Transmission of Electric Power

The transmission of electric power from a power plant to consumers is one of the most important problems of power engineering. Electric power is transmitted chiefly through overhead AC power lines, although there is a growing tendency toward the use of cables and DC lines.

Electric power has to be transmitted over long distances because it is produced by large plants with high-capacity generating units but is consumed by users with relatively low power needs who are dispersed over large areas. The tendency toward a concentration of capacities is due to economic considerations. Both the relative expense of power-plant construction and the cost of the generated power decrease as capacities increase. High-capacity power plants are sited in accordance with a number of factors, such as the presence and type of energy resources, availability of potential transportation resources, environmental conditions, and possibility of operation as part of an integrated power grid. Such plants are often remote from the main centers of power consumption. The operation of integrated electric power grids covering large areas depends on the efficient transmission of power over long distances.

A basic characteristic of a transmission line is its power capability, or the maximum power that can be transmitted, with allowance made for limitations imposed by system stability, corona losses, the heating of conductors, and other factors. The relationship between the power transmitted by an AC power line and the length of the line and the voltages is given by the equation

Here, U₁ and U₂ are the voltages at the termini of the line; Z_c is the characteristic impedance of the line; a is the phasing back ratio, which characterizes the rotation of the complexor voltage along the line per unit length of the line (the rotation is a result of the wave nature of the propagation of the electromagnetic field); l is the length of the line; and δ is the angle between the complexor voltages at the termini of the line—this angle characterizes the operating conditions and stability of the transmission line. Maximum power is obtained at δ = 90°, when sin δ = 1.

For overhead AC power lines, the maximum transmitted power can be taken to be approximately proportional to the square of the voltage and the construction costs of the line to be roughly proportional to the voltage. There thus is evident in the development of transmission systems a tendency toward higher voltages as the chief means for increasing the power capability of power lines. The maximum voltage values for power lines depend on the possible voltage surges and are limited by the dielectric strength of air. The power capability of an AC power line can also be increased by improving the design of the line or by incorporating into it various compensating devices. In lines at voltages of 300 kilovolts (kV) or more, for example, bundle, or multiple, conductors can be used. They consist of two or more electrically connected subconductors in each phase. Such an arrangement reduces the inductive reactance of the line and increases its capacitive susceptance; consequently, Z_c is lowered, and α is decreased. Another way of increasing the power capability of a power line is the construction of open-circuit lines. In such lines, the conductors of two circuits are suspended from supports in such a way that conductors of different phases are close to each other.

Many factors limiting the power capability of transmission systems are peculiar to AC lines and are absent in DC lines. The maximum power that can be transmitted through a DC power line is higher than the maximum power for a similar AC line:

Here E_out is the output voltage of the rectifier, and R_∑ is the total effective resistance of the transmission line. R_∑ includes not only the resistance of the line’s conductors but also the resistance of the rectifier and inverter. The chief restrictions on the use of DC lines are the technical difficulties encountered in providing efficient and inexpensive devices for the conversion of alternating current to direct current at the sending end and the conversion of direct current to alternating current at the receiving end. DC transmission lines are a promising means for the interconnection of large power systems that are far apart from each other. It is not necessary in this case to make special provisions for the operational stability of the systems.

The quality of electric power depends on the reliable and stable operation of the transmission system. In particular, such operation can be provided by the use of compensating devices and automatic regulating and control systems—for example, in automatic regulation of excitation, automatic voltage control, and automatic frequency control.

P. N. Iablochkov built a transmission system in St. Petersburg in 1876 to provide electric street lighting. It was the first system in the world to be designed for prolonged operation. In 1880, D. A. Lachinov and M. Deprez provided theoretical substantiation for the increasing of voltage as a means of increasing transmission capacity and distance. Extensive use by industry of electric power involving long-distance transmission, however, began only after M. O. Dolivo-Dobrovol’skii invented an economical and relatively simple method for transmitting electric power by a three-phase AC system. The voltage of three-phase systems has grown by a factor of 1.5–2 every ten to 15 years since the construction of the first such systems. The increase in voltage has permitted increases in transmission distances and power capabilities. In the 1920’s the maximum power transmission distance was about 100 km. By the 1930’s, power line lengths increased to 400 km, and by the 1970’s the transmission distances reached 1,000–1,200 km.

Along with the development of AC systems, there have occurred improvements in DC transmission techniques. The first experimental DC cable line in the world was put into operation in the USSR in 1950. It runs between the Kashira State Regional Electric Power Plant and Moscow, operates at a voltage of 200 kV, and has a power capability of 30 megawatts (MW). Accumulated experience permitted an intersystem DC line to be put into operation between 1962 and 1965. An overhead power line that operates at 800 kV, it runs between Volgograd and the Donets Coal Basin and has a power capability of 750 MW. By 1974, more than 20 DC systems were operating in various countries. In the USSR, DC power lines at voltages of ±750 kV over distances of 2,500–3,000 km are planned for construction between 1975 and 1985; power transmission at ±1,200 kV is envisioned for the future.

Since the 1960’s, considerable attention has been given to the development of qualitatively new power transmission systems, for example, “enclosed” lines in the form of a closed structure filled with a dielectric gas such as SF₆ and containing the high-voltage conductors. Cryogenic superconducting power lines are also promising. Enclosed and cryogenic lines are particularly suitable for supplying power to consumers in densely populated areas, for example, within large cities. The feasibility of power transmission through wave guides by high-frequency electromagnetic waves is also being studied.

Electric power transmission is not the only means of supplying energy to consumers over long distances. An alternative is the transporting of fuel. Comparative analysis shows that electric transmission is not always the best method of power supply. It is more advantageous, for example, to transport coal with a high calorific value (exceeding 17–19 megajoules per kilogram) by railroad, if the railroad is already in existence. In a number of cases it is preferable to transport natural gas or oil by pipeline. Analysis of power systems in a number of countries reveals two principal tendencies of development. One tendency is toward constructing power plants closer to power consumption centers when the area served by an integrated power grid lacks sources of cheap energy or when such sources are already exhausted. The other tendency is toward building power plants close to sources of cheap energy and toward the transmission of electric power to consumption centers over long distances. Supply systems for electric power, oil, and gas should be built and operated as mutually coordinated systems and should form an integrated country-wide energy system.