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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4 GENERATION, TRANSMISSION AND DISTRIBUTION OF ELECTRICAL POWER.................. 3

4.1 INTRODUCTION ............................................................................................................... 3

PART 1- POWER GENERATION .............................................................................................. 4

4.2 BASIC IDEA OF GENERATION .............................................................................................. 6

4.2.1 Changeover from D.C to A.C .................................................................................. 6

4.2.2 A.C Generator ........................................................................................................ 7

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

4.3.2 Thermal Generating Plants.................................................................................. 12

4.3.3 Nuclear plants ..................................................................................................... 13

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

4.4.2 Photovoltaics ....................................................................................................... 16 4.4.2.1 Advantages................................................................................................................. 17 4.4.2.2 Disadvantages ............................................................................................................ 18

PART 2- POWER TRANSMISSION AND DISTRIBUTION ......................................................... 19

4.5 GENERATION STATIONS .................................................................................................. 19

4.6 SWITCHGEAR ............................................................................................................... 19

4.7 CONCEPT OF ENERGY TRANSMISSION AND DISTRIBUTION ..................................................... 20

4.7.1 High-Voltage Transmission Lines......................................................................... 23

4.7.2 High-Voltage DC Lines ......................................................................................... 24

4.7.3 Substations .......................................................................................................... 24

4.7.4 Distribution system .............................................................................................. 25

4.8 POWER QUALITY ..................................................................... ERREUR ! SIGNET NON DEFINI.

4.9 CONCLUSION ............................................................................................................... 34

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4 GENERATION, TRANSMISSION AND DISTRIBUTION OF

ELECTRICAL POWER

4.1 INTRODUCTION

In this lesson a brief idea of a modern power system is outlined. Emphasis is given to

create a clear mental picture of a power system to a beginner of the course Electrical

Technology. As consumers, we use electricity for various purposes such as:

1. Lighting, heating, cooling and other domestic electrical appliances used in home.

2. Street lighting, flood lighting of sporting arena, office building lighting, powering

PCs etc.

3. Irrigating vast agricultural lands using pumps and operating cold storages for

various agricultural products.

4. Running motors, furnaces of various kinds, in industries.

5. Running locomotives (electric trains) of railways.

The list above is obviously not exhaustive and could be expanded and categorized in

detail further. The point is, without electricity, modern day life will simply come to a stop. In

fact, the advancement of a country is measured by the index per capita consumption of

electricity – more it is more advanced the country is.

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PART 1- POWER GENERATION

4.2 HISTORY OF THE ELECTRICITY GENERATION INDUSTRY

The roots of the modern electricity generating industry are to be found in the early

and middle years of the nineteenth century and in the work of men such as Benjamin

Franklin, Alessandro Volta and Michael Faraday. Faraday, in particular, was able to show the

relationship between electricity and magnetism, a relationship that makes it possible to

generate electricity with moving machinery rather than take it from chemical batteries as

was the case in his day.

The widening understanding of electricity coincided with the development of the

steam engine, and the widespread use of gas for fuel and lighting. In the USA, Thomas

Edison developed the carbon filament that produced light from electricity. Similar work was

carried out in the UK by Sir Joseph Swan.

Lighting offered the first commercial use for electricity, but it was an insufficient

foundation for an industry. What accelerated the growth of electricity generation was its use

for traction power. Electric trams for urban transport and the underground railway system in

London were the kinds of projects that stimulated the construction of large power stations

in the last two decades of the nineteenth century. Its origins may be in the nineteenth

century, but few would dispute that the growth of the electricity industry was a twentieth

century phenomenon. There is little doubt, too, that it will become the world’s most

important source of energy. Vital modern developments such as computers and

communications are impossible without it. It is worth remembering, however, that most of

the key elements necessary for electricity generation, transmission and distribution were

developed during the century before last.

4.3 THE EVOLUTION OF ELECTRICITY GENERATION TECHNOLOGIES

The earliest power stations used reciprocating steam engines to generate power.

These engines were not ideal for the purpose because they could not easily develop the high

rotational speeds needed to drive a generator effectively. This difficulty was eventually

overcome with the invention of the steam turbine by Sir Charles Parsons in 1884. Fuel for

these plants was usually coal, used to raise steam in a boiler.

