Generation and transmission of electricity over a distance. How is electricity transmitted and distributed? DC lines

Ministry of General and Vocational Education

GOU NPO Sverdlovsk region

Nizhny Tagil Professional Lyceum "Metallurg"

ESSAY

Transmission of electricity over distances

Artist: Bakhter Nikolai and Borisov Yaroslav

Supervisor: teacher of physics Reddikh Lyudmila Vladimirovna

Nizhny Tagil 2008

Introduction

Chapter 1

Chapter 2. Electric Power Generation

1 Alternator

2 MHD generator

3 Plasma generator - plasma torch

Chapter 3. Power transmission

1 Power lines

2 transformer

Chapter 4

1 Steel production in electric furnaces

2 Typical receivers of electrical energy

Conclusion

Bibliography

Introduction

The power grid complex of the Sverdlovsk region, including the Nizhny Tagil energy center, is on the verge of major transformations. In order to avoid an energy crisis in the Middle Urals, the government of the Sverdlovsk region developed and adopted the main directions for the development of the electric power industry for the next ten years. First of all, we are talking about the construction of a new generation, that is, power plants that generate electricity, and the further development of the electric grid complex - the construction and reconstruction of substations, transformer points and power transmission lines of various voltages. As far back as last year, we drew up and approved a long-term investment program until 2012, indicating specific electric power facilities to be reconstructed and those to be built.

Until 2001, there was no shortage of energy capacity in the Tagil region. But then, after many years of crisis, our industrial enterprises went, as they say, uphill, medium and small businesses began to develop actively, and electricity consumption increased significantly. Today, the deficit of energy capacities in Nizhny Tagil is over 51 megawatts. This is ... almost two Clapboards. But the comparison with Clapboard is conditional. In fact, the problem of energy capacity shortage is currently most relevant for the central part of Nizhny Tagil. Built forty years ago, the Krasny Kamen substation, on which, in fact, the energy supply of the city center depends, is morally and physically outdated and is operating at the limit of its capabilities. New consumers have, unfortunately, to be denied connection to the grid.

Nizhny Tagil needs a new substation - Substation "Prirechnaya" with a voltage of 110/35/6 kV. According to preliminary estimates, the amount of capital investment in the construction of Prirechnaya will be about 300 million rubles. The investment program of Sverdlovenergo in Nizhny Tagil also includes the reconstruction of the Soyuznaya substation, the construction of the Altaiskaya substation at Vagonka and the Demidovskiy switching point in the Galyanka area, which will radically improve the city’s energy supply system as a whole. The main event of this year is the Staratel substation, in the reconstruction of which Sverdlovenergo invested 60 million rubles. Another, also significant, event of 2007 was the commissioning of a new, second transformer at the Galyanka substation.

The beginning of the construction of a power transmission line Chernoistochinsk - Belogorye with a voltage of 110 kV and a total length of almost 18 kilometers. This object is also included in the investment program of Sverdlovenergo. The commissioning of a new high-voltage power line will make it possible to make more reliable power supply not only to the Belaya Mountain ski complex, but also to the entire adjacent territory - the villages of Uralets, Visim, Visimo-Utkinsk and other settlements. I will say more: the Belogorye project also provides for the construction of a new Belogorye substation in the village of Uralets and the reconstruction of the entire network complex of Uralets, which is at least 20 kilometers of networks with a voltage of 0.4-6 kV.

The purpose of our essay, we decided to raise the question of the transmission of electricity not only at a distance, but also its use as a necessary component in steelmaking, since our profession is inseparably linked with this electric steelmaking process.

To achieve this goal, we decided to set ourselves several important tasks: 1) to study additional literature related to the transmission of electricity and electrometallurgy; 2) get acquainted with new types of generators and transformers; 3) consider the electric current from its receipt to delivery to the consumer; 4) consider the physical and mechanical processes of steel production in electric furnaces.

Initially, people did not know how to steel and for the manufacture of various tools they used materials of native origin (copper, gold and meteoric iron). However, these methods were not enough for human needs. Often people were looking for an opportunity to get metal from ore found on the surface of the earth.

And at the turn of the second and first millennia BC, the first stage of metallurgy was born. Mankind has moved to the direct production of iron from ore by its reduction in primitive furnaces. Since "raw" blast (not heated air) was used in this process, the method was called raw blast.

The second stage of steel production (XIV-XVIII centuries) was characterized by the improvement of furnaces, the growth of the volume of cheese-blowing furnaces. The appearance of the water wheel and its use to drive bellows made it possible to intensify the blast, increase the temperature in the hearth of the furnace and accelerate the course of chemical reactions.

The third stage was the manufacture of a more advanced and productive method for obtaining low-carbon iron in a doughy state - the so-called puddling process - the process of converting cast iron into iron on the hearth of a fiery reflective (pudling) furnace.

The fourth stage (the end of the 19th and the middle of the 20th century) is characterized by the introduction into production of four methods of obtaining steel - Bessemer, Thomas, open-hearth, converter and electric steelmaking, which, by the way, we would like to talk about in our abstract, as an example of the use of electricity by a steelmaker's henchmen .

Chapter 1

We connect the wires of an electric light bulb with an electric battery. Wires, a light bulb filament formed a closed circuit - an electrical circuit. In this circuit, an electric current flows, which heats up the filament of the lamp until it glows. What is electric current? This is the directed movement of charged particles.

Chemical reactions take place in the battery, as a result of which electrons accumulate at the terminal marked with the “-” (minus) sign - particles of matter that have the smallest charge. The metal from which the wires and filament of the light bulb are made consists of atoms that form a crystal lattice. Electrons can freely pass through this lattice. The flow of electrons through conductors (the so-called substances that pass electric current) from one terminal of the battery to another - this is the electric current. The more electrons pass through the conductor, the greater the force electric current. Measure the current in amperes (A). If a current of 1 A flows through the conductor, then 6.24 * 1018 electrons fly through the cross section of the conductor every second. This number of electrons carries a charge of 1 C (coulomb).

Electric current in a circuit formed by wires, a lamp filament and a battery can be compared to the flow of liquid moving through water pipes. Connecting wires are sections of a pipe with a large cross section, a light bulb filament is a thin tube, and a battery is a pump that creates pressure. The greater the pressure, the greater the fluid flow. A battery in an electrical circuit creates a voltage (pressure). The higher the voltage, the more current in the circuit. Voltage is measured in volts (V). in order to pass a current through a pocket flashlight bulb that would make its thread glow, a voltage of 3-4 V is needed. Electrical energy is supplied to the apartments of houses at a voltage of 127 or 220 V, and the current is transmitted through power lines (power lines) under a voltage of hundreds of kilovolts ( kV). The electrical energy that is released in 1 s (power) is equal to the product of the current strength and voltage. Power at a current of 1 A and a voltage of 1 V is equal to 1 watt (W).

Not all substances freely pass electric current, for example, glass, porcelain, rubber almost do not pass electric current. Such substances are called insulators or dielectrics. Conductors are insulated with rubber, insulators for high-voltage power lines are made of glass and porcelain. However, even metals resist electric current. When moving, electrons "push" the atoms that make up the metal, make them move faster - they heat the conductor. The heating of conductors by electric current was first studied by the Russian scientist E. H. Lenz and the English physicist D. Joule. The property of electric current to heat conductors is widely used in engineering. Electric current glows the filaments of electric lamps and electric heaters, melts steel in electric furnaces.

In 1820, the Danish physicist G.-H. Oersted discovered that a magnetic needle deviates near a current-carrying conductor. Thus, the remarkable property of electric current to create a magnetic field was discovered. This phenomenon was studied in detail by the French scientist A. Ampère. He found that two parallel wires carrying current in the same direction attract each other, and if the directions of the currents are opposite, the wires repel. Ampère explained this phenomenon by the interaction of magnetic fields, which are created by currents. The effect of the interaction of wires with current and magnetic fields is used in electric motors, in electrical relays and in many electrical measuring instruments.

Another property of electric current can be detected by passing current through an electrolyte - a solution of salt, acid or alkali. In electrolytes, the molecules of a substance are split into ions - particles of molecules with positive or negative charges. The current in the electrolyte is the movement of ions. To pass current through the electrolyte, two metal plates connected to a current source are lowered into it. The positive ions move towards the electrode connected to the negative terminal. Ions are created at the electrodes. This process is called electrolysis. With the help of electrolysis, it is possible to isolate pure metals from salts, chrome and nickel plating various objects, perform the most complex processing of products that cannot be done on simple metal-cutting machines, and separate water into its constituent parts - hydrogen and oxygen.

In electrolysis baths, in a light bulb connected to a flashlight battery, the current flows all the time in one direction and the current strength does not change. This current is called direct current. However, in technology, alternating current is more often used, the direction and strength of which periodically change. The time of a complete cycle of changing the direction of the current is called a period, and the number of periods in 1 s is called the frequency of the alternating current. Industrial current, which drives machines, illuminates streets and apartments, changes with a frequency of 50 periods in 1 s. Alternating current can be easily transformed - increase and decrease its voltage using transformers.

With the invention of the telegraph and telephone, electric current is used to transmit information. At first, long and short pulses of direct current were transmitted along the wires, corresponding to the dots and dashes of Morse code. Such current pulses, or pulsating current, but with a more complex information coding system, are used in modern electronic computers (computers) to transfer numbers, commands and words from one machine device to another.

Alternating current can also be used to transmit information. Information can be transmitted by alternating current by changing the amplitude of current oscillations in a certain way. This encoding of information is called amplitude modulation (AM). It is also possible to change the frequency of alternating current oscillations so that certain information corresponds to a certain change in frequency. This encoding is called frequency modulation (FM). Radio receivers have AM and FM channels that "decode" - turn into sound - amplitude or frequency modulated oscillations of radio waves received by the antenna.

In our time, electric current has found application in all spheres of human activity. The drive of machine tools and machines, automatic control and management systems, numerous devices of research laboratories and household appliances are inconceivable without the use of electric current. Modern telephone and telegraph, radio and television, electronic computers from pocket calculators to flight control machines spaceships, - these are all devices based on the most complex electric current circuits.

Chapter 2. Electric Power Generation

.1 Alternator

Electrical energy has undeniable advantages over all other forms of energy. It can be transmitted over wires over long distances with relatively low losses and conveniently distributed among consumers. The main thing is that this energy can be easily converted into any other forms with the help of fairly simple devices: mechanical, internal (heating of bodies), light energy, etc.

Alternating current has the advantage over direct current that voltage and current strength can be converted (transformed) over a very wide range with almost no energy loss. Such transformations are necessary in many electrical and radio engineering devices. But a particularly great need for voltage and current transformation arises when transmitting electricity over long distances.

Electric current is generated in generators - devices that convert energy of one form or another into electrical energy. Generators include galvanic cells, electrostatic machines, thermobatteries, solar panels, etc. The possibilities of creating fundamentally new types of generators are being explored. For example, so-called fuel energies are being developed, in which the energy released as a result of the reaction of hydrogen with oxygen is directly converted into electrical energy. Successful work is underway to create magnetohydrodynamic generators (MHD generators). In MHD generators, the mechanical energy of a jet of hot ionized gas (plasma) moving in a magnetic field is directly converted into electrical energy.

