Alternate three-phase current (AC: Current alternative): Transport is done normally on 3 cables while three-phase current. A wire of ground with strong capacity is not necessary.
Continuous (HVDC: high direct voltage current): Transport is often done on two drivers. Ground wire are placed at the top of the pylons of reversed polarity. The return is done by are electrodes plunged in salted for the underwater cables.
Pylons: The drawings represent pylons high voltage a height proportional to the tension (of 35 m to 50 m in height). The cables are tended between two pylons and do not have to touch themselves or approach too much when the wind makes them balance on their way between pylons. The circle in which they can oscillate is represented on the diagram of left. It should be held account owing to the fact that the hot cables lengthen a little. The large cables are arranged in multiples of 3 for the three-phase current or 2 for D.C. current.
The ground wire are not essential, nor in three-phase current. Ground wire of small power are often placed above them pylons to cure imbalances between phases. The cables must be isolated from the reinforcement of the pylon by insulating discs out of glass or resin, which support the mechanical tension of the cables. Each disc allows an insulation of about 30 Kv, even in a little wet weather. One thus needs 33 discs for lines of 1 MV.Des insulators which hold the cable suspended in straight line are used when the exaggerated side swinging of wire is a weak risk.
An important potential is measured between each cable and the ground. The discharges in the ambient air (ionization of the air and loss of current) become important from 1 MV. (This maximum tension limits the average potential to 707 Kv (or 765 Kv) in alternative course). In practice maximum tension of continuous has it is of 500 Kv but tensions of 700 Kv are being studied.
The pylons often support 6 principal cables. To increase safety in the event of breakdowns on networks very charged, one multiplies the number of lines instead of putting more than 6 cables. The current pylons for the HVDC have only 2 principal cables.
Loss of energy
The cables of the line of transport heat by Joule effect (and a little by the radiation of the sun) and cool by circulation of air around the drivers. The wire cannot exceed the limiting temperature where them lengthening becomes permanent by effect of creep, a permanent destruction. Calculations make it possible to estimate this temperature and thus to approach the limit. A line with very high voltage has a larger capacity in winter in the Far North (but the white frost can pose problems).
Each functional cable is made of one or more parallel cables (normally 2, up to 6). Each cable often has a steel heart and is surrounded by aluminum which has the best conduction ratio ⁄ weight. The total power being on by 3 three-phase cables of a large pylon is about 2000 amps, thus being able to transport a power of a GW on installations of 500kV . If the loss is of 1% out of 50 km, it is of 20% out of 1000 km. The buried cables or the cables at sea are much more expensive to build and have more limited power because heat is more difficult to dissipate.
The lines must sometimes be disconnected for repairs and redundancies are thus envisaged in the networks. The maintenance of the lines is expensive. The distribution since large power stations to 200 km requires more investments than by small power stations distributed every 50 km.
To transport the variable request for electricity and to profit from safety measures in the event of breakdown of a group of cables, the power of lines (P = V X I) is decreased. The lines are often charged on average with half of their maximum capacity. The loss, which is proportional to resistance R and the square of the intensity of the transported current (E = R X I), is thus divided by four. One would need twice more lines for very high voltage to reduce the total losses to a quarter. If the loss is of 5% in normal conditions, it is of 20% when the line is used with its maximum.
The discovery of superconductors (R=0) to high temperature brought hopes to reduce the losses. Unfortunately these materials lose their supraconductivity when they transport an important power.
In all the fields of research, the media announce revolutionary projections, that it is for nuclear fusion, the wind load factors, the stabilization of the networks or supraconductivity. The experiment shows that, if it is effective to finance research, it is not wise to count on what remains still field of research to plan the energy management or to launch great investments of production (that is however done in the wind one). However, it is to better wait by hoping for revolutionary technologies rather than to spend its capital for what is not essential and which could be exceeded later a few years.
Transformer stations and maintenance
The transformer stations of the voltage (transformers) cost a high fraction of the total of the transport costs. They are all the more expensive as the potential at the entry is higher. One thus may find it beneficial to be used low potentials in dense zone of dwelling where there are many transformers.
