4 oK.

10. "Superconducting Power Cables," by D. R. Edwards and R. J. Slaughter, Elec. Times, Vol. 152, No.5, pp. 166-169, Aug. 3, 1967. Includes summary of historical development.

11. "'Design for a 750 MVA Superconducting Power Cable," by E. C. Rogers and D. R. Edwards, Elec. Rev., Vol. 181, No. 10, pp. 348-351, Sept. 8, 1967, Study made by B.Le.C. for C.E.G.B. on design of three-phase, 33-kV, 13-kA, superconducting cable. Conductors of 0.0025-cm niobium foil on O.25-cm aluminum tubing, vacuum dielectric, and liquid helium coolant.

12. "Cryogenic Power Transmission," by S. H. Minnich and G. R. Fox, Cryogenics, Vol. 9, No.3, pp. 165-176, June 1969. Based on studies by General Electric Company for Edison Electric Institute and Tennessee Valley Authority, Considers both resistive cryogenic cable with stranded aluminum conductors in liquid nitrogen or hydrogen and superconducting cable with niobium-coated tubes in liquid helium, especially for three-phase ac.

13. H Low Temperatures and Electric Power," by B. J. Maddock, W. T. Norris, D. A. Swift, and M. T. Taylor, Cryogenics, Vol. 9, No.4, pp. 291-297, August 1969. Report on a conference organized by the I.E.E. and held in London on March 24-26, 1969. It included papers on electric power systems, refrigeration, conductor materials, dielectrics, generators and motors, transformers, cables, energy storage, and transportation.

14. "French Develop Modular HYDe Thyristor Valve," Elec. Rev., Vol. 185, No. 22, pp. 790-791, Nov. 28,1969. News item on research atC.G.E.'s Marcoussis Laboratory, which includes work on low-temperature cables.

5. "Economics of Underground Transmission " by Peter I.E.E.E. Trans. on P.A. & S., Vol. 89, pp. 1 January 1970. Disc., pp.

6. Three cryogenic ac transmission lines, cooled, respectively, by liquid nitrogen,

BIBLIOGRAPHY 41

hydrogen, and helium, are compared with one another and with conventional pipetype cable. The nitrogen-cooled cable is found to be the most economical.

16. "Economic Assessment of a Liquid-Nitrogen-Cooled Cable," by S. B. Afshartous, Peter Graneau, and John Jeanmonod, I.E.E.E. Trans. on P.A. & S., Vol. 89, pp. 8-13, January 1970. Disc., pp. 14-16. Cable of tubular aluminum conductors cooled internally by liquid nitrogen and supported by dielectric spacers in high-voltage vacuum insulation.

F. Bibliographies

1. "Direct Current Bibliography-I," Direct Current, Vol. 1, pp. 50-52, September 1952. Covers years 1943 to 1952. Vol. 1, p. 97, March 1953, covers rest of 1952.

2. An Annotated Bibliography of High Voltage Direct Current Transmission, 1932-1962, compiled by Eric Bromberg, I.E.E.E. Paper CP 63-388, January 1963. Also in D-C Transmission, Publication S-155, pp. 76-214, I.E.E.E., June 1963.

3. An Annotated Bibliography of High Voltage Direct Current Transmission, 1963-1965, complied by Eric Bromberg, I.E.E.E. Publication 31 S 60, iii + 113 pp., I.E.E.E., New York, 1967.

4. High Voltage Direct Current Transnlission: An Annotated Bibliography, 1966-68, compiled by Val S. Lava, published by the Library, Bonneville Power Administration, Portland, Oregon, December 1968, ii + 90 pp.

G. Gotland Link

1. "The High Voltage D.C. Power Transmission from the Swedish Mainland to the Swedish Island of Gotland," by Ake Rusck, B. G. Rathsman, and U. Glimstedt, C.I.G.R.E., Report 406, 1950.

2. "D.C. Transmission from Swedish Mainland to Island of Gotland, by A. Rusck, B. G. Rathsman, and U. Glimstedt, Engineer, Vol. 190, No. 4931, pp. 92-93, July 28, 1950.

