Houssem Rafik El-Hana BOUCHEKARA 2009/2010 1430/1431 KINGDOM OF SAUDI ARABIA Ministry Of High Education Umm Al-Qura University College of Engineering & Islamic Architecture Department Of Electrical Engineering Fundamentals of Electrical Engineering 4. Generation, Transmission and Distribution of Electrical Power
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N FE 4. Generation Transmission and Distribution of Electrical Power
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4.2.3 Frequency, voltage & interconnected system ....................................................... 8
4.3 ELECTRIC POWER GENERATION: CONVENTIONAL METHODS .................................................... 9
4.3.1 Hydroelectric Power Generation ........................................................................... 9 4.3.1.1 Selection of Plant Capacity, Energy, and Other Design Features ............................... 11 4.3.1.2 Generator ................................................................................................................... 11
4.4 ELECTRIC POWER GENERATION: NONCONVENTIONAL METHODS ........................................... 15
4.4.1 Wind Power ......................................................................................................... 15 4.4.1.1 Benefits of Wind Power ............................................................................................. 15 4.4.1.2 Wind Power Today ..................................................................................................... 15
The purpose of the electric transmission system is the interconnection of the electric
energy producing power plants or generating stations with the loads. A three-phase AC
system is used for most transmission lines. The operating frequency is 60 Hz in the U.S. and
50 Hz in Europe, Australia, and part of Asia.
The three-phase system has three phase conductors. The system voltage is defined
as the rms voltage between the conductors, also called line-to-line voltage. The voltage
between the phase conductor and ground, called line-to-ground voltage, is equal to the line-
to-line voltage divided by the square root of three.
4.7 GENERATION STATIONS
The generating station converts the stored energy of gas, oil, coal, nuclear fuel, or
water position to electric energy. The most frequently used power plants are described in
the first part of this chapter (Power Generation).
4.8 SWITCHGEAR
The safe operation of the system requires switches to open lines automatically in
case of a fault, or manually when the operation requires it. Figure 3.7 shows the simplified
connection diagram of a generating station.
The generator is connected directly to the low-voltage winding of the main
transformer. The transformer high-voltage winding is connected to the bus through a circuit
breaker; disconnect switch, and current transformer. The generating station auxiliary power
is supplied through an auxiliary transformer through a circuit breaker; disconnect switch,
and current transformer. Generator circuit breakers, connected between the generator and
transformer, are frequently used in Europe. These breakers have to interrupt the very large
short-circuit current of the generators, which results in high cost.
Figure 3. 8: Simplified connection diagram of a generating station.
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The high-voltage bus supplies two outgoing lines. The station is protected from
lightning and switching surges by a surge arrester.
Circuit breaker (CB) is a large switch that interrupts the load and fault current. Fault
detection systems automatically open the CB, but it can be operated manually.
Disconnect switch provides visible circuit separation and permits CB maintenance. It
can be operated only when the CB is open, in no-load condition.
Potential transformers (PT) and current transformers (CT) reduce the voltage to 120
V, the current to 5 A, and insulates the low-voltage circuit from the high-voltage. These
quantities are used for metering and protective relays. The relays operate the appropriate
CB in case of a fault.
Surge arresters are used for protection against lightning and switching overvoltages.
They are voltage dependent, nonlinear resistors.
4.9 CONCEPT OF ENERGY TRANSMISSION AND DISTRIBUTION
Figure 3.8 shows the concept of typical energy transmission and distribution
systems. The generating station produces the electric energy. The generator voltage is
around 15 to 25 kV. This relatively low voltage is not appropriate for the transmission of
energy over long distances. At the generating station a transformer is used to increase the
voltage and reduce the current. In Figure 3.8 the voltage is increased to 500 kV and an extra-
high-voltage (EHV) line transmits the generator-produced energy to a distant substation.
Such substations are located on the outskirts of large cities or in the center of several large
loads.
The voltage is reduced at the 500 kV=220 kV EHV substation to the high-voltage level
and high-voltage lines transmit the energy to high-voltage substations located within cities.