The scope of each of the listed types of electric power generators is determined by their characteristics. So, electrostatic machines create a high potential difference, but are not able to create any significant current in the circuit. Galvanic cells can give a large current, but the duration of their action is not great.

The predominant role in our time is played by electromechanical induction alternators. These generators convert mechanical energy into electrical energy. Their action is based on the phenomenon of electromagnetic induction. Such generators have a relatively simple device and make it possible to obtain large currents at a sufficiently high voltage.

In the future, speaking of generators, we will mean precisely induction electromechanical generators.

There are many different types of induction generators available today. But they all consist of the same basic parts. This is, firstly, an electromagnet or a permanent magnet that creates a magnetic field, and, secondly, a winding in which a variable EMF is induced (in the generator model considered, this is a rotating frame). Since the EMF induced in series-connected turns add up, the amplitude of the induction EMF in the frame is proportional to the number of turns in it. It is also proportional to the amplitude of the alternating magnetic flux Фm = BS through each turn.

To obtain a large magnetic flux in generators, a special magnetic system is used, consisting of two cores made of electrical steel. The windings that create a magnetic field are placed in the grooves of one of the cores, and the windings in which the EMF is induced are placed in the grooves of the other. One of the cores (usually internal), together with its winding, rotates around a horizontal or vertical axis. Therefore, it is called a rotor (or armature). A fixed core with its winding is called a stator (or inductor). The gap between the stator and rotor cores is made as small as possible. This is provided highest value flux of magnetic induction.

In the generator model shown in Figure 19, a wire frame rotates, which is a rotor (though without an iron core). The magnetic field is created by a stationary permanent magnet. Of course, it would be possible to do the opposite - to rotate the magnet, and leave the frame motionless.

In large industrial generators, it is the electromagnet that rotates, which is the rotor, while the windings in which the EMF is induced are laid in the stator slots and remain motionless. The fact is that it is necessary to supply current to the rotor or divert it from the rotor winding to an external circuit using sliding contacts. To do this, the rotor is equipped with slip rings attached to the ends of its winding. The fixed plates - brushes - are pressed against the rings and connect the rotor winding with the external circuit. The strength of the current in the windings of an electromagnet that creates a magnetic field is much less than the strength of the current given by the generator to the external circuit. Therefore, it is more convenient to remove the generated current from the fixed windings, and to supply a relatively weak current through the sliding contacts to the rotating electromagnet. This current is generated by a separate DC generator (exciter) located on the same shaft.

In low-power generators, the magnetic field is created by a rotating permanent magnet. In this case, rings and brushes are not needed at all.

The appearance of EMF in the fixed stator windings is explained by the appearance of a vortex electric field in them, generated by a change in the magnetic flux during the rotation of the rotor.

If a flat frame rotates in a uniform magnetic field, then the period of the generated EMF is equal to the period of rotation of the frame. This is not always convenient. For example, to obtain an alternating current with a frequency of 50 Hz, the frame must make 50 revolutions / s in a uniform magnetic field, i.e. 3000 rpm The same rotational speed will be required in the case of rotation of a two-pole permanent magnet or a two-pole electromagnet. Indeed, the period of change of the magnetic flux penetrating the turns of the stator winding should be equal to 1/50 s. To do this, each of the rotor poles must pass the turns 50 times per second. The rotation speed can be reduced if an electromagnet with 2, 3, 4 ... pairs of poles is used as a rotor. Then the period of the generated current will correspond to the time required to rotate the rotor by 1/2, 1/3, 1/4 ... of the circle, respectively. Therefore, the rotor can be rotated 2, 3, 4 ... times slower. This is important when the generator is driven by low speed motors such as hydraulic turbines. So, the rotors of the generators of the Uglich HPP on the Volga make 62.5 rpm and have 48 pairs of poles.

2.2 MHD generator

Thermal power plants (TPPs) form the basis of modern energy. The operation of a thermal power plant is based on the conversion of thermal energy released during the combustion of fossil fuels, first into the mechanical energy of rotation of the shaft of a steam or gas turbine, and then with the help of an electric generator into electrical energy. As a result of such a double conversion, a lot of energy is wasted - it is released in the form of heat into the air, it is spent on heating equipment, etc.

Is it possible to reduce these involuntary expenditures of energy, to shorten the process of energy conversion, to exclude intermediate stages of energy conversion? It turns out you can. One of the power plants that convert the energy of a moving electrically conductive liquid or gas directly into electrical energy is a magnetohydrodynamic generator, or MHD generator for short.

As in conventional electric generators, the MHD generator is based on the phenomenon of electromagnetic induction: an electric current arises in a conductor crossing the magnetic field lines. In an MHD generator, such a conductor is the so-called working fluid - a liquid, gas or liquid metal with high electrical conductivity. Usually, MHD generators use an incandescent ionized gas, or plasma. When the plasma moves across the magnetic field, oppositely directed flows of charge carriers appear in it - free electrons and positive ions.

The MHD generator consists of a channel through which the plasma moves, an electromagnet to create a magnetic field, and electrodes that squeeze charge carriers. As a result, a potential difference arises between oppositely located electrodes, which causes an electric current in the external circuit connected to them. Thus, in the MHD generator, the energy of the moving plasma is converted directly into electrical energy, without any intermediate transformations.

The main advantage of an MHD generator compared to conventional electromagnetic generators is the absence of moving mechanical components and parts in it, such as, for example, in a turbo or hydro generator. This circumstance makes it possible to significantly increase the initial temperature of the working fluid, and, consequently, the efficiency of the generator.

The first experimental MHD generator with a power of only 11.5 kW was built in 1959 in the USA. In 1965, the first Soviet MHD generator was investigated in the USSR, and in 1971 a pilot plant was launched - a kind of power plant with an MHD generator with a capacity of 25 MW. Such power plants can be used, for example, as backup or emergency sources of electricity, as well as power supplies for such devices that require a significant consumption of electricity in a short period of time.

2.3 Plasma generator - plasma torch

If a solid is heated strongly, it will turn into a liquid. If you raise the temperature even higher, the liquid will evaporate and turn into a gas.

But what happens if you continue to increase the temperature? Atoms of matter will begin to lose their electrons, turning into positive ions. Instead of gas, a gaseous mixture is formed, consisting of freely moving electrons, ions, and neutral atoms. It's called plasma.

Nowadays, plasma is widely used in various fields of science and technology: for the heat treatment of metals, the application of various coatings on them, melting and other metallurgical operations. In recent years, plasma has been widely used by chemists. They found that the speed and efficiency of many chemical reactions greatly increases in a plasma jet. For example, by introducing methane into a hydrogen plasma jet, it can be converted into very valuable acetylene. Or arrange oil vapors into a number of organic compounds - ethylene, propylene and others, which later serve as an important raw material for the production of various polymeric materials.

Scheme of a plasma generator - plasma torch

plasma jet;

arc discharge;

Gas "spin" channels;

Refractory metal cathode;

plasma gas;

Electrode holder;

discharge chamber;

Solenoid;

Copper anode.

How to create plasma? For this purpose, a plasma torch, or a plasma generator, serves.

If you place metal electrodes in a vessel with gas and apply a high voltage to them, an electric discharge will occur. There are always free electrons in a gas. Under the action of an electric current, they accelerate and, colliding with neutral gas atoms, knock out electrons from them and form electrically charged particles - ions, i.e. ionize atoms. The released electrons are also accelerated by the electric field and ionize new atoms, further increasing the number of free electrons and ions. The process develops like an avalanche, the atoms of the substance are very quickly ionized and the substance turns into plasma.

This process takes place in an arc plasma torch. A high voltage is created in it between the cathode and the anode, which can be, for example, a metal that needs to be processed using plasma. In the space of the discharge chamber, a plasma-forming substance is most often supplied with gas - air, nitrogen, argon, hydrogen, methane, oxygen, etc. Under the action of high voltage, a discharge occurs in the gas, and a plasma arc is formed between the cathode and anode. To avoid overheating of the walls of the discharge chamber, they are cooled with water. Devices of this type are called plasma torches with an external plasma arc. They are used for cutting, welding, melting metals, etc.

The plasma torch for creating a plasma jet is arranged somewhat differently. The plasma-forming gas is blown through a system of spiral channels at high speed and “ignited” in the space between the cathode and the walls of the discharge chamber, which are the anode. Plasma, twisted into a dense jet due to spiral channels, is ejected from the nozzle, and its speed can reach from 1 to 10,000 m/s. The magnetic field, which is created by the inductor, helps to “squeeze” the plasma from the walls of the chamber and make its jet denser. The temperature of the plasma jet at the outlet of the nozzle is from 3000 to 25000 K.

Look again at this drawing. Does it remind you of something well known?

Of course, it's a jet engine. The thrust force in a jet engine is created by a jet of hot gases ejected at high speed from a nozzle. The greater the speed, the greater the traction force. What's wrong with plasma? The speed of the jet is quite suitable - up to 10 km / s. And with the help of special electric fields, plasma can be accelerated even more - up to 100 km / s. This is about 100 times the speed of gases in existing jet engines. This means that the thrust of plasma or electric jet engines can be greater, and fuel consumption can be significantly reduced. The first samples of plasma engines have already been tested in space.

Chapter 3. Power transmission

.1 Power lines

Electric energy compares favorably with all types of energy in that its powerful flows can be transmitted almost instantly over thousands of kilometers. The "channels" of energy rivers are power transmission lines (TL) - the main links of energy systems.

Currently, two types of power lines are being built: overhead, which carry current through wires above the ground, and underground, which transmit current through power cables, laid, as a rule, in trenches underground.

Power transmission lines consist of supports - concrete or metal, to the shoulders of which garlands of porcelain or glass insulators are attached. Copper, aluminum or steel-aluminum wires are stretched between the supports, which are suspended from insulators. Power transmission towers walk through deserts and taiga, climb high into mountains, cross rivers and mountain gorges.

Air serves as an insulator between the wires. Therefore, the higher the tension, the greater the distance should be between the wires. Power lines also pass through the fields, next to settlements. Therefore, the wires must be suspended at a height that is safe for people. The properties of air as an insulator depend on climate and meteorological conditions. Power line builders must take into account the strength of the prevailing winds, differences in summer and winter temperatures, and much more. That is why the construction of each new transmission line requires the serious work of prospectors for the best route, scientific research, modeling, the most complex engineering calculations, and even the high skill of builders.

The simultaneous creation of powerful power stations and electrical networks was provided for in the GOERLO plan. When electricity is transmitted over wires over a distance, energy losses are inevitable, because, passing through the wires, the electric current heats them up. Therefore, it is unprofitable to transmit low voltage current, 127 - 220 V, as it enters our apartments, over a distance of more than 2 km. To reduce losses in the wires, the voltage of the electric current, before being fed to the line, is increased at electric step-up substations. With an increase in the power of power plants, the expansion of territories covered by electrification, the alternating current voltage on transmission lines is successively increased to 220, 380, 500 and 750 kV. A power transmission line with a voltage of 1150 kV was built to connect the power systems of Siberia, Northern Kazakhstan and the Urals. There are no such lines in any country in the world: the height of the supports is up to 45 m (the height of a 15-storey building), the distance between the wires of each of the three phases is 23 m.

