Electric Power Supply

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Electric Power Supply 

the providing of electricity for all branches of the economy: industry, agriculture, transportation, municipal services, and so on.

A system for electric power supply includes power sources, step-up and step-down electric power substations, electric feeder and distribution systems, and various auxiliary devices and structures. The major portion of the electricity generated is consumed by industry—approximately 70 percent in the USSR (1977). The organizational structure of electric power supply is determined by the special characteristics of electric power generation and distribution that have been established historically in various countries. Common principles govern the design of systems in all industrially developed countries. Certain distinguishing features and local differences are due to the geographical size of a country, climatic conditions, the level of economic development, the volume of industrial production, and the distribution density of electrified installations and their power consumption.

Power sources. The principal sources of electricity are electric power plants and feeder systems of regional energy systems. District heat and power plants are used by industrial enterprises and cities to supply both electricity and heat; their capacity depends on the heat required for production purposes and heating. District heat and power plants with generators that produce current at voltages up to 20 kilovolts (kV) are constructed to supply large enterprises, such as metallurgical works, with a high heat consumption and a substantial secondary power output. The power plants are usually sited outside the boundaries of the works at a distance of 1–2 km. They are of importance for the region because they supply heat and power not only to the enterprise but also to nearby industrial and residential areas. Some consumers of electricity can ease the burden on power sources during peak hours by allowing interruption or limitation of power consumption without significant ill effect on production processes. Among such power consumers are the majority of electric furnaces, which retain well the heat already produced, and some electrolytic equipment. They make it possible to even out sharp load curves in energy systems.

The voltages of systems of electric power supply represent optimum values that have been tested in practice. In each situation the choice of voltage depends on the power being transmitted and the distance from the power source to the consumer. The voltage ranges adopted in various countries do not differ fundamentally. The voltages used in the USSR (6,10, 20, 35,110, 220, 300 kilovolts, and so on) are typical of other countries as well. The voltage ranges of some countries have intermediate values that were introduced during an earlier stage in the development of the transmission and distribution systems and continue in use, although in many cases they are not optimum values. The power supplied to large industrial enterprises, transportation, and municipal services is delivered at voltages of 110 and 220 kV (in the USA it is often 132 kV); for particularly large energy consumers 330 and 500 kV are used. In the primary distribution stages voltages of 110 or 220 kV are used. A voltage of 110 kV is used most frequently because it makes it easier to accommodate aerial transmission lines in built-up industrial and urban areas. It is expedient to distribute power among consumers at a voltage of 220 kV when such voltage matches the feed voltage. Under some conditions it is advantageous to have a mains voltage of 60–69 kV (it is used in many countries of Western Europe and in the USA).

A voltage of 35 kV is used for the feeder and distribution systems of medium-power industrial enterprises, in small and medium-size cities, and in rural electrical systems as well as for the supply of high-capacity power users in large enterprises—electric furnaces, rectifier equipment, and the like. A voltage of 20 kV is used relatively infrequently to extend systems that already carry this voltage; its use may also be advisable in areas having a low electric load density as well as in large cities and major enterprises served by a district heat and power plant with a generator voltage of 20 kV. Voltages of 6 and 10 kV are used for electric power distribution (at various supply levels) in industrial enterprises, cities, and elsewhere. Such voltages are also suitable for supplying consumers of small amounts of power located near the power source. In most cases it is advisable to make 10 kV the primary voltage, in which case electric motors can be fed from 10/6 kV step-down substations directly from a transformer or from the 6-kV winding of a 110/220 kV transformer with split secondary windings (10 and 6 kV).

System layouts. Circuits for electric power supply are designed on the principle of bringing a high-voltage source of power as close as possible to the consumers with a minimum number of intermediate switching and transformer stages. This entails the use of high-level inputs (from 35 to 220 kV) of cable and aerial power transmission lines. The step-down substations are located centrally to major power consumers, that is, at electric load centers. As a result of such siting, electric power losses are reduced, less material is required, the number of intermediate system links is decreased, and operating conditions for power consumers are improved. The elements of the supply system carry a constant load; they have reciprocal standby functions that take into account permissible overloads and a reasonable limit for the power consumption under postemergency conditions, when an element or portion of the system is recovering from a failure. In most cases provision is made for separate functioning of all elements, based on the extensive use of automatic devices and high degree of links isolation. Parallel operation is only used in case of necessity.

High-level inputs may be trunk or radial lines (Figure 1), depending on environmental conditions, building development in the area, and other factors. The simplest input layout has cable radial lines running directly into a substation transformer; it is also the most compact and reliable plan. High-level inputs may also use compact, completely enclosed units—integrated bus structures filled with sulfur hexafluoride—carrying 110 kV.

