Topic 3.1 Sources, transmission and distribution of electrical energy.
Electrical energy is universal: it is convenient for long-distance transmissions, it is easily distributed to individual consumers and, with the help of relatively simple devices, is converted into other types of energy.
These tasks are solved by the energy system, in which the energy of the fuel or falling water is converted into electrical energy, the transformation of currents and voltages, the distribution and transmission of electrical energy to consumers.
Sources of electrical energy are thermal (TPP), hydraulic (HPP) and nuclear (NPP) power plants, which have a common mode of energy production. Power lines, transformer and switchgears ensure the joint operation of power plants and the distribution of energy between consumers.
Rice. 11.1. General power supply scheme
Rice. 11.2. Mobile diesel power plant with a synchronous generator:
I - DC exciter; 2 - generator; 3 - diesel engine
Electricity transmission and distribution is based on a stepwise principle (Fig. 11.1). To reduce losses in power lines (LEGT), the voltage is increased using step-up (GGTP-1) and step-down (GPP-2) transformers installed at electrical substations. From large substations, electricity is supplied directly to facilities where the final voltage reduction is performed at transformer substations (TS). The distribution of electricity in electrical networks is carried out, as a rule, by three-phase alternating current with a frequency of 50 Hz.
In the initial period of construction in remote areas, they are used as temporary sources.
Electricity consumers... The receiver of electricity (electrical receiver) is the electrical part of a technological installation or mechanism that receives energy from the network and consumes it for the implementation of technological processes. Consuming electricity from the network, an electrical receiver, in essence, converts it into other types of energy: mechanical, thermal, light, or into electricity with other parameters (by the type of current, voltage, frequency). Some technological installations have several electrical receivers: machines, cranes, etc.
Electric receivers are classified according to the following criteria: voltage, type of current strength, its frequency, unit power, degree of reliability of power supply, operating mode, technological purpose.
By voltage electrical consumers are divided into two groups: up to 1000 V and over 1000 V.
Breed of amperage electrical receivers are subdivided: into receivers of alternating current of industrial frequency (50 Hz), direct current and alternating current with a frequency other than 50 Hz (increased or decreased).
Unit capacities individual electrical receivers and electrical consumers are different - from tenths of a kilowatt to several tens of megawatts.
By the degree of reliability electrical power supply rules for electrical installations (PUE) provide three categories:
1 Electrical receivers of category I - electrical receivers, the interruption of the supply of which with electricity is associated with a danger to people or entails large material damage (blast furnace shops, industrial steam boiler houses, hoisting and ventilation installations of mines, emergency lighting, etc.). They must work continuously.
2 Electrical receivers of the II category - electrical receivers, the interruption of the power supply of which leads to a massive undersupply of products, downtime of technological mechanisms, workers, industrial transport, disruption of the normal activities of urban and rural residents.
3 Electrical consumers of category III - all other electrical consumers that do not fit the definition of categories I and II. Electric receivers of this category allow a power outage of no more than one day.
Characteristics of electrical receivers... General industrial installations include fans, pumps, compressors, blowers, etc. This group of electrical receivers, as a rule, belongs to the first category of reliability. Some ventilation and compressor stations belong to the second category of reliability.
Variable electric drive of technological mechanisms and motors of machine tools with increased rotation speed are powered from converting installations... The modes of their operation are different and are determined by the mode of the mechanism.
The current converters are motor generators, mercury and semiconductor rectifiers, powered by three-phase AC mains of industrial frequency at voltages up to 110 kV.
To electrical installations include electric heating and electrolysis installations, installations for electrochemical, electric spark and ultrasonic processing of metals, electromagnetic installations (separators, couplings), electric welding equipment.
Electric heating installations combine electric ovens and electrothermal installations.
Electric welding equipment powered by 380 or 220 V AC industrial frequency. Electric welding equipment operates in intermittent mode. In terms of reliability, welding installations belong to the second category.
Power of electric drives hoisting-and-transport devices is determined by production conditions, its value ranges from several to hundreds of kilowatts. Electric lighting installations are mainly single-phase receivers. Electric lighting installations belong to the second category of reliability.
Electrical network diagrams. The scheme of the power network is determined by the technological process of production, the category of reliability of power supply, the mutual arrangement of the TP or power input and electrical receivers, their unit installed capacity and location. The circuit must be simple, safe and easy to use, economical, must meet the characteristics of the environment, and ensure the use of industrial installation methods.
Network diagrams can be radial, trunk and mixed - with one-way or two-way power supply.
With a radial scheme(Fig. 11.3) energy from a separate power supply unit (TP) is supplied to one sufficiently powerful consumer or to a group of electrical consumers.
