i

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, Itanagar-791109

2. UPAYAN DEBNATH

Department of Mechanical Engineering, NERIST, Itanagar-791109

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CANDIDATES 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 1

Certificate of Approval 2

Candidates Declaration 3

Contents 4

CHAPTER 1 INTRODUCTION

1.1 Overview 5

1.2 Mission 5

1.3 Corporate objectives 5

1.4.Human Resource 6

1.5 Company profile 6

CHAPTER 2 LITERATURE REVIEW

2.1 Power Plant Engineering 7

2.2 Theory of Operation 8

CHAPTER 3 COMP0NENTS OF GAS TURBINE POWER PLANT.

3.1 Starting Engine. 9

3.2 Gas Compressor. 9

3.3 Combustion Chamber. 9

3.4 Gas Turbine. 10

3.5 Load Reduction Gear Box. 10

3.6 Generators. 11

3.7 Voltage and Frequency Regulation 12

3.8 A.C.Generators 12

3.9 D.C.Generators. 16

3.10. Electric power transmission and distribution. 16

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CHAPTER 1 : INTRODUCTION

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

o To harness the vast hydro & thermal power potential

o To produce pollution free and inexhaustible power through planned development of

power generation projects.

o 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 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 To responsibly exploit the vast hydro & thermal power potential for sustainable

development of N.E Region

o To undertake execution of new hydro/thermal schemes and undertake timely

renovation & modernization of existing old hydro and thermal plants.

o 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.

o To ensure optimum utilization of installed capacity so as to achieve maximum

generation and optimum machine availability .

o 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 health and safety management systems and ISO 14001 for

environmental management systems.

o 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)

o 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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o To improve the socio economic condition of the neighbourhood by providing

infrastructure , medical schooling and the creation of productive environment

opportunities.

1.4. HUMAN RESOURCE

o 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.

o Manpower strength of the Corporation is 2784 ( as on 31st May 2013).

1.5.COMPANY PROFILE

Authorised Share Capital Rs. 5000 Crs

Installed Capacity 1130 MW

Projects Completed 7 Nos (5 Hydro , 2 Thermal)

Capacity Addition Program From Projects

Under Construction 922 MW

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 Safety

Management Systems (OHSAS)

MOU Rating for 2003-04, 2004-05, 2005-

06 Excellent

2006 - 07, 2007 - 08 Very Good

2008 - 09 Good

2009 - 10 Very Good

2010 - 11 Good

2011 - 12 Good (Provisional)

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CHAPTER 2 : LITERATURE REVIEW

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 generation of electric power. At the center

of nearly all power stations is a generator, a rotating machine that converts mechanical

power into electrical power by creating relative motion between a magnetic field and a

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 natural gas to generate electricity, and some use nuclear 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, often from combustion of a fuel, into rotational energy. Most thermal

power stations produce steam, and these are sometimes called steam power stations. Not all

thermal energy can be transformed into mechanical power, according to the second law of

thermodynamics. Therefore, there is always heat lost to the environment. If this loss is

employed as useful heat, for industrial processes or district heating, the power plant is

referred to as a cogeneration power plant or CHP (combined heat-and-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 OF OPERATION

Gases passing through an ideal gas turbine undergo three 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. 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; 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 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

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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 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, higher combustion temperatures can allow for greater

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 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 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 bearings are

a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-

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.

Axial-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

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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 at their design conditions. However, they are relatively expensive, requiring

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 and scramjets) is called 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 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 combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between.

The basic operation of the gas turbine is similar to that of the steam power plant except that

air is used instead of water. Fresh atmospheric air flows through a compressor that brings it

to higher 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 generator that may be coupled to the shaft. The energy that is not

used for shaft work comes out in the exhaust 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.

3.5. Load Reduction Gear Box: The gear ratio of a gear train, also known as its speed

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 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

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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 energy. A generator forces electric current to flow through

an external circuit. The source of mechanical energy may be a reciprocating or turbine

steam engine, water falling through a turbine or waterwheel, an internal combustion engine,

a wind turbine, a hand crank, compressed air, or any other source of mechanical energy.

Generators provide nearly all of the power for electric power grids.

An induction generator or asynchronous generator is a type of 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 = T

Where is the speed in radians per second and T is the torque in Newton metres.

As with electric 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 to zero 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.

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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.

o Series Wound Generator

Classified as a constant speed generator, they have poor voltage regulation and

few are in use.

o Shunt Wound Generator

Classified as a constant voltage generator, the output voltage can be controlled

by varying the field current. They have reasonably good voltage regulation over

the speed range of the machine.

o Brushless Excitation

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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o Cooling 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 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 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 - boost regulator which provides a fixed voltage coupled with an inverter

which provides a fixed frequency output.

Induction Generators

Induction generators are 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 inexpensive to manufacture.

o Fixed Speed Induction Generator

Fixed speed induction generators actually run over a small speed range

associated with the generator 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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o 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 and frequency.

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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 substations located near demand centers. 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 networks. These are typically 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. 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 Joule's 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.

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 but are infrequently used at present.

A transmission grid is a network of power stations, transmission lines, and substations.

Energy is usually transmitted within a grid with three-phase AC. Single-phase AC is used

only for distribution to end users since it is not usable for large 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 power 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$0.0050.02/kWh (compared to annual averaged large producer costs of US$0.010.025/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.