At the high-voltage substation the voltage is reduced to 69 kV. Sub-transmission
lines connect the high-voltage substation to many local distribution stations located within
cities. Sub-transmission lines are frequently located along major streets.
The voltage is reduced to 12 kV at the distribution substation. Several distribution
lines emanate from each distribution substation as overhead or underground lines.
Distribution lines distribute the energy along streets and alleys. Each line supplies several
step-down transformers distributed along the line. The distribution transformer reduces the
voltage to 230=115 V, which supplies houses, shopping centers, and other local loads. The
large industrial plants and factories are supplied directly by a subtransmission line or a
dedicated distribution line as shown in Figure 3.8.
The overhead transmission lines are used in open areas such as interconnections
between cities or along wide roads within the city. In congested areas within cities,
underground cables are used for electric energy transmission. The underground
transmission system is environmentally preferable but has a significantly higher cost. In
Figure 3.8 the 12-kV line is connected to a 12-kV cable which supplies commercial or
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industrial customers. The figure also shows 12-kV cable networks supplying downtown areas
in a large city. Most newly developed residential areas are supplied by 12-kV cables through
pad-mounted step-down transformers as shown in Figure 3.8.
Figure 3. 9: Concept of electric energy transmission.
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Figure 3. 10: Electric power systems consist of many subsystems. Reliability depends upon generating enough electric power and delivering it to customers without any interruptions in supply voltage. A majority of
interruptions in developed nations result from problems occurring between customer meters and distribution substations.
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4.9.1 HIGH-VOLTAGE TRANSMISSION LINES
Highvoltage and extra-high-voltage (EHV) transmission lines interconnect power
plants and loads, and form an electric network. Transmission lines are terminated at the bus
of a substation.
The physical arrangement of most extra-high-voltage (EHV) lines is similar. Figure 3.9
shows the major components of an EHV, which are:
1. Tower: The figure shows a lattice, steel tower.
2. Insulator: V strings hold four bundled conductors in each phase.
3. Conductor: Each conductor is stranded, steel reinforced aluminum cable.
4. Foundation and grounding: Steel-reinforced concrete foundation and grounding
electrodes placed in the ground.
5. Shield conductors: Two grounded shield conductors protect the phase
conductors from lightning.
At lower voltages the appearance of lines can be improved by using more
aesthetically pleasing steel tubular towers. Steel tubular towers are made out of a tapered
steel tube equipped with banded arms. The arms hold the insulators and the conductors.
Figure 3. 11: Typical high-voltage transmission line. (From Fink, D.G. and Beaty, H.W., Standard Handbook for Electrical Engineering, 11th ed., McGraw-Hill, New York, 1978.)
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4.9.2 HIGH-VOLTAGE DC LINES
High-voltage DC lines are used to transmit large amounts of energy over long
distances or through waterways. One of the best known is the Pacific HVDC Intertie, which
interconnects southern California with Oregon. Another DC system is the +400 kV Coal
Creek-Dickenson lines. Another famous HVDC system is the interconnection between
England and France, which uses underwater cables. In Canada, Vancouver Island is supplied
through a DC cable.
In an HVDC system the AC voltage is rectified and a DC line transmits the energy. At
the end of the line an inverter converts the DC voltage to AC. A typical example is the Pacific
HVDC Intertie that operates with +500 kV voltages and interconnects Southern California
with the hydro stations in Oregon.
4.9.3 SUBSTATIONS
Substations are the places where the level of voltage undergoes change with the
help of transformers. Apart from transformers a substation will house switches (called circuit
breakers), meters, relays for protection and other control equipment. Broadly speaking, a
big substation will receive power through incoming lines at some voltage (say 400 kV)
changes level of voltage (say to 132 kV) using a transformer and then directs it out wards
through outgoing lines. Pictorially such a typical power system is shown in Figure 3.10 in a
short of block diagram. At the lowest voltage level of 400 V, generally 3-phase, 4-wire
system is adopted for domestic connections. The fourth wire is called the neutral wire (N)
which is taken out from the common point of the star connected secondary of the 6 kV/400
V distribution transformer.
Figure 3. 12: Typical voltages in power system.