However, wires under high voltage are life-threatening, and it is impossible to lead them to houses, factories and plants. That is why, before transmitting electricity to the consumer, the high voltage current is reduced at step-down substations.

The AC transmission scheme is as follows. The low voltage current generated by the generator is fed to the step-up substation transformer, converted in it into a high voltage current, then along the power line it goes to the place of energy consumption, here it is converted by the transformer into a low voltage current, and then it goes to consumers.

Our country is the ancestor of another type of power lines - direct current lines. It is more profitable to transmit direct current over power lines than alternating current, since if the line length exceeds 1.5-2 thousand km, then the loss of electricity during the transmission of direct current will be less. Before introducing current into consumer homes, it is again converted to alternating.

In order to introduce high voltage current into cities and distribute it to electric step-down substations, cable power lines are laid underground. Experts believe that in the future overhead power lines will generally give way to cable ones. Overhead lines have a drawback: an electric field is created around high-voltage wires that exceeds the Earth's magnetic field. And this adversely affects the human body. This may pose an even greater danger in the future, when the voltage and current transmitted through power lines will increase even more. Already now, in order to avoid undesirable consequences, it is necessary to create “right-of-way” around power lines, where it is forbidden to build anything.

A cable line simulating future superconducting power lines has been tested. Inside a metal pipe, covered with several layers of the most perfect thermal insulation, a copper core is laid, consisting of many conductors, each of which is covered with a film of niobium. Real cosmic cold is maintained inside the pipe - a temperature of 4.2 K. At this temperature, there are no losses of electricity due to resistance.

To transmit electricity, scientists have developed gas-filled lines (GIL). GIL is a metal pipe filled with gas - sulfur hexafluoride. This gas is an excellent insulator. Calculations show that with increased gas pressure, electric current with a voltage of up to 500 kV can be transmitted through wires laid inside the pipe.

Cable power lines laid underground will save hundreds of thousands of hectares of precious land, especially in large cities.

As we have already said, such a transmission of electricity is associated with noticeable losses. The fact is that electric current heats the wires of power lines. In accordance with the Joule-Lenz law, the energy spent on heating the wires of the line is determined by the formula

Q=I 2Rt

where R is the line resistance. With very long lines, power transmission can become uneconomical. It is practically very difficult to significantly reduce the line resistance. Therefore, it is necessary to reduce the current strength.

Since the current power is proportional to the product of the current strength and voltage, in order to maintain the transmitted power, it is necessary to increase the voltage in the transmission line. Moreover, the longer the transmission line, the more profitable it is to use a higher voltage. So, in the high-voltage transmission line Volzhskaya HPP - Moscow, a voltage of 500 kV is used. Meanwhile, alternating current generators are built for voltages not exceeding 16-20 kV. Higher voltage would require complicated special measures to isolate the windings and other parts of the generators.

Therefore, step-up transformers are installed at large power plants. The transformer increases the voltage in the line as much as it reduces the current.

For the direct use of electricity in the motors of the electric motor of machine tools, in the lighting network and for other purposes, the voltage at the ends of the line must be reduced. This is achieved using step-down transformers.

Usually, a decrease in voltage and, accordingly, an increase in current strength takes place in several stages. At each stage, the voltage is getting smaller, and the area covered by the electrical network is getting wider (Fig. 4).

At a very high voltage between the wires, a corona discharge begins, leading to energy losses. Permissible amplitude AC voltage should be such that for a given cross-sectional area of ​​the wire, the energy loss due to corona discharge is negligible.

Power stations in a number of regions of the country are connected by high-voltage transmission lines, forming a common electrical network to which consumers are connected. Such a combination, called the power system, makes it possible to smooth out the "peak" loads of energy consumption in the morning and evening hours. The power system ensures uninterrupted power supply to consumers, regardless of their location. Now almost the entire territory of the country is provided with electricity by the integrated energy systems.

The loss of 1% of electricity per day for our country brings a loss of about half a million rubles.

3.2 Transformer

Alternating current compares favorably with direct current in that it is relatively easy to change its strength. Devices that convert alternating current of one voltage into alternating current of another voltage are called electrical transformers (from the Latin word "transformo" - "I will transform"). The transformer was invented by Russian electrical engineer P. N. Yablochkin in 1876.

The transformer consists of several coils (windings) wound on a frame with insulated wire, which are placed on a core made of thin plates of special steel.

An alternating electric current flowing through one of the windings, called the primary, creates an alternating magnetic field around it and in the core, crossing the turns of the other - secondary - winding of the transformer, exciting an alternating electromotive force in it. It is enough to connect an incandescent lamp to the terminals of the secondary winding, as an alternating current will flow in the resulting closed circuit. Thus, electrical energy is transferred from one winding of the transformer to another without their direct connection, only due to the connecting winding of the alternating magnetic field.

If both windings have a different number of turns, then the same voltage will be induced in the secondary winding as is brought to the primary. For example, if an alternating current of 220 V is applied to the primary winding of the transformer, then a current of 220 V will appear in the secondary winding. If the windings are different, then the voltage in the secondary winding will not be equal to the voltage supplied to the primary winding. In a step-up transformer, i.e. in a transformer that increases the voltage of an electric current, the secondary winding contains more turns than the primary, therefore the voltage on it is greater than on the primary. In a step-down transformer, on the contrary, the secondary winding contains fewer turns than the primary, and therefore the voltage across it is less.

Transformers are widely used in industry and everyday life. Power electrical transformers make it possible to transmit alternating current through power lines over a long distance with low energy losses. To do this, the voltage of the alternating current generated by the generators of the power plant is increased by means of transformers to a voltage of several hundred thousand volts and sent through power lines in various directions. At the place of energy consumption, at a distance of many kilometers from the power plant, this voltage is lowered by transformers.

Power transformers get very hot during operation. To reduce the heating of the core and windings, transformers are placed in special tanks with mineral oil. An electrical transformer equipped with such a cooling system has very impressive dimensions: its height reaches several meters, and its weight is hundreds of tons. In addition to such transformers, there are also dwarf transformers that work in radios, televisions, tape recorders, and telephones. With the help of such transformers, several voltages are obtained that feed different circuits of the device, they relay signals from one electrical circuit to another, from cascade to cascade, and separate electrical circuits.

As we have already said, the transformer consists of a closed steel core, on which two (sometimes more) coils with wire windings are put on (Fig. 5). One of the windings, called the primary, is connected to an AC voltage source. The second winding, to which the "load" is connected, i.e. devices and devices that consume electricity is called secondary. The diagram of the device of a transformer with two windings is shown in Figure 6.

The action of the transformer is based on the phenomenon of electromagnetic induction. When an alternating current passes through the primary winding, an alternating magnetic flux appears in the core, which excites the induction EMF in each winding. The core made of transformer steel concentrates the magnetic field, so that the magnetic flux exists practically only inside the core and is the same in all its sections.

The instantaneous value of the induction emf e in any turn of the primary or secondary winding is the same. According to Faraday's law, it is determined by the formula

e \u003d - F,

where Ф is the derivative of the magnetic induction flux with respect to time. If

F=F m cos wt, then

Hence,

e = wФ m sin wt,

e = E m sin wt,

where E m = wФ m - EMF amplitude in one turn.

If a circuit that consumes electricity is connected to the ends of the secondary winding, or, as they say, a transformer is loaded, then the current in the secondary winding will no longer be zero. The resulting current according to Lenz's rule should reduce the changes in the magnetic field in the core.

But a decrease in the amplitude of oscillations of the resulting magnetic flux should, in turn, reduce the induction EMF in the primary winding. However, this is impossible, because according to u 1~ e 1. therefore, when the circuit of the secondary winding is closed, the current in the primary winding automatically increases. Its amplitude increases in such a way as to restore the previous value of the oscillation amplitude of the resulting magnetic flux.

The increase in the current strength in the primary winding circuit occurs in accordance with the law of conservation of energy: the return of electricity to the circuit connected to the secondary winding of the transformer is accompanied by the consumption of the same energy from the network by the primary winding. The power in the primary circuit at a transformer load close to the nominal one is approximately equal to the power in the secondary circuit: U 1I 1~ U 2I 2.

This means that by increasing the voltage several times with the help of a transformer, we reduce the current by the same amount (and vice versa).

In modern high-power transformers, the total energy losses do not exceed 2-3%.

In order for the transmission of electrical energy to be economically profitable, it is necessary to make the heating losses of the wires as small as possible. This is achieved by the fact that the transmission of electricity over long distances is carried out at high voltage. The fact is that with an increase in voltage, the same energy can be transmitted at a lower current strength, this leads to a decrease in the heating of the wires, and, consequently, a decrease in energy losses. In practice, when transmitting energy, voltages of 110, 220, 380, 500, 750 and 1150 kV are used. The longer the transmission line, the higher the voltage used in it.

Alternators give a voltage of several kilovolts. The restructuring of generators for higher voltages is difficult - in these cases, a particularly high quality of insulation of all parts of the generator under current would be required. Therefore, when transmitting energy over long distances, it is necessary to increase the voltage using transformers installed at step-up substations.

Scheme of operation of electrical substations: step-up, converter (traction), step-down.

The transformed high voltage is transmitted through power lines to the place of consumption. But the consumer does not need high voltage. It needs to be lowered. This is achieved at step-down substations.

Step-down substations are subdivided into district, main step-down and local substations. District departments receive electricity directly from high-voltage power lines, lower the voltage and transfer it to the main step-down substations, where the voltage drops to 6.10 or 35 kV. From the main substations, electricity is supplied to local ones, where the voltage drops to 500, 380, 220V and is distributed to industrial enterprises and residential buildings.

Sometimes there is also a converter substation behind the step-up substation, where the alternating electric current is converted into direct current. This is where the rectification takes place. Direct current is transmitted over power lines over long distances. At the end of the line at the same substation, it is again converted (inverted) into alternating current, which is fed to the main step-down substations. To supply electrified vehicles and industrial installations with direct current, converter substations (in transport they are called traction) are built next to the main step-down and local substations.

electric current transformer generator

Chapter 4

.1 Steel production in electric furnaces

An electric furnace is a unit in which the heat obtained by converting electrical energy into thermal energy is transferred to the melted material. According to the method of converting electrical energy into thermal energy, electric furnaces are divided into the following groups:

) arc, in which electricity is converted into heat in the arc;

) resistance furnaces, in which heat is generated in special elements or raw materials as a result of the passage of an electric current through them;

) combined, operating simultaneously as arc furnaces and as resistance furnaces (ore-thermal furnaces);

) induction, in which the metal is heated by vortex flows excited in it by electromagnetic induction;

) electron-beam, in which with the help of an electric current in a vacuum a strictly directed flow of electrons is created, bombarding and melting the starting materials;

) plasma, in which the heating and melting of the metal are carried out by low-temperature plasma.

In an electric furnace, it is possible to obtain alloyed steel with a low content of sulfur and phosphorus, non-metallic inclusions, while the loss of alloying elements is much less. In the process of electric smelting, it is possible to accurately control the temperature of the metal and its composition, to melt alloys of almost any composition.

