The production of bulk electric power for industrial, residential, and rural use. Electric power generation generally implies large-scale production of electric power in stationary plants designed for that purpose. Historically, these plants have mostly burned fossil fuels such as coal (Fig. 1), oil, and natural gas. Hydropower, driven by the conversion of falling water to energy via hydraulic turbines, is also an electric power generating source with a legacy going back to the late 19th century. Nuclear power plants, where energy is obtained through the controlled fission of atoms, emerged in the 1950s. In more recent decades, concerns about global climate change caused by anthropogenic greenhouse gas emissions, of which a significant portion is related to electric power generation involving fossil fuels, coupled with improving economics and technologies, have led to increased interest in and utilization of alternative energy sources. These sources include wind, solar, biomass, and geothermal, as well as hydroelectric power. Because these forms of potential energy constantly and rapidly renew themselves, they are termed "renewable" energy sources, in distinction to nonrenewable fossil fuels, which exist in finite quantities within the Earth. See also: Electrical energy measurement; Electricity; Energy; Energy sources; Generator; Geothermal power; Global climate change; Nuclear fission; Nuclear power; Prime mover; Solar energy; Turbine; Wind power
An electric load (or demand) is the power requirement of any device or equipment that converts electric energy into light, heat, or mechanical energy, or otherwise consumes electric energy. The total load on any power system is seldom constant; rather, it varies widely with hourly, weekly, monthly, or annual changes in the requirements of the area served. The minimum system load for a given period is termed the base load or the unity load-factor component. Maximum loads, resulting usually from temporary conditions, are called peak loads, and the operation of the generating plants must be closely coordinated with fluctuations in the load. The peaks, usually being of only a few hours' duration, are frequently served by gas or oil combustion-turbine or pumped-storage hydro generating units. The pumped-storage type utilizes the most economical off-peak (typically 10 p.m. to 7 a.m.) surplus generating capacity to pump and store water in elevated reservoirs to be released through hydraulic turbine generators during peak periods. This type of operation improves the capacity factors or relative energy outputs of base-load generating units and hence their economy of operation. See also: Heat; Light
Generating unit sizes
The size or capacity of electric utility generating units varies widely, depending upon type of unit; duty required, that is, base-, intermediate-, or peak-load service; and system size and degree of interconnection with neighboring systems. Base-load nuclear or coal-fired units can well exceed 1000 MW each. Intermediate-duty generators, usually coal-, oil-, or gas-fueled steam units, are typically of 200 to 600 MW capacity each. Peaking units, combustion turbines or hydro, range from several tens of megawatts for the former to hundreds of megawatts for the latter. Hydro units, in both base-load and intermediate service, range in size up to 825 MW. Multiple generating units are usually sited at individual power plants. The total installed generating capacity of a system is typically 20 to 30% greater than the annual predicted peak load in order to provide reserves for maintenance and contingencies.
Voltage regulation is the change in voltage for specific change in load (usually from full load to no load) expressed as percentage of normal rated voltage. The voltage of an electric generator varies with the load and power factor; consequently, some form of regulating equipment is required to maintain a reasonably constant and predetermined potential at the distribution stations or load centers. Since the inherent regulation of most alternating-current (ac) generators is rather poor (that is, high percentagewise), it is necessary to provide automatic voltage control. The rotating or magnetic amplifiers and voltage-sensitive circuits of the automatic regulators, together with the exciters, are all specially designed to respond quickly to changes in the alternator voltage and to make the necessary changes in the main exciter or excitation system output, thus providing the required adjustments in voltage. A properly designed automatic regulator acts rapidly, so that it is possible to maintain desired voltage with a rapidly fluctuating load without causing more than a momentary change in voltage even when heavy loads are thrown on or off. See also: Alternating current; Direct current
In general, most synchronous generators have excitation systems that involve rectification of an ac output of the main or auxiliary stator windings, or other appropriate supply, using silicon controlled rectifiers or thyristors. These systems enable very precise control and high rates of response. See also: Semiconductor rectifier; Voltage regulator
Synchronization of generators
Synchronization of a generator to a power system is the act of matching, over an appreciable period of time, the instantaneous voltage of an alternating-current generator (incoming source) to the instantaneous voltage of a power system of one or more other generators (running source), then connecting them together. In order to accomplish this ideally the following conditions must be met:
- The effective voltage of the incoming generator must be substantially the same as that of the system.