Hydropower also entered the power generation mix at an early stage in the

development of the industry. Much of the key work on different turbine types used to

capture power from flowing water was carried out in the second half of the nineteenth

century.

By the beginning of the twentieth century both the spark-ignition engine and the

diesel engine had been developed. These too could be used for making electricity. And

before World War II work also began on the use of wind turbines as a way of generating

power. But until the beginning of the 1950s, steam turbine power stations burning coal, and

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sometimes oil or gas, together with hydropower stations, provided the bulk of the global

power generation capacity.

In the 1950s the age of nuclear power was born. Once the principles were

established, construction of nuclear power stations accelerated. Here, it was widely

believed, was a modern source of energy for the modern age; it was cheap, clean and

technically exciting.

Nuclear power continued to expand rapidly in the USA up to the late 1970s. In other

parts of the world, uptake was less rapid but Great Britain, France and Germany invested

heavily. In the Far East, Japan, Taiwan and South Korea worked more slowly. Russia

developed its own plants and India began a nuclear programme, as did China.

From the end of the 1970s the once lustrous nuclear industry began to tarnish. Since

then its progress has slowed dramatically, particularly in the west. In Asia, however, the

dream remains alive.

At the beginning of the same decade, in 1973 to be precise, the Arab– Israeli war

caused a major upheaval in world oil prices. These rose dramatically. By then oil had also

become a major fuel for power stations. Countries that were burning it extensively began to

seek new ways of generating electricity and interest in renewable energy sources began to

take off.

The stimulus of rising oil prices led to the investigation of a wide variety of different

alternative energy technologies such as wave power, hot-rock geothermal power and the

use of ethanol derived from crops instead of petrol or oil. However the main winners were

solar power and wind power.

Development took a long time but by the end of the century both solar and wind

technologies had reached the stage where they were both technically and economically

viable. There was considerable reason to hope that both would be able to contribute

significantly to the electricity generation mix in the twenty-first century.

One further legacy of the early 1970s that began to be felt in the electricity industry

during the 1980s was a widespread concern for the environment. This forced the industry to

implement wide-ranging measures to reduce environmental emissions from fossil-fuel-fired

power plants. Other power generation technologies such as hydropower were affected too.

The gas turbine began to make a major impact during the 1980s as an engine for

power stations. The machine was perfected during and after World War II as an aviation

power unit but soon transferred to the power industry for use in power plants supplying

peak demand.

During the 1980s the first large base-load power stations using both gas turbines and

steam turbines, in a configuration known as the combined cycle plant, were built. This

configuration has become the main source of new base-load generating capacity in many

countries where natural gas is readily available.

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The first years of the twenty-first century have seen renewed emphasis on new and

renewable sources of electricity. Fuel cells, a technically advanced but expensive source of

electricity, are approaching commercial viability. There is renewed interest in deriving

energy from oceans, from waves and currents, and from the heat in tropical seas. Offshore

wind farms have started to multiply around the shores of Europe.

The story of the twenty-first century is likely to be the contest between these new

technologies and the old combustion technologies for dominance within the power

generation industry. And while they battle for supremacy there remains one technology,

nuclear fusion, which has yet to prove itself but just might sweep the board.

4.4 BASIC IDEA OF GENERATION

Prior to the discovery of Faraday’s Laws of electromagnetic discussion, electrical

power was available from batteries with limited voltage and current levels. Although

complicated in construction, D.C generators were developed first to generate power in bulk.

However, due to limitation of the D.C machine to generate voltage beyond few hundred

volts, it was not economical to transmit large amount of power over a long distance. For a

given amount of power, the current magnitude (𝐼 = 𝑃/𝑉), hence section of the copper

conductor will be large. Thus generation, transmission and distribution of D.C power were

restricted to area of few kilometer radius with no interconnections between generating

plants. Therefore, area specific generating stations along with its distribution networks had

to be used.

4.4.1 CHANGEOVER FROM D.C TO A.C

In later half of eighties, in nineteenth century, it was proposed to have a power

system with 3-phase, 50 Hz A.C generation, transmission and distribution networks. Once

A.C system was adopted, transmission of large power (MW) at higher transmission voltage

become a reality by using transformers. Level of voltage could be changed virtually to any

other desired level with transformers – which was hitherto impossible with D.C system.