3. "High-Voltage D.C. Power Transmission-Pioneer Project," by U. Lamm, ASEA Journal, Vol. 23, No. 12, pp. 172-174, December 1950.

4. "Submarine Cable Project Will Operate at 100 kV D.C.," by B. G. Rathsman, Electric Light and Power, Vol. 29, No.8, pp. 108-109, August 1951.

5. "Gotland H.V.D.C. Link: Present Progress," by B. G. Rathsman and U. Lamm, Direct Current, Vol. 1, pp. 2-6, June 1952.

6. "Gotland D.C. Link: Layout of Plant," by I. Liden, Sy Sviden and E. Uhlmann, Direct Current, Vol. 2, pp. 2-7, June, and pp. 34-39, September 1954.

7. "The First Voltage D.C. Transmission wigh Static Converters: Some Notes on the Development," by U. Lamm, ASEA Journal, Vol. 27, No. 10, pp. 139-140, October 1954. "The Gotland D.C. Link: The Layout of the Plant," by I. Liden and E. Uhlmann, pp. 141-154.

8. "The D.C. Transmission to Gotland: Initial Experience," by S. Ekefalk, ASEA Journal, No. 10, pp. 123-126, 1956.

H.

1. "The a V..:>..::I··· "'-'l.J.UlJlJ.J.v.l. Power Link between the British and French


Supply Systems," by D. P. Sayers, M. E. Laborde, and F. J. Lane, I.E.E. Proc., Vol. 101, Part 1, pp. 284-297, September 1954. Disc., pp. 297-308.

2. "English Channel: Channel Cable," by J. H. M. Sykes, Engineer (London), Vol. 202, No. 5253, pp. 433-434, Sept. 28, 1956.

3. "The Design of the D.C. Connection across the English Channel," by I. Liden, ASEA Journal, Vol. 36, No.6, pp. 70-74, 1958.

4. "The High Voltage D.C. Transmission Scheme across the English Channel," by I. Liden, ASEA Journal, Vol. 33, No. 7-8, pp. 124-126, 1960.

5. "The Cross-Channel Cable: A Preliminary Survey," by the editor, Direct Current, Vol. 6, No.4, pp. 97-109, July 1961.

6. "D.C. Channel Link: Lydd Operational," Elec. Times, Vol. 140, No. 23, pp. 845-848, Dec. 7, 1961.

7. "Anglo-French Power Link," Engineer, Vol. 212, pp. 950-953, Dec. 8, 1961.

8. "Cross Channel Power Link," Elec. Rev. (London), Vol. 169, no. 23, pp. 907~912, Dec. 8, 1961.

9. "Some Problems in Connection with the Commissioning of the Lydd Converter Station," by L. Csuros and G. S. H. Jarrett, Direct Current, Vol. 7, No.5, pp. 114-121, May 1962.

10. "Operational Performance of the Direct Current Cross Channel Link," by J. Malaval, J. Clade, L. Csuros, and G. S. H. Jarrett, C.I.G.R.E., 1964, Paper No. 417, 11 pp. and folded chart.

11. " Some Design Aspects of the Cross-Channel Power Link," by L. A. Harris, Chapter 8 of Cory, Ref. A6, 1965.

12. "The Performance of the Lydd Convertor of the Cross Channel Connection," by G. S. H. Jarrett and L. Csuros, I.E.E. Conference Publication 22, H. V.D.C. Transmission, held at Manchester on Sept. 19-23, 1966, Part 1, Paper No.2, pp. 17-20 and folded sheet.

13. " Special Operational Tests op the Cross Channel Connection," by J. Clade, R. M. H. Middleton, and E. Uhlmann, ibid., Paper No.3, pp. 21-25. Transient conditions, such as starting, blocking, emergency power reversal, and ac faults.

14. "Service Experience with the Anglo-French D.C. Cross Channel Cable," by P. Fourcade and C. C. Barnes, ibid., Paper No.4, pp. 26-29.