Electric furnaces have significant advantages over other steel-smelting units, therefore, high-alloy tool alloys, stainless ball-bearing, heat-resistant and heat-resistant, as well as many structural steels, are smelted only in these furnaces. Powerful electric furnaces are successfully used to produce low-alloy and high-carbon open-hearth steels. In addition, various ferroalloys are obtained in electric furnaces, which are iron alloys with elements that must be removed into steel for alloying and deoxidation.

The device of electric arc furnaces.

The first electric arc furnace in Russia was installed in 1910 at the Obukhov plant. During the years of the five-year plans, hundreds of different furnaces were built. The capacity of the largest furnace in the USSR is 200 tons. The furnace consists of a cylindrical iron casing with a spherical bottom. Inside the casing has a refractory lining. The melting space of the furnace is closed with a removable vault.

The furnace has a working window and an outlet with a drain chute. The furnace is powered by three-phase alternating current. Heating and melting of the metal is carried out by electric powerful arcs burning between the ends of the three electrodes and the metal in the furnace. The furnace rests on two supporting sectors rolling over the frame. The inclination of the furnace towards the outlet and the working window is carried out using a rack and pinion mechanism. Before loading the furnace, the arch, suspended on chains, is raised to the portal, then the portal with the arch and electrodes is turned towards the drain chute and the furnace is loaded with a bucket.

Mechanical equipment of the arc furnace.

The furnace shell must withstand the load from the mass of refractories and metal. It is made welded from sheet iron with a thickness of 16-50 mm, depending on the size of the furnace. The shape of the shell determines the profile of the working space of the electric arc furnace. The most common at present is the conical casing. The lower part of the casing has the shape of a cylinder, the upper part is cone-shaped with an expansion upwards. This shape of the casing makes it easier to fill the furnace with refractory material, sloping walls increase the durability of the masonry, since it is further away from electric arcs. Cylindrical casings with water-cooled panels are also used. To maintain the correct cylindrical shape, the casing is reinforced with stiffeners and rings. The bottom of the casing is usually made spherical, which ensures the greatest strength of the casing and the minimum mass of the masonry. The bottom is made of non-magnetic steel for installation under the furnace of an electromagnetic mixing device.

From above the furnace is closed by a vault. The vault is made of refractory bricks in a metal water-cooled vault ring, which withstands the bursting forces of the arched spherical vault. V brickwork vault leave three holes for the electrodes. The diameter of the holes is larger than the diameter of the electrode, therefore, during melting, hot gases rush into the gap, which destroy the electrode and carry heat out of the furnace. To prevent this, refrigerators or economizers are installed on the vault, which serve to seal the electrode holes and cool the masonry of the vault. Gas dynamic economizers provide sealing with an air curtain around the electrode. The roof also has an opening for suction of dusty gases and an opening for an oxygen lance.

There is a loading window, framed by a cast frame, for loading the charge in a small-capacity furnace and loading alloying and fluxes into large furnaces for slag downloading, inspection, filling and repair of the furnace. Guides are attached to the frame along which the damper slides. The damper is lined with refractory bricks. To lift the damper, a pneumatic, hydraulic or electromechanical actuator is used.

On the opposite side of the casing has a window for the release of steel from the furnace. A drain chute is welded to the window. The hole for the release of steel can be round with a diameter of 120-150 mm or square 150 by 250 mm. The drain gutter has a trough-shaped section and is welded to the casing at an angle of 10-12° to the horizontal. From the inside, the gutter is lined with fireclay bricks, its length is 1-2 m.

The electrode holders are used to supply current to the electrodes and to clamp the electrodes. The heads of the electrode holders are made of bronze or steel and cooled with water, as they are strongly heated both by heat from the furnace and by contact currents. The electrode holder should tightly clamp the electrode and have a small contact resistance. The most common at present is the spring-pneumatic electrode holder. The clamping of the electrode is carried out using a fixed ring and a clamping plate, which is pressed against the electrode by a spring. The compression of the plate from the electrode and the compression of the spring occur with the help of compressed air. The electrode holder is mounted on a metal sleeve - a console, which is fastened with an L-shaped movable stand into one rigid structure. The rack can move up or down inside the fixed box rack. Three fixed racks are rigidly connected into one common structure, which rests on the platform of the furnace support cradle.

The movement of mobile telescopic racks occurs either with the help of a system of cables and counterweights driven by electric motors, or with the help of hydraulic devices. Mechanisms for moving the electrodes must ensure the rapid rise of the electrodes in the event of a collapse of the charge during the melting process, as well as the smooth lowering of the electrodes to prevent them from sinking into the metal or hitting unmelted pieces of the charge. The electrode lifting speed is 2.5-6.0 m/min, the lowering speed is 1.0-2.0 m/min.

The tilting mechanism of the furnace should smoothly tilt the furnace towards the outlet at an angle of 40-45° for the release of steel and at an angle of 10-15 degrees towards the working window for the descent of slag. The furnace bed, or cradle, on which the body is installed, rests on two to four support sectors, which roll along horizontal guides. There are holes in the sectors, and teeth in the guides, with the help of which the sectors are prevented from slipping when the oven is tilted. The inclination of the furnace is carried out using a rack and gear mechanism or hydraulically driven. Two cylinders are fixed on the fixed supports of the foundation, and the rods are pivotally connected to the supporting sectors of the furnace cradle.

There are two types of kiln loading system: through the filling window with a mulled filling machine and through the top using a bucket. Loading through the window is used only on small furnaces. When loading the furnace from above in one or two steps within 5 minutes, the lining is cooled less, the melting time is reduced; the consumption of electricity is reduced; more efficient use of the volume of the furnace. To load the furnace, the roof is raised by 150-200 mm above the furnace casing and turned to the side along with the electrodes, completely opening the working space of the furnace for introducing a bucket with charge. The arch of the furnace is suspended from the frame. It is connected to the fixed racks of the electrode holders into one rigid structure resting on a rotary console, which is mounted on a support bearing. Large furnaces have a rotary tower, in which all the mechanisms of the vault are concentrated. The tower rotates around the hinge on the rollers along an arcuate rail.

The bucket is a steel cylinder, the diameter of which is less than the diameter of the working space of the furnace. From the bottom of the cylinder there are movable flexible sectors, the ends of which are pulled together through the rings by a cable. The weighing and loading of the charge is carried out at the charge yard of the electric steel-smelting shop. The bucket on a trolley is fed into the workshop, lifted by a crane and lowered into the furnace.

With the help of an auxiliary lifting of the crane, the cable is pulled out of the eyes of the sectors, and when lifting the buckets of the sector, they open, and the charge falls into the furnace in the order in which it was placed in the bucket. When using metallized pellets as a charge, loading can be carried out continuously through a pipeline that passes into an opening in the roof of the furnace. During melting, the electrodes cut three wells in the charge, at the bottom of which liquid metal accumulates. To accelerate the melting, the furnaces are equipped with a rotary device that rotates the body in one direction and the other by an angle of 80 °. At the same time, nine wells are already cut through the electrodes in the charge. To rotate the housing, the arch is raised, the electrodes are raised above the level of the charge, and the housing is rotated using a ring gear attached to the housing and gears. The furnace body rests on rollers.

Purification of exhaust gases.

Modern large steel-smelting arc furnaces during operation emit a large amount of dusty gases into the atmosphere. The use of oxygen and powdered materials further contributes to this.

The content of dust in the gases of electric arc furnaces reaches 10 g/m^3 and significantly exceeds the norm. To trap dust, gases are sucked out of the working space of the furnaces with a powerful fan. To do this, a fourth hole is made in the roof of the furnace with a pipe for a gas exhaust. The branch pipe through a gap that allows you to tilt or rotate the furnace, approaches the stationary pipeline. Along the way, the gases are diluted with air necessary for the afterburning of CO. The gases are then cooled by water jets in a heat exchanger and sent to a system of venturi pipes where the dust is retained as a result of humidification. Fabric filters, disintegrators and electrostatic precipitators are also used. Gas cleaning systems are used, including the entire electric steelmaking shop, with the installation of smoke exhaust hoods under the roof of the shop above the electric furnaces.

Furnace lining.

Most arc furnaces have a main lining made of MgO based materials. The furnace lining creates a bath for the metal and plays the role of a heat-insulating layer that reduces heat loss. The main parts of the lining are the hearth of the furnace, the walls, the arch. The temperature in the zone of electric arcs reaches several thousand degrees. Although the lining of the electric furnace is separated from the arcs, it must still be able to withstand temperatures up to 1700°C. In this regard, the materials used for lining must have high refractoriness, mechanical strength, thermal and chemical resistance. The hearth of the steel-smelting furnace is recruited in the following order. Sheet asbestos is laid on the steel casing, on the asbestos layer of fireclay powder, two layers of fireclay bricks and the main layer of magnesite bricks. On a magnesite brick hearth, a working layer of magnesite powder is stuffed with resin and pitch - a product of oil refining. The thickness of the printed layer is 200 mm. The total thickness of the hearth is approximately equal to the depth of the bath and can reach 1 m for large furnaces. The walls of the furnace are laid out after the appropriate laying of asbestos and fireclay bricks from large-sized unfired magnesite-chromite bricks up to 430 mm long. Wall masonry can be made of bricks in iron cassettes, which ensure the welding of bricks into one monolithic block. The resistance of the walls reaches 100-150 melts. The durability of the hearth is one to two years. The furnace roof lining works in difficult conditions. It withstands high thermal loads from burning arcs and heat reflected by slag. The vaults of large furnaces are made of magnesite-chromite bricks. When typing the vault, normal and shaped bricks are used. In cross section, the vault has the shape of an arch, which ensures tight adhesion of bricks to each other. The firmness of the vault is 50 - 100 heats. It depends on the electric mode of melting, on the duration of stay in the furnace of liquid metal, the composition of the steel and slag being smelted. Currently, water-cooled vaults and wall panels are widely used. These elements facilitate lining service.

Current is supplied to the melting space of the furnace through electrodes assembled from sections, each of which is a round billet with a diameter of 100 to 610 mm and a length of up to 1500 mm. In small electric furnaces, carbon electrodes are used, in large ones, graphitized ones. Graphite electrodes are made from low-ash carbon materials: petroleum coke, tar, pitch. The electrode mass is mixed and pressed, after which the raw workpiece is fired in gas furnaces at 1300 degrees and subjected to additional graphitizing firing at a temperature of 2600 - 2800 degrees in electric resistance furnaces. During operation, as a result of oxidation by furnace gases and spraying during arcing, the electrodes burn out.

As the electrode is shortened, it is lowered into the furnace. In this case, the electrode holder approaches the arch. There comes a point when the electrode becomes so short that it cannot sustain the arc and needs to be extended. To build up the electrodes, threaded holes are made at the ends of the sections, into which an adapter-nipple is screwed in, with the help of which the individual sections are connected. The consumption of electrodes is 5-9 kg per ton of smelted steel.

An electric arc is one of the types of electric discharge in which current passes through ionized gases, metal vapors. When the electrodes come close to each other for a short time, a short circuit occurs.