Nicola Tesla suggested that constructionally simpler electrical motors (induction motors,

without the complexity of commutator segments of D.C motors) operating from 3-phase a.c

supply could be manufactured. In fact, his arguments in favor of A.C supply system own the

debate on switching over from D.C to A.C system.

4.4.2 ELECTROMAGNETISM AND ELECTROMECHANICAL ENERGY

CONVERSION

An electromechanical energy conversion device transfers energy between an input

side and an output side, as shown in Figure 3.1.

In an electric motor, the input is electrical energy drawn from the supply source and

the output is mechanical energy supplied to the load, which may be a pump, fan, hoist, or

any other mechanical load.

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An electric generator converts mechanical energy supplied by a prime mover to

electrical form at the output side. The operation of electromechanical energy conversion

devices is based on fundamental principles resulting from experimental work.

Figure 3. 1: Functional block diagram of electromechanical energy conversion devices as (A) motor, and (B) generator.

4.4.3 A.C GENERATOR

A.C power can be generated as a single phase or as a balanced poly-phase system.

However, it was found that 3-phase power generation at 50 Hz will be economical and most

suitable. Present day three phase generators, used to generate 3-phase power are called

alternators (synchronous generators). An alternator has a balanced three phase winding on

the stator and called the armature. The three coils are so placed in space that there axes are

mutually 120° apart as shown in figure 3.1. From the terminals of the armature, 3-phase

power is obtained. Rotor houses a field coil and excited by D.C. The field coil produces flux

and electromagnetic poles on the rotor surface. If the rotor is driven by an external agency,

the flux linkages with three stator coils becomes sinusoidal function of time and sinusoidal

voltage is induced in them. However, the induced voltages in the three coils (or phases) will

differ in phase by 120° because the present value of flux linkage with R-phase coil will take

place after 120° with Y-phase coil and further 120° after, with B-phase coil.

A salient pole alternator has projected poles as shown in figure 3.1(a). It has non

uniform air gap and is generally used where speed is low. On the other hand a non salient

pole alternator has uniform air gap (figure 3.1(b)) and used when speed is high.

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(a) Salient pole generator (b) Non salient pole generator

Figure 3.2: 3-phase generators.

4.4.4 FREQUENCY, VOLTAGE & INTERCONNECTED SYSTEM

The frequency of the generated emf for a p polar generator is given by 2 f = pn

where n is speed of the generator in rps or 𝑓 = 𝑝

120𝑛 when n is in rpm. Frequency of the

generated voltage is standardized to 50 HZ in many countries all over the world (Africa,

Europe, and Asia). In USA and Canada it is 60 Hz. The following table gives the rpm at which

the generators with different number of poles are to be driven in order to generate 50 Hz

voltage.

Number of poles of Generator 2 4 6 8 10

rpm at which generator to be driven 3000 500 1000 50 600

A modern power station has more than one generator and these generators are

connected in parallel. Also there exist a large number of power stations spread over a region

or a country. A regional power grid is created by interconnecting these stations through

transmission lines. In other words, all the generators of different power stations, in a grid

are in effect connected in parallel. One of the advantages of interconnection is obvious;

suppose due to technical problem the generation of a plant becomes nil or less then, a

portion of the demand of power in that area still can be made from the other power stations

connected to the grid. One can thus avoid complete shutdown of power in an area in case of

technical problem in a particular station. It can be shown that in an interconnected system,

with more number of generators connected in parallel, the system voltage and frequency

tend to fixed values irrespective of degree of loading present in the system. This is another

welcome advantage of inter connected system. Inter connected system however, is to be

controlled and monitored carefully as they may give rise to instability leading to collapse of

the system.

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All electrical appliances (fans, refrigerator, TV etc.) to be connected to A.C supply are

therefore designed for a supply frequency of 50 Hz. Frequency is one of the parameters

which decides the quality of the supply. It is the responsibility of electric supply company to

see that frequency is maintained close to 50 Hz at the consumer premises.