I. Volgograd-Donbass Link

1. "D.C. Transmission from Stalingrad Hydro-electric Station to Donbass" (in Russian), by V. P. Pimenov, A. V. Posse, A. M. Reider, S. S. Rokotian, and V. E. Turetskii, Elektricheskie Stanlsil', No. 11, 1956, pp. 12-18.

2. "Transmission of Direct Current at High Voltage according to Present-Day Concepts and the Prospects for Their Application in the U.S.S.R." (in Russian), by N. M. Mel'gunov, N.I.I.P.T., Vol. 1, pp. 21-38, 1957.

3. "The Transmission System Stalingrad Hydro-electric Station, Donbass "(in Russian), by E. S. Grois, M. L. Zelikin, V. E. Turetskii, and E. A. Man'kin, Elektrichestvo, Vol. 77, No.9, pp. 1-10 September 1957.

4. H Design Features of Stalingrad-Donbass Grois, E. K. Levitski, E. A. Man'kin, Turetski, Direct Current, Vol. 4, No. pp.

" by F. I. Butaev, E. S. Sakovitch, and V. E.

September 1958.

BIBLIOGRAPHY 43

5. "800 kV D.C. Transmission System Stalingrad-Donbass," by E. S. Groiss, A. V. Posse, and V. E. Touretski, C.I.G.R.E., 1960, Paper No. 414, 17 pp.

6. "800 kv D.C. Transmission System Stalingrad-Donbass," by E. S. Grois, A. V. Posse, and V. E. Turetskii, Engineer, Vol. 210, No. 5450, pp. 66-68, July 8, 1960. Based on C.I.G.R.E. paper.

7. "Some Problems of the Operation of the D.C. Transmission Line, Stalingrad Hydroelectric Station to Donbass "(in Russian), by N. M. Mel'gunov and V. M. K viatkovskii, Elektrichestvo, No.3, 1961, pp. 14-17.

8. "The Initial Operating Stage of the Volgograd-Donbass D.C. Transmission," by E. S. Grois, N.I.l.P.T., No.9, pp. 5-28, 1962.

9. "Initial Period of Operation of the D.C. Transmission Line between Volgograd and Donbass," by N. Chuprakov, A. Milutin, A. Posse, and V. Shashmurin, I.E.E. Conference Publication No. 22, H. V.D.C. Transmission, Manchester, Sept. 19-23, 1966, Part 2, Paper No. 93, pp. 120-125. Also in Direct Current, Vol. 11, No.4, pp. 142-145 and 148, November 1966.

10. "Operation of the Control and Protection System of the Volgograd-Donbass Link," by K. Gusakovsky, A. Posse, and A. Reider, I.E.E. Conference Publication No. 22, ibid., Part 2: Paper No. 95, pp. 130-135.

11. "Operating Experience of the Volgograd-Donbass D.C. Transmission Line and Its Applications to Extra High Voltage D.C. High Capacity Transmission," by A. M. Berkovski, F. I. Butaev, E. S. Grois, A. V. Posse, S. S. Rokotyan, and P. E. Saudler, C.I.G.R.E., Report 43-07, 1968, 7 pp.

J. New Zealand Link

1. "Report on the Possibilities of Interconnecting the Islands of New Zealand," by F. J. Lane, Direct Current, Vol. 5, pp. 12-24, June 1960.

2. "The H.V.D.C. Interconnection between the Islands of New Zealand," Direct Current, Vol. 7, pp. 32-38, February 1962.

The following series of papers (items 3 to 17) was published in New Zealand Engineering, the journal of the N.Z. Institution of Engineers, Wellington, 1965 to 1966:

3. " Economic Aspects of the Inter-island Transmission Scheme," editorial by E. B. M., Vol. 20, No.6, p. 211, June 1965.

4. "A Significant Achievement," by P. W. Blakely, Vol. 20, No.7, pp. 255-256, July 1965.

5. " Main Generating and Electrical Equipment of Benmore Power Station," by H. C. Hitchcock, VoL 20, No.1, pp. 3-13, January 1965.

6. "A Direct Current Transmission Line: The Design and Construction of the 600 MW, 500 kV d.c. Line Between Benmore and Haywards," by T. A. J. Dickens, Vol. 20, No.4, pp. 121-129, April 1965.

