I
Power Stations
Unit4
I Er.Menuka Devkota
Transmission and Distribution system
4.1 Transmission System
Introduction, necessity of TS
The electricity is generated in bulk in "the generating stations' and then transmitted over long distances to the load points. The
transmission system interconnects all the generati.Qg sta~ions and° major load centers 1n"ihe system. It forms the back bone of the
power system. Since the power loss in a transmission line is proportional to the square ofline current, the transmi~ iorllihes operate
at the highest voltage levels, usually 220 KV and upwards.
The transmission lines mainly perform the two functions
a. It transports the .energy from generating stations to bajk receiv~g stat~ons.
b. It interconnects the two or more generating stations. The neighboring substations are also interconnected through the transmission
lines.
·
The component of an electrical power system connecting all the consumers in an area to the bulk power sources is called a
distribution system. The bulk power stations are connected to the generating substations by transmission lines. They feed some
substations which are usually situated at convenient points near the load centers. The substations distribute the power to the
domestic, commercial and relatively small consumers. The consumers require large blocks of power which are usually supplied at
sub-transmission or even transmission system .
.,.
Transmission Layout, voltage level, insulation, right of way
Trapsmission-level voltages are usuaily consi_dered to-be 110 kV am:( a~ove suc4, as ·l}fkV oi Jjp;,ky~or,) Op i v _gr 76) kV
depending upon how long the transmission lines are. Lower voltages, such as 66 !,cV"}and 33 Jf.V-, are usually considered sub
transmission voltages, but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for
distribution.
A.C. and D.C applications: introduction and comparison
AC is much easi'er (consider cheaper) to step up and down in voltage than DC, and can be:done more efficiently:(typically:.currently)
than the much more ·expensive ~d compl_ex DC step-up/s_tep-,down systems. That is w4y;,:, 4C is used for·m~ s j>.2,w~i ~ sttiission
at a national level.
·
AC is, in fact, not inherently better for long-distance power transmission than DC. And this is why HVDC (high voltage DC) is
actually used
AC has ~me limitation compared with DC: inductive and capacitive losses. The larger the inducta~ce an~ ~aracihmce of_a_pJ ece of
wire is, the more difficult it 1s to keep trying to change the direction of flow of electiqnfwhich'-is:wh-~ti\C is.doing'at SQ. tiines a
second. DC on the other hand, has the current flqwing in_one direction alLthe fiw:c;, a,r!g §q is iibrimpeded by this ii!duc~ _ce, and
has negligible capacitive losses.
Straight wires in air have very little inductance and capacitive losses. Over the distances that mains power is transmitted, there are
some losses when using AC, but the losses are small: A DC system would have bett~ J rjl,ll§ ~ssiqg:e':mcienc"i~s a~{ifhas·lower
losses, but would be more expei;isive and less efficient at the step-up/step-down stages:
'
4.2 Distribution system
Introduction, necessity of DS
A distribution substation is located near or inside city/town/village/industrial area. It ~eceives powe~ §"om a transn:i,ission ~etwork.
The high voltage from the transmission line is then stepped down by a step-down transformer !O!}le'-p,rimary _distribution level
voltage. Primary distribution voltage is usually 11 kV, but can range between 2.4 kV to 33 kV depending upon region or consumer.
A typical power distribution system consists of 1. Distribution substation
2. Feeders
3. Distribution Transformers
4. Distributor conductors
5. Service mains conductors
Along with these, a distribution system also consists of switches, protection equipment, measurement equipment etc.
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I
ii
Power Stations
I Er.Menuka Devkota
"
Distribution feed~rs: The stepm~,d-down y.oltage from the,sub t f
.
. _d t 'd"' tn~ .... . ,-i., '. '1'lt:,,: , --~ , , . "1•/,,J'. •.•
Generally, no tapping are taken from the feeders so th t th. . . s .a i.o~ is farp e_ .o • is qytip;g;;!:rM~{q_rm,e~~ ~~~fae,g~1,{~0,p.dqctor~.
of a feeder conductor is its current carryi·
. a e current remams same throughout. The .mam consideration in designing
. .
.
. .
ng capacity.
D1stnbution transformer: A distribution tran :fi
1
•
electric power distribution system. It is basicall sao:e~d a so called as service tran,,~fo~ e_r?we ro~-~~~~,9 nal .~ sformatt~.n in ,~e
to 400Y/230 volts. Here it means, voltage be~een a~ own 3h-phase transformer. J?~.striJ:iution tr~sfoJil1).
~
~~ ~~e-\~.oJtage
volts.
Y one P ase and the neutral is 230 volts and phase to phase voltage is 400
Distributors: Output from a distribution transfo
·
·
· ·
·
. .