4.5 ELECTRIC POWER GENERATION: CONVENTIONAL METHODS

4.5.1 HYDROELECTRIC POWER GENERATION

Hydroelectric power generation involves the storage of a hydraulic fluid, water,

conversion of the hydraulic (potential) energy of the fluid into mechanical (kinetic) energy in

a hydraulic turbine, and conversion of the mechanical energy to electrical energy in an

electric generator.

The first hydroelectric power plants came into service in the 1880s and now

comprise approximately 20% (700 GW) of the world’s installed generation capacity (World

Energy Council, 2001). Hydroelectricity is an important source of renewable energy and

provides significant flexibility in base loading, peaking, and energy storage applications.

While initial capital costs are high, the inherent simplicity of hydroelectric plants, coupled

with their low operating and maintenance costs, long service life, and high reliability, make

them a very cost-effective and flexible source of electricity generation. Especially valuable is

their operating characteristic of fast response for start-up, loading, unloading, and following

of system load variations. Other useful features include their ability to start without the

availability of power system voltage (black start capability), ability to transfer rapidly from

generation mode to synchronous-condenser mode, and pumped storage application.

Hydroelectric units have been installed in capacities ranging from a few kilowatts to

nearly 1 GW. Multi-unit plant sizes range from a few kilowatts to a maximum of 18 GW.

As said before in a hydroelectric power station, water head is used to drive water

turbine coupled to the generator. Water head may be available in hilly region naturally in

the form of water reservoir (lakes etc.) at the hill tops. The potential energy of water can be

used to drive the turbo generator set installed at the base of the hills through piping called

pen stock. Water head may also be created artificially by constructing dams on a suitable

river. In contrast to a thermal plant, hydroelectric power plants are eco-friendly, neat and

clean as no fuel is to be burnt to produce electricity. While running cost of such plants are

low, the initial installation cost is rather high compared to a thermal plants due to massive

civil construction necessary. Also sites to be selected for such plants depend upon natural

availability of water reservoirs at hill tops or availability of suitable rivers for constructing

dams. Water turbines generally operate at low rpm, so number of poles of the alternator is

high. For example a 20-pole alternator the rpm of the turbine is only 300 rpm.

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Figure 3.3: Basic components of a hydel generating unit.

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4.5.1.1 Selection of Plant Capacity, Energy, and Other Design Features

The generating capacity of a hydroelectric plant is a function of the head and flow

rate of water discharged through the hydraulic turbines, as shown in the following equation:

𝑃 = 9.8 𝜂 𝑄𝐻 (3. 1)

Where

P is the power (kilowatts)

h is the plant efficiency

Q is the discharge flow rate (m3=s)

H is the head (m)

Flow rate and head are influenced by reservoir inflow, storage characteristics, plant

and equipment design features, and flow restrictions imposed by irrigation, minimum

downstream releases, or flood control requirements. Historical daily, seasonal, maximum

(flood), and minimum (drought) flow conditions are carefully studied in the planning stages

of a new development. Plant capacity, energy, and physical features such as the dam and

spillway structures are optimized through complex economic studies that consider the

hydrological data, planned reservoir operation, performance characteristics of plant

equipment, construction costs, the value of capacity and energy, and financial discount

rates. The costs of substation, transmission, telecommunications, and off-site control

facilities are also important considerations in the economic analysis. If the plant has storage

capability, then societal benefits from flood control may be included in the economic

analysis.

Another important planning consideration is the selection of the number and size of

generating units installed to achieve the desired plant capacity and energy, taking into

account installed unit costs, unit availability, and efficiencies at various unit power outputs.

4.5.1.2 Generator

Synchronous generators and induction generators are used to convert the

mechanical energy output of the turbine to electrical energy. Induction generators are used

in small hydroelectric applications (less than 5 MVA) due to their lower cost which results

from elimination of the exciter, voltage regulator, and synchronizer associated with

synchronous generators. The induction generator draws its excitation current from the

electrical system and thus cannot be used in an isolated power system.

The majority of hydroelectric installations utilize salient pole synchronous

generators. Salient pole machines are used because the hydraulic turbine operates at low

speeds, requiring a relatively large number of field poles to produce the rated frequency. A

rotor with salient poles is mechanically better suited for low-speed operation, compared to

round rotor machines, which are applied in horizontal axis high-speed turbo-generators.