7. "The Benmore Land Electrode," by D. G. Dell, Vol. 20, No.5, pp. 165-175, May 1965.

8. "The North Island Sea Electrode," by D. G. Dell, Vol. 20, No.6, pp. 213-222, June 1965.

Layout the Terminal," by R. J. Fyfe, Vol. 20. 8, 3 1

Direct Current Equipment at the Haywards Terminal," by J. No.9, September 1965.


11. "Valves, Valve House, and Indoor Equipment at the Converter Stations," by M. A. Louden, Vol. 20, No. 10, pp. 393-402, October 1965.

12. "Layout of the Direct Current Switchyards," by D. G . Young, Vol. 20, No. 11, pp. 472-478, November 1965.

13. "Harmonic Phomenena," by G. H. Robinson, VoI.21,No.l,pp. 16-29, January 1966.

14. "Synchronous Condenser Installation at Haywards Substation," by L. S. Y. Gock, Vol. 21, No.1, pp. 29-35, January 1966.

15. "Power Line Carrier Communications," by F. R. Swan, Vol. 21, No.2, pp. 45-55, February 1966.

16. "Commissioning and Early Operating Experience," by H. R. Gunn, Vol. 21, No.3, pp. 93-101, March 1966.

17. "The ± 250 kV d.c. Submarine Power-Cable Interconnection," by A. L. Williams, E. L. Davey, and J. N. Gibson, Vol. 21, No.4, pp. 145-160, April 1966. Also published in I.E.E. Proc., Vol. 113, No.1, pp. 121-133, January 1966.

18. "The New Zealand 500 kV High-Voltage Direct-Current Project," by P. W. Blakely, Amer. Power Con! Proc., Vol. 28, pp. 850-859, April 1966.

The following papers are from I.E.E. Conference Publication 22, Conference on High Voltage D.C. Transmission, Sept. 19-23, 1966, Manchester, Part 1:

19. "Commissioning and Early Operating Experience with the New Zealand HYDC Inter-Island Transmission Scheme," by H. R. Gunn, Paper No.5, pp. 30-38.

20. "Benmore Power Station: Special Features for H.V.D.C. Transmission," by H. C. Hitchcock, Paper No. 19, pp. 101-103.

21. "The Synchronous Condenser Installation at Haywards Sub-station for the BenmoreHaywards H.V.D.C. Transmission Scheme," by L. S. Y. Gock, Paper No. 52, pp. 265-267.

22. "Communications (Power Line Carrier Systems)," by F. R. Swan, Paper No. 63, pp. 306-311.

23. "The Cook Strait 250-kV Cables," by E. L. Davey, Paper No. 64, pp. 312-314.

24. "A Direct Current Transmission Line: The Design and Construction of the 600 MW, 500 kV D.C. Line Between Benmore and Haywards," by T. A. J. Dickens, Paper No. 72, pp. 343-346.

25. "Some Features of New Zealand's Inter-island H.V.D.C. Transmission," by R. J. Fyfe, M. A. Louden, J. Noble, and D. G. Young, Paper No. 78, pp. 375-383.

26. "The Benmore Land Electrodes for the Benmore-Haywards H.V.D.C. Transmission Scheme," by D. G. Dell, Paper No. 82, pp. 415-418.

27. "The North Island Sea Electrode for the Benmore-Haywards H.V.D.C. Transmission Scheme," by D. G. Dell, Paper No. 85, pp. 427-430.

28. "Experience with Harmonics-New Zealand H.V.D.C. Transmission Scheme," by G. H. Robinson, Paper No. 89, pp. 442-444.

29. "Operational Experience of the Benmore-Haywards HVDC Transmission Scheme," by M. T. O'Brien, C.I.G.R.E., report 14-03, 1970, 11 pp.

K. Konti-Skan Link

1. "The Conti-Skan I.E.E.E. Conference

I>-"r",,-",'. " by G. von Geijer, S. Smedsfelt, and L. Ahlgren, January 1963.