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4.9.4 DISTRIBUTION SYSTEM
Till now we have learnt how power at somewhat high voltage (say 33 kV) is received
in a substation situated near load center (a big city). The loads of a big city are primarily
residential complexes, offices, schools, hotels, street lighting etc. These types of consumers
are called LT (low tension) consumers. Apart from this there may be medium and small scale
industries located in the outskirts of the city. LT consumers are to be supplied with single
phase, 220 V, 40 Hz. We shall discuss here how this is achieved in the substation receiving
power at 33 kV. The scheme is shown in figure 3.11.
Figure 3. 13: Typical Power distribution scheme.
Power receive at a 33 kV substation is first stepped down to 6 kV and with the help
of under ground cables (called feeder lines), power flow is directed to different directions of
the city. At the last level, step down transformers are used to step down the voltage form 6
kV to 400 V. These transformers are called distribution transformers with 400 V, star
connected secondary. You must have noticed such transformers mounted on poles in cities
beside the roads. These are called pole mounted substations. From the secondary of these
transformers 4 terminals (R, Y, B and N) come out. N is called the neutral and taken out from
the common point of star connected secondary.
Voltage between any two phases (i.e., R-Y, Y-B and B-R) is 400 V and between any
phase and neutral is 230 V(=4003. Residential buildings are supplied with single phase 230V,
50Hz.
So individual are to be supplied with any one of the phases and neutral. Supply
authority tries to see that the loads remain evenly balanced among the phases as far as
possible. Which means roughly one third of the consumers will be supplied from R-N, next
one third from Y-N and the remaining one third from B-N.
The distribution of power from the pole mounted substation can be done either by
(1) overhead lines (bare conductors) or by (2) underground cables. Use of overhead lines
although cheap, is often accident prone and also theft of power by hooking from the lines
take place. Although costly, in big cities and thickly populated areas underground cables for
distribution of power, are used.
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Figure 3. 14: Power Transmission.
4.10 FAULTS AND PROTECTION OF ELECTRIC ENERGY
SYSTEMS
4.10.1 INTRODUCTION
A short-circuit fault takes place when two or more conductors come in contact with
each other when normally they operate with a potential difference between them. The
contact may be a physical metallic one, or it may occur through an arc. In the metal-to-metal
contact case, the voltage between the two parts is reduced to zero. On the other hand, the
voltage through an arc will be of a very small value. Short-circuit faults in three-phase
systems are classified as:
1. Balanced or symmetrical three-phase faults.
2. Single line-to-ground faults.
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3. Line-to-line faults.
4. Double line-to-ground faults.
Generator failure is caused by insulation breakdown between turns in the same slot
or between the winding and the steel structure of the machine. The same can take place in
transformers. The breakdown is due to insulation deterioration combined with switching
and/or lightning overvoltages. Overhead lines are constructed of bare conductors. Wind,
sleet, trees, cranes, kites, airplanes, birds, or damage to supporting structure are causes for
accidental faults on overhead lines. Contamination of insulators and lightning overvoltages
will in general result in short-circuit faults. Deterioration of insulation in underground cables
results in short circuit faults. This is mainly attributed to aging combined with overloading.
About 75 percent of the energy system’s faults are due to single-line-to-ground faults and
result from insulator flashover during electrical storms. Only one in twenty faults is due to
the balanced category.
A fault will cause currents of high value to flow through the network to the faulted
point. The amount of current may be much greater than the designed thermal ability of the
conductors in the power lines or machines feeding the fault. As a result, temperature rise
may cause damage by annealing of conductors and insulation charring. In addition, the low
voltage in the neighborhood of the fault will cause equipment malfunction.
Short-circuit and protection studies are an essential tool for the electric energy
systems engineer. The task is to calculate the fault conditions and to provide protective
equipment designed to isolate the faulted zone from the remainder of the system in the
appropriate time. The least complex fault category computationally is the balanced fault. It
is possible that a balanced fault could (in some locations) result in currents smaller than that
due to some other type of fault. The interrupting capacity of breakers should be chosen to
accommodate the largest of fault currents, and hence, care must be taken not to base
protection decisions on the results of a balanced three phase fault.