- In relation to each other the generator voltage and the system voltage should be essentially 180° out of phase; however, in relation to the bus to which they are connected, their voltages should be in phase.
- The frequency of the incoming machine must be near that of the running system.
- The voltage wave shapes should be similar.
- The phase sequence of the incoming polyphase machine must be the same as that of the system.
Synchronizing of ac generators can be done manually or automatically. In manual synchronizing, an operator controls the incoming generator while observing synchronizing lamps or meters and a synchroscope, or both. The operator closes the connecting switch or circuit breaker as the synchroscope needle slowly approaches the in-phase position.
Automatic synchronizing provides for automatically closing the breaker to connect the incoming machine to the system, after the operator has properly adjusted voltage (field current), frequency (speed), and phasing (by lamps or synchroscope). A fully automatic synchronizer will initiate speed changes as required and may also balance voltages as required, then close the breaker at the proper time, all without attention of the operator. Automatic synchronizers can be used in unattended stations or in automatic control systems where units may be started, synchronized, and loaded on a single operator command. See also: Alternating-current generator; Phase-angle measurement
The most common fossil fuels are coal, natural gas, and oil. The less common ones include peat, oil shale, and various waste or by-products such as steel mill blast furnace gas, coke-oven gas, and refuse-derived fuels. Fossil-fuel electric power generation uses the combustion heat energy from these fuels to produce electricity. See also: Coal; Coke; Fossil fuel; Natural gas; Oil shale; Peat; Petroleum
Steam power plants
A fossil-fuel steam power plant operation essentially consists of four steps (Fig. 2): (1) Water is pumped at high pressure to a boiler, where (2) it is heated by fossil-fuel combustion to produce steam at high temperature and pressure. (3) This steam flows through a turbine, rotating an electric generator (connected to the turbine shaft) which converts the mechanical energy to electricity. (4) The turbine exhaust steam is condensed by using cooling water from an external source to remove the heat rejected in the condensing process. The condensed water is pumped back to the boiler to repeat the cycle. Figure 2 also shows features to increase cycle efficiency, including preheating of the boiler feedwater by using steam extracted from the turbine, and reheating the high-pressure turbine exhaust steam before it enters the intermediate-pressure turbine.
A typical large fossil-fuel power plant consists of several major facilities and equipment (Fig. 3), including fuel handling and processing, boiler (including furnace), turbine and electric generator, condenser and condenser heat removal system, feedwater heating and pumping system, flue gas–cleaning system, and plant controls and control system. See also: Air filter; Boiler; Control systems; Cooling tower; Electrostatic precipitator; Fire-tube boiler; Gas absorption operations; Steam condenser; Steam-generating furnace; Steam-generating unit; Steam turbine; Vapor condenser
Gas turbine plants
Power plants with gas turbine–driven electric generators are often used to meet short-term peaks in electrical demand. They are generally small (up to 150–200 MW), have a low thermal efficiency, require clean fuels such as natural gas or light oils, and consequently are more expensive to operate. However, these plants have a relatively low capital cost and can be used in areas with low power demand and limited water availability. Diesel engine–driven electric generators are also used under similar conditions. See also: Diesel engine; Gas turbine
Gas turbine power plants use atmospheric air as the working medium, operating on an open cycle where air is taken from and discharged to the atmosphere and is not recycled. In a simple gas turbine plant (Fig. 4), air is compressed and fuel is injected into the compressed air and burned in a combustion chamber. The combustion products expand through a gas turbine and exhaust to the atmosphere. Variations of this basic operation to increase cycle efficiency include regeneration (where exhaust from the turbine is used to preheat the compressed air before it enters the combustion chamber) and reheating (where the combustion gases are expanded in more than one stage and are reheated between stages).