BIBLIOGRAPHY 45

2. "The Konti-Skan Project," by G. von Geijer, Direct Current. Vol. 8, pp. 149-51, June 1963.

3. "The Konti-Skan H.V.D.C. Project," by G. von Geijer, S. Smedsfelt, L. Ahlgren, and E. Andersen, C.l.G.R.E., 1964, Paper No. 408, 45 pp.

4. Direct Current, Vol. 10, pp. 10-12, February 1965. Unsigned news article.

5. "Operational Performance and Service Experience with the Konti-Skan and Gotland H.V.D.C. Projects," by S. Smedsfelt, L. Ahlgren, and V. Mets, I.E.E. Conference Publication 22, H. V.D.C. Transmission, held at Manchester, Sept. 19-23, 1966, Part 1, Paper No.1, pp. 11-16.

L. Sardinia Link

Unsigned news articles:

1. Direct Current, Vol. 10, No.1, pp. 13-14, February 1965.

2. Elec. Engineer (Australia), Vol. 44, No.6, p. 25, June 1967.

3. Elec. Times, Vol. 151, pp. 257-259, Feb. 16, 1967.

4. Elec. World, Vol. 167, p. 21, Mar. 27, 1967.

5. Trans. and Dist., Vol. 19, No.4, p. 32, April 1967.

6. Elec. News and Engg. (Don Mills, Ont.), Vol. 76, No.4, p. 28, April 1967.

7. "The Sardinian-Italian Mainland H.V.D.C. Interconnection," by M. Natale, F. J. Lane, and T. E. Calverley, I.E.E. Conference Proc. 22, H. V.D.C. Transmission, held at Manchester on Sept. 19-23, 1966, Part 1, Paper No.7, pp. 42-49.

8. "Testing and Operating Experience on the Sardinia-Italian Mainland D.C. Link," by V. CialleIIa, P. Grattarola, A. Taschini, C. J. B. Martin, and D. B. Willis, C.I.C.R.E., 1968, Paper 43-09, 21 pp.

M. Vancouver Island Link (British Columbia)

Unsigned news articles: 1. Direct Current, Vol. 10, No.1, p. 7, February 1965.

2. ASEA Journal, Vol. 38, No. 10-12, pp. 165-166, 1965.

3. Elec. News and Engg. (Don Mills, Ontario), Vol. 76, No.6, pp. 56-57, June 1967.

4. Elec. World, Vol. 170, No.4, p. 18, July 22, 1968.

5. The Engineer, Vol. 226, No. 5871, p. 161, Aug. 2, 1968.

6. "D.C.' First' Provides Power for the Future," by P. J. Croft, British Columbia Hydro and Power Authority, Progress, Summer 1967, pp. 8-10.

7. "Major Features of the Vancounver Island ±260-kV HVDC Submarine Link," by H. M. Ellis and W. Chin, Amer. Power Con! Proc., Vol. 30, pp. 1017-1034, April 1968.

8. "Vancouver Island HVDC Transmission," by Gordon H. Dunn and Lars A. Bergstrom, ASEA Journal, Vol. 42, No. 2-3, pp. 29-31, 1969.

Iii,..fl' .... r·n'"... Northwest-Southwest Intertie

"Task Force Backs 375 kV D.C. for BPA-California Tie," Elec. World, Vol. 156, pp. 34-36, Dec. 25, 1961.


2. "Layout Arrangements for EHV D-C Transmission Terminals," by E. M. Hunter and W. E. Matson, Amer. Power Conf Proc., Vol. 28, pp. 860-867, April 1966. Celilo terminal at The Dalles, Oregon.

3. "Design of the Celilo-Sylmar 800-kV DC Line (BPA Section)," by R. F. Stevens, I.E.E. Conf. Publication 22, H. V.D.C. Transmission, Manchester, Sept. 19-23, 1966, Part 1, Paper No. 74, pp. 354-358. Also I.E.E.E. Trans. on P.A. & S., Vol. 86, No.7, pp. 916-920, July, 1967. Disc., pp. 921-922.

