RU2318280C2 - Method for power transmission over cable lines - Google Patents

Abstract

FIELD: cable engineering; alternating-current power transmission over long distances.

SUBSTANCE: proposed method includes AC power transmission over coaxial cable line having annular-section conductors of enlarged sectional area and reduced external dimensions of cable whose diametrical dimensions are adequately chosen.

EFFECT: ability of electrical energy transmission over log distances exceeding hundreds of kilometers.

1 cl, 5 dwg, 5 tbl

Description

The invention relates to cable technology, to the transmission of energy over long distances by alternating and direct currents.

Known methods of power supply to consumers of the electric power system, in which alternating current electricity is converted into direct current energy, stabilize and provide energy to the load (Patents of the Russian Federation, IPC Н02J 3/28, No. 2153752, 2000 and No. 2208890, 2003).

The prior art high voltage direct current transmission is described in the monograph "Power Transmission by Direct Current." or "DC Power Transmission" (Springer Velgar), 1975.

An increase in the transmitted power along the direct current line is not accompanied by a simultaneous increase in the shift angle between the voltages at the ends of the line, as for an alternating current line.

Thanks to this, the concepts of statistical and dynamic stability, characteristic of AC power transmission, are completely removed. That is why direct current power transmission is considered as the only means for transmitting large capacities over distances of thousands of kilometers.

In addition, the independence of the transmitted power from the frequency and the angle of shear between the voltages at the ends of the lines allows the use of direct current transmission for communication systems operating at different frequencies or asynchronously.

The charging power due to the capacitive conductivity of the line has a great influence on the mode of AC lines. For extended lines of high and ultra-high voltage, a large charging power causes a number of undesirable phenomena - an increase in voltage above acceptable levels in normal and synchronization modes, loading generators and synchronous compensators of reactive power flowing from the line, etc. This necessitates the use of means of lateral compensation, which ultimately increases the cost of the line and complicates the maintenance of its regime. For cable lines, a large charging power limits their permissible length and reduces the transmitted active power.

In DC lines, charging power is absent. Due to this, there is no need to use means of lateral compensation on overhead lines, which positively affects their economic indicators, and length restrictions are removed for cable lines. This, in turn, allows the construction of long-distance DC cable lines (110 km or more). It is believed that DC cable lines should be used to cross large water spaces, such as sea straits, when other means of transport of electric energy are unacceptable.

In 1950, the first Kashira-Moscow pilot transmission line was put into operation with a cable line of about 100 km in length, 30 MW, and a voltage between the poles of 200 kV. In 1954, a direct current transmission line (DCT) was commissioned in Sweden. This PEP connected the 98 km long single-line cable line laid along the bottom of the Baltic Sea to the island of Gotland with the Swedish power system. The transmission power was 20 MW, the voltage between the poles and ground was 100 kV, and the earth played the role of the second pole.

Subsequently, intensive construction of PPT in different countries began. Transmissions were built: Volgograd - Donbass (USSR), Pacific transmission (USA), communication of energy systems of England and France through the English Channel, power transmission of Cabora - Basa, South Africa.

They built a direct current power transmission line Ekibastuz-Center with a capacity of 6 thousand MW at a distance of 2,400 km (V.A. Venikov, Yu.P. Ryzhov. Long-distance AC and DC transmissions. Moscow, Energoatomizdat, 1985, p. 161-162 )

The capacity of cable lines is determined by their natural power, and the possibility of its implementation is determined by the active section of the current-carrying core.

There is a method of transmitting electricity to a distance via a coaxial cable, in which alternating current electricity is converted into direct current energy, stabilized and output energy to the load (Patent of the Russian Federation, IPC Н02J 3/28, No. 2208890, 2003 (prototype)).

The known method has several disadvantages, namely the need for energy conversion; limited radial cable dimensions; the presence of complex equipment for converting and stabilizing current; high costs of current conversion and its selection.

The proposed invention eliminates these disadvantages.

The technical result of this invention is to increase the active section of the current-carrying core of the AC cable, matching it with the actual throughput of the cable line, improving the cooling conditions of the cable, reducing the external overall dimensions of the cable, eliminating the restrictions on the length of the AC cable lines, ensuring the transmission of electricity through the AC cable lines current over distances exceeding hundreds of kilometers.

The technical result is achieved by the fact that in the method of transmitting electricity to a distance over a coaxial cable, electric power is transmitted on alternating current through a coaxial cable, the current-carrying core of which is made with an annular cross section. The transverse dimensions of the cable section are selected from the conditions:

where R ₁ is the outer radius of the current-carrying core,

R ₀ - the inner radius of the cable, cores,

Δ = R ₁ -R ₀ - the thickness of the current-carrying core of the cable,

R ₂ is the outer radius of the insulation (inner radius of the shell),

F _A - active section of the current-carrying core of the cable line,

To _a = 1 / K _s the fill factor of the cross section of the current-carrying core with active material,

U _f - the largest working phase voltage of the cable,

E _max - permissible electric field strength in isolation at the highest working phase voltage of the cable.

