When designing the development of the electrical network, simultaneously with the development of the issue of the configuration of the electrical network, the issue of choosing its nominal voltage is being resolved. The scale of nominal line voltage of electrical networks is established by GOST 721-77 and is the following:
0.38; 3; 6; ten; 20; 35; 110; 150; 220; 330; 500; 750; 1150 kV.
When choosing the rated mains voltage, the following are taken into account general recommendations:
voltages 6 ... 10 kV are used for industrial, urban and agricultural distribution networks; most widespread received a voltage of 10 kV for such networks; the use of a voltage of 6 kV for new facilities is not recommended, but can be used during the reconstruction of an existing electrical network if there are high-voltage motors in it for such a voltage;
at present, due to the increase in the utilities sector loads, there is a tendency to increase the voltage of distribution networks in large cities up to 20 kV;
35 kV voltage is widely used to create power centers for agricultural distribution networks of 10 kV; in connection with the increase in the capacity of rural consumers, 110 kV voltage is used for these purposes;
voltages of 110 ... 220 kV are used to create regional distribution networks for public use and for external power supply to large consumers;
voltages of 330 kV and above are used to form backbone links of the UPS and to provide power to large power plants.
Historically, two voltage systems of electrical networks (110 kV and above) have been formed in our country. One system 110 (150), 330, 750 kV is typical mainly for the North-West and partly for the Center and the North Caucasus. Another 110, 220, 500 kV system is typical for most of the country. Here, the voltage of 1150 kV is taken as the next step. Electric transmission of such a voltage was built in the 80s of the last century and was intended to transfer electricity from Siberia and Kazakhstan to the Urals. Currently, the 1150 kV transmission sections are temporarily operating at 500 kV. The transfer of this transmission to the voltage of 1150 kV will be carried out later.
The rated voltage of an individual transmission line is mainly a function of two parameters: power Rtransmitted over the line and distance Lto which this power is transmitted. In this regard, there are several empirical formulas for choosing the nominal line voltage, proposed by different authors.
Still's formula
U nom \u003d, kV,
where R, kW, L, km, gives acceptable results at values L250 km and R60 MW.
Illarionov's formula
U nom \u003d 
,
where R, MW; L, km, gives satisfactory results for the entire range of rated voltages from 35 to 1150 kV.
The choice of the rated voltage of the electrical network, consisting of a certain number of lines and substations, is, in the general case, the task of a technical and economic comparison of various options. Here, as a rule, it is necessary to take into account the costs not only for the transmission line, but also for the substation. Let us explain this with a simple example.
An electrical network is being designed, consisting of two sections of length L1 and L2 (fig. 4.1, and). A preliminary assessment of the rated voltage showed that 220 kV should be taken for the head section, and 110 kV for the second section. In this case, you need to compare the two options.
In the first option (Fig.4.1, b) the entire network is performed at a voltage of 220 kV. In the second option (fig. 4.1, at) the head section of the network is performed at a voltage of 220 kV, and the second section - at a voltage of 110 kV.
In the second version, the line W2 voltage 110 kV and a 110/10 kV substation with a transformer T will be cheaper than the line W2 220 kV and 220/10 kV substations with a transformer T2 of the first option. However, a 220/110/10 kV substation with an autotransformer AT the second option will be more expensive than a 220/10 kV substation with a transformer T1 of the first option.
a B C)
Figure: 4.1. Scheme ( and) and two options ( b) and ( at) mains voltages
Final choice network voltage will be determined by comparing these cost options. If the cost difference is less than 5%, the option with a higher rated voltage should be preferred.
The rated voltages of general-purpose electrical networks of alternating current in the Russian Federation are established by the current standard (Table 4.1). Table 4.1
The International Electrotechnical Commission (IEC) recommends standard voltages above 1000 V for 50 Hz systems as shown in table. 4.2. Table 4.2 
A number of attempts are known to determine the economic zones of application of power transmissions of different voltages. Satisfactory results for the entire scale of rated voltages in the range from 35 to 1150 kV are given by the empirical formula proposed by G. A. Illarionov: 
where L is the line length, km, P is the transmitted power, MW. In Russia, two voltage systems of alternating current electrical networks (110 kV and above) have become widespread: 110-330-750 kV - in the IES of the North-West and partially in the Center - and 110-220-500 kV - in the IES of the central and eastern regions of the country ( see also section 1.2). For these OESs, the voltage of 1150 kV, introduced in GOST in 1977, was taken as the next stage. A number of constructed 1150 kV transmission sections are temporarily operating at a voltage of 500 kV. At the current stage of development of the UES of Russia, the role of backbone networks is played by networks 330, 500, 750, in a number of power systems - 220 kV. The first stage of public distribution networks is 220, 330 and partly 500 kV, the second stage is 110 and 220 kV; then the electricity is distributed over the power supply network of individual consumers (see paragraphs 4.5–4.9). The conventionality of dividing networks into backbone and distribution networks by nominal voltage is that as the load density, power of power plants and the coverage of the territory with electric networks increase, the voltage of the distribution network increases. This means that the grids that perform the functions of backbone, with the appearance of higher voltage grids in the power systems, gradually “transfer” these functions to them, turning into distribution ones. A general-purpose distribution network is always built on a stepwise principle by sequentially "superimposing" networks of several voltages. The appearance