Rice. 11.3. Radial power supply:
1- switchboard; 2 - power distribution point (RP);
3 - electrical receiver; 4 - lighting board; 5 - cable line
Radial circuits are used to supply concentrated loads of high power, with uneven placement of receivers, as well as to power receivers in explosive, fire hazardous and dusty rooms. The advantages of radial circuits are high reliability (an accident on one line does not affect the operation of receivers receiving power from another line) and ease of automation. The disadvantages of radial circuits are: low efficiency due to the significant consumption of conductive material.
With trunk circuits receivers are connected to any point on the line (trunk). The mains can be connected to the substation switchboards or to the power distribution substation (Fig.11.4):
Rice. 11.4. Trunk diagram with distribution busbar:
1- complete transformer substation (KTP);
2 - distribution busbar; 3- load
The advantages of trunk circuits are: simplification of substation switchboards; high flexibility of the network, which makes it possible to move technological equipment without reworking the network; the use of unified elements that allow installation by industrial methods.
To increase the reliability of power supply of electrical receivers according to trunk circuits, a two-way power supply of the trunk line is used (Fig.11.5):
Rice. 11.5. Circuit with two-way power supply of highways
Electric lighting network diagrams. The work lighting system provides normal illumination of the entire room and work surfaces. This system includes general and local lighting fixtures.
Emergency lighting provides illumination for continuing work or stopping the technological process and for evacuating people when the work lighting is turned off.
Group lines depending on the length and load, they can be two-, three- and four-wire. The group lines of one room should be powered so that when some of the lamps of some groups go out, the remaining groups in the work will provide the minimum illumination until the accident is eliminated. An example of a power supply circuit for the lighting network is shown in Fig. 11.6.
Rice. 11.6. Power supply circuit for electric lighting from two transformer substations:
1- switchboard; 2 - lines extending to the power RP; 3,
4 - group panels for working and emergency lighting, respectively; 5,
b - group network, respectively, of working and emergency lighting;
7- supply lines of lighting
Calculation of electrical loads. The basis for a rational solution to the complex of technical and economic issues of power supply is the correct determination of the expected electrical loads. Capital costs in the power supply scheme, non-ferrous metal consumption, power losses and operating costs depend on this.
The initial data for calculating electrical loads are the installed capacity of electrical consumers and the nature of the load change. The installed capacity (Ru) of consumer groups is understood as the total rated capacity of all electrical consumers. For example, the installed capacity of a tower crane is equal to the sum of the rated powers of all its electric motors.
As a result of the calculation, the maximum (calculated) load is determined, which serves as the basis for choosing the cross-section of live parts, power and voltage losses in networks, choosing the power of transformers and compensating devices.
For each group of electrical consumers, there is a certain relationship between the values of the calculated (Pp) and installed capacity. This ratio is called the coefficient of demand:
Knowing the installed capacity and the demand coefficient of a given group of consumers, it is possible to determine the design capacity:
The calculated reactive power (Qp) is determined by the formula:
(11.3)
where tg φ is found for the angle φ, the cosine of which is determined from the passport data of the installation.
The total rated power of the power load is defined as:
(11.4)
The lighting power must be added to the calculated power load. It is convenient to carry out calculations in tabular form (table 11.1):
Table 11.1
To reduce electricity losses, it is necessary to use higher voltages, strive to reduce the length of networks to 1000 V, and apply measures to increase the power factor.
The power factor of an electrical installation is negatively affected by the presence of lightly loaded electric motors and transformers. Therefore, first of all, organizational measures are taken to ensure that the natural power factor reaches its maximum value. If these measures are not enough, then capacitor banks and synchronous motors are used.
The method for calculating the size and location of capacitors is complicated, but in approximate calculations the value of the capacitance (kvar) is determined by the formula
(11.6)
where Qc is the capacity of the capacitor bank; Pp - rated active power of the load, kvar;
tg φр - calculated tangent.
According to the catalog data, the nearest standard capacitor is selected. Capacitor banks are installed either at the substation, or directly at the consumer.
Transformer substations... Transformer substations are used to receive electricity, convert voltage and distribute electrical energy at the facility. By purpose, the following types of transformer substations are distinguished:
main (step-up and step-down) substations designed to increase the voltage of power lines at long distances;
distribution, or simply transformer substations (TP), in which the electricity coming from the GPP is transformed from a higher voltage of 35 ... 6 kV to a lower voltage of 660/380 or 380/220 V, for which most consumers are designed.
TP equipment consists of transformers, switching and protection devices, control devices, control and electricity metering. The scheme of the TP of the type of a construction complete transformer substation with one transformer is shown in Fig. 11.7:
Rice. 11.7. An open mast substation (a) and a transformer substation circuit with one transformer (b):
1 - transformer; 2 - disconnector; 3 - fuse;
4 - distribution cabinet; 5 - spark gap
By design, open, closed, mobile substations are distinguished.