Potentials corresponding to the hauls.
The loss for a power P given varies like the reverse of the square of the voltage (E = R X P2 ⁄ V2). The loss on a line of 100 km in 500 Kv is the same one as on 10 km in 50 Kv or 4 km to 20 Kv or out of 1 km in 5 Kv or 75 m in 380 V. electricity are thus distributed in sequence on 3 or 4 cables where the voltage decreases by stage.
Let us suppose that there are 4 losses equivalent to each stage of the following example: 50 km in 500 Kv; 5 km in 50 Kv; 500m in 5 Kv; 40 m in 380V; that is to say on the whole 55 km which are seldom in straight line.
An average way of 50 km enters the power station and the user makes lose 7% to 8% of the current, part of the losses coming from the transformers. Other losses of the same order come from the electricity consumption of the generator plants for. In the same way, the wind mills do not count their consumption of electricity and their losses until network connection in the calculation of their power and their load factor.
Three-phase lines with very high voltage
For long distances, one employs lines of 500 Kv on some 40 height m pylons. Each line blocks a corridor as broad as a highway, corridor which can be with hillside and where one can cultivate (except trees) but where it would be dangerous to live. The manufacturers owe or would owe exproprier the houses in a new corridor.
A group in a line with very high voltage can transport until a GW (power of a nuclear reactor). A line of pylons (with 2 groups of cables) thus transports 2 GW in three-phase current or 3 GW uninterrupted. (That also depends on the distance from transport).
Lines buried with high voltage are 3 times more expensive and transport only one third of current , with the result that the cost per transported unit is multiplied by 10 on the cables buried or underwater, for example between England and the continent.
The advantage of the three-phase current is that the voltage can be transformed in called static apparatuses transformers. These transformer stations are economic with the purchase and maintenance. They can thus be numerous.
The power reactivates in the transmissions at very long distance
If the current moved with speed of light, it would take him 5 ms to traverse 1500 km, what to cross France in diagonal. The frequency of the current is 50 periods a second, that is to say, 20 ms per period. That is to say 5 ms for a quarter of period. The current goes from a raised potential to weaker. The voltage is not in perfect synchronism with the current but must precede it a little.
The reactive power is another means of seeing this phenomenon. The alternative course in a network is not only one oscillation of current I in synchronism with the oscillation of the voltage V. the reactive power of an alternative network is current shifted 90° compared to the voltage. This power adds losses to the network without transmitting useful output and reduces the active power which can be transmitted.
The larger the haul is, the more the power reactivates to associate is important.
In practice, there seldom was transport need at very long distance before the great projects of renewable and, in lower part of 300 km, the lines very high voltage cost less with less extreme voltages: 440 Kv or 220 Kv.
To transmit to long distance, one needs electric generators able to produce enough reactive energy, which can generate the large thermo plants and the nuclear plants but not the wind mills current or the small power stations of cogeneration. Those are built with the lower costs but should include more complicated and more expensive devices to be able to be connected to the network by improving its stability instead of compromising it.
A device which makes it possible to control the reactive power on a node of the network calls a phase-converter. It makes it possible to connect wind mills without desynchronizing the network but one needs, moreover, to add lines HT if the transported power exceeds the capacity of the existing lines.
Lines high voltage with D.C. current
Lines HT with alternative course have a maximum voltage of about 765 Kv (average voltage of the oscillation). Lines with D.C. current can have a continuous voltage more raised (let us say, 1 MV) and transmit up to 3 GW on very high pylons.
A station transformer of alternative course into D.C. current or conversely is more than 2 to 4 times more expensive than a transformer of three-phase alternative course. The losses of the cabin are raised also, say, 1% of transported energy. The loss on 1000 km of lines is then only of 2% instead of 3%. If only the losses are considered, lines HT uninterrupted have less losses starting from 300 km. If one includes the financial expenses of the stations of transformation, the length minimum necessary for a competitive price is currently of at least 500 km. The total expenses, cabins and lines, are then reduced starting from 1000 km (the percentage of loss decreases when the line does not work with its maximum).