4.10.2 NEED FOR PROTECTIVE APPARATUS
A power system is not only capable to meet the present load but also has the
flexibility to meet the future demands. A power system is designed to generate electric
power in sufficient quantity, to meet the present and estimated future demands of the users
in a particular area, to transmit it to the areas where it will be used and then distribute it
within that area, on a continuous basis.
To ensure the maximum return on the large investment in the equipment, which
goes to make up the power system and to keep the users satisfied with reliable service, the
whole system must be kept in operation continuously without major breakdowns.
This can be achieved in two ways:
• The first way is to implement a system adopting components, which should not fail
and requires the least or nil maintenance to maintain the continuity of service. By common
sense, implementing such a system is neither economical nor feasible, except for small
systems.
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• The second option is to foresee any possible effects or failures that may cause
long-term shutdown of a system, which in turn may take longer time to bring back the
system to its normal course. The main idea is to restrict the disturbances during such failures
to a limited area and continue power distribution in the balance areas. Special equipment is
normally installed to detect such kind of failures (also called ‘faults’) that can possibly
happen in various sections of a system, and to isolate faulty sections so that the interruption
is limited to a localized area in the total system covering various areas. The special
equipment adopted to detect such possible faults is referred to as ‘protective equipment or
protective relay’ and the system that uses such equipment is termed as ‘protection system’.
A protective relay is the device, which gives instruction to disconnect a faulty part of
the system. This action ensures that the remaining system is still fed with power, and
protects the system from further damage due to the fault. Hence, use of protective
apparatus is very necessary in the electrical systems, which are expected to generate,
transmit and distribute power with least interruptions and restoration time. It can be well
recognized that use of protective equipment are very vital to minimize the effects of faults,
which otherwise can kill the whole system.
4.10.3 BASIC REQUIREMENTS OF PROTECTION
A protection apparatus has three main functions/duties:
1. Safeguard the entire system to maintain continuity of supply
2. Minimize damage and repair costs where it senses fault
3. Ensure safety of personnel.
These requirements are necessary, firstly for early detection and localization of
faults, and secondly for prompt removal of faulty equipment from service. In order to carry
out the above duties, protection must have the following qualities:
Selectivity: To detect and isolate the faulty item only.
Stability: To leave all healthy circuits intact to ensure continuity or supply.
Sensitivity: To detect even the smallest fault, current or system
abnormalities and operate correctly at its setting before the fault causes
irreparable damage.
Speed: To operate speedily when it is called upon to do so, thereby
minimizing damage to the surroundings and ensuring safety to personnel.
To meet all of the above requirements, protection must be reliable which means it
must be:
Dependable: It must trip when called upon to do so.
Secure: It must not trip when it is not supposed to.
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4.10.4 BASIC COMPONENTS OF PROTECTION
Protection of any distribution system is a function of many elements and this manual
gives a brief outline of various components that go in protecting a system. Following are the
main components of protection.
Fuse is the self-destructing one, which carries the currents in a power circuit
continuously and sacrifices itself by blowing under abnormal conditions.
These are normally independent or stand-alone protective components in
an electrical system unlike a circuit breaker, which necessarily requires the
support of external components.
Accurate protection cannot be achieved without properly measuring the
normal and abnormal conditions of a system. In electrical systems, voltage
and current measurements give feedback on whether a system is healthy or
not. Voltage transformers and current transformers measure these basic
parameters and are capable of providing accurate measurement during fault
conditions without failure.
The measured values are converted into analog and/or digital signals and
are made to operate the relays, which in turn isolate the circuits by opening
the faulty circuits. In most of the cases, the relays provide two functions viz.,
alarm and trip, once the abnormality is noticed. The relays in olden days had
very limited functions and were quite bulky. However, with advancement in
digital technology and use of microprocessors, relays monitor various
parameters, which give complete history of a system during both pre-fault
and post-fault conditions.
The opening of faulty circuits requires some time, which may be in
milliseconds, which for a common day life could be insignificant. However,
the circuit breakers, which are used to isolate the faulty circuits, are capable
of carrying these fault currents until the fault currents are totally cleared.