Outdoor substations with equipment installed outdoors include mast substations with transformers installed on wooden or reinforced concrete supports. In fig. 11.7 depicts a substation with one transformer connected to a power line.
Closed transformer substations (Fig. 11.8) are located in the premises. Closed transformer substations also include complete substations KTP or SKTP (construction complete transformer substations). The electrical equipment of the KTP is located in a metal case.
Rice. 11.8. Closed transformer substation: 1 - transformer;
2 - closing contact; 3 - fuse
Mobile substations (Fig. 11.9), which can also be complete, are mounted on a road or railway platform.
Figure 11.9. Mobile complete transformer substation
Power transformer specifications... The main constructive type of power transformer with voltage up to 10 kV is a three-phase transformer with natural oil cooling. Dry-type power transformers (i.e., air-cooled) are also used. They are safe against fire and therefore they are equipped with TP in buildings with increased fire safety requirements. The industry produces three-phase power transformers for a certain power scale: 10; 16; 25; 40; 63; 100; 250; 400; 630; 1000; 1600 kVA.
Determination of the type and power of the power transformer. The choice of the type, power of TP, its location is determined by the size, nature of electrical loads and their spatial location.
The calculation is carried out in the following sequence:
the location of the TP is determined taking into account the position of hazardous areas, the location of access roads and roads. It is desirable to locate transformer substations closer to powerful consumers;
when determining the power of a transformer, it is necessary to simultaneously resolve the issue of reactive power compensation. With compensation on the 0.4 kV side, the calculated transformer power is obtained:
(11.7)
where Рр is the calculated active power of the load, kW; Qр - calculated reactive power of the load, kvar; QЭ - reactive power of the power system (as a rule, QЭ = 0.33 Rr); B is the load factor of the transformer (for a single-transformer substation B = 0.95 ... 1.0).
The nearest transformer of equal or greater power is selected from the reference data.
Power stations.
Electrical energy is generated in power plants. Various types of natural energy (fuel, nuclear, falling water, wind, sea tides, etc.) are converted into electricity at these stations. For the operation of electric generators, steam piston engines and turbines, internal combustion engines, gas and hydraulic turbines, wind turbines, etc. are used. Depending on the type of energy consumed by the prime movers, power plants are thermal, including nuclear, hydraulic, and wind. Solar stations (solar installations) are of some importance for mountainous and southern regions. However, their power is still insignificant, so they have only local significance and limited application.
City stations provide consumers not only with electricity, but also with heat and are called combined heat and power plants (CHP).
The gradual reduction of fuel resources requires the search for new ways to obtain electricity. One of the most promising is the production of electricity using thermonuclear fusion. In this direction, research is being carried out all over the world.
It should be noted that the efficiency is even large thermal power plants does not exceed 40-42%. An effective way to increase efficiency. thermal power plants is the use of so-called magnetohydrodynamic generators (MHD generators).
The concept of electrical systems... The transmission of electrical energy over long distances is advantageous at high voltages. Therefore, at power plants, transformer substations are being built, at which the voltage of the generators rises to 35, 110, 220 kV and more. At very large distances, of the order of several thousand kilometers, energy transmission can be carried out on a direct current of high voltage, which makes it possible to reduce energy losses in power lines (PTL). In places of consumption, direct current is again converted into alternating current at special conversion substations. From the busbars of the substation switchgear (RU) through power lines, energy is transmitted to regional step-down substations with a secondary rated voltage of 6-10 kV. From district step-down substations, electrical energy is usually transmitted through cable lines to city distribution points (RP), from which it is distributed between step-down substations located near consumers directly in microdistricts and residential areas.
The set of power plants, power transmission lines, substations, heating networks, connected into one by the common mode, the continuity of the production and distribution of electrical and heat energy, is called the power system.
There are a number of large power systems in Russia that unite a large number of power plants. The part of the energy system, consisting of generators, switchgears, step-up and step-down substations, power lines and electrical receivers, is called an electrical system.
In fig. 11.10 shows an approximate power supply scheme for a large city:
Long-term permissible rated current load for given conditions
Id ³ Imax / (ККп), (11.8)
where Imax is the calculated long-term maximum current load of the network element, A,
determined by the formulas:
a) for three-phase four-wire and three-wire networks
(11.9)
b) for a two-phase network with a neutral wire
, (11.10)
c) for a single-phase network
(11.11)
where Рmах - calculated maximum load, kW; Unom - nominal linear
voltage, V; UФ - rated phase voltage, V.
For networks supplying fluorescent lamps, when determining the estimated current Imax, a multiplying factor should be introduced, taking into account the power losses in ballasts (ballasts), equal to 1.25.