ENERGY CONTROL CENTER

ENERGY CONTROL CENTER

The following criteria govern the operation of an electric power
system:
• Safety
• Quality
• Reliability
• Economy
The first criterion is the most important consideration and aims to
ensure the safety of personnel, environment, and property in every aspect of
system operations. Quality is defined in terms of variables, such as frequency
and voltage, that must conform to certain standards to accommodate the
requirements for proper operation of all loads connected to the system.
Reliability of supply does not have to mean a constant supply of power, but it
means that any break in the supply of power is one that is agreed to and tolerated
by both supplier and consumer of electric power. Making the generation cost
and losses at a minimum motivates the economy criterion while mitigating the
adverse impact of power system operation on the environment.
Within an operating power system, the following tasks are performed in
order to meet the preceding criteria:
• Maintain the balance between load and generation.
• Maintain the reactive power balance in order to control the voltage
profile.
• Maintain an optimum generation schedule to control the cost and
environmental impact of the power generation.
• Ensure the security of the network against credible contingencies.
This requires protecting the network against reasonable failure of
equipment or outages.
The fact that the state of the power network is ever changing because
loads and networks configuration change, makes operating the system difficult.
Moreover, the response of many power network apparatus is not instantaneous.
For example, the startup of a thermal generating unit takes a few hours. This
essentially makes it not possible to implement normal feed-forward control.
Decisions will have to be made on the basis of predicted future states of the
system.
Several trends have increased the need for computer-based operator
support in interconnected power systems. Economy energy transactions, relianceon external sources of capacity, and competition for transmission resources have
all resulted in higher loading of the transmission system. Transmission lines
bring large quantities of bulk power. But increasingly, these same circuits are
being used for other purposes as well: to permit sharing surplus generating
capacity between adjacent utility systems, to ship large blocks of power from
low-energy-cost areas to high-energy cost areas, and to provide emergency
reserves in the event of weather-related outages. Although such transfers have
helped to keep electricity rates lower, they have also added greatly to the burden
on transmission facilities and increased the reliance on control.
Heavier loading of tie-lines which were originally built to improve
reliability, and were not intended for normal use at heavy loading levels, has
increased interdependence among neighboring utilities. With greater emphasis
on economy, there has been an increased use of large economic generating units.
This has also affected reliability.
As a result of these trends, systems are now operated much closer to
security limits (thermal, voltage and stability). On some systems, transmission
links are being operated at or near limits 24 hours a day. The implications are:
• The trends have adversely affected system dynamic performance.
A power network stressed by heavy loading has a substantially
different response to disturbances from that of a non-stressed
system.
• The potential size and effect of contingencies has increased
dramatically. When a power system is operated closer to the limit,
a relatively small disturbance may cause a system upset. The
situation is further complicated by the fact that the largest size
contingency is increasing. Thus, to support operating functions
many more scenarios must be anticipated and analyzed. In
addition, bigger areas of the interconnected system may be affected
by a disturbance.
• Where adequate bulk power system facilities are not available,
special controls are employed to maintain system integrity.
Overall, systems are more complex to analyze to ensure reliability
and security.
• Some scenarios encountered cannot be anticipated ahead of time.
Since they cannot be analyzed off-line, operating guidelines for
these conditions may not be available, and the system operator
may have to “improvise” to deal with them (and often does). As a
result, there is an ever increasing need for mechanisms to support
dispatchers in the decision making process. Indeed, there is a risk
of human operators being unable to manage certain functions
unless their awareness and understanding of the network state is
enhanced.
To automate the operation of an electric power system electric utilities
rely on a highly sophisticated integrated system for monitoring and control.Such a system has a multi-tier structure with many levels of elements. The
bottom tier (level 0) is the high-reliability switchgear, which includes facilities
for remote monitoring and control. This level also includes automatic
equipment such as protective relays and automatic transformer tap-changers.
Tier 1 consists of telecontrol cabinets mounted locally to the switchgear, and
provides facilities for actuator control, interlocking, and voltage and current
measurement. At tier 2, is the data concentrators/master remote terminal unit
which typically includes a man/machine interface giving the operator access to
data produced by the lower tier equipment. The top tier (level 3) is the
supervisory control and data acquisition (SCADA) system. The SCADA system
accepts telemetered values and displays them in a meaningful way to operators,
usually via a one-line mimic diagram. The other main component of a SCADA
system is an alarm management subsystem that automatically monitors all the
inputs and informs the operators of abnormal conditions.
Two control centers are normally implemented in an electric utility,
one for the operation of the generation-transmission system, and the other for
the operation of the distribution system. We refer to the former as the energy
management system (EMS), while the latter is referred to as the distribution
management system (DMS). The two systems are intended to help the
dispatchers in better monitoring and control of the power system. The simplest
of such systems perform data acquisition and supervisory control, but many also
have sophisticated power application functions available to assist the operator.
Since the early sixties, electric utilities have been monitoring and controlling
their power networks via SCADA, EMS, and DMS. These systems provide the
“smarts” needed for optimization, security, and accounting, and indeed are
really formidable entities. Today’s EMS software captures and archives live
data and records information especially during emergencies and system
disturbances.
An energy control center represents a large investment by the power
system ownership. Major benefits flowing from the introduction of this system
include more reliable system operation and improved efficiency of usage of
generation resources. In addition, power system operators are offered more indepth
information quickly. It has been suggested that at Houston Lighting &
Power Co., system dispatchers’ use of network application functions (such as
Power Flow, Optimal Power Flow, and Security Analysis) has resulted in
considerable economic and intangible benefits. A specific example of $ 70,000
in savings achieved through avoiding field crew overtime cost, and by leaving
equipment out of service overnight is reported for 1993. This is part of a total of
$ 340,000 savings in addition to increased system safety, security and reliability
has been achieved through regular and extensive use of just some network
analysis functions.

