INDUSTRIAL VISIT AND TRAINING REPORT A dissertation submitted in practical fulfillment of vocational training in
Submitted by 1. AVIK BAL Department of Mechanical Engineering, NERIST, NERIST, Itanagar -791109 2. UPAYAN DEBNATH Department of Mechanical Engineering, NERIST, NERIST, Itanagar -791109
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CANDIDATE’S DECLARATION
We certify that the work presented in this dissertation submitted in practical fulfillment of vocational training in Agartala Gas Tubine Power Plant, NEEPCO is an accurate record of our work carried out under the guidance of Engineers and officials of AGTP. We hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work.
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CONTENTS Cover Page
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Certificate of Approval
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Candidate’s Declaration
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Contents
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CHAPTER 1 INTRODUCTION
1.1 Overview
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1.2 Mission
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1.3 Corporate objectives
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1.4.Human Resource
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1.5 Company profile
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CHAPTER 2 LITERATURE REVIEW
2.1 Power Plant Engineering
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2.2 Theory of Operation
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CHAPTER 3 COMP0NENTS OF GAS TURBINE POWER PLANT.
3.1 Starting Engine.
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3.2 Gas Compressor.
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3.3 Combustion Chamber.
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3.4 Gas Turbine.
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3.5 Load Reduction Gear Box.
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3.6 Generators.
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3.7 Voltage and Frequency Regulation
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3.8 A.C.Generators
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3.9 D.C.Generators. 3.10. Electric power transmission and distribution. 4|Page
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CHAPTER 1 : INTRODUCTION 1.1. OVERVIEW 1.1. OVERVIEW
North Eastern Electric Power Corporation Limited (NEEPCO) , a Schedule "A" Government of India Enterprise under the Ministry of Power was set up on the 2nd of April, 1976 to plan, investigate, design, construct, generate, operate and maintain power stations in the North Eastern Region of the country. NEEPCO has an installed capacity of 1130 MW which is 47% of the total installed capacity of the N.E Region. NEEPCO's authorised share capital is Rs 5000 Crores at present and its net worth as on 31st March 2012 is Rs 4780.01 Crores. With its headquarters in the charming town of Shillong, the capital of Meghalaya, NEEPCO is a power sector enterprise with projects located in the various states of the North East. 1.2. MISSION 1.2. MISSION o o
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To harness the vast hydro & thermal power potential To produce pollution free and inexhaustible power through planned development of power generation projects. project s. To play a significant role in the integration and development of hydroelectric and thermal power in the Central Sector covering all aspects such as investigation, planning, designs, construction, operation and maintenance of hydroelectric and thermal projects which in turn would effectively promote the development of the nation as a whole.
1.3. CORPORATE 1.3. CORPORATE OBJECTIVES
For fulfillment of its mission, NEEPCO has set the following objectives for the year commensurate with the aims, programs and policies of the government evolved from time to time: o
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To responsibly exploit the vast hydro & thermal power potential for sustainable development of N.E Region To undertake execution of new hydro/thermal schemes and undertake timely renovation & modernization of existing old hydro and thermal plants. To execute on-going hydro/thermal projects as per targets set, so as to achieve commissioning of such projects as per schedule or ahead of schedule. To ensure optimum utilization of installed capacity so as to achieve maximum generation and optimum machine availability . To improve the Quality Management System, NEEPCO is already registered as an ISO: 9001:2008 Company, NEEPCO has also been accredited with OHSAS 18001 for occupational occupatio nal health and safety safet y management systems s ystems and ISO 14001 for fo r environmental management systems. To complete DPR of new schemes for hydro/thermal projects as per schedule or ahead of schedule as and when estimated by Central Electricity Authority (CEA) To promote industrial growth and prosperity of the N.E region by fulfilling the need of supply of electricity thereby improving the quality of life of the region.
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To improve the socio economic condition of the neighbourhood by providing infrastructure , medical schooling and the creation of productive environment opportunities.
1.4. HUMAN 1.4. HUMAN RESOURCE o
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An integral part of NEEPCO's employee centered policy lay thrust on knowledge up-gradation and development through seminars, workshops and training programs both in-house and external. ex ternal. Manpower strength of the Corporation is 2784 ( as on 31st May 2013).