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4.5.2 THERMAL GENERATING PLANTS

Thermal generating plants are designed and constructed to convert energy from fuel

(coal, oil, gas, or radiation) into electric power. The actual conversion is accomplished by a

turbine-driven generator.

We have seen in the previous section that to generate voltage at 50 Hz we have to

run the generator at some fixed rpm by some external agency. A turbine is used to rotate

the generator.

Turbine may be of two types, namely steam turbine and water turbine. In a thermal

power station coal is burnt to produce steam which in turn, drives the steam turbine hence

the generator (turbo set). In figure 2.2 the elementary features of a thermal power plant is

shown. In a thermal power plant coil is burnt to produce high temperature and high

pressure steam in a boiler. The steam is passed through a steam turbine to produce

rotational motion. The generator, mechanically coupled to the turbine, thus rotates

producing electricity. Chemical energy stored in coal after a couple of transformations

produces electrical energy at the generator terminals as depicted in the figure. Thus

proximity of a generating station nearer to a coal reserve and water sources will be most

economical as the cost of transporting coal gets reduced. However, these plants pollute the

atmosphere because of burning of coals.

Figure 3.4: Basic components of a thermal generating unit.

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Stringent conditions (such as use of more chimney heights along with the

compulsory use of electrostatic precipitator) are put by regulatory authorities to see that the

effects of pollution are minimized. A large amount of ash is produced every day in a thermal

plant and effective handling of the ash adds to the running cost of the plant. The speed of

alternator used in thermal plants is 3000 rpm which means 2-pole alternators are used in

such plants.

Thermal generating plants differ from industrial plants in that the nature of the

product never changes. The plant will always produce electric energy. The things that may

change are the fuel used (coal, oil, or gas) and environmental requirements. Many plants

that were originally designed for coal were later converted to oil, converted back to coal,

and then converted to gas. Environmental requirements have changed, which has required

the construction of air and water emissions control systems. Plant electrical systems should

be designed to allow for further growth. Sizing of transformers and buses is at best a matter

of guesswork. The plant electrical system should be sized at 5 to 10% the size of the

generating unit depending on the plant configuration and number of units at the plant site.

4.5.3 NUCLEAR PLANTS

As coal reserve is not unlimited, there is natural threat to thermal power plants

based on coal. It is estimated that within next 30 to 40 years, coal reserve will exhaust if it is

consumed at the present rate. Nuclear power plants are thought to be the solution for bulk

power generation.

The present day atomic power plants work on the principle of nuclear fission of 235

U.

In the natural uranium, 235

U constitutes only 0.72% and remaining parts is constituted by

99.27% of 238

U and only about 0.05% of 234

U. The concentration of 235

U may be increased to

90% by gas diffusion process to obtain enriched 235

U. When 235

U is bombarded by neutrons a

lot of heat energy along with additional neutrons are produced. These new neutrons further

bombard 235

U producing more heat and more neutrons. Thus a chain reaction sets up.

However this reaction is allowed to take place in a controlled manner inside a closed

chamber called nuclear reactor. To ensure sustainable chain reaction, moderator and control

rods are used. Moderators such as heavy water (deuterium) or very pure carbon 12

C are used

to reduce the speed of neutrons. To control the number neutrons, control rods made of

cadmium or boron steel are inserted inside the reactor. The control rods can absorb

neutrons. If we want to decrease the number neutrons, the control rods are lowered down

further and vice versa. The heat generated inside the reactor is taken out of the chamber

with the help of a coolant such as liquid sodium or some gaseous fluids. The coolant gives up

the heat to water in heat exchanger to convert it to steam as shown in figure 3.4. The steam

then drives the turbo set and the exhaust steam from the turbine is cooled and fed back to

the heat exchanger with the help of water feed pump. Calculation shows that to produce

1000 MW of electrical power in coal based thermal plant, about 6 × 106

Kg of coal is to be

burnt daily while for the same amount of power, only about 2.5 Kg of 235

U is to be used per

day in a nuclear power stations.

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The initial investment required to install a nuclear power station is quite high but

running cost is low. Although, nuclear plants produce electricity without causing air

pollution, it remains a dormant source of radiation hazards due to leakage in the reactor.