The lines with D.C. current are not an technological innovation suitable significantly to change the electric transport costs into lower part of 1500 km but they have other advantages. Transport by D.C. current avoids synchronization between independent networks. The D.C. current is currently a solution interesting for interconnections of independent networks.
What is innovating in the HVDC is the use of thyristors of power. These transistors are not rectifiers because an additional entry orders to let pass the current or not. The current can reach 5000 amps but the tension cannot exceed 6 Kv. Thyristors (IGCT) are put in series. One needs 40000 IGCT cooled for a line HVDC.
The underwater cables have a price much more raised than that of the air lines (20 times). The high price of the stations transformer of continuous into alternate is thus a less great part of the cost, which facilitates the choice of the continuous one.
The network subdivided in independent networks
So that a total electrical communication functions in the United States, the engineers divided the American network into independent zones, each one having a clean synchronized frequency. The internal distances from transmission are enough reduced not to destabilize each one of these networks.
Statistics show the places where the frequency and the voltage tend to deviate from that of the network and this information (these alarms) is used to choose actions for better distributing the local transmissions and generations and minimizing these incidents.
The transmission between independent zones is done by lines with D.C. current. This additional transmission is used as auxiliary safety in the event of breakdown of a unit but remains limited enough. An exception is the superabundant hydraulic electricity transmission in Quebec to the United States. The electricity transmission between networks is well measured and the transport costs are allotted to those which cause them.
The description of lines transporting of the D.C. current (up to 1 MW to 320 Kv). One can connect on a new technology HVDC, stations intermediate VSC (Voltage Converters Source). One can thus graft deviations towards local catches, such as wind farms. These stations VSC consume at least 3% of the power at each end.
Progressive interconnection of the networks
At the end of XXIe and the beginning of the XXe century, the USA ges of electricity multiply, as well at the domestic level as industrial (in particular the electrification of the tramwà ys, subways and railroads). In each big city are established companies of electricity. These last build powerplants and small lans, each one using of the frequencies and the different levels of tension. The operators tardily realize of the interest to use a single frequency (indispensa corn with the interconnection of the networks), and one sees appearing 2 standards of frequency finally: 60 Hz on the majority of the American continent and the 50 Hz almost everywhere in the rest of the world.
In first half of the XXe century the urban networks of the industrialized countries increased at end to electrify the campaigns. In parallel, these networks were inter-connected between them at the regional level at end to garner economies of scale on the size of the power stations 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. with the fur and amesure of the increase in the powers called and distances from the lines of interconnection, the operating voltage of the lines with also increased (1st line to 220 Kv built into 1923 with the United States, that to 380 Kv in 1930 in Germany). The appearance in 1937 of the first turboalternator cooled with hydrogen, 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 placement of the standards of the moment (in terms of tension and frequency). In the years 1950, the European companies coordinate each other to standardize the tensions of the grid systems to 400 Kv, which allows in 1967 the first interconnection of the French networks, German and Switzerland has laufenbourg (Swiss).
second half of the XXe century to known moreover a reinforcement of the interconnections intrà - national and a significant development of the transnational interconnections, with a principal aim to create capacities of mutual help between operators and to improve the stability of the electric systems overall, like, in a more specific way, to create capacities of energy exchange on the long run.
Europe, with its strong population density and an elevated level of economic development and industrialist, presents an electrical communication at the same time dense and with a grid. the installation of physical interconnections under these conditions, to need the adoption of common rules of safety enters the owners of the various systems, often national to prevent the risks of incident of great width. today, it is the ENTSO-E, in the past UCTE, which carries out this coordination in Europe.
Finally more recently, within the framework of the construction of the interior market of electricity, European commission with selected to encourage the development of the transborder capacities of interconnection, at end to increase the potentials of exchange and the commercial interconnection of the national markets.
For the XXIe century, the networks are confronted important new challenges:
to accommodate simultaneously, without significantly decreasing safety and quality of operation of the network, the stable production units and commandables (electricity hydroelectric or resulting from thermo plants) like less foreseeable sources and often not or very little commandables, like solar energy or wind energy. These energy sources are the subject in many developed countries of development programs at an intensive pace.
to facilitate the interaction between the consumers and the electric system in particular to adapt the request to production capacities when that is necessary.
to be more sparing in nonrenewable resources than they are materials for their construction as of the losses than they involve.
to accommodate the new USA ges as the electric vehicle.