1.5.COMP 1.5.COMPANY ANY PROFILE PROFIL E
Authorised Share Capital
Rs. 5000 Crs
Installed Capacity
1130 MW
Projects Completed
7 Nos (5 Hydro , 2 Thermal)
Capacity Addition Program From Projects 922 MW Under Construction Projects Under Construction
5 Nos (3 Hydro , 2 Thermal)
Projects Under Survey & Investigation
2118 MW (4 Hydro , 1 Thermal , 2 Solar)
Manpower
2784 (as on 31/05/2013)
Certifications Received ISO 9001:2008
Quality Management Systems (QMS)
ISO 14001:2004
Environmental Management Systems (EMS)
ISO 18001:2007
Occupational Health and Management Systems (OHSAS)
MOU Rating for 2003-04, 2004-05, 2005Excellent 06 2006 - 07, 2007 - 08
Very Good
2008 - 09
Good
2009 - 10
Very Good
2010 - 11
Good
2011 - 12
Good (Provisional)
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Safety
CHAPTER 2 : LITERATURE REVIEW 2.1.POWER 2.1.POWER PLANT ENGIEERING
A power station (also referred to as a generating station, power plant, powerhouse or generating plant) is an industrial facility for the the generation of electric electric power. At power. At the center of nearly all power stations is a generator, generator, a rotating machine that converts mechanical power into electrical power by creating relative motion between a magnetic field and a conductor. conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on which fuels are easily available, cheap enough and on the types of technology that the power company has access to. Most power stations in the world burn fossil fuels such as coal, oil, and and natural gas to generate electricity, and some use nuclear power, but power, but there is an increasing use of cleaner renewable sources such as solar, wind, wave and hydroelectric. In thermal power stations, mechanical power is produced by a heat engine that transforms thermal energy, energy, often from combustion of a fuel, fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power po wer stations. Not all thermal energy can be transformed into mechanical power, according to the second the second law of thermodynamics. thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, heating, the power plant is referred to as a cogeneration cogeneration power power plant or CHP (combined heat-and-power) heat-an d-power) plant Gas turbine plants use the dynamic pressure from flowing gases (air and combustion products) to directly operate the turbine. Natural-gas fuelled (and oil fueled) combustion turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants.
2.2.THEORY 2.2.THEORY OF OPERATION
Gases passing through an ideal gas turbine undergo three three thermodynamic processes. thermodynamic processes. These are isentropic compression, isobaric (constant pressure) combustion and isentropic expansion. Together, these make up the Brayton cycle. In a practical gas turbine, fresh air from ambient atmosphere are first accelerated in either a centrifugal or axial compressor. axial compressor. The The air is screened and filtered to remove all dust particles and other impurities. These gases are then slowed using a diverging nozzle known as a diffuser; diffuser; these processes increase the pressure and temperature of the flow. In an ideal system, this is isentropic. However, in practice, energy is lost to heat, due to friction and turbulence. Gases then pass from the compressor (or diffuser) to a combustion chamber, or similar device, where heat is added. In an ideal system, this occurs at constant pressure (isobaric heat addition). As there is no change in pressure the specific the specific volume of the gases increases. In practical situations this process is usually accompanied by a slight loss in pressure, due to friction. Finally, this larger volume of gases is expanded and accelerated by nozzle guide vanes before energy is extracted by a turbine. In an ideal system, these gases are expanded 7|Page
isentropically and leave the turbine at their original pressure. In practice this process is not isentropic as energy is once again lost to friction and turbulence. If the device has been designed to power a shaft as with an industrial generator or a turboprop, the turboprop, the exit pressure will be as close to the entry pressure as possible. In practice it is necessary that some pressure remains at the outlet in order to fully expel the exhaust gases. In the case of a jet engine only enough pressure and energy is extracted from the flow to drive the compressor and other components. The remaining high pressure gases are accelerated to provide a jet that can, for example, be used to propel an aircraft.
Brayton cycle
As with all cyclic heat engines, engines, higher combustion temperatures can allow for greater efficiencies. efficiencies. However, temperatures are limited by ability of the steel, nickel, ceramic, or other materials that make up the engine to withstand high temperatures and stresses. To combat this many turbines feature complex blade cooling systems. As a general rule, the smaller the engine, the higher the rotation rate of the shaft(s) must be to maintain tip speed. Blade-tip speed determines the maximum pressure ratios that can be obtained by the turbine and the compressor. This, in turn, limits the maximum power and efficiency that can be obtained by the engine. In order for tip speed to remain constant, if the diameter of a rotor is reduced by half, the rotational speed must double. For example, large large Jet engines operate around 10,000 rpm, while micro turbines spin as fast as 500,000 rpm. More sophisticated turbines (such as those found in modern jet modern jet engines) engines) may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers. Thrust and journal and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or bearings, or oilcooled ball cooled ball bearings.