The invention is illustrated in figures 1-4 and in tables (1-5).

Figure 1 shows the dependence of the ratio of the active current to the total current at the ends of the cable line with adjustable reactors on the wavelength of the line for various ratios of permissible current in terms of cable heating to natural current:

1 - ratio I _max / I _n = 0.1; 2 - ratio I _max / I _n = 0.2; 3 - ratio I _max / I _n = 0.4; 4 - ratio I _max / I _n = 0.6; 5 - ratio I _max / I _n = 0.8; 6 - ratio I _max / I _n = 0.9; 7 - ratio I _max / I _n = 1,0 ...

Figure 2 shows the dependence of the maximum wavelength of the cable line on the ratio of the allowable heating total current to natural current; in the presence of adjustable compensation for excess reactive power at both ends of the cable line (solid curves); in the absence of compensation at the receiving end of the cable line (dashed curves): where curves 8 and 10 are obtained at I _a = 0; and curves 9 and 11 are obtained at I _a / I _max = 0.9.

Figure 3 (a) shows the relationship of the ratio of the active current to the total current, permissible under the condition of limiting the heating of the current-carrying core of the cable, from its active section for various voltage classes: 12 - voltage 220 kV; 13 - voltage 330 kV; 14 - voltage 500 kV; with a wavelength of line 0.628 rad.

Figure 3 (b) shows the same dependencies, in the presence of CSR (controlled shunt reactor) at the end of the line: 12 - voltage 220 kV; 13, 14 - voltage of 330 kV and 500 kV; 15 - voltage 750 kV, 16 - voltage 1150 kV.

Figure 4 shows the dependences of the ratio of the required power Q CSR to the permissible transmitted power P _add on the active section of the current-carrying core of cables of optimal design with an allowable current density J _add = 2 A / mm ² and different voltage classes: 17 - voltage 220 kV; 18 - voltage 330 kV; 19 - voltage 500 kV; 20 - voltage 750 kV and 21 - voltage 1150 kV.

Consider the essence of the method of transmission of electricity. With small cross sections of the current-carrying core, when the power transmitted by cable is acceptable for heating the wires, it is much less than the natural power of the cable line (see Tables 1 and 2), it becomes difficult to ensure the mode of electric power transmission.

The wave impedances of the cable lines are low and decrease with increasing active section F _{A of the} current-carrying core, and with increasing working voltage of the cable Z increase. In this case, the propagation velocity of an electromagnetic wave along cable lines is approximately 1.5–2 times less than along air lines. Accordingly, the wavelengths of cable lines are 1.5–2 times longer than air lines, with the same physical length of these lines.

In table 1 and table. 2 also shows the values of the natural currents of the cable lines and the permissible currents for heating the wires. The active section F _{A of the} current-carrying core of the cable line is determined by the maximum current I I _MAX transmitted through the line and the permissible current density in the current-carrying core J _ADP

As can be seen, the currents allowed by the heating of wires in cable lines with ordinary sections of current-carrying conductors are significantly less than the natural currents of the same lines. When the cable line is equal to the wavelength, the maximum current in all modes from idle to natural current flows at both ends of the line.

When the cable line is idling, reactive (capacitive) current flows at its ends (at the ends of the cable line), and in the natural power transmission mode I _MAX = I _N , i.e. in all sections, the cable line current is the same and there is no reactive current.

In all intermediate modes, the maximum current at the ends of the cable line is determined by the combination of active and reactive currents. In this case, the maximum current cannot be more than the permissible current for heating the cable.

The ratios I _A / I _max depending on the wavelength of the cable lines for various ratios I _A / I _n are shown in FIG.

The transmission of electrical energy through cable lines is possible only up to a certain wavelength thereof, depending on the ratio I _max / I _n , when the capacitive current of the cable at the ends of the line reaches a limit on the heating limit of the cable. Moreover, the larger the ratio I _max / I _n , the longer the cable line can transmit electric energy, since with increasing I _max / I _n the capacitive component of the current in the line decreases.

With a ratio of I _max / I _n > 0.5, the maximum wavelength of the line exceeds 0.95 rad, which for a cable line at a frequency of 50 Hz corresponds to a physical line length of l = 550 km. However, with small ratios I _max / I _n characteristic of cables of a traditional design, the permissible lengths of cable lines are much smaller.

With an increase in the ratio I _max / I _{n, the} maximum wavelength of the cable line increases, reaching λ _PR = π / 2 for I _max / I _n = 1 (Fig. 2), when sinλ _PR = 1. For the maximum wavelength of the cable line, power transmission is not possible.