Also the used fuel rods are to be carefully handled and disposed off as they still remain

radioactive.

The reserve of 235

U is also limited and cannot last longer if its consumption continues

at the present rate. Naturally search for alternative fissionable material continues. For

example, plutonium (239

Pu) and (233

U) are fissionable. Although they are not directly

available. Absorbing neutrons, 238

U gets converted to fissionable plutonium 239

Pu in the

atomic reactor described above. The used fuel rods can be further processed to extract 239

Pu

from it indirectly increasing the availability of fissionable fuel. Effort is also on to convert

thorium into fissionable 233

U.

Figure 3.5: Nuclear power generation.

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4.6 ELECTRIC POWER GENERATION: NONCONVENTIONAL

METHODS

4.6.1 WIND POWER

The wind is a free, clean, and inexhaustible energy source. It has served humankind

well for many centuries by propelling ships and driving wind turbines to grind grain and

pump water. Wind power is the conversion of wind energy into a useful form of energy, such

as electricity, using wind turbines.

4.6.1.1 Benefits of Wind Power

Wind power has many benefits that make it an attractive source of power for both

utility-scale and small, distributed power generation applications. The beneficial

characteristics of wind power include:

Clean and inexhaustible fuel—Wind power produces no emissions and is not

depleted over time. A single one megawatt (1 MW) wind turbine running for

one year can displace over 1,500 tons of carbon dioxide, 6.5 tons of sulfur

dioxide, 3.2 tons of nitrogen oxides, and 60 pounds of mercury (based on

the U.S. average utility generation fuel mix).

Local economic development—Wind plants can provide a steady flow of

income to landowners who lease their land for wind development, while

increasing property tax revenues for local communities.

Modular and scalable technology—Wind applications can take many forms,

including large wind farms, distributed generation, and single end-use

systems. Utilities can use wind resources strategically to help reduce load

forecasting risks and stranded costs.

Energy price stability—By further diversifying the energy mix, wind energy

reduces dependence on conventional fuels that are subject to price and

supply volatility.

Reduced reliance on imported fuels—Wind energy expenditures are not

used to obtain fuels from abroad, keeping funds closer to home, and

lessening dependence on foreign governments that supply these fuels.

4.6.1.2 Wind Power Today

Wind power is the world’s fastest growing source of electricity. Generating

capacity grew at an average annual rate of 25% between 1990 and 2000, exceeding less

than 2% annual growth in each of nuclear, oil and natural gas, and an average annual

decline of 1% in coal consumption over this period. As of the end of 2002 total global

wind generating capacity exceeds 31,000 MW, and provides about 65 billion kWh of

electricity annually.

Generating capacity is mainly concentrated in just five countries;

Germany (36%), the U.S. (18%), Spain (14%), Denmark (10%) and India (6%) together

account for 84% of the total (see Figure 4).

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At the end of 2008, worldwide nameplate capacity of wind-powered generators was

121.2 giga watts (GW). In 2008, wind power produced about 1.5% of worldwide electricity

usage; and is growing rapidly, having doubled in the three years between 2005 and 2008.

Several countries have achieved relatively high levels of wind power penetration, such as

19% of stationary electricity production in Denmark, 11% in Spain and Portugal, and 7% in

Germany and the Republic of Ireland in 2008. As of May 2009, eighty countries around the

world are using wind power on a commercial basis.

Figure 3. 6: Wind power generation

Wind power is non-dispatchable, meaning that for economic operation, all of the

available output must be taken when it is available. Other resources, such as hydropower,

and standard load management techniques must be used to match supply with demand. The

intermittency of wind seldom creates problems when using wind power to supply a low

proportion of total demand. Where wind is to be used for a moderate fraction of demand

such as 40%, additional costs for compensation of intermittency are considered to be

modest.

4.6.2 PHOTOVOLTAICS

Photovoltaics are best known as a method for generating electric power by using

solar cells to convert energy from the sun into electricity. The photovoltaic effect refers to

photons of light knocking electrons into a higher state of energy to create electricity. The

term photovoltaic denotes the unbiased operating mode of a photodiode in which current

through the device is entirely due to the transduced light energy. Virtually all photovoltaic

devices are some type of photodiode.