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CHAPTER 3:COMP0NENTS OF GAS TURBINE POWER PLANT.
Function of the Components used in Gas Turbine Power Plant. 3.1. Starting Engine: A starting engine is used as the main prime mover at initial starting of operation. The compressor is initially rotated using a 360 KW 2300 RPM motion. Once the turbine starts producing power the rotary motion of the turbine is used to rotate the compressor by a connected common shaft. 3.2. Gas Compressor: It is a mechanical device that increases the pressure of a gas by reducing its volume. Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa). Many large snowmaking operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines. AxialAxial-flow compressors are dynamic rotating compressors that use arrays of fan -like airfoils to progressively compress the working fluid. They are used where there is a requirement for a high flow rate or a compact design. The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary. The rotating airfoils, also known as blades or rotors, accelerate the fluid. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. Axial compressors are almost always multi -staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach 9|Page
number. number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation. Axial compressors can have high efficiencies; around 90% polytropic 90% polytropic at their design conditions. However, they are relatively expensive, r equiring a large number of components, tight tolerances and high quality materials. Axial -flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants. In Agartala Gas Turbine Power Plant each unit employs a 17 stage axial gas compressor. 3.3. Combustion Chamber: The combustion chamber in gas turbines and jet engines (including ramjets (including ramjets and scramjets) and scramjets) is is called the combustor. the combustor.
The combustor is fed with high pressure air by the compression system, adds fuel and burns the mix and feeds the hot, high pressure exhaust into the turbine components of the engine or out the exhaust nozzle. Different types Different types of combustors exist, mainly:
Can type: Can combustors are self contained cylindrical combustion chambers. Each "can" has its own fuel injector, liner,interconnectors,casing. Each "can" get an air source from individual opening. Cannular type: Like the can type combustor, can annular combustors have discrete combustion zones contained in separate liners with their own fuel injectors. Unlike the can combustor, all the combustion zones share a common air casing. Annular type: Annular combustors do away with the separate combustion zones and simply have a continuous liner and casing in a ring (the annulus).
3.4. Gas Turbine: A gas turbine, also called a combustion turbine, is a type of internal internal combustion engine. engine. It has an upstream rotating compressor coupled to a downstream turbine, and turbine, and a combustion a combustion chamber in- between. between.
The basic operation of the gas turbine is similar to that of the steam the steam power plant except that air is used instead of water. Fresh atmospheric air flows through a compressor a compressor that brings it to higher pressure. Energy pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature flow. This high-temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure , producing a shaft work output in the process. The turbine shaft work is used to drive the compressor and other devices such as an electric an electric generator that may be coupled to the shaft. The energy that is not used for shaft work comes out in the exhaust the exhaust gases, so gases, so these have either a high temperature or a high velocity. The purpose of the gas turbine determines the design so that the most desirable energy form is maximized. o f a gear a gear train, also train, also known as its speed 3.5. Load Reduction Gear Box: The gear ratio of ratio, is the ratio of the angular velocity of the input gear to the angular velocity of the output gear. The gear ratio can be calculated directly from the numbers of teeth on the gears in the gear train. The torque ratio of the gear train, also known as its mechanical its mechanical advantage, is determined by the gear ratio. The speed ratio and mechanical advantage are defined so they yield the same number in an ideal linkage. In Agartala Gas Turbine Power Plant each 10 | P a g e
unit employs a load reduction gear box which reduces the speed from 5000 rpm to 3000 rpm to synchronize with the grid frequency (standard grid frequency for India is 50 Hz). Generator: In electricity generation, an electric generator is a device that converts mechanical energy to electrical to electrical energy. A energy. A generator forces electric current to flow through an external circuit. circuit. The source of mechanical energy may be a reciprocating or turbine steam engine, water engine, water falling through a turbine a turbine or waterwheel, an waterwheel, an internal internal combustion engine, a wind turbine, turbine, a hand hand crank, crank, compressed air, air, or any other source of mechanical energy. Generators provide nearly all of the power for electric electric power grids.