A more stringent limitation of the length of AC cable lines can be obtained based on the limitation of the ratio of the transmitted active current to the permissible total current under the condition of limiting the heating of the current-carrying core I _A / I _max

With large ratios I _A / I _{max, the} maximum wavelength of the cable line is much less than under the condition that there is no active current. However, with relatively large cross sections of the current-carrying cable core, when I _{max is} close to I _{n, the} limiting wave lengths of cable lines are close in both cases (see Fig. 2, solid lines).

The range of variation of the I _MAX / I _H ratio for cables of optimal design is limited to 0.5 ÷ 0.95 (values near the upper limit are typical for cables of higher voltage classes). The corresponding ultimate wavelengths of the cable lines of FIG. 1 vary from 0.5 to 1.5 rad, which at a frequency of 50 Hz corresponds to the physical lengths of the lines 288 ÷ 860 km. It should be noted that the wavelength of cable lines is limited as well as overhead lines, by the requirement to limit the increase in voltage on the line in idle mode.The values of the allowable physical length of the lines are presented in Table 3.

Table 3 Voltage class, kV The maximum length of the cable line, l _PR , km no reactor at the end with a reactor at the end 50 Hz 60 Hz 50 Hz 60Hz 110-220 300 250 600 500 330 250 210 500 400 500-1150 180 150 362 300

Accepting the limitation of wavelengths in the presence of a shunt reactor at the end, it is possible to obtain the dependence of the ratio I _A / I _max on the active section of the current-carrying core (Figs. 3a and 3b).

As can be seen from Fig. 3 (a, b), at the maximum allowable lengths of cable lines, with an allowable current under the condition of limiting the heating of the current-carrying core I _max and with its active cross section F _A > 2000 mm ^{2, the} active component of the current at the ends of the cables is quite large and small different for different voltage classes. When F _A <2000 mm ^2, this ratio is much smaller and noticeably different for cable lines of different voltage classes, especially when taking into account the difference in allowable wavelengths of cable lines (Fig.3b). With a large active section of the current-carrying cable core, when the current allowed by the condition of limiting its heating approaches the natural current of the cable line, the capacitive current of the cable does not limit the length of the cable line.

More stringent is the limitation of the length of the line associated with an increase in voltage on the line in idle mode. Moreover, for cable lines this restriction is more stringent than for overhead, due to the reduced propagation speed of electromagnetic waves along cable lines.

The use of cables with a constant thickness of the conductive layer of the current-carrying core Δ, permissible under the condition of limiting its heating, removes these restrictions, it is only necessary to ensure a sufficiently large ratio

The length of the cable line can be increased when connected to it at intermediate points of controlled shunt reactors (CSR). In this case, by the limited length of the cable line according to Table 3, we should understand the section between two points with controlled shunt reactors (CSR). When laying cable lines under water, such intermediate points can be organized on islands or artificial platforms.

When laying cable lines by land, the creation of intermediate points with CSR is no more complicated than for overhead lines.

The ratio of the required power of the CSR to the permissible (maximum) transmitted power is equal to:

As can be seen, for a given wavelength of cable lines, the ratio of the required rated power of the CSR to the maximum transmitted power decreases with an increase in the active section of the current-carrying cable core (Fig. 4).

характерна для классов напряжения 220 и 330 кВ. Для высших классов напряжения эта зависимость значительно слабее.Most violent addiction

typical for voltage classes 220 and 330 kV. For higher voltage classes, this dependence is much weaker.

Assuming the permissible wavelength of the line section between two CSRs, λ _DOP = 0.628 rad, corresponding to the physical length of the line l _DOP = 362 km at a frequency of 50 Hz, J _add = 2 A / mm ² , Δ = 15 mm and E _max = 8 kV / mm , we obtain the following dependences of the ratio of the required rated power of the CSR to the maximum power transmitted through the cable from the physical length of the lines at a frequency of 50 Hz (see Table 4).

As you can see, an increase in the cable voltage class leads to a decrease in the ratio of the required power of the CSR to the maximum power transmitted along the line.

A decrease in this ratio is achieved with the maximum active section of the current-carrying core of the zone of optimal sections (see Table 5).

In the latter case, the required CSR power for transmitting the maximum power on cable lines is greater than on overhead lines, mainly due to an increase in the wavelength of cable lines compared to overhead lines for a given physical length of lines.

Claims (1)

A method for transmitting electric power over a distance through a coaxial cable with a central current-carrying core, characterized in that the electric power is transmitted on alternating current through a coaxial cable, the current-carrying core of which is made with an annular cross section, and the transverse dimensions of the cable section are selected from the conditions

where R ₁ is the outer radius of the current-carrying core; R ₀ is the inner radius of the cable, cores; Δ = R ₁ -R ₀ is the thickness of the current-carrying core of the cable; R ₂ is the outer radius of insulation (inner radius of the shell); F _a - active section of the current-carrying core of the cable line; To _and - the fill factor of the cross section of the conductive core with active material; U _f - the largest working phase voltage of the cable; E _max - permissible electric field strength in isolation at the highest working phase voltage of the cable.

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