of the next voltage step is associated with an increase in the power of power plants and the expediency of supplying it at a higher voltage. The transformation of the network into a distribution network leads to a reduction in the length of individual lines due to the connection of new substations to the network, as well as to a change in the values \u200b\u200band directions of power flows along the lines. With the existing density of electrical loads and a developed network of 500 kV, abandoning the classic scale of rated voltages with a step of about two (500/220/110 kV) and a gradual transition to a scale step of about four (500/110 kV) is a technically economically sound solution. This tendency is confirmed by the experience of technically advanced foreign countries, when intermediate voltage networks (220-275 kV) are limited in their development. This technical policy is most consistently pursued in the power systems of Great Britain, Italy, Germany and other countries. So, in the UK, the transformation of 400/132 kV is increasingly used (the 275 kV network is conserved), in Germany - 380/110 kV (the development of the 220 kV network is limited), in Italy - 380/132 kV (the 150 kV network is conserved), etc. ... The most widespread as distribution networks are 110 kV, both in the UPS with a voltage system of 220–500 kV and 330–750 kV. The specific gravity of 110 kV lines is about 70% of the total length of 110 kV overhead lines and above. This voltage is used to supply power to industrial enterprises and power centers, cities, electrification of railway and pipeline transport; they are the top tier of rural power distribution. The voltage of 150 kV was developed only in the Kola power system and is not recommended for use in other regions of the country. Voltages 6-10-20-35 kV are intended for distribution networks in cities, rural areas and industrial enterprises. The predominant distribution is 10 kV; 6 kV networks maintain a significant specific gravity in length, but, as a rule, do not develop and, if possible, are replaced by 10 kV networks. This class adjoins the voltage of 20 kV available in GOST, which has received limited distribution (in one of the central districts of Moscow). The 35 kV voltage is used to create a 10 kV central heating network in rural areas (35 / 0.4 kV transformation is used less often).
Each electrical network is characterized by the rated voltage for which its equipment is designed. The rated voltage ensures the normal operation of electrical consumers (ED), should give the greatest economic effect and is determined by the transmitted active power and the length of the transmission line.
GOST 21128-75 introduced a scale of nominal phase-to-phase voltages of electrical networks and receivers up to 1000 V AC: 220.380, 660 V.
GOST 721-77 introduced a scale of nominal phase-to-phase voltages of alternating current electrical networks over 1000 V:
0,38, 3, 6, 10, 20, 35, 110, 150, 220, 330, 500, 750, 1150.
Table 2.1. the classification of electrical networks is presented, which shows the division into networks of lower (LV), medium (MV), higher (HV), ultra-high (EHV) and ultra-high (UHV) voltage.
The load of the electric drive does not remain constant, but changes depending on the change in the operating mode (for example, in accordance with the course of the technological process of production), therefore the voltage in the network nodes constantly deviates from the nominal value, which reduces the quality of electricity and entails losses. Studies have shown that for most electrical receivers, the stable zone is limited by the values \u200b\u200bof voltage deviations
Studies have shown that for most electrical receivers, the stable zone is limited by the values \u200b\u200bof voltage deviations
As a rule, the voltage at the beginning of the line is greater than the voltage at the end and differs by the amount of voltage loss
To bring the consumer voltage U 2 closer to the rated voltage of the electrical network and provide high-quality energy, the rated voltages of the network voltage generators are set by GOST 5% more than the nominal
Since the primary windings of step-up transformers must be directly connected to the same terminals of the generators, their rated voltages
The primary windings of step-down transformers are consumers in relation to the networks from which they are powered, therefore the condition must be met
Recently, the industry has been producing step-down transformers with a voltage of 110-220 kV with a primary winding voltage of 5% higher than the nominal grid voltage
The secondary windings of both step-down and step-up transformers are sources in relation to the network they supply. The rated voltages of the secondary windings are 5-10% higher than the rated voltage of this network
This is done in order to compensate for the voltage drop in the supply network. In fig. Figure 2.1 presents a stress diagram that clearly illustrates the above.
2.2. Neutral modes of electrical networks
The zero point (neutral) of three-phase electrical networks can be grounded tightly (Figure 2.2, a), grounded through a high-resistance resistance (Figure 2.2, b), or isolated from the ground (Figure 2.2, c).
The neutral mode in electrical networks up to 1000 V is determined by the safety of network maintenance, and in networks above 1000 V - by uninterrupted power supply, efficiency and reliability of electrical installations. The electrical installation rules (PUE) allow the operation of electrical installations with voltages up to 1000 V both with solidly grounded and insulated neutral.
End of work -
This topic belongs to the section:
LECTURE 1. GENERAL CHARACTERISTICS OF ELECTRIC ENERGY TRANSMISSION AND DISTRIBUTION SYSTEMS. MODELING ELECTRICAL SYSTEMS
Plan ... Basic concepts and definitions ...
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All topics in this section:
Characteristics of the electric power transmission system
The basis of the system for the transmission of electrical energy from power plants producing it to large areas of power consumption or distribution nodes of EPS are developed
Characteristics of electrical power distribution systems
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High-voltage networks with compensated neutral
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High-voltage networks with solid-grounded neutral
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