An induction generator or asynchronous generator is a type of AC AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as mini hydro power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls. To operate an induction generator must be excited with a leading voltage; this is usually done by connection to an electrical grid, or sometimes they are self excited by using phase correcting capacitors. The primary supply of all the world's electrical energy is generated in three phase synchronous generators using machines with power ratings up to 1500 MW or more. Though the variety of electric generators is not as great as the wide variety of electric motors available, they obey similar design rules and most of the operating principles used in the various classes of electric motors are also applicable to electric generators. The vast majority of generators are AC machines (Alternators) with a smaller number of DC generators (Dynamos).
3.6. Voltage and Frequency Regulation
Most generator applications require some way controlling the output voltage and in the case of AC machines a method of controlling the frequency. Voltage and frequency regulation is normally accomplished in very large machines carrying very high currents, by controlling the generator excitation and the speed of the prime mover which drives the generator.
Stand Alone (Island) Systems In smaller, stand alone systems particularly those designed to capture energy from intermittent energy flows such as wind and wave power the voltage and frequency control may be carried out electronically. In principle these control systems are similar to Motor Controls and the the various components are outlined in that section. Grid Connected Systems In grid connected systems the generator voltage and frequency are locked to the grid system. Changing the energy output from the prime mover does not affect the frequency and voltage but will cause the output current to increase resulting in an equivalent change in the generator output power. When connecting a generator to the grid, it's speed should be run up so that it's output frequency matches the grid frequency before the connection is made.
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Generator Power Handling
The mechanical shaft power P in Watts applied to a generator is given by: P = ωT
Where ω is the speed in radians per second and T is the torque in Newton metres. As with electric motors, motors, the maximum power handling capability of the generator is determined by its maximum permissible temperature. Generator Load
Voltage and frequency regulation correct for minor deviations in the generator output as noted above but large changes in the load demand (current) can only be accommodated by adjusting the torque of the prime mover driving the generator since generally, in electric machines, torque is proportional to current or vice versa. Generator Types 3.7. AC Generators (Alternators)
Stationary Field Synchronous AC Generator
In a stationary field generator, the stator in the form of fixed permanent magnets (or electromagnets fed by DC) provides the magnetic field and the current is generated in the rotor windings. When the rotor coil is rotated at constant speed in the field between the stator poles the EMF generated in the coil will be approximately sinusoidal, the actual waveform being dependent on the size and shape of the magnetic poles. The peak voltage occurs when the moving conductor is passing the centre line of the magnetic pole. It diminishes diminis hes to zero zer o when the conductor is in the space between the poles and it increases to a peak in the opposite direction as the conductor approaches the centre line of the opposite pole of the magnet. The frequency of the waveform is directly proportional to the speed of rotation. The magnitude of the wave is also proportional to the speed until the magnetic circuit saturates when rate of voltage increase, as the speed increases, slows dramatically. o
Generator Speed and Frequency
The output frequency is proportional to the number of poles per phase and the rotor speed in the same way as a synchronous motor. The alternating current output generated in the rotor can be connected to external circuits via slip rings and does not need a commutator. The high speed generator needs fewer poles, simplifying the design and reducing the costs. 12 | P a g e
Rotating Field Synchronous AC Generator
The power handling capacity of a brushed machine is usually constrained by the current handling capability of the slip rings in an AC machine (or even more by the commutator in a DC machine). Since the generator load current is generally much higher than the field current, it is usually desirable to use the rotor to create the field and to take the power off the generator from the stator to minimise the load on the slip rings. By interchanging the fixed and moving elements in the above example a rotating field generator is created in which the EMF is instead generated in the stator windings. In this case, in its simplest form, the field is provided by a permanent magnet (or electromagnet) which is rotated within a fixed wire loop or coil in the stator. The moving magnetic field due to the rotating magnet of the rotor will then cause a sinusoidal current to flow in the fixed stator coil as the field moves past the stator conductors. If the rotor field is provided by an electromagnet, it will need direct current excitation fed through slip rings. It does not need a commutator. If instead of a single coil, three independent stator coils or windings , spaced 120 degrees apart around the periphery of the machine, are used, then the output of these windings will be three phase alternating current. Series Wound Generator
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Classified as a constant speed generator, they have poor voltage regulation and few are in use. Shunt Wound Generator
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Classified as a constant voltage generator, the output voltage can be controlled by varying var ying the field current. curr ent. They have h ave reasonably reason ably good voltage regulation over the speed range of the machine. Brushless Excitation
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Rotating field machines are used for the high power generating plant in most of the world's national electricity grid systems. The field excitation power needed for these huge machines can be as much as 2.5% of the output power ( 25 KW in a 1.0 MW generator) though this reduces as the efficiency improves with size so that a 500 MW generator needs 2.5 MW (0.5%) of excitation power. If the field voltage is 1000 Volts, the required field current will be 2500 Amps. Providing such excitation through slip rings is an engineering challenge which has been overcome by generating the necessary power within the machine itself by means of a pilot, three phase, stationary field generator on the same shaft. The AC current generated in the pilot generator windings is rectified and fed directly to the rotor windings to supply the excitation for the main machine.