Hydroelectric power can be defined as the generation of electricity by flowing water; potential energy from the weight of water falling through a vertical distance is converted to electrical energy. The amount of electric power P that can be generated is given by the equation below,
where Q is the volume flow of water, h is the height through which the water falls, and k is a constant equal to 0.102 when Q is in cubic meters and h is in meters.
A typical hydroelectric development consists of a dam to divert or store water; waterways such as a forebay, canals, tunnels, and penstocks to deliver the water to the hydraulic turbine and a draft tube, tunnel, or tailrace to return the water to the stream; hydraulic turbines and governors; generators and exciters; electrical controls to provide protection and to regulate frequency, voltage, and power flow; a powerhouse to enclose the machinery and equipment; transformers and switching equipment; and a transmission line to deliver the power to the load center for ultimate distribution (Fig. 5). See also: Dam; Hydraulic turbine; Hydroelectric generator
Pumped storage is a process for converting large quantities of electrical energy to potential energy by pumping water to a higher elevation where it can be stored indefinitely, then released to pass through hydraulic turbines and generate electrical energy on demand. Storage is desirable, as the consumption of electricity is highly variable according to the time of day or week, as well as seasonally. Consequently, there is excess generating capacity at night and on weekends. This excess capacity can be used to generate energy for pumping, hence storage. Normally, pumping energy can be obtained cheaply at night or on weekends, and its value will be upgraded when used for daytime peak loads.
In a typical operation, water is pumped at night or on weekends from a lower to an upper reservoir, where it is stored. The water can be retained indefinitely without deterioration or significant loss. During the daylight hours when the loads are greatest, stored water is released to flow from the upper to the lower reservoir through hydraulic turbines which generate electricity. No water is consumed in either the pumping or the generating cycles. A hydroelectric pumped-storage development is similar to a typical hydro installation, except that it has two reservoirs of essentially equal size situated to maximize their difference in elevation. Also, since it is a closed system, it need not be located directly on a large stream but must have a source for initial filling and to make up for the losses of evaporation and seepage. The second principal difference is the pump-turbine which is capable of rotation in either direction and acts as a pump in one direction and a turbine in the other. See also: Energy storage; Water power
Alternative power sources
The collection of solar energy takes many forms, but one of its most desirable configurations is direct conversion to electricity via photovoltaic cells, or solar cells. Usually many cells are arrayed together as a flat panel, called a solar panel. The device configuration is modular, making solar panels convenient from a mass-production viewpoint. Solar cells are typically made of thin semiconductor material. When the material is struck by sunlight, electrons are freed, producing an electric current. The direct-current (dc) power is passed through a dc load, into a storage battery, or converted to alternating current (ac) for general use in electric utility grids. See also: Electron; Solar cell; Solar energy; Sun
Electricity generation by wind turbines consists of several major subsystems. Among the most important is the mechanical system and electromechanical rotary converter. A wind energy conversion system is designed to rotate at either constant or variable speed as the wind varies. The variable-speed system usually offers high wind-collection efficiency; however, constant-speed units permit simpler electrical systems. There are a multitude of aeroturbine designs available, with each striving to enhance the wind to mechanical power conversion effectiveness. See also: Wind power
The natural emissions of steam (geysers), hot springs, and volcanoes represent potential sources of electricity production. The hydrothermal approach converts high-temperature (above 210°C or 410°F) water by direct-flash technology. In this process, hot water is extracted from the Earth and its pressure is dropped, causing the water to vaporize (flash-boil). The steam is applied to conventional steam turbine technology. Because much of the geothermal potential occurs at lower temperatures, below 210°C, a binary process is also used to capture this energy. In the binary process, hot geothermal energy is used to vaporize a secondary fluid, such as a hydrocarbon like isobutane or isopentane, which boils at a lower temperature. The heat from the geothermal source is transferred to the secondary fluid through a heat exchanger, which in turn produces steam for a conventional steam cycle. See also: Geothermal power