There is a large current flowing. The ends of the electrodes become white hot. When the electrodes are moved apart, an electric arc occurs between them. From the hot cathode, thermionic emission of electrons occurs, which, heading towards the anode, collide with neutral gas molecules and ionize them. Negative ions go to the anode, positive to the cathode. The space between the anode and cathode becomes ionized and conductive. The bombardment of the anode by electrons and ions causes its strong heating. The anode temperature can reach 4000 degrees. The arc can burn on direct and alternating current. Electric arc furnaces run on alternating current. Recently, a DC electric arc furnace has been built in Germany.

In the first half of the period, when the electrode is the cathode, the arc burns. When the polarity is reversed, when the charge - metal becomes the cathode, the arc goes out, since in the initial period of melting the metal is not yet heated and its temperature is insufficient for electron emission. Therefore, in the initial period of melting, the arc burns restlessly, intermittently. After the bath is covered with a layer of slag, the arc stabilizes and burns more evenly.

Electrical equipment.

The electrodes are used to supply current to the working space of the furnace and the formation of an electric arc. The electrodes can be carbon and graphite. In electric steelmaking, mainly graphite electrodes are used. Carbon electrodes are commonly used on small furnaces.

Electrical equipment of arc furnaces includes main current circuit equipment, control and measuring, protective and signal equipment, as well as an automatic regulator of the electrode movement mechanism, electric drives of the furnace mechanisms and an electromagnetic metal mixing plant.

The operating voltage of electric arc furnaces is 100 - 800 V, and the current strength is measured in tens of thousands of amperes. The power of a separate installation can reach 50 - 140 MVA*A. Electric arc furnace shop substation is supplied with voltage up to 110 kV. The primary windings of furnace transformers are fed with high voltage. The electrical equipment of the arc furnace includes the following devices:

Air disconnector designed to disconnect the entire electric furnace installation from the high voltage line during melting. The disconnector is not intended for switching current on and off, therefore, it can only be used with raised electrodes and no arcs. Structurally, the disconnector is a three-phase chopping type switch.

The main circuit breaker is used to disconnect under load an electrical circuit through which a high voltage current flows. When the charge is not packed tightly in the furnace at the beginning of melting, when the charge is still cold, the arcs burn unsteadily, the charge collapses and short circuits occur between the electrodes. In this case, the current strength increases sharply. This leads to large overloads of the transformer, which can fail. When the current exceeds the set limit, the switch automatically turns off the installation, for which there is a maximum current relay.

A furnace transformer is needed to convert high voltage to low voltage (from 6-10 kV to 100-800 V). The high and low voltage windings and the magnetic circuits on which they are placed are located in a tank with oil, which serves to cool the windings. Cooling is created by forced pumping of oil from the transformer casing to the heat exchanger tank, in which the oil is cooled by water. The transformer is installed next to the electric furnace in a special room. It has a device that allows you to switch the windings in steps and thus stepwise regulate the voltage supplied to the furnace. So, for example, a transformer for a 200-ton domestic furnace with a capacity of 65 MVA * A has 23 voltage steps, which are switched under load, without turning off the furnace.

The section of the electrical network from the transformer to the electrodes is called a short network. The feeders coming out of the wall of the transformer substation, using flexible, water-cooled cables, supply voltage to the electrode holder. The length of the flexible section should allow the required tilt of the furnace and turn off the roof for loading. Flexible cables are connected to water-cooled copper busbars mounted on the sleeves of the electrode holders. The tubes are directly connected to the head of the electrode holder, which clamps the electrode. In addition to the indicated main nodes of the electrical network, it includes various measuring equipment connected to the current lines through current or voltage transformers, as well as automatic control devices for the melting process.

Automatic regulation.

In the course of melting, various amounts of energy are required to be supplied to the electric arc furnace. You can change the power supply by changing the voltage or arc current. Voltage regulation is performed by switching the transformer windings. The current strength is controlled by changing the distance between the electrode and the charge by raising or lowering the electrodes. In this case, the arc voltage does not change. The lowering or raising of the electrodes is carried out automatically by means of automatic regulators installed on each phase of the furnace. In modern furnaces, a predetermined electrical mode program can be set for the entire melting period.

Device for electromagnetic stirring of metal.

To mix metal in large arc furnaces, to speed up and facilitate the technological operations of slag downloading, an electric winding is installed in the box under the bottom of the furnace, which is cooled by water or compressed air. The stator windings are fed from a two-phase generator with a low frequency current, which creates a traveling magnetic field that captures the pool of liquid metal and causes the lower layers of metal to move along the bottom of the furnace in the direction of the field. The upper layers of the metal, together with the slag adjacent to it, move in reverse side. Thus, it is possible to direct the movement either towards the working window, which will facilitate the exit of the slag from the furnace, or towards the drain hole, which will favor the uniform distribution of alloying and deoxidizing agents and the averaging of the metal composition and its temperature. This method has recently been of limited use, since metal is actively mixed with arcs in heavy-duty furnaces. Steel melting in the main electric arc furnace.

Raw materials.

The main material for electric smelting is steel scrap. Scrap should not be highly oxidized, since the presence of a large amount of rust introduces a significant amount of hydrogen into the steel. Depending on the chemical composition scrap must be sorted into appropriate groups. The main amount of scrap intended for melting in electric furnaces should be compact and heavy. With a small bulk mass of scrap, the entire portion for melting is not placed in the furnace. We have to interrupt the melting process and load the charge. This increases the duration of melting, leads to increased power consumption, and reduces the productivity of electric furnaces. Recently, metallized pellets obtained by the direct reduction method have been used in electric furnaces. The advantage of this type of raw material containing 85-93% iron is that it is not contaminated with copper and other impurities. It is advisable to use pellets for smelting high-strength structural alloy steels, electrical, ball-bearing steels.

Alloyed wastes are generated in the electric steel-smelting shop in the form of unfilled ingots, sprues; in the peeling department in the form of shavings, in rolling shops in the form of trim and scrap, etc.; in addition, a lot of alloyed scrap comes from machine-building plants. The use of alloyed metal waste allows you to save valuable alloying, increases the economic efficiency of electric melting. Soft iron is specially smelted in open-hearth furnaces and converters and is used to control the carbon content in the electric smelting process.

4.2 Typical receivers of electrical energy

The consumers of the group under consideration create a uniform and symmetrical load in all three phases. Load shocks occur only at start-up. The power factor is quite stable and usually has a value of 0.8-0.85. For the electric drive of large pumps, compressors and fans, synchronous motors operating with a leading power factor are most often used.

Hoisting and transport devices operate in intermittent mode. These devices are characterized by frequent load shocks. due to sudden changes in load, the power factor also varies significantly, on average from 0.3 to 0.8. In terms of uninterrupted power supply, these devices should be classified (depending on the place of work and installation) as consumers of the 1st and 2nd categories. In lifting and transport devices, both alternating (50 Hz) and direct current are used. In most cases, the load from the handling devices on the AC side should be considered symmetrical in all three phases.

Electrical lighting installations

Electric lamps are a single-phase load, however, due to the low power of the receiver (usually no more than 2 kW) in the electrical network, with the correct grouping of lighting devices, it is possible to achieve a fairly uniform load in phases (with asymmetry of no more than 5-10%).

The nature of the load is uniform, without shocks, but its value varies depending on the time of day, year and geographical location. The current frequency is common industrial, equal to 50 Hz. The power factor for incandescent lamps is 1, for gas discharge lamps 0.6. It should be borne in mind that higher harmonics of the current appear in wires, especially zero wires, when gas-discharge lamps are used.

Short-term (several seconds) emergency interruptions in the power supply of lighting installations are permissible. Long breaks (minutes and hours) in food for some types of production are unacceptable. In such cases, redundant power supply from a second current source is used (in some cases even from an independent DC source). In those industries where the shutdown of lighting threatens the safety of people, special emergency lighting systems are used. For lighting installations of industrial enterprises, voltages from 6 to 220 V are used.

Converter installations

To convert a three-phase current into a direct or three-phase current of an industrial frequency of 50 Hz into a three-phase or single-phase current of low, high or high frequency, converter stops are built on the territory of an industrial enterprise.

Depending on the type of current converters, converter stops are divided into:

) semiconductor converter installations;

) converter units with mercury rectifiers;

) converter units with motor-generators,

) converter stops with mechanical rectifiers.

According to their purpose, the converter installations will be folded for power

) engines of a number of machines and mechanisms;

) electrolysis baths;

) intra-factory electric transport;

) electrostatic precipitators;

) DC welding installations, etc.

Converter installations for electrolysis purposes are widely used in non-ferrous metallurgy to produce electrolytic aluminum, lead, copper, etc. In such installations, an industrial frequency current of 6-35 kV, as a rule, using silicon rectifiers, is converted into a direct current of the voltage required by the technological conditions ( up to 825 V).

A break in the power supply of electrolysis plants does not lead to severe accidents with damage to the main equipment and can be tolerated for several minutes, and in some cases for several hours. Here, a power interruption is mainly associated with underproduction. However, due to the back emf. electrolysis baths, in some cases, there may be a movement of the released metals back into the bath solution and, consequently, an additional cost of electricity for a new isolation of the same metal. Electrolysis plants must be supplied with electrical energy, like category 1 receivers, but allowing short-term power interruptions electrolysis plants gives a fairly uniform and phase-symmetrical load graph. The power factor of electrolysis plants is approximately 0.85-0.9.

Converter installations for intra-industrial electric transport (hauling, lifting, different kinds movement of goods, etc.) are relatively small in terms of power (from hundreds to 2000-3000 kW). The power factor of such installations ranges from 0.7-0.8. The load on the AC side is symmetrical in phases, but changes sharply due to current peaks during the operation of traction motors. An interruption in the power supply of the receivers of this group can lead to damage to products and even equipment (especially in metallurgical plants). Stopping the operation of transport in general causes serious complications in the operation of the enterprise, and therefore this group of consumers must be supplied with electricity, like receivers of the 1st or 2nd category, allowing a short interruption in power supply. These installations are powered by alternating current of industrial frequency with a voltage of 0.4-35 kV.

Converter installations for powering electrostatic precipitators (with mechanical rectifiers) up to 100-200 kW are widely used for gas purification. These installations are powered by alternating current of industrial frequency from special transformers with a voltage of 6-10 kV on the primary winding, and up to 110 kV on the secondary. Power factor of these settings is 0.7-0.8. The load on the high voltage side is symmetrical and uniform Power interruptions are acceptable, their duration depends on the technological process of production In industries such as chemical plants, these installations can be classified as category 1 and 2 receivers.

Electric motors of production mechanisms

This type of receiver is found on all industrial enterprises For the electric drive of modern machine tools, all types of engines are used. The power of motors is extremely diverse and varies from fractions to hundreds of kilowatts and more. In machine tools where high speeds of rotation and its regulation are required, DC motors powered by rectifiers are used. Mains voltage 660-380/220 V with a frequency of 50 Hz Power factor varies widely depending on the process safety conditions (possible injury to operating personnel) and due to possible damage to products, especially when processing large expensive parts.

Electric furnaces and electrothermal installations

According to the method of converting electrical energy into thermal energy, it can be divided into:

) resistance furnaces;

) induction furnaces and installations;

) electric arc furnaces;

) ovens with mixed heating.

