在电力系统的许多工作位置上和许多用户的家中都要安装许多各种型号的仪表。有了这些仪表，就能连续监控供电的工作状态。
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Electric Power Systems
Introduction of Electric Power Systems
As the power industry grows, so do the economic and engineering problems connected with the generating, transmission and distribution systems used to produce and handle the vast quantities of electrical energy consumed today. These systems together form an electrical power system.
It is important to note that the industry that produces electrical energy is unique in that it manufactures its product at the very instant that it is required by the customer. Energy for the generation of electricity can be stored in the form of coal and oil, and of water in reservoirs and lakes, to meet future requirements, but this does not decrease the need for generator capacity to meet the customers’ demands.
It is obvious that the problem of the continuity of service is very important for an electrical power system. No service can be completely protected from the possibility of failure and clearly the cost of the system will depend on its requirements. However a net reliability gain is obtained by employing a certain number of generating units and by using automatic breakers for the separation into sections of the bus bars in generating stations and of the transmission lines in a national or international grid system. In fact a large system comprises numerous
generating stations and loads interconnected by high-capacity transmission lines. An individual unit of generation or set of
transmission ‘lines can usually cease to function without interrupting the general service.
The most usual system today for generation and for the general transmission of power is the three-phase system. In favor of this system are its simplicity and its simplicity and its saving with respect to other a.c. system. In particular, for a given voltage between conductors, with a given power transmitted, with a given distance, and with a give line loss, the three-phase system requires only 75 per cent of the copper or aluminium needed in the single-phase system. Another important advantage of the three-phase system is that three-phase motors are more efficient than single-phase ones. The sources of energy ofr large-scale electricity generation are:
1. steam obtained by means of a conventional fuel (coal, oil or natural gas),
the combustion of city refuse or the employment of nuclear fuel; 2. water;
3. diesel power from oil.
There are other possible sources of energy such as direct solar heat, wind power, tidal power, etc., but none of these has yet gone beyond the pilot-plant stage.
In large steam power plants, the thermal energy stored in steam is converted into work by means of turbines. A turbine consists essentially
of a shaft or rotor fixed in bearings and enclosed in a cylindrical casing. The rotor is made to turn smoothly by means of jets of steam from nozzles around the periphery of the turbine cylinder. These steam jets strike blades attached to the shaft. Central power stations employ condensing turbines in which the steam passes into a condenser after leaving the turbine. Condensation is effected by the circulation of large quantities of cold water
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through the tubes of the condenser,thus increasing the expansion ratio of the steam and the consequent efficiency and work output of the turbine. The turbines are connected directly to large electricity generators.
In turbines the action of the steam is kinetic. There is progressive expansion of the steam from the high pressure and relatively small volume at which it leaves.
Steam is made by heating water in a boiler. The usual boiler has a furnace in which fuel is burned, and the heat given off during combustion is conducted through the metal walls of the boiler to
generate steam at a pressure within the boiler vessel. In nuclear plants, steam is generated with the aid of a reactor in which the controlled fission of uranium or plutonium supplies the necessary heat for the vaporization of water. Thus the reactor rep; aces the steam generator of conventional plants.
Use is made of the energy possessed by water in hydroelectric stations. In order to transform this energy into work, hydraulic turbines are used. Modern hydraulic turbines may be divided into two classes: impulse turbines and pressure or reaction turbines. Of the former, the Pelton wheel is the only type used in important installations; of the latter, the Francis turbine or one of its modifications is universally employed.
In an impulse turbine, the whole head of water is converted into kinetic energy before the wheel is reached, as the water is supplied to the wheel through a nozzle. In the pressure or reaction turbine the wheel (or runner) is provided with vanes into which water is directed by means of a series of guide vanes around the whole periphery. The water leaving these guide vanes is under pressure and supplies energy partly in the kinetic form and partly in the pressure form.
The diesel engine is an excellent prime mover for electricity generation in plant below about 10,000 KVA. It has the advantage of low fuel cost, a brief warming-up period and low standing losses. Moreover it requires little cooling water. Diesel generation is generally chosen for small power requirements by municipalities, hotels and factories; hospitals often keep and independent diesel generator for emergency supply.
The transmission of electrical energy by means of lines is a great problem in electrical power systems. Transmission lines are essential for three purposes:
1. To transmit power from a hydroelectric site to a load center perhaps a
considerable distance away;
2. For the bulk supply of power from steam stations to load centers a
relatively short distance away;
3. For interconnection purposes to transfer energy from one system to
another in case of emergency.
The transmission voltage is determined largely by economic factors. In fact, in a transmission line, if the distance, the power and the power loss are fixed, the total weight of the conductor varies inversely as the square of the transmission voltage. For the economic transmission of power over considerable distances the voltage must therefore be high. Naturally with higher voltages the insulation cost also rises and to find the optimum voltage we must strike a balance between this cost and the saving through the reduction of the cross-section of the conductors.
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For high voltages, overhead-line construction is generally used with suspension-type insulators. Steel tower, called pylons serve to carry the insulators, with each conductor suspended from the bottom of a group or string of insulator units. The following types of conductor are those most commonly used: stranded copper conductors, hollow copper conductors and ACSR (aluminum cable, steel reinforced) conductors.
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