Transmission of electricity energy over long distances. Transmission of electricity over a distance. High voltage as a way to reduce losses
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.
The transmission of electricity over a distance using a resonant single-wire system is characterized by low economic costs compared to traditional technologies. At the same time, there are practically no losses in the wires (hundreds of times less than with the traditional method of transmitting electrical energy). Significantly - up to 10 times the cost of laying cables is reduced. Provides a high level of electrical safety for the environment and humans.
Description:
One of the most urgent problems of modern energy is the transmission of electricity over a distance with low economic costs and energy conservation.
In practice, for the transmission of electrical energy over long distances, as a rule, three-phase systems are used, the implementation of which requires the use of at least 4 wires which has the following significant disadvantages:
– large losses of electrical energy in the wires, the so-called joule losses,
– the need to use intermediate transformer substations, compensating for energy losses in the wires,
– the occurrence of accidents due to a short circuit of wires, including due to dangerous weather phenomena (strong wind, ice on wires, etc.),
– high flow non-ferrous metals,
– high economic costs for laying three-phase electrical networks (several million rubles per 1 km).
The disadvantages noted above can be eliminated through the use of a resonant single-wire system for the transmission of electrical energy, based on the ideas of N. Tesla, modified taking into account the modern development of science and technology. At present, the technology of a resonant single-wire system for the transmission of electrical energy has been developed.
Resonant single-wire waveguide the electrical energy transmission system at an increased frequency of 1-100 kHz does not use active conduction current in a closed circuit. There is no closed circuit in the resonant waveguide single-conductor line, there are no traveling waves of current and voltage, but there are standing (stationary) waves of reactive capacitive current and voltage with a phase shift of 90°. At the same time, due to the absence of active current and the presence of a current node in lines there is no need and need to create a high-temperature conduction mode in such a line, and the Joule losses become insignificant due to the absence of closed active conduction currents in the line and insignificant values of open capacitive current near the nodes of stationary current waves in the line.
The proposed technology is based on the use of two resonant circuits with a frequency of 0.5-50 kHz and a single-wire line between the circuits (see Figure 1) with a line voltage of 1-100 kV when operating in the voltage resonance mode.
The line wire is the guiding channel along which the electromagnetic energy moves. The energy of the electromagnetic field is distributed around conductor lines.
Rice. 1. Electric circuit of a resonant single-wire power transmission system
1 - high frequency generator; 2 - resonant circuit of the step-up transformer; 3 - single-wire line; 4 - resonant circuit of the step-down transformer; 5 - rectifier; 6 - converter.
As calculations and experiments show, with this method of transmitting electrical energy, there are practically no losses in the wires (hundreds of times less than with the traditional method of transmitting electrical energy) and this technology is safe for the environment and humans.
To harmonize a conventional power supply system with the proposed system, matching devices have been developed and converters, which are installed at the beginning and at the end of a single-wire line and allow the use of standard AC or DC electrical equipment at the input and output.
At present, the technology of power transmission with a capacity of up to 100 kW has been developed. The transmission of electricity of greater power requires the use of electronic devices (transistors, thyristors, diodes, etc.) of increased power and reliability. It is necessary to carry out additional studies to solve the problem of power supply for objects that consume electricity with a power of more than 100 kW.
Advantages:
– electrical energy is transferred using reactive capacitive current in resonant mode,
– unauthorized use of energy is difficult,
– reduction of costs for the construction of power lines,
– the possibility of replacing overhead power lines with single-conductor cable lines,
– significant savings in non-ferrous metals, because the cable section is 3-5 times smaller than the sections of a traditional three-phase power transmission system, the content of aluminum and copper in the wires can be reduced by 10 times,
– a significant reduction in the turning radius of lines, which is very important when laying cables in urban areas,
– a significant (up to 10 times) reduction in the cost of laying cables,
– there is no phase-to-phase short circuit,
– provides a high level of electrical safety for the environment and humans,
– power losses in a single-wire line are small,
- electricity can be transmitted over long and ultra-long distances,
– short circuits are not possible in a single-wire cable and a single-wire cable cannot cause a fire,
– no need for maintenance,
– the presence of a low magnetic field,
– no influence of weather conditions,
– the natural landscape is not disturbed,
- no right-of-way
– there are practically no losses in the wires (hundreds of times less than with the traditional method of transmitting electrical energy).