According to the heating method, resistance furnaces are divided into indirect furnaces and direct furnaces. The heating of the material in indirect furnaces occurs due to the heat generated by the heating elements when an electric current passes through them. Furnaces of indirect heating are installations with voltage up to 1000 V and are powered in most cases from networks of 380 V with an industrial frequency of 50 Hz. Furnaces are produced with one- and three-phase power from units to several thousand kilowatts. The power factor in most cases is 1.

In furnaces of direct action, heating is carried out by the heat released in the heated product when an electric current passes through it. Furnaces are made single- and three-phase with power up to 3000 kW; Power is supplied by industrial frequency current 50 Hz from 380/220 V networks or through step-down transformers from higher voltage networks. The power factor lies in the range from 0.7 to 0.9. Most of the resistance furnaces in terms of uninterrupted power supply belong to the 2nd category of electrical energy receivers.

Furnaces and installations for induction and dielectric heating are divided into melting furnaces and installations for hardening and through heating of dielectrics.

The melting of metal in inertial furnaces is carried out by the heat that occurs in it during the passage of an induction current.

Melting furnaces are manufactured with and without a steel core. Core furnaces are used for melting non-ferrous metals and their alloys. The furnaces are powered by industrial frequency current 50 Hz, voltage 380 V and higher, depending on the power. Core furnaces are available in single-, two- and three-phase capacities up to 2000 kVA. The power factor ranges from 0.2-0.8 (furnaces for melting aluminum have cos (?) = 0.2 - 0.4, for melting copper 0.6-0.8). Coreless furnaces are used for smelting stainless steel and, less often, non-ferrous metals. The power supply of industrial furnaces without a core can be carried out with an industrial frequency current of 50 Hz from networks with a voltage of 380 V and above and an increased frequency current of 500-10,000 Hz from thyristor or electric machine converters. The drive motors of the converters are powered by industrial frequency current.

Furnaces are produced with power up to 4500 kVA, their power factor is very low: from 0.05 to 0.25. All melting furnaces belong to category 2 electrical energy receivers. Installations for hardening and through heating, depending on the purpose, are fed at frequencies from 50 Hz to hundreds of kilohertz.

The power supply of high and high frequency installations is produced, respectively, from thyristor or machine inductor-type converters and lamp generators. These installations belong to the receivers of electrical energy of the 2nd category.

In installations for heating dielectrics, the material to be heated is placed in the electric field of a capacitor and heating occurs due to displacement currents. This group of installations is widely used for gluing and drying wood, heating press powders, soldering and welding plastics, sterilizing products, etc. Power is supplied by current with a frequency of 20-40 MHz and higher. With regard to the uninterrupted power supply, installations for heating dielectrics belong to the receivers of electrical energy of the 2nd category.

Electric arc furnaces according to the method of heating are divided into furnaces of direct and indirect action. In direct-acting furnaces, the heating and melting of the metal is carried out by the heat generated by the electric arc burning between the electrode and the melted metal. Arc furnaces of direct action are divided into a number of types, characteristic of which are steelmaking and vacuum.

Steel-smelting furnaces are powered by industrial frequency current of 6-110 V through step-down transformers. Furnaces are produced with three-phase power up to 45000 kVA per unit. Power factor 0.85-0.9. In the process of operation during the melting of the charge in arc steel-smelting furnaces, frequent operational short circuits (SC) occur. exceeds the nominal by 2.5-3.5 times. Short circuits cause a decrease in voltage on the substation buses, which adversely affects the operation of other electrical energy receivers. In this regard, the joint operation of arc furnaces and other consumers from a common substation is permissible if, when powered from a powerful power system, the total power of the furnaces does not exceed 40% of the power of the step-down substation, and when powered from a low-power system, 15-20%

Vacuum arc furnaces are manufactured with power up to 2000 kW. Power is supplied by direct current with a voltage of 30-40 V. Electric machine converters and semiconductor rectifiers connected to an alternating current network of 50 Hz are used as sources of electrical energy.

Metal heating in indirect furnaces is carried out by heat generated by an electric arc burning between carbon electrodes Indirectly heated arc furnaces used for smelting copper and its alloys. The power of furnaces is relatively small (up to 500 kVA); the power is supplied by a current of industrial frequency of 50 Hz from special furnace transformers. In terms of uninterrupted power supply, these furnaces belong to category 1 electrical energy receivers, which allow short-term interruptions in power supply.

Electric furnaces with mixed heating can be divided into ore-thermal and electroslag remelting furnaces.

In ore-thermal furnaces, the material is heated by heat, which is released when an electric current passes through the charge and the arc burns. Furnaces are used to produce ferroalloys, corundum, smelting iron, lead, sublimation of phosphorus, smelting copper and copper-nickel matte. Power is supplied by industrial frequency current through step-down transformers. The power of some furnaces is very high, up to 100 MVA (yellow phosphorus sublimation furnace). Power factor 0.85-0.92. With regard to uninterrupted power supply, furnaces for ore-thermal processes are classified as category 2 electrical energy receivers.

In electroslag remelting furnaces, heating is carried out due to the heat released in the slag when a current passes through it. The slag is melted by the heat of the electric arc. Electroslag remelting is used to obtain high-quality steels and special alloys. The furnaces are powered by an industrial frequency current of 50 Hz through step-down transformers, usually from 6-10 kV networks with a secondary voltage of 45-60 V. Furnaces are usually single-phase, but can also be three-phase. Power factor 0.85-0.95. With regard to the reliability of power supply, electroslag remelting furnaces belong to the category 1 electrical energy receivers.

When supplying power to workshops that have vacuum electric furnaces of all types, it must be taken into account that a break in the power supply of vacuum pumps leads to an accident and the rejection of expensive products. These furnaces should be attributed to the receivers of electrical energy of the 1st category.

Electric welding installations

How receivers are divided into installations operating on alternating current and direct current. Technologically, welding is divided into arc and contact, according to the method of work - into manual and automatic.

DC electric welding units consist of an AC motor and a DC welding generator. With such a system, the welding load is distributed evenly over the three phases in the AC supply network, but its schedule remains variable. The power factor of such installations in the nominal mode of operation is 0.7-0.8; at idle, the power factor drops to 0.4. Among the DC welding units, there are also rectifier installations.

AC electric welding machines operate at an industrial AC frequency of 50 Hz and are a single-phase load in the form of welding transformers for arc welding and resistance welding machines. AC welding gives a single-phase load with intermittent operation, uneven phase loading and, as a rule, a low power factor (0.3-0.35 for arc and 0.4-0.7 for resistance welding). Welding installations are powered by 380-220 V mains. Welding transformers at construction and installation sites are characterized by frequent movements in the supply network. This circumstance must be taken into account when designing the supply network. From the point of view of power supply reliability, welding installations belong to the receivers of electrical energy of the 2nd category.

Conclusion

The advances in automation made it possible to create a project for a continuous metallurgical plant, where disparate processes would be connected into a single flow system. It turns out that the central place in the whole process is still occupied by a blast furnace. Is it possible to do without a domain?

The problem of homeless production, or, as it is called, direct production of iron, has been solved for many decades. Significant progress has been made in this direction. There is reason to believe that in the 1970s fairly large installations for the direct reduction of iron with a daily output of 500 tons will come into operation. But even so, blast-furnace production will retain its positions for more than a decade.

A domainless process can be imagined, for example, as follows. In rotary tube furnaces, iron ore is converted into iron. With the help of magnets, iron grains are separated from the rest of the mass - and the pure product is ready for further processing. Finished products can be stamped from iron powder. From it you can cook steel of various grades, adding the necessary additives (alloying elements).

With the commissioning of gigantic power plants, Soviet metallurgy will receive a lot of cheap electricity. This will create favorable conditions for the development of electrometallurgical production and for an even wider application of electricity at all subsequent stages of processing iron alloys.

The successes of atomic physics prompted the idea of ​​the so-called radiation metallurgy. Academician IP Bardin (1883-1960) expressed a bold, almost fantastic idea for the future development of metallurgy. “I think,” he said, “that at first a person will “design” alloyed steels of the required composition using radioactive influence, without introducing rare and expensive alloying additives into them, but creating them directly in a ladle of molten steel. From iron atoms, maybe sulfur, phosphorus, under the influence of a stream of rays in the molten metal, purposeful nuclear transformations will occur.

Future generations of researchers will have to work on solving this and other fascinating problems. Ferrous metallurgy is waiting for new discoverers.

In this essay, in our opinion, we have achieved our goal and considered the transmission of electricity over distances and its use as a necessary component in the electric steelmaking process. And also we, it seems to us, have fulfilled all the tasks set by us, namely: we studied additional literature that helped us in writing this work; got acquainted with new types of generators and transformers; considered the path of electric current from its receipt to delivery to the consumer; and, finally, studied the physical and mechanical processes occurring in the electric furnace.

Bibliography

1. Babich V. K., Lukashkin N. D., Morozov A. S. et al. / Fundamentals of metallurgical production (ferrous metallurgy). Textbook for secondary vocational schools - M.: Metallurgy, 1988. 272 ​​p.

Barg I. G., Valk H. Ya., Komarov D. T.; Ed. Barga I. G. / Improving the maintenance of 0.4-20 kV power networks in the rural area - M .: Energia, 1980. - 240 p., ill.

Bornatsky I. I., Blashchuk N. M., Yargin S. A., Strok V. I. / Assistant steelmaker of a wide profile: A textbook for secondary vocational schools - M .: Metallurgy, 1986. 456 p.

Zubkov B.V., Chumakov S.V. / Encyclopedic Dictionary of a Young Technician - M .: Pedagogy, 1980. - 512 p., ill.

Myakishev G. Ya., Bukhovtsev B. B. / Physics: Proc. for 10 cells. avg. school - M.: Enlightenment, 1990. - 223 p.: ill.

Myakishev G. Ya., Bukhovtsev B. B. / Physics: Proc. for 10 cells. avg. school - 9th ed., revised. - M.: Enlightenment, 1987. - 319 p., 4 sheets. ill.: ill.

Chigray I.D. Assistant steelworker converter. M.: Metallurgiya, 1977. 304 p.

It's no secret that electricity in our home comes from power plants, which are the main sources of electricity. However, there can be hundreds of kilometers between us (consumers) and the station, and through all this long distance the current must somehow be transmitted with maximum efficiency. In this article, we, in fact, will consider how electricity is transmitted at a distance to consumers.

Electricity transportation route

So, as we have already said, the starting point is the power station, which, in fact, generates electricity. To date, the main types of power plants are hydro (HPP), heat (TPP) and nuclear (NPP). In addition, there are solar, wind and geothermal electric. stations.

Further from the source, electricity is transmitted to consumers, which may be located at long distances. To carry out the transmission of electricity, you need to increase the voltage using step-up transformers (the voltage can be increased up to 1150 kV, depending on the distance).

Why is electricity transmitted at high voltage? Everything is very simple. Recall the formula for electrical power - P = UI, then if you transfer energy to the consumer, then the higher the voltage on the power line - the lower the current in the wires, with the same power consumption. Thanks to this, it is possible to build power lines with high voltage, reducing the cross section of wires, compared to power lines with low voltage. This means that construction costs will be reduced - the thinner the wires, the cheaper they are.