Solar cells produce direct current electricity from light, which can be used to power

equipment or to recharge a battery. The first practical application of photovoltaics was to

power orbiting satellites and other spacecraft, but today the majority of photovoltaic

modules are used for grid connected power generation. In this case an inverter is required

to convert the DC to AC

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Cells require protection from the environment and are usually packaged tightly

behind a glass sheet. When more power is required than a single cell can deliver, cells are

electrically connected together to form photovoltaic modules, or solar panels. A single

module is enough to power an emergency telephone, but for a house or a power plant the

modules must be arranged in multiples as arrays. Although the selling price of modules is

still too high to compete with grid electricity in most places, significant financial incentives in

Japan and then Germany, Italy and France triggered a huge growth in demand, followed

quickly by production. In 2008, Spain installed 45% of all photovoltaics, but a change in law

limiting the feed-in tariff is expected to cause a precipitous drop in the rate of new

installations there, from an extra 2500 MW in 2008 to an expected additional 375 MW in

2009.

World solar photovoltaic (PV) installations were 2.826 gigawatts peak (GWp) in

2007, and 5.95 gigawatts in 2008, a 110% increase. The three leading countries (Germany,

Japan and the US) represent nearly 89% of the total worldwide PV installed capacity.

According to Navigant Consulting and Electronic Trend Publications, the estimated PV

worldwide installations outlooks of 2012 are 18.8GW and 12.3GW respectively. Notably, the

manufacture of solar cells and modules had expanded in coming years.

Figure 3. 7: Solar power generation.

4.6.2.1 Advantages

Solar power is pollution-free during use. Production end-wastes and emissions are

manageable using existing pollution controls. End-of-use recycling technologies are under

development.

PV installations can operate for many years with little maintenance or intervention

after their initial set-up, so after the initial capital cost of building any solar power plant,

operating costs are extremely low compared to existing power technologies.

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Solar electric generation is economically superior where grid connection or fuel

transport is difficult, costly or impossible. Long-standing examples include satellites, island

communities, remote locations and ocean vessels.

When grid-connected, solar electric generation replaces some or all of the highest-

cost electricity used during times of peak demand (in most climatic regions). This can reduce

grid loading, and can eliminate the need for local battery power to provide for use in times

of darkness. These features are enabled by net metering. Time-of-use net metering can be

highly favorable, but requires newer electronic metering, which may still be impractical for

some users.

Grid-connected solar electricity can be used locally thus reducing

transmission/distribution losses (transmission losses in the US were approximately 7.2% in

1995).

Compared to fossil and nuclear energy sources, very little research money has been

invested in the development of solar cells, so there is considerable room for improvement.

Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40%

and efficiencies are rapidly rising while mass-production costs are rapidly falling.

4.6.2.2 Disadvantages

Photovoltaics are costly to install. While the modules are often warranted for

upwards of 20 years, an investment in a home-mounted system is mostly lost if you move.

Solar electricity is seen to be expensive. Once a PV system is installed it will produce

electricity for no further cost until the inverter needs replacing. Current utility rates have

increased every year for the past 20 years and with the increasing pressure on carbon

reduction the rate will increase more aggressively. This increase will (in the long run) easily

offset the increased cost at installation but the timetable for payback is too long for most.

Solar electricity is not available at night and is less available in cloudy weather

conditions from conventional silicon based-technologies. Therefore, a storage or

complementary power system is required. However, the use of germanium in amorphous

silicon-germanium thin-film solar cells provides residual power generating capacity at night

due to background infrared radiation. Fortunately, most power consumption is during the

day, so solar does not need to be stored at all as long to the extent that it offsets peak and

"shoulder" consumption.

Apart from their own efficiency figures, PV systems work within the limited power

density of their location's insolation. Average daily insolation (output of a flat plate collector

at latitude tilt) in the contiguous US is 3-7 kilowatt·h/m² and on average lower in Europe.

Solar cells produce DC which must be converted to AC (using a grid tie inverter)

when used in current existing distribution grids. This incurs an energy loss of 4-12%.

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PART 2- POWER TRANSMISSION AND DISTRIBUTION

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

31

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

32

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

or power station – don’t miss it.