Electrical communication

One calls electrical communication the whole of the Infrastructure S allowing to forward the electrical energy production centres (powerplants), to the Consommateur S electricity.

The network consists of electric lines exploited at various levels of tension, connected between them in electric stations. The electric stations make it possible to distribute the electricity and to make it pass from one tension to the other thanks to the Transformateur S.

An electrical communication must also ensure dynamic management of the production unit - transport - consumption, implementing adjustments having for goal to ensure the stability of the unit.

History

An electrical communication being composed of consumption and production machine tools, as well as structures (lines, transformers) to connect them, the electrical communications appeared only towards the end XIXe century, when each element had reached a sufficient technological maturity.

First networks with D.C. current

At the time of first half of the XIXe century, the inventive develop of many types of electrical motors to D.C. current, but their use in an industrial way will be allowed only after the invention of the Dynamo (generating of D.C. current) by Zénobe Gram in 1869, which will quickly be improved. With the International exhibition of Electricity of Paris of 1881, Marcel Deprez present for the first time an electric installation of energy distribution supplied with 2 Dynamo S. At the autumn 1882, the first electrical communications appear simultaneously with New York and Bellegarde, in France. They are very local and use the D.C. current.

Thomas Edison played a determining role in the development of electricity: it founds in 1878 Edison Electric Light Co (which will become in 1892 General Electric), deposits the patent of the electric bulb in 1879, then creates the electrical communication of New York. This last, the purpose of which was primarily lighting, develops quickly: from a power of 1200 bulbs in 1882, it passes to 10000 bulbs the following year. This network, which suffers from many breakdowns, is made up small powerplants (30 kw) and of a distribution network with 110 V. It however is very limited because the routing of electricity is not possible that on a few kilometers.

At this period the first experiments of transport of electrical energy develop and are carried out in particular by Marcel Deprez, which uses D.C. current. They are however relative failures because they do not allow the transport of industrial powers (Deprez succeeded in 1882 transporting 400 W out of 57 km of distance, but with a total output from only 30%). The engineers Lucien Gaulard and John Gibbs work as for them on the Alternative course. Although the Transformateur is known since 1837, they develop in 1884 a using transformer of strong power of the alternative course Triphasé, which makes it possible to change easily the level of tension. The same year they show the interest of the Transformateur by bringing into service a 80 km length line fed in alternative course under 2000 V.

Victory of the three-phase alternative course

George Westinghouse, engineer and American entrepenor who created his own company of electricity, is interested by the technology of the Alternative course. In 1887, it buys the Brevet S of the transformer of Gaulard and recruiting Nikola Tesla which invents the three-phase Alternateur in 1891. This same year the first three-phase installation is installation around Frankfurt, with a line of 175 km.

In the United States the networks in D.C. current continue their development, but are limited in the face: each central can feed in electricity only one zone approximately 5 km in diameter, which poses problem apart from the cities. In parallel small urban networks in alternative course are constituted. A severe opposition at that time makes rage in the United States between Edison (defender of the D.C. current) and Tesla (defender of the Alternative course). Edison insists in particular on the risk (quite real) of the Alternative course for the living beings, and goes until financing the macabre invention of the Electric chair.

The decisive battle between D.C. current and alternative proceeds around a project of power supply of the industry of Buffalo by a hydaulic power station of 75 MW located at Niagara Falls, 32 away km. Edison proposed a project in D.C. current while Tesla and Westinghouse proposed a system in alternative course. The contract was given to Westinghouse. In 1896, the startup of the first industrial line in three-phase current was a success total and led to universally impose the three-phase alternative course like means of transport of electrical energy, adapted better to transport on long distances.

Progressive interconnection of the networks

At the end of the XIXe and with the beginning of the XXe century, the uses of the electricity multiply, as well at the domestic level as industrial (in particular the electrification of the Tramway S, Métro S and railroads). In each big city is established companies of electricity. These last build powerplants and small lans, each one using of the Fréquence S and the levels of different tension. The operators tardily realize of the interest to use a Fréquence single (essential to the interconnection of the networks), and one sees appearing finally 2 standards of Fréquence: the 60 Hz on the majority of contains American and the 50 Hz almost everywhere in the rest of the world.

In first half of the 20th century the urban networks of the industrialized countries increased in order to electrify the campaigns. In parallel, these networks were inter-connected between them at the regional level in order to garner economies of scale on the size of the central of production, and to better develop geographically localized energy resources, like the hydraulic production located in the mountainous areas, moved away from the great centers of consumption. Progressively by the increase in the powers called and the distances from the lines of interconnection, the operating voltage of the lines also increased (1st line to 220 Kv built into 1923 in the United States, that to 380 Kv in 1930 in Germany). The appearance in 1937 of the first turboalternator cools with the Hydrogène, of a power of 100 MW, opens the way of the powerplants of strong power.