On these subjects, the prospectivists announce an intelligent network (Smart grid) more flexible and able to better integrate the energy sources clean and sure, but diffuse and noncontinuous such as the wind one and the solar one.
An electrical communication is first of all defined 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 sinusoidal three-phase alternative course. This decisive choice rises from a whole of reasons which we present here.
Need for transporting electricity has a high tension
Exit of the powerplant to the meter of the end user, 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 involves losses due to the Joule effect, which depend on intensity I, the tension U and resistance R of the line. For three-phase current one obtains : Pjoule loss = RI² = R * (P² electric ⁄ 3U²)
For the same electric output transmitted by the line and to equal resistance, the losses by Joule effect thus decrease like the square of the tension: they are divided by four when the double tension. thus, a line of a hundred km with a resistance of 10 O on which 400 MW circulate would involve approximately 4 MW of Joules loss if it were exploited to 200 Kv, but only 1 MW if it were exploited to 400 Kv.
The stake of these losses can be measured with the very important amounts of energy that represents: for France, on the 509 TWh produced in 2005, approximately 25 TWh were lost continuation aces phenomena (due to Joule effect, of effect crowns or losses avid), 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 the various levels of tension, within sight of the hoped profit (relating to the reduction in the losses by Joule effect). One arrives thus has a multi-layer structure of the electrical communications, with the networks transporting of great quantities of energy exploited ades tensions of several hundred kilovolts, and the tension decreasing with the fur and amesure which the transported powers decrease.
Alternative course or continuous
The transport of important powers on long distances requires high tensions. One thus needs transformers to pass from a tension has another, but they function only with alternative course. The changes of tension on a continuous system acourant is not as effective (more losses) as into alternate (transformer). The profit of the rise in tension would be contreba launched by the more important losses at the time of the phases of lowering of the tension. Moreover the cut of the currents in the circuit breakers is facilitated by passed repetitive Ge to zero of the alternative course. This last produces 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 at end to limit the effects on the tension of them
the creation of an effect of skin which concentrates the current with the periphery of the electric cables, thus increasing the Joules losses and requiring in certain cases of the specific measures.
The alternative course was essential in almost all the networks, but the D.C. current remains still interesting for certain particular projects where the recourse ades expensive stations of conversion is necessary.
Why a sinusoidal tension
the most convenient solution to produce in an industrial way of electrical energy is the drive of an alternator by a turbine, the whole in rotation around an axis. In a natural way these installations produce sinusoidal tensions.
In opposite direction and also naturally, these sinusoidal tensions allow the regular drive of an electrical motor.
This facility of use to the production and the USA Ge in the revolving machines constitute the two large assets of the sinusoidal tension.
A system single-phase current or three-phase current
It is completely possible to carry out a network only while single-phase current. The reasons which resulted in adopting the three-phase network 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 single-phase current sees its instantaneous power passing by a zero value achaque 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 three-phase systems balanced ensure a constant power instantaneous, that is to say without acoup, which is important in electromechanics.
The transport of the same electric output in three-phase current (without neutral) requires a section of conducting cables twice weaker than in single-phase current. The economy which results from this on the cost from realization from the lines is notable.
The three-phase currents can produce revolving magnetic fields by distributing in a specific way windings on a rotor. However the electric machines which produce and use these currents function in an optimal way in three-phase mode.
A distribution of electricity while three-phase current with wire of neutral makes it possible to propose for the same network two different tensions of use:
maybe between a phase and the neutral: for example 230 V in Europe
maybe between two phases: for example 400 V in Europe
Frequency of the electrical communications
To choose the frequency of a network is determining because one cannot return any more behind once the network to reached a certain size.
A high frequency is particularly interesting for the transformers, 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 inductances (standard electrical motor, or line of transport agrande distance), have a better output with low frequencies. It is at the end of the XIXe century that this question arose, but the low dimension of the networks allowed acette time 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 with EG had chosen the 50 Hz, this frequency diffused small appetite. One preserves today this history and the current networks are exploited either to 50 Hz, or to 60 Hz.