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It is known that large thermal power plants are built near coal deposits or large gas pipelines, hydroelectric power plants are built on large rivers, and nuclear power plants are built no closer than 30-50 km from large cities where the main consumers of electricity are located. In other words, electricity is produced far from where it is consumed. Therefore, it must be transferred to the places of its consumption, for which power transmission lines (TL) serve.
But did you know that with a typical generator power of a power plant of 500 MW and a voltage of 10 kV, the current in the wires is 50 thousand amperes? Such a current, according to the Joule-Lenz law, with a power line resistance of only 1 ohm, will emit as much heat every second as a million electric kettles switched on at the same time!
According to the Joule-Lenz law Q = I2Rt, there are two possibilities to reduce power losses: reduce the resistance of the power line (R) or reduce the current strength (I) in it.
Let's consider the first possibility. To reduce resistance, you must either reduce the length of the wires (and the energy will not reach the consumer), or increase their thickness (and then they will become heavy and can break off the supports). As you can see, the first possibility is not feasible in practice.
Consider now the second possibility. When studying the transformer (see § 10-h), we noted that the transformer increases the voltage, while simultaneously lowering the current strength by the same number of times. Therefore, before the current from the generator enters the power line, it is transformed (transformed) into a high voltage current. By increasing the voltage from 10 kV to 1000 kV, that is, by a factor of 100, we will reduce the current by the same number of times. According to the Joule-Lenz law, the amount of heat that is uselessly released in the wires will decrease by a factor of 100 100, that is, immediately by 10,000 times.
The figure on the previous page shows that the electricity generated by generator 1 is fed through thick wires 2 to transformer 3. After the voltage rises, the current is transmitted to consumers through relatively thin wires 4. For this, special strong supports 5 with garlands of insulators 6 are used.
When electricity reaches the place of consumption through wires 4, a step-down transformer 7 is used, from which energy is supplied to consumers 9. Energy can also be supplied to other transformers that lower the voltage even more.
As a rule, the energy supplied to the city via a high-voltage line passes through three to four step-down transformers. They step down the voltage in a cascade to produce the various voltages required by both industrial and domestic consumers. This is conditionally shown in the diagram.For many years, scientists have been struggling with the issue of minimizing electrical costs. There are different ways and proposals, but the most famous theory is the wireless transmission of electricity. We propose to consider how it is carried out, who is its inventor and why it has not yet been brought to life.
Theory
Wireless electricity is literally the transmission of electrical energy without wires. People often compare the wireless transmission of electrical energy to the transmission of information such as radios, cell phones, or Wi-Fi Internet access. The main difference is that radio or microwave transmission is a technology aimed at restoring and transporting exactly information, and not the energy that was originally spent on transmission.
Wireless electricity is a relatively new area of technology, but one that is growing rapidly. Methods are now being developed to efficiently and safely transfer energy over a distance without interruption.
How does wireless electricity work
The main work is based precisely on magnetism and electromagnetism, as is the case with radio broadcasting. Wireless charging, also known as inductive charging, is based on a few simple principles of operation, in particular, the technology requires two coils. A transmitter and receiver that together generate an alternating, non-constant current magnetic field. In turn, this field causes a voltage in the receiver coil; this can be used to power a mobile device or charge a battery.
If you direct an electric current through a wire, then a circular magnetic field is created around the cable. Despite the fact that the magnetic field affects both the loop and the coil, it manifests itself most strongly on the cable. When you take a second coil of wire that does not have an electric current passing through it, and place the coil in the magnetic field of the first coil, the electrical current from the first coil will be transmitted through the magnetic field and through the second coil, creating an inductive coupling.
Let's take an electric toothbrush as an example. In it, the charger is connected to an outlet that sends an electric current to a coiled wire inside the charger, which creates a magnetic field. There is a second coil inside the toothbrush, when the current starts to flow and, thanks to the formed magnetic field, the brush starts charging without it being directly connected to the 220 V power supply.