4. "The Celilo (The Dalles) Convertor Station for the Pacific H.V.D.C. Intertie," by G. D. Breuer, E. M. Hunter, P. G. Engstrom, and R. F. Stevens, ibid., Paper No. 80, pp. 394-402.

5. "Technology and Economics of EHV D.C. with Application to PNW-PSW lntertie," by R. J. Mather and E. F. Weitzel, Bonneville Power Administration report, presented at the symposium of the Assn. of Amer. Railroads, Denver, Nov. 15-16, 1966.

6. "The Pacific Northwest-Pacific Southwest Intertie," by E. V. Lindseth, Civil Engg .. Vol. 36, No. 12, pp. 46-47, December 1966.

7. "Long Distance Transmission of H.V.D.C. in Western U.S.A." (in German), by H. Dommel, S.E. V. Bull. (Switzerland), Vol. 58, pp. 60-68, Jan 21, 1967.

8. "Reclamation's Mead Substation," by N. B. Bennett, Jr., Power Engg., Vol. 72, No.1, pp. 32-35, January 1968. Southern terminal of second dc intertie near Hoover Dam.

9. " Some Design Considerations of the Celilo-Sylmar 800 k V D-C Line (Los Angeles Department of Water and Power Section)," by L. L. Burnside and W. M. Mahoney, Western Water & Power Symposium Proc., Los Angeles, Apr. 8-9, 1968, pp. D25-33.

10. "800 kV DC Transmission Lines of the Bonneville Power Administration,n by S. A. Annestrand, E. J. Harrington, M. N. Marjerrison, and R. F. Stevens, Western Water & Power Symposiun1 Proc., Los Angeles, Apr. 8-9, 1968, pp. D35-52.

11. "Final Design Criteria Established for E.H.V. D.C. Transmission Line and Terminal Facilities," by J. L. Mulloy and Edward York, Jr., Amer. Power Can! Proc., Vo1. 30, pp. 1035-1044, Chicago, April 1968.

O. Kingsnorth Link

Unsigned news articles:

1. Engineering, VoL 201, p. 484, Mar. 11, 1966.

2. Elec. Rev.,Vo!. 178, No. 10, pp. 378-379, Mar. 11, 1966.

3. Elec. World, Vol. 165, No. 15, pp. 19-20, Apr. 11, 1966.

4. Direct Current, Vol. 11, p. 53, May 1966.

5. English Electric Jour., Vol. 21, No.3, pp. 2-3, May-June 1966.

6. Elec. Times, Vol. 149, No. 10, pp. 361-362, Mar. 10,1966.

7. Elec. Times, Vol. 150, p. 479, Sept. 29. 1966.

8. "Kingsnorth-London D.C. Transmission Interconnector," by W. Casson, I.E.E. Conference Publication 22, High Voltage DC Transmission, Manchester, Sept. ] 9-23, 1966, Part 1, Paper 9, pp. 56-57.

9. "The Kingsnorth, Beddington, Willesden D.C. Link" by T. Calverley, A. Gavrilovic, Mott, C.l.G.R.E., Report 43-04, 1968, 14 pp.

BIBLIOGRAPHY 47

P. Nelson River, Manitoba

1. "The ±450 kV Direct Current Transmission System for the Nelson River Project," by L. A. Bateman, L. S. Butler, and R. W. Haywood, C.I.G.R.E., Report 43-02, 1966, 9 pp.

2. "The Nelson River Transmission System," by E. M. Scott, Trans. Canadian Elec. Assn., Vol. 6, Part 2, Paper No. 67-SP 131, Mar. 21, 1967, 16 pp.

3. "The Selection of ±450-kV HYDC Transmission for the Nelson River," by E. M. Scott, American Power Conference Proc., Vol. 29, pp. 966-977, Chicago, April 1967. Abstracted in Elec. Rev., Vol. 180, pp. 711-712, May 12, 1967.

4. "Why ±450-kV H.V.D.C. Was Selected for the Nelson River Transmission Medium," by E. M. Scott, Elec. News and Engg., Vol. 76, No.6, pp. 50-55, June 1967.

5. "English Electric Wins Nelson River Contract," Elec. Rev. (London), Vol. 181, No.7, p. 229, Aug. 18, 1967.

