EHV AC CHAPTER I. INTRODUCTION High voltage is used in electric power distribution,in cathode ray tubes,to generate x-rays and particle beams and high power vacuum tubes and other scientific and industrial applications.Many Industrialcentered countries of the world thrive for a vast amount of energy in which electrical energy forms a major portion.There are se ve ral ty pe s of ener gy su ch as na tu ra l ga s for domest ic an d in du st ri al cons umpt io n, wh ic h form a co ns ider ab le proport io n of th e tota l energy consumption. Thus,we can say that theelectrical energy is not the only form in which energy is consumed but it is also an important part nevertheless.Today the world is on the verge of consuming the major portion of its natural resources in a very short spam of time and is already looing for alternative sources ofenergy excluding hydro and thermal to cater for the rapid rate of consumption which is outpacing the discovery of new resources.This will however does not slow down with respect to time and therefore there exists a need to reduce the rate at which energy consumption is increasing.Therefore,a tremendous need for energy is very urgent in many developing countries and their relation to other countries are sometimes based on the energy re!uirements."sually the Hydroelectric and coal oroil-fired stations are located very far from the load centres for various reasons which re!uires the transmission of the generated electric power over very long distances.This led to the re!uirement of very high voltages for transmission. #xtra High $oltage %#H$& '.(. transmission may be considered to come when the first )*+++ $ line was put into service in weden. 'fter then,all the industriali/edcount ries all over the world have adopted the higher voltage levels. 0a te r it was ob served that the impact of such hi gh vo ltage levels on the environment needed careful attention because the use of high surface voltage gradients on conductors led to the interference problems from powerfre!uency to T$ fre !ue ncies. Thus cor ona effects,elec trosta tic fields in the line vic ini ty , losses,audible noise, carrier interference and radio interference became recogni/ed as steadystate problems which governs the line conductor design, line height, and phase-spacing used to e ep the interfering fields wi thin prescribed limits."se o fsynchronous condensers due to high line charging currents at load end only became impractical to control voltages at the sending-end and receiving-end buses."se of hunt compensating reactors for voltage control at no load and switched capacitors at load conditions became necessary. . 'll these are still 1
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EHV AC
CHAPTER I. INTRODUCTION
High voltage is used in electric power distribution,in cathode ray tubes,to generate
x-rays and particle beams and high power vacuum tubes and other scientific and
industrial applications.Many Industrialcentered countries of the world thrive for a
vast amount of energy in which electrical energy forms a major portion.There are
several types of energy such as natural gas for domestic and industrial
consumption, which form a considerable proportion of the total energy
consumption. Thus,we can say that theelectrical energy is not the only form in
which energy is consumed but it is also an important part nevertheless.Today the
world is on the verge of consuming the major portion of its natural resources
in a very short spam of time and is already looing for alternative sources of
energy excluding hydro and thermal to cater for the rapid rate of consumptionwhich is outpacing the discovery of new resources.This will however does not
slow down with respect to time and therefore there exists a need to reduce the rate
at which energy consumption is increasing.Therefore,a tremendous need for energy
is very urgent in many developing countries and their relation to other countries are
sometimes based on the energy re!uirements."sually the Hydroelectric and coal or
oil-fired stations are located very far from the load centres for various reasons
which re!uires the transmission of the generated electric power over very long
distances.This led to the re!uirement of very high voltages for transmission.
#xtra High $oltage %#H$& '.(. transmission may be considered to come when
the first )*+++ $ line was put into service in weden. 'fter then,all the
industriali/edcountries all over the world have adopted the higher voltage levels.
0ater it was observed that the impact of such high voltage levels on the
environment needed careful attention because the use of high surface voltage
gradients on conductors led to the interference problems from powerfre!uency to
T$ fre!uencies. Thus corona effects,electrostatic fields in the line vicinity,
losses,audible noise, carrier interference and radio interference became recogni/ed
as steadystate problems which governs the line conductor design, line height, and phase-spacing used to eep the interfering fields within prescribed limits."se of
synchronous condensers due to high line charging currents at load end only became
impractical to control voltages at the sending-end and receiving-end
buses."se of hunt compensating reactors for voltage control at no load and
switched capacitors at load conditions became necessary.. 'll these are still
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categori/ed as steady-state problems.However, the single major problem
considered with e.h.v. voltagelevels is the overvoltages during switching
operations which is commonly nown as switching-surge
overvoltages.