Story
Wireless power transmission as an alternative to the transmission and distribution of electric lines was first proposed and demonstrated by Nikola Tesla. In 1899, Tesla presented a wireless transmission to power a field of fluorescent lamps located twenty-five miles from a power source without the use of wires. But at the time, it was cheaper to wire 25 miles of copper wire rather than build the custom electrical generators that Tesla's experience requires. He was never granted a patent, and the invention remained in the bins of science.
While Tesla was the first person to demonstrate the practical possibilities of wireless communication back in 1899, today, there are very few devices on sale, these are wireless brushes, headphones, phone chargers and more.
Wireless Technology
Wireless power transmission involves the transmission of electrical energy or power over a distance without wires. Thus, the core technology lies on the concepts of electricity, magnetism and electromagnetism.
Magnetism
It is a fundamental force of nature that causes certain types of material to attract or repel each other. Earth's poles are considered the only permanent magnets. The current flow in the loop generates magnetic fields that differ from oscillating magnetic fields in the speed and time required to generate alternating current (AC). The forces that appear in this case are shown in the diagram below.
This is how magnetism appearsElectromagnetism is the interdependence of alternating electric and magnetic fields.
Magnetic induction
If a conducting loop is connected to an AC power source, it will generate an oscillating magnetic field in and around the loop. If the second conducting loop is close enough, it will pick up some of this oscillating magnetic field, which in turn generates or induces an electric current in the second coil.
Video: how is the wireless transmission of electricity
Thus, there is an electrical transfer of power from one cycle or coil to another, which is known as magnetic induction. Examples of such a phenomenon are used in electrical transformers and generators. This concept is based on Faraday's laws of electromagnetic induction. There, he states that when there is a change in the magnetic flux connected to the coil, the EMF induced in the coil is equal to the product of the number of turns of the coil and the rate of change of the flux.
power clutch
This part is necessary when one device cannot transmit power to another device.
A magnetic link is generated when an object's magnetic field is capable of inducing an electrical current with other devices within its reach.
Two devices are said to be mutually inductively coupled or magnetically coupled when they are designed such that a change in current occurs when one wire induces a voltage at the ends of the other wire through electromagnetic induction. This is due to the mutual inductance
Technology
The principle of inductive coupling The two devices, mutually inductively coupled or magnetically coupled, are designed such that the change in current when one wire induces a voltage at the ends of the other wire is produced by electromagnetic induction. This is due to mutual inductance.
Inductive coupling is preferred due to its ability to operate wirelessly as well as shock resistance.Resonant inductive coupling is a combination of inductive coupling and resonance. Using the concept of resonance, you can make two objects work depending on each other's signals.
As you can see from the diagram above, resonance provides the inductance of the coil. The capacitor is connected in parallel to the winding. Energy will move back and forth between the magnetic field surrounding the coil and the electric field around the capacitor. Here, radiation losses will be minimal.
There is also the concept of wireless ionized communication.
It is also feasible, but here you need to make a little more effort. This technique already exists in nature, but there is hardly any reason to implement it, since it needs a high magnetic field, from 2.11 M/m. It was developed by the brilliant scientist Richard Volras, the developer of the vortex generator, which sends and transmits heat energy over great distances, in particular with the help of special collectors. The simplest example of such a connection is lightning.
Advantages and disadvantages
Of course, this invention has its advantages over wired methods, and disadvantages. We invite you to consider them.
The advantages include:
- Complete absence of wires;
- No power supplies needed;
- The need for a battery is eliminated;
- Energy is transferred more efficiently;
- Significantly less maintenance required.
The disadvantages include the following:
- Distance is limited;
- magnetic fields are not so safe for humans;
- wireless transmission of electricity, using microwaves or other theories, is practically impossible at home and with your own hands;
- high installation cost.
Electricity transmission. The path from the power plant to the consumer. Reducing losses in the transmission of electricity.
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 example:
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 example:
If transformation is not applied, i.e., electricity is transmitted at a voltage of 220 V, then in order to reduce losses to a given value, the cross section of the wires would have to be increased to approximately 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 example:
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 example:
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. The letters in the symbols of insulators indicate the following: 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, bare wires 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.Table 1.1.
Minimum allowable values of wires of overhead power lines with a voltage of more than 1000 V
Table 1.1Overhead 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