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Cooling
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The efficiency of a very large generator can be as high as 98% or 99% but for a 1000 MW generator, an efficiency loss of just 1% means 10 Megawatts of losses must be dissipated, mostly in the form of heat. To avoid overheating, special cooling precautions must be taken and two forms of cooling are usually employed simultaneously. Cooling water is circulated through copper bars in the stator windings and hydrogen is passed through the generator casing. Hydrogen has the advantages that its density is only about 7% of the density of air resulting in fewer wind age losses due to the rotor churning up the air in the machine and its thermal capacity is 10 times that of air giving it superior heat removal capability. o
Permanent Magnet AC Generators
Smaller versions of both of the above machines can use permanent magnets to provide the machine's magnetic field and since no power is used in providing the field this means that the machines are simpler and more efficient . The drawback however is that there is no simple way to control such machines. Permanent magnet synchronous generators (PMSGs) are typically used in low cost "gensets" to provide emergency power. The voltage and frequency output of the permanent magnet generator are proportional to the speed of rotation and though this may not be a problem for applications powered by fixed speed mechanical drives, many applications applicati ons such as wind turbines, require a fixed voltage and frequency output but are powered by variable speed prime movers. In these cases, complex feedback control systems or external external power conditioning may be required to provide the desired stabilised output. Generally the output will be rectified and the varying output voltage fed through the DC link to a buck a buck - boost regulator which provides a fixed voltage coupled with an inverter an inverter which provides a fixed frequency output.
Induction Generators
Induction generators are essentially essentially induction motors which are run slightly above the synchronous speed associated with the supply frequency. They have no means of producing or generating voltage unless they are connected to an external source of excitation. The squirrel cage construction is used for small scale power generation because it is simple, robust and a nd inexpensive to manufacture. manu facture. o
Fixed Speed Induction Generator
Fixed speed induction generators actually run over a small speed range associated with the generator slip. slip. They receive their excitation from the electricity supply grid and can only be run in parallel with that supply. When used on line, they are fine for returning power to the grid from which they derive their excitation current but useless as standby generators when the electric grid goes down. Their limited speed range restricts the possible applications.
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Variable Speed - Self Excited Induction Generator(SEIG)
Small scale electricity generating systems are quite often stand alone applications, remote from the electricity supply grid, utilising widely fluctuating energy sources such as wind and water power for their source of energy. The fixed speed induction generator is not suitable for such applications. Variable speed induction generators need some form of self-excitation as well as power conditioning to be able to make practical use of their unregulated voltage and frequency output.
Operation
Self-excitation is obtained by connecting capacitors across the stator terminals of the generator. When driven by an external prime mover, a small current will be induced in the stator coils as the flux due to the residual magnetism in the rotor cuts the windings and this current charges the capacitors. As the rotor turns, the flux cutting the stator windings will change to the opposite direction as the orientation of the remanent magnetic field turns with the rotor. The induced current in this case will be in the opposite direction and will tend to discharge the capacitors. At the same time the charge released from the capacitors will tend to reinforce the current increasing the flux in the machine. As the rotor continues to turn the induced EMF and current in the stator windings will continue to rise until steady state is attained, depending on the saturation of the magnetic circuit in the machine. At this operating point the voltage and current will continue to oscillate at a given peak value and frequency determined by the characteristics of the machine, the air gap , the slip, the load and the choice of capacitor sizes. The combination of these factors sets maximum and minimum limits on the speed range over which self excitation occurs. The operating slip is generally small and the variation of the frequency depends on the operating speed range. If the generator is overloaded the voltage will collapse rapidly providing a measure of built in self-protection.
Control
In variable-speed operation, an induction generator needs a converter to adapt the variable frequency output of the generator to the fixed frequency of the application or the electricity supply grid. During operation the only controllable factor available in a self excited induction generator to influence the output is the mechanical input from the prime mover, so the system is not amenable for effective feedback control. To provide a controllable output voltage and frequency, external AC/DC/AC converters are required. A three-phase diode bridge is used to rectify the generator output current providing a DC link to a three-phase thyristor inverter which converts the power from the DC link to the required voltage vol tage and frequency. frequenc y.