Important electric quantities
The great electrical communications require the constant monitoring of certain parameters at end 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 has to supervise are the frequency, 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 has a premature wear of the material, then if they are frankly too high a breakdown of the insulator has (case of the buried cables, domestic cables, or insulators of the electric lines). Overpressures very high on naked drivers can lead ades startings with close objects, for example of the trees.
to contrario, too low tensions compared to the specified beach lead has a faulty operation of many installations, that it is in the consumers, or on the network in itself (faulty operation of protections). Moreover, low tensions on the grid systems of electricity were the cause of great incidents which were responsa corns of the cut of several million hearths.
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 privilege an exploitation rather in high tension because that limits the losses joules in the network.
Problems of the intensity
The intensity is a particularly important parameter has to supervise because it can involve the destruction of expensive material (transformers and cables), or endanger the safety of the goods and the 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. at end to facilitate the exploitation of the electrical communications, certain works can be exploited has 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.
The problem created by a too high intensity 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 electrical insulator, 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 transformers: rollings up of the transformers are in general immersed in an oil bath which plays the electrical part of insulator but also of coolant air cooler. In the event of too high intensity, oil cannot evacuate enough any more heat and rollings up are likely to worsen by overheating.
for the air electric lines (absence of insulating sleeve): the drivers warming up by Joule effect, they also will lengthen by the thermal phenomenon of dilation, the electric line being maintained achaque end by a pylon, this lengthening và to materialize by a reduction height between the line and ground, which leads has a starting (electric arc creating a short-circuit) within sight of the important tensions used in these networks. Fortunately protections are installed on the lines to avoid such startings which are of course extremely dangerous.
Intensity of short-circuit
The intensity of short-circuit (shortened Icc) is a theoretical size which corresponds to the current that one could measure in a point of the network if this point were connected directly to the ground. It is equal to the current circulating in a work at the time of a three-phase defect frankly to the ground. The Icc are provided mainly by the groups of production. It is high in the nodes of the network which are the electric stations (on the European network 400 Kv, the values are about 30 to 50 kà). The Icc become increasingly weak with the fur and amesure which the levels of tension decrease and which one moves away from the electric stations.
The materials used in the electric stations are designed to resist has a maximum value of Icc: beyond, it there at the risk of breakage of material in the event of short-circuit (caused by the lightning, the white frost, a rupture of material...) Breakings of this nature are in particular caused by powerful electrodynamic phenomena which take place when drivers are subjected ades current exceptionally strong.
An electrical communication with however any interest Icc to have raised one. Indeed, that allows the damping of the disturbances emitted by big industries (problem of the flickers), as well as a reduction of the voltage drops at the time of the short-circuit on the network. For the consumer, the Icc correspond to the maximum intensity which the network can provide: one Icc sufficient is thus indispensa corn with the starting of the large electrical motors. In a general way, one Icc high maintains a good quality of the electric wave provided to the customers.
Structure of the electrical communications structure with a grid: the electric stations are connected between them by many electric lines, bringing a great safety of food. radial or buckled structure (the red stations represent the contributions of energy): the safety of food, although lower than that of the structure with a grid, remains high. tree structure (the red stations represent the contributions of energy): the safety of food is weak since a defect on the line or the red station cuts the whole of the customers downstream.
Each type of structure has specificities and very different modes of exploitation. The wide-area networks of energy use all these types of structure. In the highest levels of tension, one uses the structure with a grid: it is the grid system. In the lower levels of tension, the buckled structure is used in parallel of the structure with a grid: it is the network of distribution. Lastly, for the low levels of tension, the tree structure is almost exclusively used: it is the distribution network.
The grid system
The purpose of the grid systems are with high voltage (HTB) (of 50 Kv to 400 Kv) and are to transport the energy of the great production centres towards the areas consuming electricity. The forwarded great powers impose electric lines of strong capacity of transit, as well as a structure with a grid (or inter-connected). The mesh networks guarantee a very good safety of food, because the loss of any element (electric line, transformer or group of production) does not involve any cut of electricity if the owner of the grid system complies with the rule known as of the N-1 (possibility of losing any element of the network without unacceptable consequences for the consumers).