1.1 ROLE OF EHV AC TRANSMISSION
Industrial-minded countries of the world re!uire a vast amount of energy of which
electrical energy forms a major fraction. There are other types of energy such as oil
for transportation and industry, natural gas for domestic and industrial
consumption, which form a considerable proportion of the total energy
consumption. Thus, electrical energy does not represent the only form in which
energy is consumed but an important part nevertheless. It is only 12+ years since
the invention of the dynamo by 3araday and 14+ years since the installation of thefirst central station by #dison using dc. 5ut the world has already consumed major
portion of its natural resources in this short period and is looing for sources of
energy other than hydro and thermal to cater for the rapid rate of consumption
which is outpacing the discovery of new resources. This will not slow down with
time and therefore there exists a need to reduce the rate of annual increase in
energy consumption by any intelligent society if resources have to be preserved for
posterity. 'fter the end of the econd 6orld 6ar, countries all over the world
have become independent and are showing a tremendous rate of industrial
development, mostly on the lines of 7orth-'merican and #uropean countries, the"...8. and 9apan. Therefore, the need for energy is very urgent in these
developing countries, and national policies and their relation to other countries are
sometimes based on energy re!uirements, chiefly nuclear. Hydro-electric and coal
or oil-fired stations are located very far from load centres for various reasons
which re!uires the transmission of the generated electric power over very long
distances. This re!uires very high voltages for transmission. The very rapid strides
taen by development of dc transmission since 1:2+ is playing a major role in
extra-long-distance transmission, complementing or supplementing e.h.v. ac
transmission. They have their roles to play and a country must mae intelligent
assessment of both in order to decide which is best suited for the country;seconomy. This boo concerns itself with problems of e.h.v. ac transmission only.
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CHAPTER II. DESIGN METHODOLOGY
3ig 1<#.H.$ Tower
3ollowing parameters should be carefully considered while designing a #.H.$ ac
transmission line.
• (ollecting preliminary line design data and
gathering available climatic data.
• electing 8eliability level.• (alculating climatic loading on components.
• (alculating loads corresponding to security
re!uirements.
• (alculating loads according to safety
re!uirements.
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• electing appropriate correction factors.
• =esigning the components for the above load
and strengths.
2.1 A. Reliability Levels• 8eliability 0evel ≥ 1
• Higher the 8eliability higher is the safety
factor and hence, more is the cost.
2.2 . Sele!ti"# "$ T%a#s&issi"# V"lta'e
• tandard voltage >>,11+,1)4,44+,++ ?$
• election criterion of #conomic $oltage
1. @uantum of power is evaluated
4. 0ength of line). $oltage regulation
. Aower loss in transmission
2. Initial and operating cost
2.( C. E!"#"&i! V"lta'e "$ T%a#s&issi"#
"$ P")e%
# B 5.5 √(km/1.61+(load in kva)/150)Table 1* E!"#"&i! V"lta'e "$
T%a#s&issi"# "$ P")e%
#(C7CMI( $C0T'D#
=8CA
=IT'7(#%miles&
E>2$ 411>
2++$ 11)
)2$ 2:1+
2.+ D. Ty,es "$ T")e%s
• Type ' Tower <
Tangent tower with suspension string• Type 5 Tower
mall angle tower with tension string
• Type ( Tower
Medium angle tower with tension
string
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• Type = Tower
0arge angle tower with tension string
• Type # Tower
=ead end tower with tension string
•pecial Tower
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3ig < Tower tructure
2.- E. Sele!ti"# "$ T")e% St%!t%e
• ingle circuit towerF=ouble circuittower
• 0ength of insulator assembly
• Minimum clearances between ground
conductors,and between conductors
and ground
• 0ocation of ground wires with respect
to outermost conductor
• Minimum clearance of the lowest
conductor above ground level
2./ 0. Sele!ti"# "$ C"#!t"% Sie
• Mechanical 8e!uirement
• #lectrical 8e!uirement
1. Mechanical 8e!uirement
• Tensile strength%for tension&
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• train strength%for vibration&
4. #lectrical 8e!uirement
• (ontinuous current rating
• hort time current carrying rating
• $oltage drop• Aower loss
• 0ength of line
• (harging current
2.3 G. I#slat"% Re4i%e&e#t
Insulators are used to support line
conductor and it also provides clearance
from ground and structure.
Insulator material are as followsG
• Toughened glass• 3iber class
• High grade electric porcelain
Types of Insulators are categori/ed as G
• =isc type
• Strut type
• on! rod in"ulator
#
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CHAPTER III. PROLEMS ENCOUNTERED
The problems encountered while using such high voltages are different from those
encountered at lower voltages. These problems are<
%a& Increase in (urrent =ensity due to increase in line loading by using series
%e& High electrostatic field under the line.%f& witching urge Cvervoltages which cause more havoc to air-gap insulation
than lightning or power fre!uency voltages.
%g& Increased hort-(ircuit currents and possibility of ferro resonance conditions.
%h& "se of gapless metal-oxide arresters which replaces the conventional gap-type
ilicon (arbide arresters, for both lightning and switching-surge duty.
%i& hunt reactor compensation and use of series capcitors, resulting in possible
subsynchronous resonance conditions and high shortcircuit currents.
%j& Insulation coordination based upon switching impulse levels.
%& ingle-pole reclosing to improve stability, but causing problems with arcing.