The circuit breakers are the main isolating devices in a distribution system,
which can be said to directly protect the system.
The operation of relays and breakers require power sources, which shall not
be affected by faults in the main distribution. Hence, the other component,
which is vital in protective system, is batteries that are used to ensure
uninterrupted power to relays and breaker coils.
The above items are extensively used in any protective system and their design
requires careful study and selection for proper operation.
4.10.5 SUMMARY
Power System Protection – Main Functions
1. To safeguard the entire system to maintain continuity of supply. 2. To minimize damage and repair costs. 3. To ensure safety of personnel.
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Power System Protection – Basic Requirements
1. Selectivity: To detect and isolate the faulty item only. 2. Stability: To leave all healthy circuits intact to ensure continuity of supply. 3. Speed: To operate as fast as possible when called upon, to minimize damage,
production downtime and ensure safety to personnel. 4. Sensitivity: To detect even the smallest fault, current or system abnormalities
and operate correctly at its setting.
Power System Protection – Speed is Vital!!
The protective system should act fast to isolate faulty sections to prevent:
Increased damage at fault location. Fault energy = 𝐼2 𝑅𝑓 𝑡, where t is time in
seconds.
Danger to the operating personnel (flashes due to high fault energy sustaining for a long time).
Danger of igniting combustible gas in hazardous areas, such as methane in coal mines which could cause horrendous disaster.
Increased probability of earth faults spreading to healthy phases.
Higher mechanical and thermal stressing of all items of plant carrying the fault current, particularly transformers whose windings suffer progressive and cumulative deterioration because of the enormous electromechanical forces caused by multi-phase faults proportional to the square of the fault current.
Sustained voltage dips resulting in motor (and generator) instability leading to extensive shutdown at the plant concerned and possibly other nearby plants connected to the system.
Power System Protection – Qualities
1. Dependability: It MUST trip when called upon. 2. Security: It must NOT trip when not supposed to.
Power System Protection – Basic Components
1. Voltage transformers and current transformers: To monitor and give accurate feedback about the healthiness of a system.
2. Relays: To convert the signals from the monitoring devices, and give instructions to open a circuit under faulty conditions or to give alarms when
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the equipment is protected, is approaching towards possible destruction. 3. Fuses: Self-destructing to save the downstream equipment being protected. 4. Circuit breakers: These are used to make circuits carrying enormous currents,
and also to break the circuit carrying the fault currents for a few cycles based on feedback from the relays.
5. DC batteries: These give uninterrupted power source to the relays and breakers that is independent of the main power source being protected.
4.11 POWER QUALITY, RELIABILITY, AND AVAILABILITY
“Power quality” is an ambiguous term that means many things to many people.
From a customer perspective, a power quality problem might be defined as any electric
supply condition that causes appliances to malfunction or prevents their use. From a utility
perspective, a power quality problem might be perceived as noncompliance with various
standards such as RMS voltage or harmonics. In fact, power is equal to the instantaneous
product of current and voltage, and formulating a meaningful definition of power quality is
difficult.
Power quality is better thought of as voltage quality. This is why the IEEE group
focused on the subject is called the Working Group on Distribution Voltage Quality. The best
a utility can do is to supply customers a perfect sinusoidal voltage source. Utilities have no
control over the current drawn by end uses and should be generally unconcerned with
current waveforms. Load current can affect voltage as it interacts with system impedances,
but voltage is the ultimate measure of power quality.
In this book, power quality is defined as the absence of deviation from a perfect
sinusoidal voltage source. Perfect power quality is a perfect sinusoid with constant
frequency and amplitude. Less than perfect power quality occurs when a voltage waveform
is distorted by transients or harmonics, changes its amplitude, or deviates in frequency.
According to this definition, customer interruptions are power quality concerns since
they are a reduction in voltage magnitude to zero. Reliability is primarily concerned with
customer interruptions and is, therefore, a subset of power quality. Although there is
general agreement that power quality includes reliability, the boundary that separates the
two is not well defined. Sustained interruptions (more than a few minutes) have always
been categorized as a reliability issue, but many utilities have classified momentary
interruptions (less than a few minutes) as a power quality issue. The reasons are (1)
momentary interruptions are the result of intentional operating practices, (2) they did not
generate a large number of customer complaints, and (3) they are difficult to measure.