The purpose of the distribution networks are to feed the whole of the consumers. There exist two pennies levels of tension:
the networks amoyenne tension (from 3 to 33 Kv);
the networks abasse tension (from 110 to 600 V), on which are connected the domestic users.
Contrary to the distribution and grid systems, the distribution networks have a great diversity of technical solutions at the same time according to the countries concerned, like according to the population density.
The networks amoyenne tension (MT) have in a very majority way a tree structure, which authorizes simple and inexpensive protections: to apartir of a source station (itself supplied with the network of distribution), electricity traverses an artery (or framework) on which are directly connected branches of derivation to the end of which the stations MT/BT of public distribution are, which feed the networks low tension (BT) on which the smallest consumers are connected. the tree structure of these networks implies that a defect on an electric line MT entrainerà inevitably the cut of the customers supplied with this line, even if more or less fast possibilities of help exist.
The frameworks of the networks amoyenne tension (MT) European are made up only of the 3 phases, whereas in North America the wire of neutral is also distributed (3 phases + 1 neutral). MT derivations as for them can be made up of 1 wire (case of Australia where the current return is carried out by the ground) with 4 wire (case of the United States), or systematically 3 wire (3 phases) like the French network.
The air MT networks are majority in rural area, where the tree structure largely prevails. On the other hand in urban area the constraints of obstruction, esthetics and safety lead has a massive use of the buried cables. The underground grids being subjected potentially to long unavailabilities in the event of damage (several tens of hours), it is appealed ades structures in double derivation or ades débouclées radial structures provided with automatic apparatuses of refeeding, allowing a better safety of food.
Networks BT result from the structure of the MT networks: in North America the networks single-phase currents are current (1 neutral + 1 phase), while in Europe the three-phase distribution with wire of neutral is very majority (1 neutral + 3 phases). the tree structure is laaussi by far most widespread, because it is at the same time simple, cheap, and allows an easy exploitation.
Materials used in the electrical communications
The electrical communication is made up not only of material high voltage (known as material of power), but also of many peripheral functions such as the téléconduite or the protective system.
Materials of power
The electric lines connect the stations between them. inside a station, one finds for each level of tension a play of bar which connects the transformer departures lines and departures.
Electric lines line with high voltage.
The purple cable is the earth wire. These pylons support 2 terns: red and blue. Each tern consists of 3 phases. Each phase is supported by an insulator.
The electric lines provide the function transport of energy on the long distances. They consist of 3 phases, and each phase can be made up of a beam of several drivers (from 1 to 4) spaced few centimetres at end to limit the effect crowns which involves on-line losses, different from the Joule losses. The whole of these 3 electric phases constitutes a tern.
An electric pylon can support several terns: in France never more than 4, seldom more than 2, but of another country like Germany or Japan make support alor pylon up to 8 terns. The pylons all are carefully connected to the ground by an effective ground network. The pylons support the drivers by insulators out of glass or porcelain which resist the high tensions of the electric lines. Generally the length of an insulator depends directly on the tension of the electric line which it supports. The insulators are always provided with spark-gaps which consist of two metal points facing. Their distance is sufficient so that in normal circumstances the behavior of tension can be guaranteed. Their utility appears when the lightning strikes the electric line: an electric arc và then to be established on the level of the spark-gap which contournerà the insulator. If there were no spark-gap, overpressure between the pylon and the struck down electric line would destroy the insulator systematically.
An earth wire, made up of only one driver, overhangs sometimes the electric lines. It is attached directly to the pylon, and does not transport any energy: it is connected to the network of ground and its goal is to attract the lightning at end which it does not strike the 3 phases of the line, thus avoiding the hollows of tension disturbing the customers. in the center of the earth wire one places sometimes a cable fiberoptic which is used for the communication of the owner. If one decides to install fiberoptic on an already existing earth wire, one then uses a robot which viendrà to roll up in spiral fiberoptic around the earth wire.