,. -,
-· · -·
.
condµctor for power ~upply to the end consumers~~:
tiStrl:ut?br con~uctor. Tappmg
~ 1tj1frotn,!.@'c;ljstrjbqfot
laces throughout its len th s
l
.
en . ou,g a stn utor is not conS tant as tappmg are taken at various
~ervice mains: It is a s;all ca~1:::~~ t~°!:~~:~h~~~':gbthtis the mdaint conshideration_whi~e,,desigg_ing a 1:lisJ:fJJJutor ~f9Agl,!..ctor.
u or con uc or at t e nearest pole to the consumer's end.
Distributor 2 ·
Di s tri butor 1
Service
m a ins
t..o c o n -s.ume r s
Feeder
Fe.eder
Distribution Layout, voltage level
Primary Distribution
It is that part of an AC distribution system which operates at somewhat higher voltages than general residential consumer utilization.
Commonly used primary distribution voltages in most countries !\Te 1) .k\:, 6.6 kV~.andf3~ kV.
distril::m_ion~hatJd[~Jarge
consumers such as factories and industries. It also feeds small substation from where secondary distribution is carried out. Primary
distribution is carried out by 3-phase, 3-wire system.
J>.ritll;y'
Secondary Distribution
This part directly suppli"es to the residential end consumers. Domestic consumers are fed with $ingl~:pli~efsuppJyJiti, o y oJ!S (120
volts in USA and some other countries). Three phase supply may also be provided at 400 volts for big properties, commercial
bn°ildings, small factories etc. Secondary transmission in most countries is carried out ~,y .3-phase,4:.~fr~~sys tein:
,-
Topologies or Connection schemes of distribution system
An electric power distribution system can be classified according to its feeder connection schemes or topologies as follows •
•
•
•
Radial distribution system
Parallel feeder distribution
Ring main distribution system
Interconnected distribution
Radial Distribution System
This system is used only when subs~tion' of-,generatihg $tation.is, lb~ate,c!h tt tn~,centf~ o'(iui~t ~'tlµ~~~~!.~ Y~i~ sy~te_~: / i~~er~nt
feeders radiate from a substation or a generating station and feed the distributors at one end. Thu~ i\th,e m~ ~~~ _!!S~nsti.Q'Qf:a 'r~dial
distribution system is that ~ -:PoWer,i.tlow is only one 'direction; _Single line d!agram of a typical radial distnbutlon system lS as
in
shown in the figure below. It is the sim,plest system and has the lo~yst mitial cost:
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Power Stations
I Er.Menuka Devkot a
HV/MV
-------
'sub.stat~:on
~ V" feeaets
( ciY'ert'.leaa),
LVfeeclers
(cable }
·
Although
thisfeeder
system
is result
simplest
and least
expensive
.
. drawback of a radial distribution system is
a fault in the
will
in supply
failure
to ' it. is not
d h'igh1Yrehable.
A ?IaJor
. - di,.;..;b
;)Lll ··u1
associate consumers as there won't be any alternative feeder to feed
· ors.'
P ara II eI F eeders Distribution System
·
-- ·
The above-mentioned disadvantage of a radial s stem can be . . .
.
.
. ' ..
system is much more as the number of fr d •
bl
mmnmzed by mtrodupmg parallel feeders. Th• 1D1tral cost of this
forload sharing where the load is higheree ers IS ou ed. Such system may be used where reliability Oftlie supply iS important or
feeder 1
feeder-2
Ring Main Distribution System
A similar level of system.-reliab,i11ty to that of the ~,~ct:rtc 7.,,ie~:.'/,.-:om
parallel feeders can be achieved by using ring
distribution · system. Here, • each distribution
tr~sformer is fed with two feeder~ but i~ different
paths. The feeders in this system form a loop which
starts from the substation bus-bars, runs through the
load area feeding distribution transformers and returns Distributor
to the i;;ubstation bus-bars. The following figure shows
a typical single line diagram of a ring main
distribution system.
Ring main distribution system is the most preferred due to its following advantages.
•
•
There are fewer voltage fluctuations at consumer's terminal.
The system is very reliable as each distribution transformer is fed with two feeders. That means, in the event of a fault in
any section of the feeder, the continuity of the supply is ensured from the alternative path,
Interconnected Distribution System
When a ring main feeder is e,,ergized by two or more substatioos or gen<111tini: stations, it is ~ailed .,'li(i il\_le,c.,,.Cct,;iJ-\listribution
system. This system ensures feij•BililY ln•an event of ""'1smiSSion failm:e. Also, ap.y area;fOd U'.Qm Q~; gene,ati)ig si:;,i;cin, during
peak load boucs can be fed from the otbe, generating station o, substation foe meeting power requirements from increased load.
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