6. "The ±450 kV Direct Current Transmission System for the Nelson River Project," by L. A. Bateman, R. W. Haywood, and L. S. Butler, C.I.G.R.E., Report 43-02, 1968, 9 pp.

7. "Manitoba's Kettle Simmers Year 'Round," Engg. News Record, Vol. 181, No.7, pp. 34-36, 41, Aug. 15, 1967.

8. "Nelson River D.C. Transmission Project," by L. A. Bateman, R. W. Haywood, and R. F. Brooks, I.E.E.E. Trans. on P.A. & S., Vol. 88, pp. 688-693, May 1969. Disc., pp. 693-694. Also in I.E.E.E. Publ. 68 C57-PWR, October 1968.

Q. Miscellaneous Projects

1. "The Exploitations of Yugoslav Water Resources and the Possibility of Using H.V.D.C. Transmission," by H. von Bertele, Direct Current, Vol. 2, No.5, pp. 107-109, June 1955.

2. "The Present Status of High-Voltage D.C. Power Transmission in Japan," by Naohei Yamada, Appendix I to "Report on the Work of the Study Committee No. 10: D.C. Transmission at E.H.V.," C.I.G.R.E., 1956, Paper No. 407, pp. 2-7.

3. "Electric Power Transmission by H.V.D.C. Submarine Cables across the Adriatic Sea from Yugoslavia to Southern Italy," by M. Visentini, Asta, and F. Trimani, C.I.G.R.E., Report 210, 1958, Vol. 2, 22 pp.

4. "The Introduction ofH.V.D.C. Transmission into a Predominantly A.C. Network," by W. Casson, F. H. Last, and K. W. Huddart, Elec. Rev.) Vol. 178, No.8, pp. 290-295, Feb. 25, 1966.

5. "The Economics of D.C. Transmission Applied to an Interconnected System," by W. Casson, F. H. Last, and K. W. Huddart, I.E.E. Conf. Publication 22, H. V.D.C. Transmission, Manchester, Sept. 19-23, 1966, Part 1, Paper No. 13, pp. 75-83. Reinforcement of an ac system with dc links which do not increase required circuit-breaker interrupting ratings.

6. "High Capacity D.C. Transmission in the U.S.S.R.," by A. Berkovsky, N. Chouprakov, T. Izrailevich, A. Kolpakova, and S. Rokotjan, ibid., Part 2, Paper No. 94, pp. 126-129.

7. "Introductory Lecture," by F. J. Lane, ibid., transmission of 4500 MW by 500-k V dc and New York, U.S.A., by the Atlantic route, 92 IniIes of submarine cable.

pp. 7-25. Describes proposed to Boston

miles of land line and


8. " Preliminary Studies of Power Transmission from the Churchill Falls Development," by G. W. Clayton and D. T. McGillis, Trans. Canadian Elec. Assn., Vol. 6, Part 2, Paper No. 67-SP 132, March, 1967. Also in Amer. Power Con! Proc., Vol. 29, pp. 954-965, Chicago, April 1967.

':J.'" Problems in Designing a Direct Vpltage Power Transmission System," by S. P. Jackson, 1.E.E.E. Region 3 Con/. Record, 1967, pp. 309-314. 300-mi submarine cable network.

10. "Underwater D.C. Line Proposed in Alaska," Elec. World, Vol. 169, No.6, p. 20, February 1968. Line from Snettisham powerplant to Juneau by submarine cable is proposed.

11. "SeveralAspects of the System Studies of the DC Alternative for Power Transfer from Churchill Falls," by G. A. Baril, A. Lacoste, H. Persoz, J. D. Ainsworth, and J. P. Bowles, C.I.G.R.E., 1968, Paper No. 43-06, 18 pp.

12. "High Voltage Direct Current Plans for an Integrated Power System in the U.S.A.," by D. B. Giesner, Direct Current (new series), Vol. 1, No.3, pp. 121-123, February 1970. Based on Transmission Study 190, U.S. Department of the Interior, 1968.

02. chapter 1

Documents