Transformers of power
a small transformer MT/BT
One finds on the electrical communications two types of transformers of power:
the auto-transformers which do not have insulation between the primary education and the secondary. They have a fixed report/ratio of transformation when they are in service, but which can be changed if the auto-transformer is put except service.
the transformers with rulers in load are able to change their report/ratio of transformation when they are in service. They are used to maintain a tension constant with the secondary (the lowest tension) and play a big role in the maintenance of the tension.
The transformers being particularly expensive materials, their protection is ensured by various redundant mechanisms.
The electric stations are the nodes of the electrical communication. They are the points of connection of the electric lines. The stations of the electrical communications can have 2 finalities:
interconnection between the of the same lines level of tension: that makes it possible to distribute energy on the various lines resulting from the station
the energy conversion: the transformers make it possible to pass from a level of tension has another.
Moreover, the electric stations ensure of the strategic functions:
to ensure the protection of the network: a complex system of protection allows that a defect on only one work does not involve the setting not under tension of many works, which would be likely to put a vast zone not under tension. This protection is ensured by sensors which provide an image of the tension and current ades relay of protection, which work out tripping orders adestination of the circuit breakers
to allow the normal exploitation of the network: presence of several plays of bar and coupling at end to be able to take different diagram electric
to ensure the monitoring of the network: the tension of the network and the intensity in the lines are supervised in the electric stations, via transmitters of tension and current.
Protection of the electrical communications
Any electrical communication has protective systems to disconnect the system of production in the event of defect on the line. The objective is to protect the 3 components from an electrical communication:
bodies of production (alternator)
grid systems (air lines, transformer, plays of bar)
the distribution networks (final customers)
Material of control and monitoring
control is carried out since regional or national load dispatching centers. Those have instruments of remote control including ⁄ understanding devices allowing:
to order the switchgear (circuit breakers, disconnecting switches)
to know the position of these bodies.
to measure a certain number of sizes (tension, intensity, frequency)
to announce dysfunctions (has tears)
In addition to the elements above allowing remote control, one also finds devices local, being able to carry out in an automatic way of the intended operations asauvegarder the operation of the electric system where to restore the service when that at stopped summer.
An important network of reliable and protected ways of telecommunication is necessary to exchange this information between the load dispatching center and the stations which it exploits.
The material of monitoring is intended for the analysis a posteriori of the incidents. It includes ⁄ understands primarily consignors of state charged to raise the position of the switchgear, and perturbographes which, thanks to a system of memory, restore the evolution of the tensions and the currents during the course of the incidents. When sensitive customers are aproximity of the station, of the qualimeters, intended amesurer the short cuts, can also be installed. The abundant data by this equipment are consulted on the spot. By convenience, they can be transmitted remotely, but the reliability requested from the transmission channels used is less important than in the preceding case.
balance production - consumption
Electricity is one of rare energies which it is not possible to store agrande scale (one excludes the systems of batteries or the stoppings considered as reserves from electromechanical energy to weak inertia). Permanently, the operators of the networks must make sure of balance between supply and. In the event of imbalance, one observes mainly two phenomena:
a consumption higher than the production: the risk of unballasting frequencemetric or black out is not excluded, (fast loss of synchronism on the alternators), like in the case of the massive unballasting of Italy in 2003
a production higher than consumption: there can be in this case an acceleration of the synchronous machines which produce electricity and a racing which can also lead has a black out via frequencemetric protections. This situation is known insular electric systems where in particular wind overproduction involves sometimes high frequencies on the networks, for example 54 Hz in Guadeloupe at the time of the summer 2008 with a strong wind production in addition to the centralized production of the island.
The interconnections between country make it possible to better distribute the risk of black out on a country scale, the countries being interdependent the ones with the others in management of supply-demand balance: one speaks here about mutualized primary reserve.
Massive appearance of the production decentralized on the final networks (distribution networks) also led to atenir account of this production not centralized in the total balance of the networks, in particular for the problems of behavior to the tension. The emergence of the intelligent networks or smart grids must in particular contribute to make cohabit the total balance of the grid system (frequency, tension), with the local balance of the distribution networks.