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3.8. DC Generators (Dynamos)
Direct Current (DC) Generator
The stationary field AC generator described above can be modified to deliver a unidirectional current by replacing the slip rings on the rotor shaft with a suitable commutator to reverse the connection to the coil each half cycle as the conductor passes alternate north and south magnetic poles. The current will however be a series of half sinusoidal pulses just like the waveform from a full wave rectifier as shown below.
The output voltage ripple can be minimised by using multipole designs. The construction of a DC generator is very similar to the construction of a DC motor.The rotor consists of an electromagnet providing the field excitation. Current to the rotor is derived from the stator or in the case of very large generators, from a separate exciter rotating on the same rotor shaft. The connection to the rotor is through a commutator so that the direction of the current in the stator windings changes direction as the rotor poles pass between alternate north and south stator poles. The rotor current is very low compared with the current in the stator windings and most of the heat is dissipated in the more massive stator structure.In self excited machines, when starting from rest, the current to start the electromagnets working is derived from the small residual magnetism which exists in the electromagnets and surrounding magnetic circuit. 3.9. Electric power transmission and distribution. Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to electrical electri cal substations subs tations located l ocated near demand centers. cente rs. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become transmission trans mission networks. networ ks. These are typically typicall y referred to as "power grids" or just "the grid."
Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.
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A key limitation in the distribution of electric power is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated control system is required to ensure electric generation very closely matches the demand. If the demand for power exceeds the supply, generation plants and transmission equipment can shut down which, in the worst cases, can lead to a major regional blackout. blackout. To reduce the risk of such failures, electric transmission networks are interconnected into regional, national or continental wide networks thereby providing multiple redundant alternative routes for power to flow should (weather or equipment) failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network. Transmission efficiency is greatly improved by devices that increase the voltage, (and thereby proportionately reduce the current) in the line conductors, thus allowing power to be transmitted with acceptable losses. The reduced current flowing through the line reduces the heating losses in the conductors. According to to Joule's Law, Law, energy losses are directly proportional to the square of the current. Thus, reducing the current by a factor of 2 will lower the energy lost to conductor resistance by a factor of 4. This increase of voltage is usually achieved in AC circuits by using a step-up transformer step-up transformer . HVDC systems require relatively costly conversion equipment which may be economically justified for particular projects such as submarine cables and longer distance high capacity point to point transmission tran smission but are infrequently infrequen tly used at present. A transmission grid is a network of power stations, stations, transmission lines, and substations. Energy is usually transmitted within a grid with with three-phase AC. Single-phase AC. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase induction polyphase induction motors. In the 19th century, two-phase transmission was used but required either four wires or three wires with unequal currents. Higher order phase systems require more than three wires, but deliver marginal benefits. The price of electric power station capacity is high, and electric demand is variable, so it is often cheaper to import some portion of the needed power than to generate it locally. Because loads are often regionally correlated (hot weather in the Southwest portion of the US might cause many people to use air conditioners), electric power often comes from distant sources. Because of the economic benefits of load sharing between regions, wide area transmission grids now span countries and even continents. The web of interconnections between power producers and consumers should enable power to flow, even if some links are inoperative. The unvarying (or slowly varying over many hours) portion of the electric demand is known as the base load and is generally served by large facilities (which are more efficient due to economies of scale) with fixed costs for fuel and operation. Such facilities are nuclear, coal-fired or hydroelectric, while other energy sources such as concentrated solar thermal and geothermal power have the potential to provide base load power. Renewable energy sources such as solar photovoltaics, wind, wave, and tidal are, due to their intermittency, not considered as supplying "base load" but will still add power to the grid. The remaining or 'peak' power demand, is supplied by peaking by peaking power plants, plants, which are typically smaller, faster-responding, and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas.
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Long-distance transmission of electricity (thousands of kilometers) is cheap and efficient, with costs of US$0.005 – 0.02/kWh 0.02/kWh (compared to annual averaged large producer costs of US$0.01 – 0.025/kWh, 0.025/kWh, retail rates upwards of US$0.10/kWh, and multiples of retail for instantaneous suppliers at unpredicted highest demand moments). Thus distant suppliers can be cheaper than local sources. Multiple local sources (even if more expensive and infrequently used) can make the transmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.
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