Layouts for distribution systems at 6–20 kV may involve trunk, radial, or composite lines (Figure 2) modified according to the degree of reliability required. The first stages of electric power supply for large enterprises usually use trunk lines having high-capacity conductors at 6–10 kV, from which the shop transformer stations are fed through distribution stations. Looped, double-feeder, and multifeeder layouts, which are varieties of trunk lines, are used in municipal systems carrying 6 or 10 kV.

The circuits of large central substations carrying 110–220 kV (in large factories and in cities with an extended electrical network and a large number of connections) usually have a double-bus system. In large bus structures carrying voltages of 6 and 10 kV, where it may be necessary to divide the supply or to isolate consumers (for example, in large converter substations), a double-bus system permits some aggregates to be shifted to a lower voltage while the normal voltage is maintained for other users. The substation circuits most often used in consumers’ installations have a single system of sectionalized buses with (when needed) automation for the section circuit breakers or inputs. When frequent operational changeovers, examinations, or switch tests are required, it is convenient to have circuits with a bypass (auxiliary) bus system that allows inspection or repair of any operating bus system and any switching procedure without interrupting the power supply. Such circuits are used, for example, in the large electric-furnace substations of industrial enterprises. Simple substation circuits without primary-voltage buses are widely used in high-level input substations carrying 210 and 220 kV and in transformer substations carrying 6 and 10 kV, which are supplied over line-to-transformer circuits (see Figures 1 and 2). Transformer substations have load-break switches on the 10 and 6 kV side; a dead-break connector between transformers is used for radial feeds.

In large consumer installations it is expedient to build systems with high-capacity conductors carrying 10 and 6 kV (instead of a large number of cables) and cable trestles and galleries (in place of surface lines and large tunnels) as well as to run cables at 110 and 220 kV (instead of aerial lines).

Reliability. The reliability of electric power supply depends on the requirements for uninterrupted operation imposed by the power users. The minimum degree of reliability is determined by the permissible amount of loss incurred by production in the event of an interruption in supply. There are three categories of reliability for power users. The first applies to users supplied by no fewer than two independent, automatic standby sources. Equipment so supplied is necessary in plants where the necessity of uninterrupted operation is greater than normal (for example, continuous chemical production). In such cases the best circuits have independent sources from different geographical areas. The permissible power interruption for some production is no longer than 0.15–0.25 sec; thus the necessary high-speed restoration of power is an important condition. An additional third source is provided in the supply circuit for particularly critical power users. The second category applies to power users that can tolerate a supply interruption for the time needed to connect a hand-switched standby. Power users in the third category can tolerate supply interruptions up to 24 hr in duration—the time needed to replace or repair a defective element in the system.

Quality of electric power. Systems of electric power supply often have power users that impose severe short-term loads during operation that adversely affect the operation of other power users, the overall operating conditions of the system, and the quality of the electric power supplied. Among these are valve-type converters, arc furnaces, electric welding equipment, and electric locomotives, whose operation produces sharp load variations, voltage fluctuations, a reduced power factor, high harmonics, and nonsymmetrical voltages. The quality indicators of electric power may be improved by increasing the short-circuit rating at the point in the system where power users having adverse characteristics are connected. In order to create such conditions, the reactance of the feed lines is reduced by omitting series reactors or reducing their reactance, by eliminating current conductors from the circuit, and by other measures. In such cases there should be a corresponding increase in the capacity of the disconnecting devices used.

Problems concerning the improvement of the quality of electric power supplied are worked out jointly in the designing of a system of power supply and an electric drive. Good results are obtained by separating the supplies to power users that impose severe short-term loads and those imposing normal loads via connections to different transformers and different taps on split transformers or taps on double reactors. The quality may also be improved by using electric drives having lower reactive power consumption and by using multiphase rectifier circuits. When such measures prove inadequate, special equipment may be used: synchronous equalizers with high-speed excitation and a large reactive power overload factor (3–4) that operate in a tracking mode governed by the reactive power of consumers; synchronous motors having a normal load that are connected to buses in common with valve-type converters and that have the requisite available power and high-speed excitation with a high level of overexcitation; static sources of reactive power that exhibit rapid response times and zero lag and that vary reactive power smoothly; longitudinal capacitive compensation, which permits instantaneous and continuous automatic voltage control with zero lag; and electric power filters for suppressing higher harmonics.

Electric Power System 

an aggregate of electric power plants interconnected for parallel operation, power transmission lines, transformer substations, and consumers of electric power. An electric power system has standby equipment for common use and a centralized supervisory office for day-to-day coordination of the operations of the power plants, substations, and distribution systems.

Electric power systems are often associated with heat and power systems that include district heat and power plants and district heat supply systems. A heat and power system in conjunction with centralized power transmission and distribution provides centralized heat supply for cities and industrial centers. With respect to science and technology, a transition to the broader concept of a heat and power system implies not only consideration of the electrical part of the system and the electrical and electromechanical processes occurring in it, but also consideration of the associated mechanical and thermomechanical processes in turbines, boilers, and pipelines.