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STRUCTURE OF THE POWER SYSTEM

STRUCTURE OF THE POWER SYSTEM

An interconnected power system is a complex enterprise that may be
subdivided into the following major subsystems:
• Generation Subsystem
• Transmission and Subtransmission Subsystem
• Distribution Subsystem
• Utilization Subsystem

Generation Subsystem

This includes generators and transformers.
Generators – An essential component of power systems is the threephase
ac generator known as synchronous generator or alternator. Synchronous

generators have two synchronously rotating fields: One field is produced by the
rotor driven at synchronous speed and excited by dc current. The other field is
produced in the stator windings by the three-phase armature currents. The dc
current for the rotor windings is provided by excitation systems. In the older
units, the exciters are dc generators mounted on the same shaft, providing
excitation through slip rings. Current systems use ac generators with rotating
rectifiers, known as brushless excitation systems. The excitation system
maintains generator voltage and controls the reactive power flow. Because they
lack the commutator, ac generators can generate high power at high voltage,
typically 30 kV.
The source of the mechanical power, commonly known as the prime
mover, may be hydraulic turbines, steam turbines whose energy comes from the
burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal
combustion engines burning oil.
Steam turbines operate at relatively high speeds of 3600 or 1800 rpm.
The generators to which they are coupled are cylindrical rotor, two-pole for
3600 rpm, or four-pole for 1800 rpm operation. Hydraulic turbines, particularly
those operating with a low pressure, operate at low speed. Their generators are
usually a salient type rotor with many poles. In a power station, several
generators are operated in parallel in the power grid to provide the total power
needed. They are connected at a common point called a bus.
With concerns for the environment and conservation of fossil fuels,
many alternate sources are considered for employing the untapped energy
sources of the sun and the earth for generation of power. Some alternate sources
used are solar power, geothermal power, wind power, tidal power, and biomass.
The motivation for bulk generation of power in the future is the nuclear fusion.
If nuclear fusion is harnessed economically, it would provide clean energy from
an abundant source of fuel, namely water.
Transformers – The transformer transfers power with very high
efficiency from one level of voltage to another level. The power transferred to
the secondary is almost the same as the primary, except for losses in the
transformer. Using a step-up transformer will reduce losses in the line, which
makes the transmission of power over long distances possible.
Insulation requirements and other practical design problems limit the
generated voltage to low values, usually 30 kV. Thus, step-up transformers are
used for transmission of power. At the receiving end of the transmission lines
step-down transformers are used to reduce the voltage to suitable values for
distribution or utilization. The electricity in an electric power system may
undergo four or five transformations between generator and consumers.