A difficulty of the development of the electrical communications is the legacy of the past, because the infrastructures are conceived to last several tens of years. The electrification of the campaigns was easy because of absence of any former network, thus allowing the implementation of the standard of the moment (in term of tension and Fréquence). At the urban level on the other hand the problem was complex because several networks not interconnectables coexisted, leading to the multiplication of the cables. The networks in D.C. current thus remained very a long time locally: until 1965 with Paris, and 2005 with New York!

In the years 1950, the European companies coordinate each other to standardize the tensions grid systems to 400 Kv, which allows in 1967 the first interconnection of the French networks, German and Switzerland with Laufenbourg (Swiss).

History of the electrical communications in France

The electrification of the French territory is carried out during first half of the 20th century: of 7 000 communes electrified in 1919, they are 36 528 with the being in 1938. In parallel, the close networks are inter-connected gradually:

the networks of Paris are it in 1907 to 12 Kv
those of the Pyrenees in 1923 to 150 Kv
finally the near total of the French territory is inter-connected in 1938 to 220 Kv, but of great areas remain isolated.

Even during the second world war, the grid system of electricity believes of 30% and with the Release it is densest in the world. In 1946, office plurality of the electric lines of more than 100 Kv reached 12 400 km, whereas it was only of 900 km in 1923.

April 8th, 1946 the state nationalizes the companies of electricity, by gathering the production companies, distribution and transport in a single establishment: EDF. Until 1950, EDF will have to organize the cuts of electricity, following the shortage of central of production. The Fréquence to 50 Hz spreads in France (it was for example of 25 Hz on most of the Mediterranean coastline). The network with 225 Kv replaces the networks with 110,120 and 150 Kv. In 1956, it is decided to generalize for the distribution low tension the couple of tension 220/380 V to replace old couple 127/220 V (in 1986 the standardized tension will be couple 230/400 V). The network 400 Kv, decided at the European level, develops in France of coordination with the nuclear plane , in particular as from the years 1970-1980.

General information

An electrical communication is first of all defines by the type of electric current which it uses. Once fixed, this choice engages the future and is full of consequences because the modifications are very delicate a posteriori. Then, at the time of the exploitation of the networks, certain electric quantities must be supervised regularly to make sure that the conditions of operating are respected strictly.

Strategic choices of the electric wave

The current electrical communications use a Alternative course sinusoidal Triphasé . This decisive choice rises from a whole of reasons which we present here.

Need for transporting electricity to a high tension

Exit of the Powerplant to the meter of the end user, the electricity must forward on an electrical communication. These networks often have the same structure of one country to the other, because the transport of strong powers on long distances imposes the minimization of the Joule effect.

The electricity transmission generates losses which had with the Joule effect, which depend on the intensity I , of the tension U and of the resistance R of the line. For three-phase current one obtains: $P_ {losses Joule} = R I^2 \ = R \ frac {P_ {electric} ^2} {3U^2} \,$

For same a electric output transmitted by the line, the losses by Joule effect decrease very quickly as soon as one works with high tensions. For example, a line of a hundred km with a resistance of 3Ω on which 400 MW circulate would generate approximately 4 MW of Joules loss if it were exploited to 200 Kv, but only 1 MW if it were exploited to 400 Kv. These losses represent very important amounts of energy: for France, that represents for the year 2005 approximately 25 TWh of losses (due to Joule effect, of effect crowns or of losses with vacuum) on the 509 TWh produced, that is to say 5% of the French electric production.

The costs of construction of a line to 400 Kv, 20 Kv or 220 V are however very different. It is thus necessary to find an optimum technico-economic between different the levels from tension, within sight of the hoped profit (relating to the reduction in the losses by Joule effect). One arrives thus at a multi-layer structure of the electrical communications, with the networks transporting of great quantities of energy exploited with tensions of several hundreds of Kv, and the tension decreasing as the transported powers decrease.

Alternative course or continuous?

The transport of important powers on long distances requires high tensions. One thus needs Transformateur S to pass from a tension to another, but they function only with Alternative course. A network containing D.C. current could be exploited only with only one constant tension, which is not possible considering the diversity of the uses of electrical energy and the losses Joules that such a system would involve. Moreover the cut of the currents in the circuit breakers is facilitated by the repetitive passage to zero of the Alternative course. This last generates operational requirements nevertheless, in particular the 2 following ones:

the existence of inductive and capacitive effects in the electric lines which it is necessary to compensate for in order to limit the effects on the tension of them;
the creation of a Effect of skin which concentrates the current with the periphery of the electric cables, thus increasing the losses Joules and requiring in certain cases of the specific measures.

Although the Alternative course was essential universally in all the networks, the D.C. current remains used for certain particular projects where the recourse to expensive stations of conversion are necessary (example of the underwater interconnections or those of very long distance).

Why a tension sinuosïdale?

The most convenient solution to produce in an industrial way of electrical energy is the drive of a Alternateur by a Turbine, the whole in rotation around an axis. In a natural way these installations produce sinusoidal tensions .

A system single-phase current or three-phase current?