Heat and power systems may be classified according to installed capacity, the presence of interconnections with other systems, the system’s structure, the power generated, the territory covered, the load distribution, and the physical configuration. Systems are divided (in a first approximation) into three groups by installed capacity: systems with a capacity of more than 5 giga-watts (GW), those between 1 and 5 GW, and those with less than 1 GW (the last group also includes independent power supply systems, such as those in mobile units, for example, ships and aircraft). The structure of a heat and power plant and the installed capacity depend on the type and capacity of the system’s power plants (fossil-fuel-fired steam power plants, hydroelectric power plants, atomic power plants, and other types). The physical configuration of a heat and power system and its switching can be diverse (the physical configuration of a system refers to the relative locations of the system’s power plants, the principal transmission and distribution systems, or, in the case of an interconnected system, the individual subsystems; switching refers to the connections between power plants and the centers of power consumption). The electrical sections of individual heat and power systems are interconnected by main line connections that transmit power in one direction from one system to another by inter-system lines designed for the exchange of electric power.

The operation of an electric power system or a heat and power system is characterized by an operating mode—a group of processes that determine at any moment of time the values of the power, voltage, current, and frequency as well as other quantities that vary during operation of the system. A distinction is made between the steady-state and transient operating modes of a heat and power system. In the steady-state mode the power, voltage, current, and so on are practically constant; in the transient mode they vary either as a result of a control action, that is, a directed action by personnel or by automatic equipment (normal transient processes), or as a result of random disturbances that disrupt the operating mode of the system (emergency transient processes). A corresponding distinction is made between the normal operating mode, in which a heat and power system functions according to prescribed conditions with normal quality indicators for the electric power, and the emergency mode, in which the operation of the system differs from the normal mode when emergencies occur and the quality indicators are abnormal. The postemergency mode is defined as the state of the system after the emergency has been eliminated.

The quality of operation of an electric power system depends primarily on the reliability of the power supply and the quality indicators of the electric power. The reliability of a heat and power system as a whole is determined mainly by the stability of the electric power system and its ability to counteract the development of emergencies (system viability). To a considerable extent the reliable operation of a heat and power system is assured by counteremergency automation, which includes automatic regulation of excitation and a protective relay system as well as preventive protection, by which reports are made on the condition of the system’s elements and on the possible danger of a failure. Counteremergency automation includes automatic isolation for frequency variation and, in a number of cases, for voltage variation (that is, the disconnection of some consumers when there is a dangerous change in the operating parameters), automatic switch-in of standby facilities, automatic reconnection of system elements, automatic elimination of asynchronous operation for a portion of the system, and other measures.

The primary task of a heat and power system is to provide a centralized power supply with integrated day-to-day supervision of the generating, transmitting, and distribution processes for electric power. In the USSR control of a system’s operation is handled by the supervisory services of the regional power administrations, which are subordinate to the Integrated Supervisory Offices of the heat and power system. Day-to-day supervision of the functioning of integrated heat and power systems is accomplished by the Central Supervisory Office of the Integrated Electric Power System of the USSR.

Achievement of the optimum level of electrification in a country with the most economical and reliable electric power supply requires the solution of many scientific problems, including the optimization of development and the day-to-day supervision of the operation of heat and power systems. The solution of such problems requires the extensive use of the systems approach, systems analysis, and the methods of cybernetics.

The creation of a heat and power system provides an economically advantageous increase in the capacity of electric power plants and power units, improves the reliability of electric power supply by means of more flexible manipulation of system reserves, and reduces the total (combined) maximum load as a result of the noncoincidence of the daily load peaks in different regions, thereby reducing the required capacity for an integrated power system. It makes it possible to establish the most efficient operating modes for different types of electric power plants and units, reduces the amount of fuel that must be transported, and facilitates the extensive use of hydroelectric resources that are often far removed from the principal consumers of electric power.

In the countries of Western Europe and in the USA, interconnections between electric power systems are also being intensified. However, the formation of an integrated electric power grid on a national scale does not conform to the capitalist method of production. Electric power supply, which is provided by individual electric power systems that are interconnected only for the reciprocal sale of electric power, often does not provide the quality of electricity required. This is reflected in the nonconformity of technological development with current technological, economic, and social conditions. In the USA, for example, efforts are being made to overcome this disparity by creating associations of private companies to pursue the joint development and operation of electric power systems.

In the USSR the development of electric power systems is inseparably tied to the concentration of electric power generation and the centralization of power distribution. By 1970 construction of the Integrated Electric Power Grid of the European part of the USSR was practically completed. It includes 61 regional heat and power systems and seven interconnected power systems. Interconnected heat and power systems have been created for Siberia and Middle Asia. The Mir international power system, which connects the systems in the member countries of the Council for Mutual Economic Assistance, has undergone extensive development.