Today, momentary interruptions are an important customer issue and most distribution
engineers consider them a reliability issue. Therefore, this book defines reliability as all
aspects of customer interruptions, including momentary interruptions.
Availability is defined as the percentage of time a voltage source is uninterrupted. Its
complement, unavailability, is the percentage of time a voltage source in interrupted. Since
availability and unavailability deal strictly with interruptions, they are classified as a subset
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of reliability. The hierarchy of power quality, reliability, and availability is shown in Figure
2.1.
Figure 3. 15: Availability is a subset of reliability and reliability is a subset of power quality. Power quality deals with any deviation from a perfect sinusoidal voltage source. Reliability deals with interruptions. Availability
deals with the probability of being in an interrupted state.
Perfect power quality is characterized by a perfect sinusoidal voltage source without
waveform distortion, variation in amplitude or variation in frequency. To attain near-perfect
power quality, a utility could spend vast amounts of money and accommodate electrical
equipment with high power quality needs. On the other hand, a utility could spend as little
money as possible and require customers to compensate for the resulting power quality
problems. Since neither extreme is desirable, utilities must find a balance between cost and
the level of power quality provided to customers. Concerns arise when power quality levels
do not meet equipment power quality needs.
Power quality concerns are becoming more frequent with the proliferation of
sensitive electronic equipment and automated processes. A large part of this problem is due
to the lack of communication between electric utilities and product manufacturers. Many
manufacturers are not aware of common power quality problems and cannot design their
equipment accordingly.
Power quality problems can be divided into many categories such as interruptions,
sags, swells, transients, noise, flicker, harmonic distortion and frequency variation.
Waveforms corresponding to these power quality problems are shown in Figure 2.2. Each of
these categories is a complex topic in its own right, but can only be addressed briefly in this
section. The reader is referred to chapter references for more detailed treatment.
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Figure 3. 16: Common power quality problems. Perfect power quality corresponds to an undistorted sinusoidal voltage with constant amplitude and constant frequency. Most electrical appliances can accommodate slight disturbances in power quality, with some being much more sensitive than others. Utilities usually attempt to
keep voltage quality within tolerances defined in industry standards such as ANSI, IEEE, and IEC.
Interruptions — Interruptions are the loss of service voltage to a customer and can
be momentary or sustained in nature.
Sags — Voltage sags are temporary RMS reductions in voltage typically lasting from a
half cycle to several seconds. Sags result from high currents, typically due to faults or
starting motors, interactingwith system impedances.
Swells — Voltage swells are temporary RMS increases in voltage typically lasting
from a half cycle to several seconds. Swells are commonly caused by the de-energizing of
large loads or asymmetrical faults (a line to ground fault will cause a voltage rise in the other
two phases). Swells can cause insulation breakdown in sensitive electronic equipment if
voltage increases are high enough for a long enough period of time. Equipment tolerance to
swells, like sags, is described by voltage tolerance envelopes like the ITIC Curve.
Transients — Voltage transients are sudden nonrecurring changes in voltage
magnitude. An impulse transient, most commonly caused by lightning, is described by the
time to reach its peak value and the time to decay to half of its peak value.
Noise — Noise can be broadly defined as unwanted voltage signals with broadband
spectral content. Common causes include power electronics, control circuits, arcing
equipment, loads with solid state rectifiers, and switched mode power supplies. Noise
problems are often exacerbated by improper grounding.
Flicker — Voltage flicker refers to low frequency variations in RMS voltage that cause
visible changes in the brightness of incandescent lighting. These voltage variations are
caused by the cycling of large loads such as refrigerators, air conditioners, elevators, arc
furnaces, and spot welders.
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4.12 CONCLUSION
In this lesson, a brief idea of generation, transmission and distribution of electrical
power is given - which for obvious reason is neither very elaborative nor exhaustive.
Nonetheless, it gives a reasonable understanding of the system for a beginner going to
undertake the course on electrical technology. If you ever get a chance to visit a substation