Accordingly, electricity is transmitted from the station to a step-up transformer (if necessary), and after that, with the help of power lines, electricity is transmitted to the CRP (central distribution substations). The latter, in turn, are located in cities or in close proximity to them. At the CRP, the voltage drops to 220 or 110 kV, from where electricity is transmitted to substations.

Further, the voltage is lowered again (already up to 6-10 kV) and the distribution of electrical energy takes place among transformer points, also referred to as TP. Electricity can be transmitted to transformer points not through power lines, but through an underground cable line, because. in urban areas it will be more appropriate. The fact is that the cost of the right-of-way in cities is quite high and it will be more profitable to dig a trench and lay a cable in it than to take up space on the surface.

From transformer points, electricity is transmitted to multi-storey buildings, buildings of the private sector, garage cooperative, etc. We draw your attention to the fact that the voltage at the transformer substation drops again, already to the usual 0.4 kV (380 volt network).

If we briefly consider the route of electricity transmission from the source to consumers, then it looks like this: power plant (for example, 10 kV) - step-up transformer substation (from 110 to 1150 kV) - power transmission line - step-down transformer substation - TP (10-0.4 kV) - residential buildings.

In this way, electricity is transmitted through wires to our house. As you can see, the scheme for the transmission and distribution of electricity to consumers is not too complicated, it all depends on how large the distance is.

You can clearly see how electrical energy enters cities and reaches the residential sector in the picture below:

Experts talk about this issue in more detail:

How does electricity travel from a source to a consumer?

What else is important to know?

I also wanted to say a few words about the points that intersect with this issue. Firstly, research has been going on for quite a long time on how to carry out the transmission of electricity without wires. There are many ideas, but the most promising solution to date is the use of wireless WI-Fi technology. Scientists from the University of Washington found that this method is quite real and began to study the issue in more detail.

Secondly, today the power transmission line transmits alternating current, not direct current. This is due to the fact that converter devices, which first rectify the current at the input, and then again make it variable at the output, have a rather high cost, which is not economically feasible. However, the throughput of DC power lines is still 2 times higher, which also makes us think about how it is more profitable to implement it.

So we considered the scheme for transmitting electricity from a source to a house. We hope you understand how electricity is transmitted at a distance to consumers and why high voltage is used for this.

Electricity is not a storage resource. To date, there are no effective technologies that allow accumulating the energy generated by generators, so the transmission of electricity to consumers is an urgent task. The cost of the resource includes the cost of its production, losses during transportation, and the cost of installing and maintaining power lines. At the same time, the efficiency of the power supply system directly depends on the transmission scheme.

High voltage as a way to reduce losses

Despite the fact that in the internal networks of most consumers, as a rule, 220/380 V, electricity is transmitted to them through high-voltage mains and is reduced at transformer substations. There are good reasons for such a scheme of work, the fact is that the largest share of losses is due to the heating of the wires.

The power loss is described by the following formula: Q \u003d I 2 * R l,

where I is the strength of the current passing through the line, R L is its resistance.

Based on the above formula, we can conclude that it is possible to reduce costs by reducing the resistance in the power line or by lowering the current strength. In the first case, it will be necessary to increase the cross-section of the wire, this is unacceptable, since it will lead to a significant increase in the cost of power transmission lines. Choosing the second option, you will need to increase the voltage, that is, the introduction of high-voltage power lines leads to a decrease in power losses.

Power line classification

In the energy sector, it is customary to divide power lines into types depending on the following indicators:

1. Design features of lines carrying out the transmission of electricity. Depending on the execution, they can be of two types:

1. Voltage. Depending on the magnitude of the voltage, power lines are usually classified into the following types:

1. Separation by type of current in the transmission of electricity, it can be variable or constant. The first option is more common, since power plants are usually equipped with alternators. But to reduce load energy losses, especially at a long transmission distance, the second option is more efficient. How electricity transmission schemes are organized in both cases, as well as the advantages of each of them, will be discussed below.
2. Classification depending on the purpose. For this purpose, the following categories have been adopted:

• Lines from 500.0 kV for extra long distances. Such overhead lines interconnect separate energy systems.
• Power transmission lines for main purposes (220.0-330.0 kV). With the help of such lines, the transmission of electricity generated at powerful hydroelectric power stations, thermal and nuclear power plants, as well as their integration into a single energy system, is carried out.
• Power transmission lines 35-150 kV are distributive. They serve to supply electricity to large industrial sites, connect district distribution points, etc.
• Power lines with voltage up to 20.0 kV are used to connect groups of consumers to the electrical network.

Electricity transmission methods

There are two ways to transfer electricity:

• direct transmission method.
• Converting electricity into another form of energy.

In the first case, electricity is transmitted through conductors, which are a wire or a conductive medium. This transmission method is used in overhead and cable power lines. Converting electricity to another form of energy opens up the prospect of wireless consumer supply. This will make it possible to abandon power lines and, accordingly, the costs associated with their installation and maintenance. Below are promising wireless technologies that are being improved.

Unfortunately, at the moment, the possibilities of transporting electricity wirelessly are very limited, so it is too early to talk about an effective alternative to the direct transmission method. Research work in this direction let us hope that a solution will be found in the near future.

The scheme of transmission of electricity from the power plant to the consumer

The figure below shows typical circuits, of which the first two are open-circuited, the rest are closed-circuited. The difference between them is that open configurations are not redundant, that is, they do not have redundant lines that can be activated when the electrical load increases critically.

Designations:

1. Radial scheme, at one end of the line there is a power plant producing energy, at the other end - a consumer or a switchgear.
2. The main version of the radial scheme, the difference from the previous version is the presence of taps between the initial and final points of transmission.
3. Main circuit with power supply at both ends of the power line.
4. Ring configuration type.
5. Trunk with redundant line (double trunk).
6. Complicated configuration option. Similar schemes are used when connecting responsible consumers.

Now let's consider in more detail the radial scheme for the transmission of generated electricity through AC and DC power lines.

Designations:

1. A generator that generates electricity with a sinusoidal characteristic.
2. Substation with step-up three-phase transformer.
3. Substation with a transformer that steps down the voltage of three-phase alternating current.
4. Branch for transmission of electricity to a switchgear.
5. Rectifier, that is, a device that converts three-phase alternating current into direct current.
6. The inverter unit, its task is to form a sinusoidal voltage from a constant voltage.

As can be seen from the diagram (A), electricity is supplied from the energy source to a step-up transformer, then electricity is transported over considerable distances using overhead power lines. At the end point, the line is connected to a step-down transformer and from it goes to the distributor.

The method of transmitting electricity in the form of direct current (B in Fig. 6) differs from the previous scheme by the presence of two converter units (5 and 6).

Closing the topic of the section, for clarity, we present a simplified version of the city network scheme.

Designations:

1. A power plant where electricity is produced.
2. A substation that boosts voltage to provide high efficiency transmission of electricity over long distances.
3. High voltage power lines (35.0-750.0 kV).
4. Substation with step-down functions (output 6.0-10.0 kV).
5. Electricity distribution point.
6. Feeding cable lines.
7. Central substation at industrial facility, serves to lower the voltage to 0.40 kV.
8. Radial or trunk cable lines.
9. Introductory shield in the workshop.
10. District distribution substation.
11. Cable radial or trunk line.
12. Substation that lowers the voltage to 0.40 kV.
13. Introductory shield of a residential building, for connecting the internal electrical network.

Transmission of electricity over long distances

The main problem associated with such a task is the growth of losses with an increase in the length of the power transmission line. As mentioned above, in order to reduce energy costs for the transmission of electricity, the current strength is reduced by increasing the voltage. Unfortunately, this solution gives rise to new problems, one of which is corona discharges.

From the point of view of economic feasibility, losses in overhead lines should not exceed 10%. Below is a table that shows the maximum length of lines that meet the conditions of profitability.

Table 1. Maximum length of power transmission lines, taking into account profitability (no more than 10% losses)

* - at the moment, the ultra-high-voltage overhead line has been switched to work with a voltage of half the nominal voltage (500.0 kV).

DC alternative

As an alternative to AC power transmission over a long distance, direct voltage overhead lines can be considered. Such power lines have the following advantages:

• The length of the overhead line does not affect the power, while its maximum value is significantly higher than that of power lines with alternating voltage. That is, with an increase in electricity consumption (up to a certain limit), you can do without modernization.
• Static stability can be ignored.
• There is no need to synchronize the associated power systems in frequency.
• It is possible to organize the transmission of electricity through a two-wire or single-wire line, which greatly simplifies the design.
• Less impact electromagnetic waves to means of communication.
• There is practically no reactive power generation.

Despite the listed capabilities of DC power lines, such lines are not widely used. This is primarily due to the high cost of the equipment required to convert the sinusoidal voltage to DC. DC generators are practically not used, with the exception of solar power plants.

With inversion (the process is completely opposite to rectification), everything is also not simple, it is necessary to drink high-quality sinusoidal characteristics, which significantly increases the cost of equipment. In addition, one should take into account problems with the organization of power take-off and low profitability with an overhead line length of less than 1000-1500 km.

Briefly about superconductivity.

The resistance of wires can be significantly reduced by cooling them to ultra-low temperatures. This would bring the efficiency of electricity transmission to a qualitatively new level and increase the length of lines for the use of electricity at a great distance from the place of its production. Unfortunately, the technologies available today cannot allow the use of superconductivity for these purposes due to economic inexpediency.

Let us briefly consider the power supply system, which is a group of electrical devices for the transmission, conversion, distribution and consumption of electrical energy. The chapter will expand the horizons of those who want to learn how to properly use the home electrical network.

Electricity supply carried out according to standard schemes. For example, in fig. 1.4 shows a radial single-line power supply circuit for transmitting electricity from a step-down substation of a power plant to a consumer of electricity with a voltage of 380 V.

From the power plant, electricity with a voltage of 110-750 kV is transmitted through power lines (TL) to the main or regional step-down substations, at which the voltage is reduced to 6-35 kV. From switchgears, this voltage is transmitted via overhead or cable transmission lines to transformer substations located in close proximity to consumers of electrical energy. At the substation, the voltage is reduced to 380 V, and electricity is supplied directly to the consumer in the house via overhead or cable lines. At the same time, the lines have a fourth (neutral) wire 0, which makes it possible to obtain a phase voltage of 220 V, as well as to provide protection for electrical installations.
This scheme allows you to transfer electricity to the consumer with the least loss. Therefore, on the way from the power plant to consumers, electricity is transformed from one voltage to another. A simplified example of transformation for a small section of the power system is shown in fig. 1.5. Why use high voltage? The calculation is complicated, but the answer is simple. To reduce heating losses of wires during transmission over long distances.

Losses depend on the amount of current flowing and the diameter of the conductor, and not on the applied voltage.

For instance:
Let us assume that from a power plant to a city located at a distance of 100 km from it, it is necessary to transmit 30 MW via one line. Due to the fact that the wires of the line have electrical resistance, the current heats them up. This heat is dissipated and cannot be used. The energy spent on heating is a loss.