Transmission and Subtransmission Subsystem

An overhead transmission network transfers electric power fromgenerating units to the distribution system which ultimately supplies the load.
Transmission lines also interconnect neighboring utilities which allow the
economic dispatch of power within regions during normal conditions, and the
transfer of power between regions during emergencies.
Standard transmission voltages are established in the United States by the
American National Standards Institute (ANSI). Transmission voltage lines
operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161
kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line. Transmission voltages
above 230 kV are usually referred to as extra-high voltage (EHV).
High voltage transmission lines are terminated in substations, which are
called high-voltage substations, receiving substations, or primary substations.
The function of some substations is switching circuits in and out of service;
they are referred to as switching stations. At the primary substations, the
voltage is stepped down to a value more suitable for the next part of the trip
toward the load. Very large industrial customers may be served from the
transmission system.
The portion of the transmission system that connects the high-voltage
substations through step-down transformers to the distribution substations is
called the subtransmission network. There is no clear distinction between
transmission and subtransmission voltage levels. Typically, the subtransmission
voltage level ranges from 69 to 138 kV. Some large industrial customers may
be served from the subtransmission system. Capacitor banks and reactor banks
are usually installed in the substations for maintaining the transmission line
voltage.

Distribution Subsystem

The distribution system connects the distribution substations to the
consumers’ service-entrance equipment. The primary distribution lines from 4
to 34.5 kV and supply the load in a well-defined geographical area. Some small
industrial customers are served directly by the primary feeders.
The secondary distribution network reduces the voltage for utilization
by commercial and residential consumers. Lines and cables not exceeding a few
hundred feet in length then deliver power to the individual consumers. The
secondary distribution serves most of the customers at levels of 240/120 V,
single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V,
three-phase, four-wire. The power for a typical home is derived from a
transformer that reduces the primary feeder voltage to 240/120 V using a threewire
line.
Distribution systems are both overhead and underground. The growth
of underground distribution has been extremely rapid and as much as 70 percent
of new residential construction is via underground systems.

Load Subsystems

Power systems loads are divided into industrial, commercial, and
residential. Industrial loads are composite loads, and induction motors form a
high proportion of these loads. These composite loads are functions of voltage
and frequency and form a major part of the system load. Commercial and
residential loads consist largely of lighting, heating, and cooking. These loads
are independent of frequency and consume negligibly small reactive power.

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HISTORY OF ELECTRIC POWER SYSTEMS

HISTORY OF ELECTRIC POWER SYSTEMS

Over the past century, the electric power industry continues to shape
and contribute to the welfare, progress, and technological advances of the
human race. The growth of electric energy consumption in the world has been
nothing but phenomenal. In the United States, for example, electric energy sales
have grown to well over 400 times in the period between the turn of the century
and the early 1970s. This growth rate was 50 times as much as the growth rate
in all other energy forms used during the same period. It is estimated that the
installed kW capacity per capita in the U.S. is close to 3 kW.

Edison Electric Illuminating Company of New York inaugurated the
Pearl Street Station in 1881. The station had a capacity of four 250-hp boilers
supplying steam to six engine-dynamo sets. Edison’s system used a 110-V dc
underground distribution network with copper conductors insulated with a jute
wrapping. In 1882, the first water wheel-driven generator was installed in
Appleton, Wisconsin. The low voltage of the circuits limited the service area of
a central station, and consequently, central stations proliferated throughout
metropolitan areas.

The invention of the transformer, then known as the “inductorium,”
made ac systems possible. The first practical ac distribution system in the U.S.
was installed by W. Stanley at Great Barrington, Massachusetts, in 1866 for
Westinghouse, which acquired the American rights to the transformer from its
British inventors Gaulard and Gibbs. Early ac distribution utilized 1000-V
overhead lines. The Nikola Tesla invention of the induction motor in 1888
helped replace dc motors and hastened the advance in use of ac systems.

The first American single-phase ac system was installed in Oregon in
1889. Southern California Edison Company established the first three phase 2.3
kV system in 1893.

By 1895, Philadelphia had about twenty electric companies with
distribution systems operating at 100-V and 500-V two-wire dc and 220-V
three-wire dc, single-phase, two-phase, and three-phase ac, with frequencies of
60, 66, 125, and 133 cycles per second, and feeders at 1000-1200 V and 2000-
2400 V.