It is completely possible to carry out a network only while running Monophasé. The reasons which resulted in adopting the network Triphasé are the technical and economic advantages important that it presents:

an alternator of very strong power cannot function by producing a single-phase current because the fluctuating power which results from it causes a destruction of the tree of connection between the alternator and the mechanical energy source which puts it in rotation. Indeed, a system Monophasé sees its instantaneous power passing by a zero value to each oscillation of the wave of tension (when the tension or the intensity passes by zero). The instantaneous power is thus variable. On the contrary, the balanced three-phase systems ensure a instantaneous power constant, i.e. " without with coup" , which is important in electromechanical.

the transport of the same electric output in Triphasé (without neutral) requires twice less conducting cables that in Monophasé. The economy which results from this on the cost from realization from the lines is notable.
the currents Triphasé S can generate revolving magnetic fields by distributing in a specific way the windings on a Rotor. However the electric machines which produce and use these currents function in an optimal way in mode Triphasé.
a distribution of electricity while running Triphasé with wire of neutral makes it possible to propose for the same network two different tensions of use:

is between a phase and the neutral : for example 230 V in Europe
are between two phases: for example 400 V in Europe

Frequency of the electrical communications

To choose the Fréquence of a network is determining because one cannot return any more behind once the network reached a certain size.

A high frequency is particularly interesting for the Transformateur S, thus making it possible to reduce their size. The electric bulbs are they adapted also better to the high frequencies (appearance of flickerings with low frequencies). Other applications, particularly those calling upon the Inductance S (standard electrical motor, or line of long-distance haulage), have better a output with low frequencies. It is at the end of the 19th century that this question arose, but the low dimension of the networks at that time made it possible to adjust the frequency according to the use which one was to make, and of the frequencies of 16 Hz to 133 Hz coexisted.

It is Westinghouse, probably with the councils of Tesla, which gradually imposed the 60 Hz on the United States. In Europe, after AEG chose the 50 Hz, this frequency was diffused gradually. One preserves today this history and the current networks are exploited either to 50, or to 60 Hz.

Important electric quantities

The great electrical communications require the constant monitoring of certain parameters in order to maintain the network, as well as the consumption and generating stations which are connected there, in the fields of application envisaged. The principal sizes to be supervised are the Fréquence, the tension, the intensity in the works, and the power of short-circuit.

Monitoring of the tension

A great electrical communication has multiple levels of tension. Each level of tension is conceived for a quite specific beach of use. Tensions slightly too high lead to a premature wear of the material, then if they are frankly too high with a " Breakdown " insulator (case of the buried cables, domestic cables, or Insulating S of the electric lines). Very high overpressures (for example caused by the the Lightning) on conducting " nus" (i.e. without insulator, which is the case of the electric lines) can lead to starting with close objects, for example of the trees.

A contrario, of the too low tension compared to the specified beach lead to a faulty operation of much of installation, that it is in the consumers (for example driving ), or on the network in him even (faulty operation of the protections). Moreover, low tensions on the grid systems of electricity were the cause of great incidents which were responsible for the cut of several million hearths (e.g. of the Greek blackout on July 12th, 2004 or of January 12th, 1987 in France).

Although the beaches of use of the materials specify a margin from 5 to 10% compared to the nominal voltage, the large operators of networks currently priviligient an exploitation rather in high tension.

Intensity and problems of the IMAP

The intensity is a parameter supervised permanently on the transformer air, underground electric lines and . The problem created by a high intensity (i.e. a high transmitted power) is a heating by important Joule effect. The consequence of this heating appears different manner according to the works considered:

for the electric cables (presence of a insulating sleeve): the heat produced by the cable must be evacuated via the electric Isolant, which is often bad driver of heat. Moreover, the cables being often underground, this heat is evacuated all the more badly: in the event of too high intensity, the risk is the physical destruction of the cable by overheating.
for the Transformer S : rollings up of the Transformateur S are in general immersed in an oil bath which also plays the part of electrical insulator but of coolant, and which is him even cools by Aéroréfrigérant too high S. In the event of intensity, oil cannot more evacuate enough heat and rollings up are likely to worsen by overheating.
for the air lines (absence of insulating sleeve): the drivers warming up by Joule effect, they also will lengthen by the thermal phenomenon of Dilatation; the Electric line being maintained at each end by a pylon, this lengthening will materialize by a reduction height between the line and the ground. In the event of excessive intensity, the Electric line will not respect any more the minimal height of safety, even will be able to come into contact with the ground, thus creating an electric arc.

The intensity is a parameter particularly important to supervise because it can involve the destruction of expensive material (the Transformateur S and cables), or endanger the safety of the goods and people (case of the air lines). The IMAP (Acceptable Maximum Intensity permanently) is the maximum intensity to which a work can be exploited without time limit. In order to facilitate the exploitation of the electrical communications, certain works can be exploited with an intensity higher than the IMAP but during one limited time. Moreover, certain works are provided with particular protections which put them in safety if the intensity exceeds a certain value for one definite length of time.

Intensity of short-circuit

Structure of the electrical communications

The electrical communications can be organized according to several types of structures exposed below:

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