It is impossible to reduce losses to zero. But they need to be limited. Therefore, the permissible losses are normalized, i.e., when calculating the wires of the line and choosing its voltage, it is assumed that the losses do not exceed, for example, 10% of the useful power transmitted over the line. In our example, this is 0.1-30 MW = 3 MW.

For instance:
If you do not apply transformation, i.e., transmit electricity at a voltage of 220 V, then to reduce losses to set value the cross section of the wires would have to be increased to about 10 m2. The diameter of such a "wire" exceeds 3 m, and the mass in the span is hundreds of tons.
Applying transformation, that is, increasing the voltage in the line, and then, reducing it near the location of consumers, they use another way to reduce losses: they reduce the current in the line. This method is very efficient, since the losses are proportional to the square of the current. Indeed, when the voltage is doubled, the current is halved, and the losses are reduced by 4 times. If the voltage is increased by a factor of 100, then the losses will decrease by a factor of 100 to the second power, that is, by a factor of 10,000.

For instance:
As an illustration of the effectiveness of voltage boosting, I will point out that a 500 kV three-phase AC transmission line transmits 1000 MW per 1000 km.

Power lines

Electrical networks are designed for the transmission and distribution of electricity. They consist of a set of substations and lines of various voltages. At power plants, step-up transformer substations are built, and electricity is transmitted over long distances through high-voltage power lines. In places of consumption, step-down transformer substations are being built.

The basis of the electrical network is usually underground or overhead high voltage power lines. The lines running from the transformer substation to the input distribution devices and from them to power distribution points and to group shields are called the supply network. The supply network, as a rule, consists of underground low-voltage cable lines.

According to the principle of construction, networks are divided into open and closed. An open network includes lines that go to electrical receivers or their groups and receive power from one side. An open network has some disadvantages, namely that in the event of an accident at any point in the network, the power supply to all consumers beyond the emergency section is stopped.

A closed circuit may have one, two or more power supplies. Despite a number of advantages, closed networks have not yet received wide distribution. At the place where the network is laid, there are external and internal.

Ways to make power lines

Each voltage corresponds to certain methods of wiring. This is because the higher the voltage, the more difficult it is to insulate the wires. For example, in apartments where the voltage is 220 V, wiring is carried out with wires in rubber or plastic insulation. These wires are simple and cheap.

An underground cable designed for several kilovolts and laid underground between transformers is incomparably more complicated. In addition to increased requirements for insulation, it must also have increased mechanical strength and corrosion resistance.

For direct power supply to consumers are used:

♦ overhead or cable transmission lines with a voltage of 6 (10) kV to power substations and high-voltage consumers;
♦ cable transmission lines with voltage 380/220 V for direct power supply of low-voltage power receivers. To transmit a voltage of tens and hundreds of kilovolts over a distance, overhead power lines are created. The wires rise high above the ground, air is used as insulation. The distances between the wires are calculated depending on the voltage that is planned to be transmitted. On fig. 1.6 shows on the same scale supports for overhead power lines with voltages of 500, 220, 110, 35 and 10 kV. Notice how the dimensions increase and the designs become more complicated with increasing operating voltage!

Rice. 1.6.

For instance:
The 500 kV line pole has a height of a seven-story building. The height of the wire suspension is 27 m, the distance between the wires is 10.5 m, the length of the garland of insulators is more than 5 m. The height of supports for river crossings reaches 70 m. Let's consider the power transmission line options in more detail.

Overhead power lines
Definition.
An overhead power line is a device for transmitting or distributing electricity through wires located in the open air and attached with the help of traverses (brackets), insulators and fittings to supports or engineering structures.

In accordance with the "Electrical Installation Rules", overhead lines are divided into two groups by voltage: voltage up to 1000 V and voltage over 1000 V. For each group of lines, the technical requirements for their device are established.

Overhead power lines 10 (6) kV are most widely used in rural areas and in small towns. This is due to their lower cost compared to cable lines, lower building density, etc.

For wiring overhead lines and networks use a variety of wires and cables. The main requirement for the material of wires of overhead power lines is low electrical resistance. In addition, the material used for the manufacture of wires must have sufficient mechanical strength, be resistant to moisture and airborne chemicals.

Currently the most commonly used aluminum and steel wires, which allows saving scarce non-ferrous metals (copper) and reducing the cost of wires. Copper wires are used on special lines. Aluminum has low mechanical strength, which leads to an increase in the sag and, accordingly, to an increase in the height of the supports or a decrease in the length of the span. When transmitting small amounts of electricity over short distances, steel wires are used.

For insulation wires and fastening them to power line poles serve line insulators, which, along with electrical strength, must also have sufficient mechanical strength. Depending on the method of fastening on the support, pin insulators are distinguished (they are mounted on hooks or pins) and suspended (they are assembled into a garland and attached to the support with special fittings).

Pin insulators used on power lines with voltage up to 35 kV. They are marked with letters indicating the design and purpose of the insulator, and numbers indicating the operating voltage. On 400 V overhead lines, pin insulators TF, ShS, ShF are used. Letters in legend insulators are as follows: T- telegraph; F- porcelain; WITH- glass; ShS- pin glass; CHF- pin porcelain.

Pin insulators are used for hanging relatively light wires, while depending on the conditions of the route, various types of wire fastening are used. The wire on the intermediate supports is usually fixed on the head of the pin insulators, and on the corner and anchor supports - on the neck of the insulators. On the corner supports, the wire is placed on the outside of the insulator with respect to the angle of rotation of the line.

Suspension insulators used on overhead lines 35 kV and above. They consist of a porcelain or glass plate (insulating piece), a ductile iron cap and a rod. The design of the socket of the cap and the head of the rod provides a spherical hinged connection of the insulators when completing the garlands. Garlands are assembled and hung from supports and thus provide the necessary insulation for the wires. The number of insulators in a string depends on the line voltage and the type of insulators.

The material for knitting aluminum wire to the insulator is aluminum wire, and for steel wires, mild steel. When knitting wires, a single fastening is usually performed, while a double fastening is used in populated areas and at increased loads. Before knitting, a wire of the desired length is prepared (at least 300 mm).

head knit performed with two knitting wires of different lengths. These wires are fixed on the neck of the insulator, twisting together. The ends of the shorter wire are wrapped around the wire and tightly pulled four to five times around the wire. The ends of another wire, longer ones, are placed on the head of the insulator crosswise through the wire four to five times.

To perform side knitting, they take one wire, put it on the neck of the insulator and wrap it around the neck and the wire so that one end passes over the wire and bends from top to bottom, and the other from bottom to top. Both ends of the wire are brought forward and again wrapped around the neck of the insulator with the wire, swapping relative to the wire.

After that, the wire is tightly attracted to the neck of the insulator and the ends of the knitting wire are wrapped around the wire from opposite sides of the insulator six to eight times. In order to avoid damage to aluminum wires, the knitting point is sometimes wrapped with aluminum tape. It is not allowed to bend the wire on the insulator by strong tension of the binding wire.

Wire tying performed manually using pliers. At the same time, special attention is paid to the tightness of the binding wire to the wire and to the position of the ends of the binding wire (they should not stick out). Pin insulators are attached to supports on steel hooks or pins. Hooks are screwed directly into wooden supports, and pins are installed on metal, reinforced concrete or wooden traverses. For fastening insulators on hooks and pins, transitional polyethylene caps are used. The heated cap is pushed tightly onto the pin until it stops, after which the insulator is screwed onto it.

The wires are suspended on reinforced concrete or wooden supports using suspension or pin insulators. For overhead power lines, uninsulated ones are used. An exception is the inputs to buildings - insulated wires pulled from the power transmission line support to insulators mounted on hooks directly on the building.

Attention!
The lowest permissible height of the lower hook on the support (from ground level) is: in power lines with voltage up to 1000 V for intermediate supports from 7 m, for transitional supports - 8.5 m; in power lines with a voltage of more than 1000 V, the height of the lower hook for intermediate supports is 8.5 m, for corner (anchor) supports - 8.35 m.

The smallest permissible wires of overhead power lines with a voltage of more than 1000 V, selected according to the conditions of mechanical strength, taking into account the possible thickness of their icing, are given in Table 1.1.

Minimum allowable values ​​of wires of overhead power lines with a voltage of more than 1000 V
Table 1.1

Overhead power lines with voltage up to 1000 V and up to 10 kV and their supports to the objects are presented in Table. 1.2.

Table 1.2

Currently, electricity is generated mainly by powerful power plants located far from consumers.

As a result, it becomes necessary to transmit it over long distances.

In principle, electromagnetic energy can be transferred from a source to a consumer in the microwave frequency range (SHF) and in the optical frequency range. It is in this form that electromagnetic energy from the Sun arrives at the Earth. The spectrum of the Sun's radiation will fade from extremely low frequencies, on the order of a few Hertz, to ultraviolet and even X-ray frequencies. However, at the present state of the art, it is practically difficult to transmit large amounts of electricity through free space. Therefore, at present, electricity is transmitted through open transmission lines using aluminum and copper wires or using shielded cables.

At the same time, in cases where electrical energy is generated at relatively low frequencies (50 or 60 Hz), it is economically more profitable to transmit it using high-voltage power lines. As already noted, in this case, the electromagnetic field propagates in the dielectric surrounding the metal wire, and only a small part of the energy penetrates the wire and is spent on heating it. For the transmission of electricity over long distances, conductive channels made of metallic aluminum or copper wires are currently mainly used. In this case, both open overhead lines and shielded underground cables are used. In both cases, electromagnetic energy propagates in the dielectric surrounding the conductor, and only a small part of it (fractions of a percent) is lost to heat the conductor. When using open conductors, some of the transmitted energy is radiated into free space.

The energy radiated into free space is negligible (fractions of a percent) if the length of the transmission line is much less than half the wavelength, equal to 6000 km at a frequency of 50 Hz, and increases almost linearly as the length of the transmission line increases.

As noted above, the transmission of electricity is currently produced using alternating voltage. This is due to the possibility of using transformers to change the value of the alternating voltage.

In practice, the electromagnetic field penetrates the wire metal to a depth of several hundred nanometers. In the general case, the amount of losses in the wires depends on the power of the transmitted electricity, the concentration of impurities in the metal of the wires, and the temperature. Naturally, the hotter the wire, the greater the loss in it.

Therefore, the wires have to be chosen the thicker, the greater the power transmitted through them and the more impurities in the metal of the wires. Oxidation of wires in a humid environment leads to the formation of a dielectric film on their surface and also naturally increases losses.

A serious problem when using open transmission lines over long distances is the increase in losses caused by increased radiation of electricity into free space.

It must be remembered that when transmitting electricity at direct current (at f \u003d 0 Hz), the electromagnetic field also propagates along the wires at a speed close to the speed of light. In this case, energy losses due to radiation into free space are sharply reduced. In this case, the energy losses in the wires practically do not decrease. They can be significantly reduced by using superconductors. However, at present, the transmission of electricity using superconductors is practically not used, mainly due to the fact that they need to be cooled to a very low temperature. At the same time, the energy required for cooling the conductors exceeds the losses of electricity during its transmission through shielded wires.