The subsequent consolidation of electric companies enabled the
realization of economies of scale in generating facilities, the introduction of
equipment standardization, and the utilization of the load diversity between
areas. Generating unit sizes of up to 1300 MW are in service, an era that was
started by the 1973 Cumberland Station of the Tennessee Valley Authority.
Underground distribution at voltages up to 5 kV was made possible by
the development of rubber-base insulated cables and paper-insulated, leadcovered
cables in the early 1900s. Since then, higher distribution voltages have
been necessitated by load growth that would otherwise overload low-voltage
circuits and by the requirement to transmit large blocks of power over great
distances. Common distribution voltages presently are in 5-, 15-, 25-, 35-, and
69-kV voltage classes.

The growth in size of power plants and in the higher voltage equipment
was accompanied by interconnections of the generating facilities. These
interconnections decreased the probability of service interruptions, made the
utilization of the most economical units possible, and decreased the total reserve
capacity required to meet equipment-forced outages. This was accompanied by
use of sophisticated analysis tools such as the network analyzer. Central control
of the interconnected systems was introduced for reasons of economy and
safety. The advent of the load dispatcher heralded the dawn of power systems
engineering, an exciting area that strives to provide the best system to meet the
load requirements reliably, safely, and economically, utilizing state-of-the-art
computer facilities.

Extra higher voltage (EHV) has become dominant in electric power
transmission over great distances. By 1896, an 11-kv three-phase line was
transmitting 10 MW from Niagara Falls to Buffalo over a distance of 20 miles.
Today, transmission voltages of 230 kV, 287 kV, 345 kV, 500 kV, 735 kV, and
765 kV are commonplace, with the first 1100-kV line already energized in the
early 1990s. The trend is motivated by economy of scale due to the higher
transmission capacities possible, more efficient use of right-of-way, lower
transmission losses, and reduced environmental impact.

In 1954, the Swedish State Power Board energized the 60-mile, 100-kV
dc submarine cable utilizing U. Lamm’s Mercury Arc valves at the sending and
receiving ends of the world’s first high-voltage direct current (HVDC) link
connecting the Baltic island of Gotland and the Swedish mainland. Currently,
numerous installations with voltages up to 800-kV dc are in operation around
the world.

In North America, the majority of electricity generation is produced by
investor-owned utilities with a certain portion done by federally and provincially
(in Canada) owned entities. In the United States, the Federal Energy Regulatory
Commission (FERC) regulates the wholesale pricing of electricity and terms and
conditions of service.

The North American transmission system is interconnected into a large
power grid known as the North American Power Systems Interconnection. The
grid is divided into several pools. The pools consist of several neighboring
utilities which operate jointly to schedule generation in a cost-effective manner.
A privately regulated organization called the North American Electric
Reliability Council (NERC) is responsible for maintaining system standards and
reliability. NERC works cooperatively with every provider and distributor of
power to ensure reliability. NERC coordinates its efforts with FERC as well as
other organizations such as the Edison Electric Institute (EEI). NERC currently
has four distinct electrically separated areas. These areas are the Electric
Reliability Council of Texas (ERCOT), the Western States Coordination
Council (WSCC), the Eastern Interconnect, which includes all the states and
provinces of Canada east of the Rocky Mountains (excluding Texas), and
Hydro-Quebec. These electrically separate areas exchange with each other but
are not synchronized electrically.

The electric power industry in the United States is undergoing
fundamental changes since the deregulation of the telecommunication, gas, and
other industries. The generation business is rapidly becoming market-driven.

The power industry was, until the last decade, characterized by larger, vertically
integrated entities. The advent of open transmission access has resulted in
wholesale and retail markets. Utilities may be divided into power generation,
transmission, and retail segments. Generating companies (GENCO) sell directly
to an independent system operator (ISO). The ISO is responsible for the
operation of the grid and matching demand and generation dealing with
transmission companies as well (TRANSCO). This scenario is not the only
possibility, as the power industry continues to evolve to create a more
competitive environment for electricity markets to promote greater efficiency.

