Electric Power Supply System

(Russian, elektricheskaia set’), an aggregate of equipment used to transmit and distribute electricity from sources to consumers. General-purpose power supply systems transmit and distribute approximately 98 percent of the total electricity generated. They link electric power plants with consumers of electricity within electric power systems and interconnect individual systems by overhead or cable power transmission lines. Electric power supply systems ensure reliable, centralized power supply with the required quality and excellent economy to widely dispersed locations of consumption. Some electric power supply systems are self-contained and are not connected to transmission lines, for example, the systems used in aircraft, ships, and automobiles.

Electric power supply systems may be classified according to several characteristics. Depending on their purpose, they may be classified as feeder systems, used to transmit electric power, and distribution systems, which distribute power from central substations to consumers (urban, industrial, agricultural, and other users). Classification by voltage divides systems into two groups: those carrying voltages up to 1 kilovolt (kV) and those carrying voltages of more than 1 kV. Electric power supply systems may also be classified by type of current (direct and alternating), by plant location (overhead and cable), by layout (circular and radial), and by normal operating mode (open and closed). In addition to transmission lines, electric power supply systems also have power substations, which are used for the conversion and distribution of electric power and for controlling operation of the system (raising and lowering voltages, converting three-phase alternating current to direct current and vice versa, and providing a number of outgoing lines that differ from incoming lines). Voltage is usually lowered or raised in several steps. For each voltage step there is a separate network of transmission lines and substations through which electricity is fed to the network operated at the next voltage step. The resulting multistage system consists of several interconnected networks carrying different voltages.

The great majority of electric power supply systems carry alternating current. In the USSR the nominal voltages for such systems have been standardized as follows: 12, 24, 36, 48, 60, 127, 220, 380, and 660 volts (V) and 3, 6, 10, 20, 35, 110, 150, 220, 330, 500, and 750 kV. Systems operating at voltages of less than 220 V are used for supplying power to low-power consumers (household lighting fixtures, electrical equipment, and the like). Low voltages are carried where usage conditions are hazardous; for example, a maximum of 36 V is used for localized illumination of work positions in industrial enterprises, and 12 V is supplied in mines. Electric power supply systems with voltages from 380 V to 10 kV serve high-demand consumers, primarily large electric motors. Electric power supply systems with voltages of 6 kV and higher are used primarily for power transmission and distribution and require a subsequent step-down in voltage. Feeder systems and a large number of distribution systems use overhead transmission lines. However, electricity is frequently supplied through cable lines in densely built-up areas, in regions with a severe climate (frequent freezing rain, wind, or thunderstorms), and on valuable agricultural tracts. Cable lines are usually laid underground, but underwater and aboveground cables are also used. In the USSR, AC feeder cable lines carry a maximum of 500 kV and handle up to 0.5 gigawatt (GW). Cable power supply systems capable of carrying 750 kV have also been built, for example, in France.

DC distribution systems are used mainly to supply electricity for urban transportation, for some railroads, and for certain electrochemical enterprises. DC feeder systems are used for the following applications: the transmission of power in excess of 5 GW over very long distances (more than 1,500 km) without intermediate take-offs (such as the Ekibastuz-Central Zone line in the USSR: voltage, 750 kV; distance, 2,500 km; transmitted power, 6 GW); integration of AC systems operated at different frequencies (adopted in Japan and Canada); integration (for limited carrying capacities) of large interconnected electric power systems (such as the ±400 = kV Volgograd-Donbass line and the ±400 = kV Pacific transmission system in the USA); and power transmission via underwater cables (such as the 100 = kV line from Sweden to the island of Gotland and the ± 100 = kV line between Great Britain and France). The total length of all DC feeder lines in the world is less than 1 percent of the total length of AC feeder lines.

The growth in unit power ratings of electric power plants and the siting of the largest power plants in the Asian section of the USSR calls for an intensive increase in the carrying capacity of electric power supply systems as well as an increase in the maximum transmission distance. These demands determine the primary trends in the development of new electric power supply systems. During the 1970’s the maximum nominal voltage for overhead AC power supply systems was increased to 750 kV, both in the USSR and the USA (with carrying capacities of 2.5 GW per circuit). The next voltage step to follow will be 1,150 kV (approximately 6 GW), and a step to 1,500 kV is envisioned (up to 15 GW). Construction of overhead lines and outdoor substations for AC operations at still higher voltages is impeded mainly by the required drastic increase in the overall dimensions of supporting structures, by the limited capabilities of air insulation, and by ecological factors. The estimated maximum possible voltage for overhead DC power supply systems is ±1,100 kV with a carrying capacity up to 15 GW. Further increase of the carrying capacity of electric power supply systems calls for engineering solutions based on new principles, such as the use of conductors with gas insulation (sulfur hexafluoride or Freon) laid in hermetically sealed pipes with diameters up to 3 m. In 1977 the carrying capacity of such power supply systems (operated at a voltage of 500 kV) was 6.5 GW. In principle, it is possible to construct lines with gas insulation for voltages up to 3,000 kV and carrying capacities of 180 GW.