(.1 A. Ove%v"lta'es
Cvervoltages in #H$ systems are caused by switching operations.overvoltages due
to the release of internally trapped electromagnetic and electrostatic energy in an
e.h.v. system which can further cause serious damages to the e!uipment
insulation.These damages could, under many circumstances, be more severe than
lightning damage.Therefore,urge diverters and resistances are included purposely
while maing switching operations as well as other schemes to reduce the danger
to a considerable extent. Investigation of switching overvoltages has ac!uired
very great importance since the transmission voltages are on the increase and linelengths and capacity of generating stations are also increased. It is assumed that the
short-circuit capacity of sources is mainly responsible for a large amount of
damage to the insulation.
(.2 . Li'5t#i#' St%"6es T" Li#es
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0ightning is considered as one of the major source of danger and damage to e.h.v.
transmission lines, resulting in the loss of transmission up to a few hours to
complete destruction of a line. This entails a lot of expense to power utilities and
consumers.0ightning protection methods are based on sound scientific and
engineering principles and practiceGhowever, extensive damages do tae place in
power systems in spite of this.Thus,no transmission line design can be considered
lightning-proof, nor do designers aim for this goal. 0ine outages, or a line being
taen out of service for a short time,and line designs against these are based on
statistical procedures. 'n acceptable design would be to allow a certain number 7t
outages per year per 1++ m of line, or other time durations and other line lengths.
The possibility of an outage depends on so many factors which are statistical in
nature that a worstcase design is neither practical nor economical.It is evident that
the number of stroes contacting a tower or ground wire along the span can
only depend on the number of thunderstorm days in a year called the ;eraunic
level; or also called ;isoeraunic level;, and denoted by Il. Cn the basis of a vastamount of experience from all over the world, it is estimated that the number of
lightning stroes occurring over 1 s!. m. per year at any location is fairly well
everal measures are being taen to overcomes theproblems faced while using
#.H.$ ac transmission line.
0ightning arresters which are also called ;surge absorbers; because they are also
meant for switching surge protection.Their purpose is to protect primarily the
major e!uipment such as transformers, rotating machines,shunt reactors, and even
entire substations. 0ess expensive protective devices such as rod gaps can be used
for protection of transformer and circuitbreaer bushings and open contacts.6hen
substations are to be protected,they are located at the entrance of the incoming andoutgoing lines.Modern surge absorbers re!uired for e.h.v. levels are designed to
offer protection to the e!uipments and lines for both lightning overvoltages and
currents as well as switching surge over-voltages where the energy involved is
much higher.'rresters can be classified into three important types and categorised
according to their internal structure.They are G
%1& Dap type arrester without current-limiting functions.
%4& Dap type arrester with current-limiting capability.
%)& Dapless metal oxide varistors. The Dap type arrester is commonly nown by
different names such as Thyrite, Magnavalve, 'utovalve,Miurite.The non-linear
resistance material is usually sintered ilicon-(arbide %i(& and is designed to
dissipate the energy in short duration lightning-stroe current and the current at
power fre!uency that will follow this current when the series gap conducts.The
current is then finally interrupted at a power-fre!uency /ero.The second type of
arrester nown as the current-limiting gap type, is the one in which a magnetic
action on the arc between the gap establishes a lengthening of the arc withconse!uent large resistance which is capable of limiting the current.In such type of
design,the power-fre!uency current can be extinguished before it reaches a current
/ero.uch an arrester can perform switching surge function also.The non-linear
'll the e.h.v. e!uipment voltage levels are governed by tandard
specification.#!uipment manufacturers produce goods not only meant for domestic
use but also for export so that a design or testing engineer must be familiar withthese specifications from all over theworld.ome of the major specifications are
I.#.(., I..I., $.=.#., .'.#., '.7..I., 5..., (..'.., 9apan and others.
-.1 Te!5#"l"'y se i# EHV AC t%a#s&issi"#*
3'(T technologies have a small footprint and minimal impact on the
environment. Aroject implementation times are considerably faster and investment
costs substantially lower than the alternative of building more transmission lines or new power generation facilities.
3'(T consists of three technology branches<
• series compensation
• dynamic shunt compensation
• dynamic energy storage
-.2 Se%ies !"&,e#sati"#< eries compensation was invented by '55 in 1:2+. It
made possible the world;s first ++ $ transmission in the 1:2+s by resolving the
issue of how to transmit power over long distances.
In this article,we have outlined the design methodology of #H$ ac transmission
lines including the various factors re!uired for their designs.This paper also
presents the problems encountered while using #H$ ac transmission line and it
presents the various techni!ues adopted to overcome these problems.#H$
transmission lines uses #H$ cables which are used for high voltages and power when necessary.The future wor where #H$ ac transmission will prove effective
follows<
• upport is going for a national #H$ interstate transmission grid
• 3or "H$,fewer lines would be needed and for voltage above E>2 $
more lines would be needed.
• It is understood,however,that the completion of this grid re!uires
collaboration among staeholders,especially transmission owners