The industry now faces new challenges and problems associated with the
interaction of power system entities in their efforts to make crucial technical
decisions while striving to achieve the highest level of human welfare.

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AC Motor-Control

AC Motor-Control

 

Power distribution systems used in large commercial and
industrial applications can be complex. Power may be
distributed through switchgear, switchboards, transformers,
and panelboards. Power distributed throughout a commercial
or industrial application is used for a variety of applications such
as heating, cooling, lighting, and motor-driven machinery. Unlike
other types of power distribution equipment, which are used
with a variety of load types, motor control centers primarily
control the distribution of power to electric motors.

Wherever motors are used, they must be controlled. In Basics
of Control Components you learned how various control
products are used to control the operation of motors. The most
basic type of AC motor control, for example, involves turning
the motor on and off. This is often accomplished using a motor
starter made up of a contactor and an overload relay.
The contactor’s contacts are closed to start the motor
and opened to stop the motor. This is accomplished
electromechanically using start and stop pushbuttons or other
pilot devices wired to control the contactor.

The overload relay protects the motor by disconnecting power
to the motor when an overload condition exists. Although the
overload relay provides protection from overloads, it does not
provide short-circuit protection for the wiring supplying power
to the motor. For this reason, a circuit breaker or fuses are also
used.

Typically one motor starter controls one motor. When only a
few geographically dispersed AC motors are used, the circuit
protection and control components may be located in a panel
near the motor.

  motor control center (MCC).

In many commercial and industrial applications, quite a few
electric motors are required, and it is often desirable to control
some or all of the motors from a central location. The apparatus
designed for this function is the motor control center (MCC).
Motor control centers are simply physical groupings of
combination starters in one assembly. A combination starter is
a single enclosure containing the motor starter, fuses or circuit
breaker, and a device for disconnecting power. Other devices
associated with the motor, such as pushbuttons and indicator
lights, may also be included.

tiastar

tiastar (pronounced tie-star) is the trade name for Siemens
Motor Control Centers

Some of the advantages of using tiastar motor control centers are:

• Ruggedness and reliability
• Reduced time needed for installation and startup
• Space saving design
• Excellent component selection
• Simplicity in adding special components
• Ease of future modifications.

The TIA

The TIA portion of the tiastar name stands for Totally
Integrated Automation. TIA is more than a concept. It is a
strategy developed by Siemens that emphasizes the seamless
integration of automation, networking, drive, and control
products. The TIA strategy has been the cornerstone of
development for a wide variety of Siemens products.
TIA is important not just because it simplifies the engineering,
startup, and maintenance of systems developed using Siemens
products, but also because it lowers the life-cycle costs for
systems incorporating these products. Additionally, by reducing
engineering and startup of systems, TIA helps Siemens
customers reduce time to market and improve overall financial
performance.

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AC Motor-Construction

AC Motor Construction

Three-phase AC induction motors are commonly used in
industrial applications. This type of motor has three main parts,
rotor, stator, and enclosure. The stator and rotor do the work,
and the enclosure protects the stator and rotor.

The stator

The stator is the stationary part of the motor’s electromagnetic
circuit. The stator core is made up of many thin metal sheets,
called laminations. Laminations are used to reduce energy
loses that would result if a solid core were used.

Stator laminations are stacked together forming a hollow
cylinder. Coils of insulated wire are inserted into slots of the
stator core.
When the assembled motor is in operation, the stator windings
are connected directly to the power source. Each grouping of
coils, together with the steel core it surrounds, becomes an
electromagnet when current is applied. Electromagnetism is
the basic principle behind motor operation.

The rotor

The rotor is the rotating part of the motor’s electromagnetic
circuit. The most common type of rotor used in a three-phase
induction motor is a squirrel cage rotor. Other types of rotor
construction is discussed later in the course. The squirrel cage
rotor is so called because its construction is reminiscent of the
rotating exercise wheels found in some pet cages.

A squirrel cage rotor core is made by stacking thin steel
laminations to form a cylinder.

Rather than using coils of wire as conductors, conductor bars
are die cast into the slots evenly spaced around the cylinder.
Most squirrel cage rotors are made by die casting aluminum to
form the conductor bars. Siemens also makes motors with die
cast copper rotor conductors. These motor exceed NEMA
Premium efficiency standards.
After die casting, rotor conductor bars are mechanically and
electrically connected with end rings. The rotor is then pressed
onto a steel shaft to form a rotor assembly.

The enclosure

The enclosure consists of a frame (or yoke) and two end
brackets (or bearing housings). The stator is mounted inside
the frame. The rotor fits inside the stator with a slight air
gap separating it from the stator. There is no direct physical
connection between the rotor and the stator.

Bearings

Bearings, mounted on the shaft, support the rotor and allow
it to turn. Some motors, like the one shown in the following
illustration, use a fan, also mounted on the rotor shaft, to cool
the motor when the shaft is rotating.

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AC.Motors-Introduction

AC.Motors-Introduction

 

AC motors.are.used.worldwide.in.many.applications.to.

transform.electrical.energy.into.mechanical.energy..There.are. many.types.of.AC.motors,.but.this.course.focuses.on.three- phase AC induction motors,.the.most.common.type.of.motor. used.in.industrial.applications..

An.AC.motor.of.this.type.may.be.part.of.a.pump.or.fan.or.

connected.to.some.other.form.of.mechanical.equipment.such. as.a.winder,.conveyor,.or.mixer..Siemens.manufactures.a.wide. variety.of.AC.motors..In.addition.to.providing.basic.information. about.AC.motors.in.general,.this.course.also.includes.an. overview.of.Siemens.AC.motors. 

 

NEMA Motors

National Electrical Manufacturers Association (NEMA). NEMA
develops standards for a wide range of electrical products,
including AC motors. For example, NEMA Standard Publication
MG 1 covers NEMA frame size AC motors, commonly referred
to as NEMA motors.

Above NEMA Motors

 In addition to manufacturing NEMA motors, Siemens also
manufactures motors larger than the largest NEMA frame
size. These motors are built to meet specific application
requirements and are commonly referred to as above NEMA
motors.

IEC Motors 

Siemens also manufactures motors to International
Electrotechnical Commission (IEC) standards. IEC is another
organization responsible for electrical standards. IEC standards
perform the same function as NEMA standards, but differ
in many respects. In many countries, electrical equipment
is commonly designed to comply with IEC standards. In the
United States, although IEC motors are sometimes used,
NEMA motors are more common. Keep in mind, however,
that many U.S.-based companies build products for export to
countries that follow IEC standards.

 

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Electric Circuits

Electric Circuits

Although static electricity has its useful applications, more useful by far is a
system that can continuously separate negative from positive charges, then extract
energy from them as they move around to recombine. This is the principle of the
electric circuit. Figure .1 shows a very simple electric circuit, consisting of a
battery with its two ends connected by a single wire.

Figure 5.1. A very simple electric circuit, consisting of a battery with its two ends

connected by a metal wire. (Don’t try this at home unless you don’t mind running

down the battery very quickly.)

The battery uses chemical energy to separate negative from positive charges,
always maintaining a slight excess positive charge on its “+” end and a slightconductors, these excess charges just sit there and do nothing of interest. Connect
the two ends together with a metal wire, however, and the electrons will move along
the wire in order to recombine with the protons. Along the way, they will collide
with the atoms in the wire, creating a kind of “friction” that makes the wire get hot.
The battery, meanwhile, keeps replenishing the supply of electrons at its negative
end, until its internal chemical reaction has gone to completion. In summary, this
circuit converts chemical energy in the battery into electrical energy, which is then
converted into thermal energy in the wire.

Figure2  shows a slightly more complicated circuit, consisting of a battery, a
pair of wires, and an ordinary incandescent light bulb. This circuit is essentially a
flashlight. Because the filament of the bulb offers significantly more resistance to
the flow of electrons than do the wires leading to it, the electrons will flow much
more slowly in this circuit than in the previous one. Instead of creating thermal
energy uniformly along the wires, this circuit concentrates the thermal energy at the
point of greatest resistance, the filament. The filament becomes so hot that it glows.
The bulb around the filament keeps oxygen out, preventing chemical reactions of the hot metal filament with oxygen.

Figure 2. A “flashlight” circuit, consisting of a battery connected to a light bulb

by a pair of wires. (The connections inside the bulb simply route the electric current

through the filament.)

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